Measurement of the Top Quark Pair Production Cross Section with the ATLAS Detector at the LHC. Andrea Bangert

Transcription

1 Measurement of the Top Quark Pair Production Cross Section with the ATLAS Detector at the LHC Andrea Bangert September 4, 2008

2 Contents 1 The ATLAS Detector CERN CERN Accelerators The Large Hadron Collider The Injection Chain Layout of the LHC LHC Magnets Cryogenics Radiofrequency Acceleration ATLAS Overview of the ATLAS Detector Physics Goals Kinematic Quantities Coördinate System Transverse Momentum and Transverse Energy Missing Transverse Energy Azimuthal and Polar Angles Rapidity and Pseudorapidity Primary Vertex and Impact Parameter Invariant and Transverse Mass Inner Detector Geometry Pixel Detector Semiconductor Tracker Transition Radiation Tracker Calorimeters Electromagnetic Calorimeter The Hadronic Calorimeter Cryostats and Support Structures Magnets

3 1.7.1 Central Solenoid Toroid Magnets Muon Spectrometer Geometry Alignment Tracking Chambers Muon Trigger Backgrounds Trigger Level 1 Trigger Level 2 Trigger Event Filter The Grid Detector Simulation Full Simulation Atlfast Luminosity Definition Machine Parameters The ALFA Detector Physics Processes

4 List of Figures 1.1 The LHC ring Main dipole cross section Magnet cooling system Diagram of the ATLAS experiment Particle signatures in the detector components The ATLAS coördinate System m(z ee) and m T (W eν) The Inner Detector ATLAS trigger and DAQ systems

5 List of Tables 1.1 LHC machine parameters

6 Chapter 1 The ATLAS Detector 1.1 CERN At the end of the Second World War a number of visionary scientists 1 proposed the creation of a European laboratory of atomic and nuclear physics. The Conseil Européen pour la Recherche Nucléaire (CERN) was established provisionally in 1952 with a mandate to develop a world class organization dedicated to fundamental research in nuclear and subnuclear physics. The founding states 2 ratified the CERN convention, and in 1954 the European Organization for Nuclear Research was formed. The goals delineated in the convention included promotion international collaboration among the European states, performance of fundamental research in nuclear and subnuclear physics for strictly peaceful purposes, and dissemination of information including the results of said research [73]. Geneva was chosen as the site of the future laboratory. CERN is governed by 20 European member states 3. Member states con- 1 French physicists Pierre Auger, Louis de Broglie, Raoul Dautry, and Lew Kowarski, the Italian Edoardo Amaldi, the Dane Niels Bohr, and the American Isidor Rabi were among the proponents of the new, collaborative international laboratory [73]. 2 The twelve founding states included Belgium, Denmark, France, the Federal Republic of Germany, Greece, Italy, the Netherlands, Norway, Sweden, Switzerland, the United Kingdom, and Yugoslavia [73]. 3 The CERN member states include Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, the Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland, and the United Kingdom. Observer states include India, Israel, Japan, the Russian Federation, Turkey and the USA. Nonmember states which are involved in CERN activities include Algeria, Argentina, Armenia, Australia, Azerbaijan, Belarus, Brazil, Canada, Chile, China, Colombia, Croatia, Cuba, Cyprus, Estonia, Georgia, Iceland, Iran, Ireland, Lithuania, Mexico, Mon- 5

7 tribute to the capital and operating costs of the CERN physics program, are represented in the council, and are responsible for making decisions about the organization and its activities. CERN dedicates the greatest portion of its budget to the construction and running of the accelerator facilities. The organization contributes only partly to the cost of design, construction, and operation of the experiments. Financial support for the experiments is provided primarily by funding agencies and institutions from both member and nonmember states. CERN employs 2,500 people. The scientific and technical staff designs, constructs, and oversees the operation of the particle accelerators and the extraction and preparation of data. 8,000 visiting scientists travel to CERN in order to perform research. These scientists represent 580 institutions and are of 85 different nationalities [73]. It is thanks to CERN that Europe has recovered the leading role in fundamental research which it enjoyed prior to World War II CERN Accelerators Synchrocyclotron and Proton Synchrotron The first CERN accelerator was constructed at the site in Geneva in The Synchrocyclotron provided a beam energy of 600 MeV and closed in 1990 after a remarkable 33 years of service [73]. In November of 1959 the Proton Synchrotron (PS) accelerated a proton beam to an energy of 28 GeV, and for a brief period enjoyed the status of the highest energy particle accelerator in existence. The Proton Synchrotron represents the world s most versatile accelerator [73]; since construction the intensity of the proton beam has increased by a factor of The PS is now the first circular machine in the accelerator chain which supplies protons to the Large Hadron Collider (Section 1.2). The first proton-proton collider was called the Intersecting Storage Rings. This machine was 300 m in diameter and was constructed in France on a piece of land adjoining the original Swiss laboratory site. In January 1971 the Proton Synchrotron was used to feed protons into the two interconnected rings, where collisions between the beams were engineered. tenegro, Morocco, New Zealand, Pakistan, Peru, Romania, Serbia, Slovenia, South Africa, South Korea, Taiwan, Thailand, Ukraine, and Vietnam [73]. 6

8 Super Proton Synchrotron The Super Proton Synchrotron (SPS) was the first of CERN s giant rings. It was constructed in a subterranean tunnel 7 km in circumference which crosses the French-Swiss border. The initial beam energy was 300 GeV; today the machine provides beams of 450 GeV. Although the Super Proton Synchrotron was initially constructed to collide protons, in 1979 measures were initiated which would convert it into a proton-antiproton collider. The first pp collisions occurred two years later, and the discovery of the W and Z bosons was announced by the UA1 and UA2 experiments in Carlo Rubbia and Simon van der Meer 4 received the Nobel prize in physics in 1984 in recognition of the discovery. Today the SPS is the last accelerator in the chain which provides proton beams to the Large Hadron Collider (Section 1.2). The Large Electron Positron Collider The Large Electron Positron (LEP) collider was designed to produce electronpositron collisions at a center of mass energy of 200 GeV. The construction of the collider represented an extremely ambitious industrial engineering project requiring excavation of a tunnel 27 km in circumference at an average depth of 100 m below the surface [73]. Four caverns were excavated in order to house the detectors, and tunnels were provided in order to enable the transfer of beams from the Super Proton Synchrotron to LEP. The excavation began in September 1983 and was completed in February 1988; the first collisions occurred in August The collider was shut down in 2000 in order to allow construction of the Large Hadron Collider within the LEP tunnel. 1.2 The Large Hadron Collider The Large Hadron Collider (LHC) is a superconducting proton-proton collider situated at CERN. The LHC is housed in a tunnel which was originally excavated in order to contain the LEP collider. The LEP tunnel is 27 km in circumference, lies an average of 100 meters below the ground [73] and crosses the border between Switzerland and France at two points. Figure 1.1 depicts the LEP ring and the four LHC detectors ATLAS, LHCb, CMS and ALICE. The LHC is designed to collide two proton beams with a center of mass 4 Carlo Rubbia was responsible for the conversion of the Super Proton Synchrotron into a proton-antiproton collider. Simon van der Meer invented the technique called stochastic cooling, which allowed the accumulation of antiprotons into a beam [73]. 7

