7 Physics at Hadron Colliders

Transcription

1 7 Physics at Hadron Colliders The present and future Hadron Colliders - The Tevatron and the LHC Test of the Standard Model at Hadron Colliders Jet, W/Z, Top-quark production Physics of Beauty Quarks (T. Kuhr) Search for Supersymmetry (Kasakow, de Boer) Search for the Higgs Boson and for Physics beyond the SM Extracted from lecture of Karl Jakobs, CERN Summer School 2004 Further reading: Collider Physics Barger, Phillips The Higgs Hunters Guide Hochenergiephysik Perkins

2 1. What is the origin of mass? 7.1 INTRODUCTION 7.1 Important open questions of particle physics e t γ, g, W Does the Higgs particle exist? as proposed by P. Higgs (1964) All properties of the Higgs particle are known, once its mass is fixed. The mass is a free parameter in the Standard Model Constraints (from theory and experiment): GeV/c 2 (exp.) < m H < ~ 1000 GeV/c 2 (theo.)

3 2. The question of unification: Is there a universal force, a common origin of the different interactions? Famous example: J.C.Maxwell (1864) Unification of electricity and magnetism : Glashow, Salam and Weinberg Unification of the electromagnetic and weak interactions electroweak interaction (prediction of W- und Z-bosons) Higgs mechanism is a cornerstone of the model

4 3. Are there new, yet unknown types of matter? Will we meet supersymmetry (SUSY) on the way towards unification? Quark Top Electron Wino Higgsino Squark Stop Selectron W H Motivation for SUSY: (i) Unification of forces seems possible (ii) Supersymmetry provides a candidate for dark matter in the universe SUSY? energy (GeV)

5 Many other open questions: Are quarks and leptons really elementary? Why are there three families? Are there additional families of (heavy) quarks and leptons? Are there additional gauge bosons? What is the origin of matter-antimatter asymmetry in the universe? What is the origin of CP-violation?

6 7.1.2 Why a hadron collider? e + e - colliders are excellent machines for precision physics!! -e+ e - are point-like particles, no substructure clean events - complete annihilation, centre-of-mass system, kinematic fixed

7 Proton proton collision are more complex

8 Main drawbacks of e + e - circular accelerators: 1. Energy loss due to synchrotron radiation (basic electrodynamics: accelerated charges radiate, x-ray production via bremsstrahlung, synchrotron radiation ) - Radiated power (synchrotron radiation): Ring with radius R and energy E - Energy loss per turn: - Ratio of the energy loss between protons and electrons: Future accelerators: pp ring accelerators (LHC, using existing LEP tunnel) or e + e - linear accelerators (under study / planning)

9 2. Hard kinematic limit for center-of-mass energy from the beam energy: s = 2 E beam

10 Proton-antiproton collider The Tevatron collider at Fermilab : Run I, 2 experiments CDF und DØ, s = 1.8 TeV Chicago L dt = 125 pb : Upgrade program Machine: new injector + Antiproton recycler CDF TeVatron D0 Higher luminosity higher event rates + Upgraded, improved detectors Since March 2001: Run II, s = 1.96 GeV Currently running Ldt = 2.5 fb -1 (1.5 fb -1 ready for analysis)

11 Impressions from Fermilab

12 The CDF-Experiment CDF New in Run II : Tracking system Silicon vertex detector (SVXII) Intermediate silicon layers Central outer tracker (COT) End plug calorimeter Time of flight system Front-end electronics Trigger and DAQ systems 730 Physicists from 13 nations, 61 Institutions; 23 from Germany

13 The DØ Experiment 21 countries, 91 institutions 670 physicists (40 from Germany) New for Run II Inner detector magnetic field added Preshower detectors Forward muon detector Front-end electronics Trigger and DAQ

14 7.1.4 The Large Hadron Collider (LHC) Proton-proton accelerator in the LEP-tunnel at CERN p p 7 TeV 7 TeV - Highest energies per collision - Conditions as at times of s after the big bang Four experiments: ATLAS, CMS (pp physics) LHC-B (physics of b-quarks) ALICE (Pb-Pb collisions) Constructed in an international collaboration Startup late 2007

15 Important components of the accelerator superconducting dipole magnets - challenge: magnetic field of 8.4 Tesla - in total 1300 magnets, each 15 m long - operation temperature of 1.9 K Eight superconducting accelerator structures, acceleration gradient of 5 MV/m In full production LHC is the largest cryogenic system in the world