9 Figure 1.1: Diagram of the Super Proton Synchrotron (SPS) and the LHC ring. The four detectors are ATLAS, LHCb, CMS, and ALICE. energy s = 14 TeV at a luminosity L = cm 2 s 1 (Section 1.12). At design luminosity bunches will contain up to protons and a 40 million bunch collisions will occur per second [1]. In addition, the machine is designed to collide heavy ions with a center of mass energy of 5.5 TeV per nucleon pair 5 and a luminosity of cm 2 s 1 [1]. The LHC machine parameters are listed in Table 1.1. At design luminosity 350 MJ of energy is stored in each proton beam. This represents an increase of two orders of magnitude with respect to previous colliders [74]. Excellent collimation systems are needed to protect machine and detectors from the beam halo. Furthermore, it is essential that safety systems can reliably abort the beams if necessary The Injection Chain The Proton Synchrotron (Section 1.1) is the first circular machine in the injector chain and provides 25 GeV proton beams. The beams are transferred from the PS to the Super Proton Synchrotron, which ramps the energy up to 450 GeV before injecting the beams into the LHC. After the LHC rings have 5 Collisions of lead nuclei will thus occur with a center of mass energy of 1150 TeV [74]. 8

10 Machine Parameters Reference Circumference 27 km [1] Operating temperature 1.9 K [74] Dipole field at 7 TeV 8.33 T [74] Stored energy in magnets 11 GJ [74] Proton energy during injection 0.45 TeV [74] Proton energy during collisions 7 TeV [74] Protons per bunch [1] Number of bunches 2808 [74] Bunch crossing rate 40 MHz [2] Orbit frequency khz [74] Beam current 0.56 A [74] Nominal bunch spacing ns [74] Design luminosity cm 2 s 1 [1] Stored energy per beam 350 MJ [74] Power radiated per beam 3.8 kw [74] Inelastic events per crossing 23 [1] Normalized emittance 3.75 µm [74] Nominal bunch width at interaction point 16 µm [4] β at interaction point 0.5 m [74] Crossing angle at interaction point 300 µrad [74] Table 1.1: LHC machine parameters. been filled, the beams are ramped to the nominal energy of 7 TeV during a time period of about 28 minutes Layout of the LHC The LHC ring is divided into eight arcs and eight straight sections. Each straight section is 528 m in length. Four of the straight sections house the ATLAS, LHCb, CMS, and ALICE detectors. The detectors are situated at collision points; these are the only locations where the proton beams cross. Two straight sections house the momentum collimation system and a betatron collimation system designed to remove beam halo. Two 400 MHz radio frequency systems occupy a further section, while the final straight section contains two beam abort systems which allow the beams to be extracted safely and directed into absorbers [74]. The arc sections of the LHC ring contain the main dipole magnets and short straight sections where other types of magnets are situated. The main dipole 9

11 magnets are 14.2 m long. The short straight sections are 6.6 m in length and contain the main quadrupole magnets, sextupole magnets designed to correct the chromaticity 6, and the orbit correction dipoles. They can also contain skew quadrupoles or octupoles which provide Landau damping 7. A lattice period is composed of six dipole magnets and two short straight sections; the lattice period is m long. Each arc contains 23 lattice periods [74] LHC Magnets The LHC employs more than 7,000 superconducting magnets. These range in size from 10 cm long octupole or decapole correctors to the 14 m main dipoles. The LHC also utilizes more than 100 warm magnets. A further 500 conventional warm magnets are employed in the 2.6 km long tunnel between the Super Proton Synchrotron and the LHC [74]. The LHC utilizes quadrupole magnets to provide alternating gradient focussing 8 [49]. The two proton rings are incorporated into a single magnetic structure which consists of two sets of coils in a common yoke and cryostat. The cross section of one of the main dipole magnets is displayed in Figure 1.2 [74] Cryogenics In order to bend the trajectory of extremely high energy protons so that it remains within the confines of the beampipe it is necessary that the superconducting magnets produce an 8.3 Tesla field. This extremely high field strength can only be achieved using niobium-titanium (NbTi) superconducting magnets if the magnets operate at extremely low temperatures. A temperature of 1.9 K is achieved by immersing the LHC magnets in a bath of superfluid helium. The cryogenic engineering needed to produce the necessary 100 tons of superfluid helium is unprecedented [74]. The heat capacity of the NbTi superconductor at a temperature of 1.9 K 6 The chromaticity of the beam indicates the range of energies of the constituent protons. A high degree of chromaticity can cause instabilities to arise when the beam is bent or focussed. 7 Landau damping is provided by strong octupole magnets and helps to alleviate beam instabilities [74]. 8 Two or more quadrupole magnets are arranged to focus alternately horizontally and vertically. The net efect of such a series of magnets on a proton beam is to cause the beam to converge. All current cyclotrons employ alternating gradient focussing. 10

12 Figure 1.2: Cross section of a main dipole magnet [74]. Figure 1.3: The magnet cooling scheme [74]. 11

13 is an order of magnitude smaller than the heat capacity of the same material at 4.5 K. Heat sources include synchrotron light radiated by the proton beams, image currents, energy dissipated during the development of electron clouds, and energy loss due to nuclear scattering [74]. If heat enters the system at 1.9 K the temperature rises much more quickly than it would at 4.5 K, which means that the magnets are extremely susceptable to quenching [74]. The LHC magnets are cooled with pressurized superfluid helium, which can be used to combat the very small heat capacity of the superconductor at low temperatures and thus to avoid quenching effects. Superfluid helium has a low viscosity, which allows it to permeate very small spaces. The insulation of the magnet coils was designed to be porous so that the superfluid helium can come in contact with the superconductor. The helium has a very large specific heat and a thermal conductivity which peaks at 1.9 K. The helium in the coil can therefore absorb undesired thermal loads and transport them away from the magnet. In order to catalyze the transition to superfluid helium one reduces the pressure on the helium bath. At a pressure of 50 mbar and a temperature of 2.17 K, gaseous helium condenses to form superfluid helium. The temperature can be further reduced to 1.9 K by lowering the pressure on the bath to 15 mbar. The superconducting magnets, however, are immersed in a helium bath at atmospheric pressure; this precludes lowering the pressure as a means of cooling the system. A linear heat exchanger containing superfluid helium at 15 mbar is therefore used to cool the pressurized helium which is in contact with the magnets. The diagram in Figure 1.3 depicts the magnet cooling scheme. The LHC ring is cooled by 8 cryogenic plants. Four of these refrigerators were constructed for LEP and refurbished; four were built for the LHC. The plants use liquid nitrogen as a precooler. A single cryogenic plant can cool one sector of the LHC in 15 days; warming a sector requires the same length of time [74] Radiofrequency Acceleration Two sets of superconducting radiofrequency cavities allow independent control of the two proton beams. The cavities are constructed of copper with a thin niobium deposit on the internal surface and operate at 400 MHz. While the beams are coasting, each radiofrequency system must provide a potential 12

14 difference of 16 MV. One such system consists of 8 single cell cavities, each of which provides 2 MV with a gradient of 5.5 MV/m. The radiofrequency hardware required by the LHC is much smaller than the apparatus required by LEP because the synchrotron power loss experienced by protons is much less than that experienced by electrons. However, controlling beam loading and radiofrequency noise is a challenge. The LHC is the first hadron collider to experience the effects of synchrotron radiation. The synchrotron radiation at design luminosity will produce 3.6 kw of power per beam 9 [74]. This quantity of radiation is quite small when compared to that produced by high energy lepton colliders, however it is radiated into a vulnerable cryogenic environment. Synchrotron radiation has heavily influenced the design of the vacuum and cryogenic systems employed by the LHC. 1.3 ATLAS Overview of the ATLAS Detector The ATLAS experiment is one of the four particle detectors situated at collision points about the LHC ring (see Figure 1.1). The purpose of the experiment is to provide a varied selection of information about the products of the proton collisions. All components must operate successfully in the harsh radiation environment associated with the high LHC design luminosity of L = cm 2 s 1. During each second of data-taking 10 to 100 interesting events must be selected by analyzing collision products while 1 billion uninteresting events must be discarded [70]. The various components of the ATLAS experiment occupy a series of concentric cylinders. The central portion is a tracking chamber called the inner detector, surrounded by a superconducting solenoidal magnet. In order of increasing radial position, subsequent components are the liquid argon electromagnetic calorimeter, the hadronic calorimeter, and the muon spectrometer, which is partially contained within a system of toroid magnets. ATLAS is about 50 m in length and 20 m in height; it is as tall as a five story building [70] and weighs about 7000 metric tons. The design, fabrication, validation and installation of the ATLAS experiment is the culmination 9 A circular accelerator with a ring of radius R collides particles of mass m and energy E. The energy loss per revolution due to synchrotron radiation is E synch 1 R ( E m )4 [51]. 13

15 of fifteen years of labor of several thousand physicists, engineers, technicians, and students. Its construction cost participating nations a total of 1.2 billion Swiss Francs [69]. Figure 1.4: The innermost portion of the ATLAS experiment is a tracking chamber enclosed by a superconducting solenoid. The inner detector is surrounded by a liquid argon electromagnetic calorimeter, a hadronic calorimeter, toroid magnets, and muon chambers [2]. Particle Signatures The various components of the ATLAS experiment provide complementary information about the decay products of a high energy collision. The inner detector records the tracks of charged particles such as e ±, µ ±, π ± and K ±. The presence of a strong magnetic field causes the tracks to be deflected, which quantifies the momentum 10 and determines the sign of the charge. Neutral particles such as photons, neutrons, π 0 and K 0 leave no tracks in the inner detector. Electrons and photons loose their energy in the electromagnetic calorimeter via bremsstrahlung and photon pair production, producing an electromagnetic shower. The electromagnetic calorimeter thus provides electron and 10 Let p represent the magnitude of the momentum of a particle of charge q. The quantity R represents the radius of curvature of the particle trajectory under the influence of a magnetic field of magniture B. The cyclotron formula then states that p = q B R [9]. 14

16 Figure 1.5: The most central component records the tracks of charged particles; neutral particles do not interact in the inner detector. Electrons and photons produce an electromagnetic shower in the electromagnetic calorimeter. Hadrons deposit energy primarily in the hadronic calorimeter. Muons produce tracks in the inner detector but do not interact strongly in the calorimeters, instead continuing to the muon spectrometer [70]. 15

17 photon identification and measures energy and direction of flight. A fraction of the energy of charged hadrons is deposited in the electromagnetic calorimeter, but most of the hadron energy is deposited subsequently in the hadronic calorimeter. The hadronic calorimeter provides measurements of jet energy and direction of flight. Muons are charged particles and therefore produce tracks in the inner detector. However, muons do not interact strongly with the heavy nuclei in the electromagnetic or hadronic calorimeters. Instead they continue on to the muon chambers. The muon chamber measures the curvature of the muon track in a high magnetic field and thus determines the muon momentum 11 and the sign of the muon charge. Muons are nominally the only particles which continue through the detector to reach and be detected by the muon chambers. The hermetic calorimeter system allows reconstruction of the missing transverse energy carried by neutrinos, which do not interact with the detector material (see Section 1.4.3). Figure 1.5 shows a diagram of particle signatures in the various detector components. Particle Lifetimes Particles which participate in typical electromagnetic or strong decays such as π 0, ρ 0, and ρ ±, as well as extremely massive particles such as Z 0, W ±, and the top quark decay almost instantaneously. Only the products of such decays exit the beampipe and are visible in the detector. In contrast, particles with a lifetime τ O(10 12 s) such as the beautiful and charming mesons B 0, B ±, D 0, and D ± and the tau lepton may travel a distance cτ 100 µm before decaying. This results in a secondary decay vertex which is displaced from the primary interaction vertex. The displacement can be measured by extremely precise silicon detectors situated close to the beampipe and can be used to identify particles of interest. Quasi-stable particles with lifetime τ O(10 10 s) include muons, charged pions and kaons π ± and K ±, and neutral hadrons such as the neutron or the Λ. Stable particles include electrons e ±, protons, and the photon. Both sta- 11 According to the cyclotron formula the trajectory of a muon with large momentum will not curve very much under the influence of a magnetic field, whereas the trajectory of a muon with very little momentum will be highly curved. Furthermore the trajectories of a µ + and a µ will curve in opposite directions. 16

18 ble and quasi-stable particles will interact with components of the detector which are more distant from the beam axis such as calorimeters and muon spectrometer [50] Physics Goals The most important goal of the ATLAS collaboration is to perform measurements which will lead to an understanding of the mechanism of electroweak symmetry breaking [5]. Although detection of the Higgs boson will provide an experimental challenge, the ATLAS detector was designed to be sensitive to the signature of the Higgs over the full range of permitted masses [5]. If Supersymmetry exists at the electroweak scale, plentiful production of squarks and gluinos is expected. The challenge in this case would not be the discovery of Supersymmetric signatures, but would instead consist of determining which of many competing models is favored [5]. The ATLAS detector is also intended to search for particles predicted by technicolor theories, for as yet undiscovered gauge bosons, and for evidence of composite quarks and leptons. Precision measurements of the W boson and top quark masses will increase constraints on the mass of the Standard Model Higgs. Measurements of the triple gauge boson couplings, the investigation of CP violation in B decays, and detailed understanding of the QCD processes which form the dominant backgrounds to searches for new phenomena are also important components of the ATLAS physics program [5]. 1.4 Kinematic Quantities Coördinate System The ATLAS experiment utilizes a right-handed coördinate system. The positive x axis is defined to be horizontal and to point from the origin of the detector towards the center of the LHC ring, while the positive y-axis points upwards [2]. The z axis is superimposed upon the trajectory of the counterclockwise proton beam. The x and y axes define the plane perpendicular to the beam axis, which nominally intersects the x-y plane at the point (x, y) = (0, 0) Transverse Momentum and Transverse Energy The momentum vector p = (p x, p y, p z ) of an object in the detector is uniquely defined by its magnitude p, polar angle θ, and azimuthal angle φ, 17

19 Figure 1.6: The ATLAS coördinate system. 18

20 p = ( p sin θ cos φ, p sin θ sin φ, p cos θ) The polar angle θ is defined to be the angle between the z axis and the momentum vector p (see Figure 1.6). The component of p along the z axis is therefore p z = p ẑ = p cos θ. The transverse momentum p T is the projection of the momentum vector p onto the x-y plane, p T = (p x, p y ). The azimuthal angle φ is defined to be the angle between the x axis and the transverse momentum p T such that p T ˆx = p T cos φ = p x = p sin θ cos φ The magnitude p T of the transverse momentum is therefore p T = p T = p sin θ The x and y components of the momentum vector p are thus p x = p T cos φ and p y = p T sin φ, so that p = ( p T cos φ, p T sin φ, p cos θ) In analogy to the transverse momentum p T, the transverse energy is defined to be E T = (E x, E y ) and has magnitude E T = E T = E sin θ Missing Transverse Energy The colliding protons have equal and opposite longitudinal momenta and the transverse momentum of the proton-proton system is approximately zero. The sum of the transverse energies of the initial state partons in a protonproton collision is also zero 12. If energy is conserved then the net transverse energy of the products of the collision will also be zero. However, not all of the final state particles may be observed in the detector. Neutrinos do not interact with the material of the detector, and thus depart the system carrying a quantity of transverse energy E T (ν). The quantity and direction 12 If no initial state radiation occurs then the transverse energy of each of the two incoming partons is approximately zero. However, if initial state radition occurs then the parton which emitted the radiation recoils against the emitted gluon and the transverse energy of each of the partons involved is thus nonzero. In this case only the vector sum of the transverse energies of the incoming partons is zero. 19

21 of the tranverse energy carried by the neutrino can be reconstructed if the assumption is made that transverse energy is conserved and that any missing transverse energy should be attributed to unheralded departure of the neutrino. This reconstruction assumes that the calorimeter is hermetic. The missing transverse energy E/ T is defined to be the vector opposite in direction and equal in magnitude to the vector sum of the transverse energy in all calorimeter towers with η < 3.6. Let ˆn i be a unit vector perpendicular to the beam axis and pointing towards the i-th calorimeter tower. The missing transverse energy is then E/ T = Σ i E Ti ˆn i = (E/ x, E/ y ) The index i runs over all calorimeter towers fulfilling the requirement placed on η. The magnitude of the missing transverse energy is E/ T = E/ T = (E/ x ) 2 + (E/ y ) Azimuthal and Polar Angles The azimuthal angle φ sweeps out the x-y plane, and is defined to be tan φ = p x p y The polar angle θ is the angle between the beam axis and the momentum vector p and is defined to be tan θ = p T p z Rapidity and Pseudorapidity The proton is a composite object. The colliding partons within the proton carry some fraction of the proton momentum, and the partonic momenta are not known a priori. At a hadron collider the laboratory frame and the partonic center of mass frame thus do not coincide: instead, the partonic center of mass frame is boosted along the beam axis. This longitudinal boost is most easily taken into account by describing four-momenta in terms of the rapidity y rather than the scattering angle θ. The rapidity of an object in the detector is defined to be y = 1 2 ln E + p z E p z 20

22 Differences in rapidity remain invariant under an arbitrary boost along the z axis 13 [19]. The transverse momentum p T is also invariant under a boost along the z axis [51]. In situations where it is necessary to transform between various systems which are boosted parallel to the z axis it is thus expedient to use rapidity y and transverse momentum p T to describe the particle kinematics rather than the momentum vector p. Furthermore, in most high energy hadron collisions 14 the distribution of the final state particles is approximately uniform in rapidity 15. In the massless limit, E p such that y 1 2 ln 1 + cos θ 1 cos θ = ln [cot θ 2 ] The pseudorapidity η of an object in the detector is defined to be [50] η = ln [cot θ 2 ] = ln [tan θ 2 ] The pseudorapidity is thus a one-to-one transformation of the polar angle π θ 0 for < η <. In the low mass limit m p T the pseudorapidity is equal to the rapidity. The polar angle θ and hence the pseudorapidity are experimentally accessible quantities. However, the variable of physical significance is the rapidity [19]. A distance R in η φ space is defined by R = ( η) 2 + ( φ) 2 The separation R between two objects is invariant under longitudinal boosts [50]. 13 The rapidity of a nonrelativistic particle is equal to the velocity parallel to the z axis, y 1 2 ln m + mv z m mv z v z Nonrelativistic velocities transform additively under boosts. The nonlinear change of variable from velocity to rapidity ensures that this additivity applies to relativistic particles as well, albeit only in the favored z direction [19]. 14 Most high energy hadron collisions are minimum bias events. 15 In e + e collisions, in contrast, the most interesting events tend to be distributed uniformly with respect to the solid angle Ω rather than uniformly with respect to the rapidity [19]. 21

23 Figure 1.7: Invariant mass distribution for Z e + e and transverse mass distribution for W eν. Data was collected by the CDF collaboration at the Fermilab Tevatron using an integrated luminosity L = 2.4 fb 1 [84] Primary Vertex and Impact Parameter In a reconstructed event the interaction vertex z P V is defined to be the origin of the physics coördinate system, whereas the center of the detector is the origin of the detector coördinate system [?]. The longitudinal impact parameter z 0 is defined to be the z coordinate at the point of closest approach Invariant and Transverse Mass Invariant Mass Consider the resonant production of a Z boson qq Z ll. The square of the dilepton invariant mass m ll is m 2 ll = (p 1 + p 2 ) 2 where p 1 and p 2 are the four-momenta of leptons l 1 and l 2. Then m ll = (p 1 + p 2 ) 2 = (E 1 + E 2 ) 2 ( p 1 + p 2 ) 2 The distribution of the dilepton invariant mass m ll peaks near the pole mass m Z of the Z boson 16. If the momenta of the decay products can be 16 Consider the s-channel process q 1 q 2 V q 3 q 4. The Breit-Wigner resonance R(ŝ) represents the propagator of an unstable particle V of mass M and width Γ [50], R(ŝ) = 1 (ŝ M 2 ) 2 + Γ 2 M 2 The amplitude for the process develops a kinematic peak near the pole mass M, (p 1 + p 2 ) 2 = (p 3 + p 4 ) 2 M 2 22

24 completely reconstructed, the invariant mass is the most effective observable which can be used to discover a resonance such as the Z boson [50]. In the rest frame of the Z the distribution of the energy of each of the decay products peaks at E m Z / 2. The distribution of the transverse momentum of each lepton also demonstrates an enhancement at p T = m Z / 2 which is termed the Jacobian peak [50]. Although the invariant mass is not affected by possible motion of the Z boson, the distribution of the transverse momenta of the decay products is shifted if the Z is not produced at rest in the lab frame. One can generalize the invariant mass to a multibody system such as a top quark decay, t W b jjb, m jjb = Transverse Mass (p j1 + p j2 + p b ) 2 = (E j1 + E j2 + E b ) 2 ( p j1 + p j2 + p b ) 2 Let us now consider the leptonic decay W lν of the W boson. The momentum of the neutrino cannot be completely reconstructed and it is thus not possible to measure the invariant mass. It is, however, permissible to ignore the longitudinal momentum of the system and to consider only the motion in the transverse plane. In analogy to the dilepton invariant mass m ll, the transverse mass m T lν of the decay products of the W boson is then defined to be [51] m T lν = (E T l + E T ν ) 2 ( p T l p T ν ) 2 If one assumes that the missing transverse energy of the system is due entirely to the departure of the energetic neutrino, then E T (ν) = E/ T and p T (ν) = E/ T. Then m T lν = (E T l + E/ T ) 2 ( p T l E/ T ) 2 The definition of the transverse mass is analogous to the definition of the invariant mass but neglects the contributions from the longitudinal momenta of the lepton and the neutrino. In general the transverse mass is less than the invariant mass, 0 m T lν m lν. In the rest frame of the W boson the distributions of the transverse energies of 23

25 lepton and neutrino are enhanced at E T l E/ T m W / 2. The Breit-Wigner resonance in the distribution of the invariant mass for m lν = m W ensures that the transverse mass also demonstrates a Jacobian peak at m T lν = m W [51]. In the narrow width approximation the distribution of the transverse mass m T lν drops sharply at m W. In praxis, the distribution extends beyond the W mass because the width Γ W of the W boson is finite [50]. The concept of transverse mass can be extended to a three-body decay such as a top quark decay t W b lνb, m T lνb = (E T l + E/ T + E T b ) 2 ( p T l E/ T p T b ) Inner Detector The inner detector is the component of ATLAS situated closest to the beampipe (see Figure 1.8). High resolution semiconductor pixel and strip detectors occupy the most central portion of the inner detector. The transition radiation tracker encloses the semiconductor detectors and provides continuous tracking. A thin superconducting solenoid surrounds the inner detector cavity and maintains the 2 T solenoidal magnetic field. Full tracking coverage is provided over the pseudorapidity range η < 2.5. The pixel detector delivers three precision measurements, while the semiconductor tracker measures four spatial points and the transition radiation tracker typically provides thirty-six measurements. Pattern recognition algorithms are utilized to determine helical particle trajectories and then to calculate direction of flight, momentum and the sign of the charge. In addition, the inner detector provides electron identification, measures the impact parameter d 0, and provides vertexing capabilities which allow identification of B hadrons and τ leptons. The transition radiation tracker provides additional discrimination between electrons and hadrons. The precision provided by the various detector elements is comparable, and no single detector component dominates the resolution associated with the momentum measurement. This results in robust performance capabilities Geometry The inner detector occupies a cylinder 7 m in length and 115 cm in radius, permeated by a magnetic field of 2 T directed parallel to the beam axis. The barrel portion of the detector occupies the region z < 80 cm while two endcaps extend from 80 cm < z < 350 cm. The high resolution semiconductor 24

26 Figure 1.8: The ATLAS Inner Detector. pixel and strip detectors are contained within the radius r = 56 cm while the transition radiation tracker extends from 56 cm < r < 115 cm. In the barrel region the detector elements are arranged in concentric cylinders parallel to the beamline. In the end caps, the detectors are mounted radially on disks perpendicular to the z axis. The pixel detector and semiconductor tracker are operated cold. Heat generated by electronics and leakage current is removed. Mechanical structures which support the elements of the detector were chosen to have small coefficients of thermal expansion [2] Pixel Detector The most precise measurements are delivered by the high granularity pixel detector, which is situated very close to the beam pipe. The detector contains 140 million pixels, each of which is µm 2 in dimension [70]. The number of pixel layers is constrained by the high density of the material and the considerable cost of the technology. Each charged track traverses three pixel layers, which provides three precision measurements [2]. The three barrel layers occupy concentric cylinders arrayed about the beam pipe and are situated at radii r 4 cm, r 10 cm, and r 13 cm. The thickness of each barrel layer is about radiation lengths [2]. The solenoidal magnetic field which permeates the inner detector is parallel to the beamline. The particle trajectory is approximately radial, so v B is proportional to ˆr ẑ = ˆφ and charged particles are deflected tan- 25

27 gentially 17. In the barrel region the pixels are placed such that their longest dimension is parallel to the z axis while the shortest dimension is tangential. Precision measurements of the Rφ coördinate can thus be made while the measurement in the z direction is less precise. The resolution provided by the pixel detector is (Rφ) z = µm 2 [2]. The readout chips used in the pixel detector are of large area; each chip serves an array of pixels and provides an individual circuit for each pixel. The chips are radiation hard and must withstand more than 300 kgy of ionizing radiation and a neutron equivalent dose of at least neutrons /cm 2 during ten years of LHC operation [2]. The pixel detector determines the resolution associated with the measurement of the impact parameter z 0. It also enables identification of secondary vertices, which provide a means of recognizing B hadrons and tau leptons. The measurement of secondary vertices is enhanced by the innermost layer of the pixel detector, which is situated at a radius of 4 cm from the beam line. The lifetime of this layer is limited due to radiation damage and will be determined by the luminosity profile achieved during operation of the LHC. The design of the pixel detector allows replacement of the B layer [2] Semiconductor Tracker The semiconductor tracker surrounds the pixel detector. This component of the inner detector contributes to the measurement of momentum, impact parameter and vertex position, and also provides good pattern recognition. In the barrel the readout strips are 80 µm in width and several centimeters in length. Each p-on-n silicon sensor utilizes 768 readout strips and has a surface area of cm 2. Each module contains four sensors. Two sensors are bonded to form a strip 12.8 cm in length; two such strips are separated by a heat transport plate and fixed together at a 40 mrad angle [2]. In the barrel portion of the inner detector the strips are placed such that their longest dimension is parallel to the z axis. The endcap modules are similar in construction, but utilize tapered strips which are situated radially. The four barrel layers are located at r = 30.0 cm, r = 37.3 cm, r = 44.7 cm, and r = 52.0 cm. Typically eight strip layers are crossed by each track, which provides four precision measurements of the Rφ and z coördinates [2]. The 17 F = q( v B) 26

28 spatial resolution is (Rφ) z = µm 2. Two tracks can be distinguished if they are separated by more than 200 µm. The surface area of the semiconductor tracker is 61 m 2. The tracker must withstand levels of radiation which will alter the fundamental characteristics of the silicon sensors. Modules containing both silicon wafers and front end electronics have been irradiated to the level expected after ten years of LHC operation and have been shown to provide the required functionality [2] Transition Radiation Tracker The transition radiation tracker is a collection of drift tube detectors. The tubes are gas filled straws 4 mm in diameter which contain a gold plated tungsten and rhenium wire of 30 µm diameter. The gas mixture contains 70% xenon, 20% CO 2 and 10% CF 4 ; the total gas volume is 3 m 3 [2]. The use of xenon allows detection of transition radiation photons and enhances the electron identification capabilities of the ATLAS experiment. There are two independent thresholds which allow the detector to discriminate between tracking hits and transition radiation. In the barrel the rate for hits above the lower threshold varies between 6 MHz and 18 MHz as a function of the radius. The maximum rate for hits above the higher threshold is 1 MHz [2]. Because the diameter of the wires is small and the sensory wires are isolated within small gas volumes, the transition radiation tracker can operate at the high rates observed at the LHC. The tracker occupies a cylindrical volume of inner radius 56 cm and outer radius 107 cm. In the barrel region the straws are parallel to the beam pipe; in the endcap sections the straws are positioned radially. The barrel contains 50,000 straws while the endcaps contain 320,000 straws. The spatial resolution on measurements performed by the tracker is about 170 µm for a single straw at a hit rate of 12 MHz [2]. The large number of spatial measurements per track allows a total precision of better than 50 µm at LHC design luminosity, which includes a systematic error of about 30 µm due to alignment. The transition radiation tracker typically measures 36 spatial points per track. The gas tube technology thus achieves continuous tracking with less material density and lower cost than that required by the pixel and silicon strip detectors. The measurements performed by the transition radiation tracker at large radii contribute significantly to the momentum measurement because a large number of spatial points can be determined. 27

29 1.6 Calorimeters The ATLAS calorimetry consists of electromagnetic and hadronic barrel calorimeters, electromagnetic and hadronic endcap calorimeters, and two forward calorimeters. The electromagnetic calorimeter provides electron and photon identification and measurements with excellent position and energy resolution. The hadronic calorimeter provides accurate measurements of jet energy and reconstruction of the missing transverse energy E/ T Electromagnetic Calorimeter The lead and liquid argon electromagnetic sampling calorimeter provides excellent energy and position resolution for electrons and photons within the pseudorapidity range η < 3.2. The electromagnetic calorimeter occupies a cylinder m in length with an outer radius of 2.25 m. It consists of thin lead plates of a thickness which varies as a function of the pseudorapidity η, separated by liquid argon gaps which in the barrel calorimeter are of constant thickness 2.1 mm. Presampling Detector The total amount of material traversed by a particle before entering the calorimeter is 2.3 radiation lengths X 0 at η = 0 and increases as a function of the pseudorapidity. Within the range η < 1.8 the electromagnetic calorimeter is therefore preceded by a presampling detector. The presampler is installed directly behind the cryostat wall and is used to correct for energy lost by electrons and photons while traversing the inner detector, cryostats, and solenoid coil prior to entering the calorimeters. The granularity of the presampler is η φ = Geometry The electromagnetic calorimeter is divided into a barrel portion and two endcaps. The barrel calorimeter is divided into two half barrels separated at z = 0 by a 6 mm gap. Each half barrel extends over the pseudorapidity range 0 < η < The endcap calorimeter consists of two coaxial wheels which occupy pseudorapidity ranges < η < 2.5 and 2.5 < η < 3.2. The amount of material in front of the calorimeter is maximal at the boundary between the barrel and endcap calorimeters and is equal to about 7 radiation lengths in this region. Due to the large amount of material traversed 28

30 before entering the calorimeter, the pseudorapidity range 1.37 < η < 1.52 is not suitable for performing precision measurements. The region η < 2.5 is devoted to the performance of precision measurements. In this region the electromagnetic calorimeter is divided into three layers. The first layer acts as a preshower detector. This layer has a constant thickness of 6 radiation lengths as a function of η. It has a granularity of η φ = , and thus provides a precise measurement of the pseudorapidity η. It also enhances the ability to distinguish between photons, pions and electrons [2]. The second layer is segmented into towers of size η φ = (At η = 0 this corresponds to 4 4 cm 2.) The first and second layers have a total thickness of about 24 radiation lengths which decreases as a function of η. The final layer has a granularity of η φ = and a thickness of 2 radiation lengths, which increases as a function of η. Cryostat The barrel electromagnetic calorimeter occupies a barrel cryostat. In order to minimize the material in front of the electromagnetic calorimeter, the central solenoid and the liquid argon calorimeter share a vacuum vessel The Hadronic Calorimeter The hadronic calorimeter surrounds the electromagnetic calorimeter and provides measurements of jet energy and missing transverse energy. It is required to contain hadron showers induced by protons, neutrons, pions, and kaons, and to prevent hadrons from continuing through the calorimeter to enter the muon chambers. Highly energetic hadrons react within the steel absorber plates, producing a hadronic shower. Protons and neutrons are ejected from the nuclei of the absorber plates and less massive hadrons are produced. The scintillating tiles then emit a quantity of light proportional to the energy of the incident hadron. Because the hadron shower can begin in the electromagnetic calorimeter, information from both calorimeters must be combined in order to determine the energy of the incident particle [70]. At η = 0 the hadronic calorimeter has a thickness of 11 interaction lengths λ. This has been shown by measurements and simulation to reduce the hadronic punch-through to well below the rates for decay muons [2]. A thickness of 29

31 10λ for the active material is sufficient to provide good resolution on measurement of the properties of high energy jets [2]. A good resolution on measurements of jet energy together with the large range in pseudorapidity allow determination of the missing transverse energy E/ T (see Section 1.4.3). Geometry The barrel calorimeters occupy the range η < 1.7. The hadronic endcap calorimeter extends from 1.5 < η < 3.2 while the dense forward calorimeter occupies the range 3.1 < η < 4.9. The electromagnetic endcap calorimeter, the hadronic endcap calorimeter and the forward calorimeter are housed within the same cryostat. Scintillator-tile Calorimeter The hadronic barrel calorimeter is a sampling calorimeter comprised of plastic scintillator tiles of 3 mm thickness embedded in steel absorbers. It occupies a cylinder of inner radius 2.28 m, outer radius 4.25 m, and length m. It is segmented into a barrel which covers η < 1.0 and an extended barrel which occupies the region 0.8 < η < 1.7. The scintillating tile calorimeter consists of three consecutive layers which at η = 0 are 1.4λ, 4.0λ, and 1.8λ thick, respectively. The granularity of the first two layers is η φ = ; that of the third layer is η φ = The electromagnetic calorimeter has a thickness of 1.2λ. The hadronic barrel calorimeter has a thickness of 9.2λ at η = 0 and the outer support has a thickness of 1.5λ [2]. Hadrons can initiate a shower in the electromagnetic calorimeter, and signals from both electromagnetic and hadronic calorimeters must be combined in order to obtain the full hadron energy. Liquid Argon Endcaps Scintillating tiles are damaged by excessive exposure to radiation, whereas the liquid argon technology is intrinsically radiation hard. The radiation emanating from the collision point is most intense at large rapidities and is therefore most intense in the endcap regions. In the extremely hostile radiation environment of the endcaps, the liquid argon technology is used for both electromagnetic and hadronic calorimeters. The hadronic endcap calorimeter has a parallel plate geometry and is composed of copper plates with liquid argon gaps. Each endcap consists of two coaxial wheels of 2.03 m radius. The wheel closest to the interaction point 30

32 is constructed of 25 mm copper plates while the wheel furthest from the interaction point is constructed of 50 mm plates. The 8.5 mm gap between consecutive plates is split by three parallel electrodes into four drift chambers. The first wheel weighs 67 metric tons, while the second weighs 90 tons. Forward Calorimeters The forward calorimeter extends the coverage of the calorimetry in η, provides uniformity of coverage and reduces the level of radiation experienced by the muon spectrometer. The front face of the liquid argon forward calorimeter is 4.7 m from the interaction point, and this calorimeter must operate in an extremely hostile radiation environment. The forward calorimeter is partitioned into three sections. The first is composed of a copper matrix with regularly spaced longitudinal grounded tubes equipped with high voltage rods. The sensitive medium is liquid argon and fills the gaps, which are as small as 250 µm. The second and third sections utilize a tungsten matrix. The net density of a typical section of the tungsten matrix forward calorimeter is 14.5 g/cm 3 ; this high density design prevents leackage of energy into neighboring elements of the detector Cryostats and Support Structures One set of cryostats house the liquid argon hadronic endcaps, the electromagnetic endcaps, and the forward calorimeters. The iron flux-return yoke of the central solenoid is integrated into the tile calorimeter support structure. The total weight of the calorimeter system, including the liquid argon electromagnetic calorimeter, hadronic scintillatortile calorimeter, electromagnetic and hadronic endcaps, forward calorimeters, and the iron solenoid flux-return yoke, is about 4000 metric tons. 1.7 Magnets The magnetic system consists of a thin superconducting solenoid which provides a magnetic field to the inner detector, as well as a system of large superconducting air-core toroids which provide a magnetic field to the muon system. The magnets are cooled via forced flow of helium at a temperature of 4.5 K. 31

33 1.7.1 Central Solenoid The central solenoid is 5.3 m in length, 1.2 m in radius, and surrounds the cavity in which the inner detector is housed. The solenoid provides a central field of 2 T parallel to the beam line with a peak field of 2.6 T at the surface of the superconductor. The solenoid is provided with an 8 ka power supply. It stores 38 MJ of energy, and is equipped with a quench protection system designed to dissipate stored energy without overheating the coil Toroid Magnets A system of three large air-core toroids provides a magnetic field of 4 T to the muon spectrometer. Each of the toroids consists of eight racetrack, doublepancake coils of aluminum stabilized NbTi superconductor which are housed in separate cryostats and are provided with a 21 ka power supply. The coils are arranged radially with eight-fold symmetry about the beam axis. The two end-cap toroids are located within the barrel toroid and line up with the central solenoid. The barrel toroid with its two end-cap magnets is 26 m in length and 20 m in diameter, and generates a large magnetic volume within a light structural framework. The peak magnetic field at the superconductor is 3.9 T in the barrel region and 4.1 T in the endcap toroids. The barrel toroid provides about 2 Tm of bending power in the pseudorapidity range 0.0 < η < 1.3 and 6 Tm in the range 1.6 < η < 2.7. The endcap toroid provides 4 Tm of bending power in the range 0.0 < η < 1.3 and 8 Tm in the range 1.6 < η < 2.7. The bending power is lower in the transition region 1.3 < η < 1.6, where the barrel and toroid magnets overlap. The barrel toroid and the two endcaps store a total of 1492 MJ of energy, and are equipped with control systems which enable fast and slow energy dumps. 1.8 Muon Spectrometer The calorimeter system is surrounded by the muon spectrometer, which is instrumented with trigger (section 1.8.4) and tracking chambers (section 1.8.3). The trigger chambers provide trigger capabilities and identify bunch crossings [2]. The tracking chambers determine the sign of the muon charge and perform high-precision measurements of the muon momentum by measuring the deflection of the track under the influence of the toroidal magnetic field. 32

34 Electrons and photons are absorbed by the electromagnetic calorimeter and do not enter the hadronic calorimeter [70]. A muon of energy 5 GeV will penetrate five meters of steel, whereas a hadron jet will typically be absorbed before it traverses 1.5 meters of the same material. Muons are thus the only charged particle which can traverse all calorimeter layers to enter the muon chambers. Any energetic particle emerging from the calorimeter whose track originates close to the collision point is thus identified as a muon [70]. The spectrometer instrumentation is designed to operate under a large particle flux. It must also function under difficult background conditions including the products of the primary collision as well as soft neutrons and photons produced during secondary interactions in the calorimeters, the shielding material, and the beam pipe [2] Geometry In the region η < 1 the muon chambers are divided into three concentric cylindrical layers of radii r 5 m, r 7.5 m, and r 10 m, centered on the beam axis. In the pseudorapidity region 1 < η < 2.7 the endcap chambers consist of four disks concentric with the beam axis and situated between 7 m and 23 m from the interaction point. The outer radius of the muon spectrometer is 11 m; the outermost layer of the muon chambers is about 46 m in length. All of the barrel chambers and a portion of the endcap chambers are supported by the barrel toroid structure. The endcap chambers are mounted on support structures at either end of the ATLAS cavern. Deformations and changes in the position of the elements of the muon spectrometer are monitored constantly via optical alignment systems. Track-based alignment will also be performed. Within the region η < 1.0 the magnetic field is provided by the barrel toroid. The pseudorapidity range 1.0 < η < 1.4 represents the transition region in which the magnetic deflection is attributed to both barrel and endcap magnets. Within the region 1.4 < η < 2.7 the necessary field is provided by the toroid endcap magnets. The toroidal field is roughly tangential (ˆφ) and is thus typically orthogonal to the muon trajectory [2]. Within the barrel region the muon trajectory tends to be radial. The deflection is thus primarily in the direction of v B or ˆr ˆφ = ẑ, ie parallel to the beam pipe. In the endcap region the velocity has a large z component; v B therefore has a large radial component, and the deflection tends to be radial. 33

35 1.8.2 Alignment The survey of the chambers at installation insured an accuracy of approximately 1 mm with respect to the relative spatial positioning of the projective towers [2]. Physics considerations require an accuracy of 30 µm with respect to the position of chambers within a projective tower. However, the muon spectrometer is 22 m in height and 46 m long. Due to the large dimensions of the system it is impossible to stabilize the dimension and position of the chambers globally within the desired accuracy of 30 µm [2]. Chamber deformations and spatial positions are monitored by an optical alignment system. Displacements of up to 1 cm can be corrected for during the offline reconstruction. Track based alignment will be used to calibrate the optical survey. The relative alignment of the inner detector, the calorimeters and the muon spectrometer will be determined by measuring the trajectories of muons with large transverse momenta Tracking Chambers Precision measurements of the track coördinates in the direction of the deflection are provided by monitored drift tubes. In the barrel region the monitored drift tubes measure the z coördinate; in the transition and endcap regions they measure the radial coordinate. In the innermost layer cathode strip chambers are installed at large pseudorapidities (2 < η < 2.7), because these are best equipped to deal with the high flux and the demanding background conditions [2]. The barrel chambers are rectangular in shape with surface area between 2 and 10 m 2. The endcap chambers are trapezoidal in shape and are of area 1 to 10 m 2. In the two lowest barrel layers the calorimeter supports necessitate individually shaped tracking chambers. The muon spectrometer incorporates a total of 1194 monitored drift chambers and 32 cathode strip chambers [2]. Monitored Drift Tubes The monitored drift tubes are aluminum tubes of 30 mm diameter which contain a central tungsten-rhenium wire of 50 µm diameter. The tube lengths vary from 70 cm to 630 cm. The tubes contain a mixture of 93% argon and 7% CO 2 at a pressure of 3 bar and contain a total gas volume of 800 m 3. The 34

36 maximum drift time is about 700 ns and the single wire resolution is about 80 µm [2]. Support structures maintain the positions of the drift tubes. In the barrel region the tubes are placed horizontally and the wires sag slightly under the influence of gravity. In this region the support structures are therefore designed to bend each tube slightly such that the cylindrical shell remains parallel to the wire [2]. Mechanical deformations are monitored by an optical system, hence the name monitored drift tubes. Cathode Strip Chambers The cathode strip chambers are multiwire proportional chambers with cathode strip readout. The anode-cathode spacing is equal to the pitch of the anode wire [2]. The gas used in the cathode strip chambers is a mixture of 30% argon, 50% CO 2 and 20% CF 4 with a total volume of 1.1 m 3 [2]. The precision coördinate is measured by determining the charge induced on the segmented cathode due to the avalanche which forms on the anode. The electron drift time is 30 ns, the time resolution is 7 ns, and the neutron sensitivity is low [2]. The resolution of the measurement delivered by the cathode strip chambers is about 60 µm [2]. The spatial resolution is sensitive to the inclination of incoming tracks. The chambers are therefore positioned such that a linear track originating at the interaction point will be normal to the chamber surface at the point of incidence [2] Muon Trigger The trigger system covers the pseudorapidity range η < 2.4. This system triggers on muons with transverse momentum greater than a cutoff pt cut. It also distingushes between bunch crossings, which occur every 25 ns. Finally, it provides a measurement of the coördinate orthogonal to that measured by the precision tracking chambers. A typical resolution for this second measurement is 10 mm. Three layers of resistive plate chambers are implemented in the barrel; thin gap chambers are utilized in the endcaps. Resistive Plate Chambers The resistive plate chambers are gas detectors which provide a spatial and temporal resolution of 1 cm 1 ns. A narrow gap is constructed using two parallel 2 mm thick bakelite plates separated by insulating polycarbonate spacers. The spacers are 2 mm thick and define the size of the gas gap. A 35

37 frame of the same polycarbonate material is used to seal the four edges of the gas gap. The chambers contain primarily tetrafluoroethane (C 2 H 2 F 4 ) with a small admixture of SF 6, which allows operation at a relatively low voltage [2]. A uniform 4.5 kv/mm electric field causes a primary ionization electron to initiate an avalanche, resulting in a pulse of about 0.5 pc. The signal is read out by metal strips positioned along the side of the detector. The η strips are parallel to the wires in the monitored drift tubes and measure the coördinate in the direction of the deflection; φ strips provide a measurement of the second coördinate. Thin Gap Chambers The thin gap chambers resemble multiwire proportional chambers, except that the cathode-anode separation is shorter than the pitch of the anode wire [2]. The anode wires are positioned parallel to the wires in the monitored drift tubes. Readout strips which measure the second coördinate are situated orthogonal to the anode wires. The gas gap is 2.8 mm wide, the pitch of the anode wire is 1.8 mm, and the diameter of the wire is 50 µm. The operational voltage is 3.1 kv. The thin gap chambers are filled with a highly quenching mixture containing 55% CO 2 and 45% n-pentane (n-c 5 H 12 ). The mixture is flammable and necessitates safety precautions. The total volume of gas in the chambers is 16 m Backgrounds The performance of the muon system depends strongly on the level of background recorded in the active elements of the detector. The main source of background is attributed to particles produced during interactions of primary hadrons from the proton-proton collisions with the material of the detector, in particular with the material in the calorimeters, toroid magnets, collimators and beam pipe. The particles which reach the muon detector are thermal neutrons, low energy photons produced during neutron capture, and charged particles. The photons interact with the detector material via the Compton effect and can produce a signal in the sensitive volume of the detector. Neutrons have a much lower probability to produce a signal but are responsible for the presence of the photons. The charged particles consist of muons, charged pions, protons, electrons and positrons. The hadrons and muons have a typical momentum of 100 MeV and contribute a counting rate of a few Hz/cm 2 [3]. Due to the uncertainty associated with the cross section, 36

38 composition, and spectra of minimum bias events this background is not well understood. The most important consequences of this background are the high occupancy in the muon detector, reduced lifetime, and high fake rate for the level 1 muon trigger. 1.9 Trigger The bunch crossing rate for the LHC is 40 MHz. At design luminosity L = cm 2 s 1 the interaction rate is about 1 GHz [2]. The rate at which events are selected must be reduced to about 100 Hz before permanent storage. This requires a rejection factor of 10 7 against minimum bias events. A high efficiency for the selection of interesting physics events must nonetheless be achieved because the cross sections for the processes of interest are quite small. The trigger is a collection of hardware and software designed to perform the decision making process which selects the events to be recorded for later analysis. The ATLAS trigger and data acquisition system consists of three levels of online event selection. Each trigger level refines the decision made during the previous step by applying additional selection criteria. The level 1 trigger reduces the event rate from 1 GHz to about 40 khz. The level 2 trigger further reduces this rate to about 1 khz. The event filter selects events at a rate of about 100 Hz which corresponds to an output rate of about 100 MB/s [2]. Figure 1.9 presents a diagram of the trigger and data acquisition system Level 1 Trigger The level 1 trigger selects events based on information delivered by a subset of detector components. Muons with large transverse momenta are identified by the muon trigger chambers, including the resistive plate chambers in the barrel region and the thin gap chambers in the endcaps. Calorimeter decisions are based on information with reduced granularity delivered by the electromagnetic and hadronic barrel and endcap calorimeters as well as the forward calorimeters. Trigger information is provided for sets of transverse momentum thresholds which typically include 6 to 8 different p T thresholds for each type of object. The calorimeter trigger targets electrons and photons with high transverse momenta, jets, hadronically decaying tau leptons, events with large total transverse energy, and events with large missing transverse energy E/ T. The 37

39 Figure 1.9: The ATLAS trigger and data acquisition system. The level 1 trigger selects events based on information delivered by a subset of detector components including the electromagnetic and hadronic calorimeters and the muon trigger chambers. The level 1 trigger reduces the event rate from 1 GHz to about 40 khz. During the time necessary to compute and distribute the level 1 trigger decision, information from all detector channels is held in pipeline memory. Events selected by the level 1 trigger are accepted into readout drivers (RODs) and thence into readout buffers (ROBs). Derandomizers average out the high instantaneous rate of data transfer in order to reflect the available bandwidth. The level 2 trigger utilizes region of interest information provided by the level 1 trigger to selectively access the data needed in order to make a decision. The level 2 trigger reduces the rate of data transfer to about 1 khz. All data pertaining to the bunch crossing selected by the level 1 trigger is held in the readout buffers until the event is either rejected by the level 2 trigger or it is accepted and transferred to the event filter. The process of moving the data from the readout buffers to the event filter is termed event building. The last stage of the online selection is the event filter, which selects events at a rate of about 100 Hz [2]. Events selected by the event filter will be written to data storage. 38