16 Proton proton collisions at the LHC Proton proton: 2835 x 2835 bunches Separation: 7.5 m ( 25 ns) protons / bunch Crossing rate of p-bunches: 40 Mio. / s Luminosity: L = cm -2 s -1 ~10 9 pp collisions / s (superposition of 20 pp-interactions per bunch crossing: pile-up) ~1600 charges particles in the detector high particle densities high requirements for the detectors

17 Luminosity The rate of produced events for a given physics process is given by: N = L σ dimensions: s -1 = cm -2 s -1 cm 2 L = Luminosity σ = cross section Luminosity depends on the machine: important parameters: number of protons stored, beam focus at interaction region,. In order to achieve acceptable production rates for the interesting physics processes, the luminosity must be high! L = cm -2 s -1 design value for Tevatron Run II L = cm -2 s -1 planned for the initial phase of the LHC (1-2 years) L = cm -2 s -1 LHC design luminosity, very large!! One experimental year has ~ 10 7 s (1000 x larger than LEP-2, 50 x Tevatron Run II design) Integrated luminosity at the LHC: 10 fb -1 per year, in the initial phase 100 fb -1 per year, later, design

18 The CMS experiment Solenoidal magnetic field (4T) in the central region used to measure tracks in the inner detector and muons in an instrumented iron return yoke High resolution silicon strip and pixel detectors - 9,7 Mio. channels, 210 m 2 Measurement of energy in a Lead- Tungstate crystal calorimeter (very good resolution for photons)

19 The ATLAS experiment Solenoidal magnetic field (2T) in the central region (momentum measurement) High resolution silicon detectors: - 6 Mio. channels (80 µm x 12 cm) -100 Mio. channels (50 µm x 400 µm) space resolution: ~ 15 µm Diameter Barrel toroid length End-cap end-wall chamber span Overall weight 25 m 26 m 46 m 7000 Tons Energy measurement down to 1 o to the beam line Independent muon spectrometer (supercond. toroid system)

20 7.2 Physics at Hadron Colliders Protons are complex objects: Partonic substructure: Quarks and Gluons Hard scattering processes: (large momentum transfer) quark-quark quark-gluon scattering or annihilation gluon-gluon However: hard scattering (high P T processes) represent only a tiny fraction of the total inelastic pp cross section Total inelastic pp cross section ~ 70 mb (huge) Dominated by events with small momentum transfer

21 Variables used in the analysis of pp collisions θ p p T Transverse momentum (in the plane perpendicular to the beam) p T = p sinθ Pseudo-rapidity: θ = 90 o η= 0 θ = 10 o η 2.4 θ = 170 o η -2.4

22 Inelastic low - P T pp collisions Most interactions are due to interactions at large distance between incoming protons small momentum transfer, particles in the final state have large longitudinal, but small transverse momentum < p T > 500 MeV (of charged particles in the final state) dn dη 7 - about 7 charged particles per unit of pseudorapidity in the central region of the detector (5 at the Tevatron) - uniformly distributed in Φ These events are called Minimum-bias events

23 The hard scattering process: Proton beam can be seen as beam of quarks and gluons with a wide band of energies The proton constituents (partons) carry only a fraction 0 < x < 1 of the proton momentum x 1 p ŝ x 2 p The effective centre-of-mass energy is smaller than s of the incoming protons To produce a mass of: LHC Tevatron 100 GeV: x ~ TeV: x ~

24 From where do we know the x-distributions? The structure of the proton is investigated in Deep Inelastic Scattering experiments: Today s highest energy machine: the HERA ep collider at DESY/Hamburg Scattering of 30 GeV electrons on 900 GeV protons: Test of proton structure down to m HERA ep accelerator, 6.3 km circumference

25 How do the x-values of the proton look like? Parton density functions (pdf): u- and d-quarks at large x-values Gluons dominate at small x!! Uncertainties in the pdfs, in particular on the gluon distribution at small x Sea Quarks Valence Quarks

26 Calculation of cross sections σ = a,b dx 2 2 a dx b fa (xa, Q ) fb (x b, Q ) σ ab (xa, x b) ˆ Sum over initial partonic states a,b σˆ hard scattering cross-section ab f i (x, Q 2 ) parton density function Example: W-production: (leading order diagram) u W + σ (pp W) ~ 150 nb ~ σ tot (pp) d + higher order QCD corrections (perturbation theory)

27 Cross Sections and Production Rates Rates for L = cm -2 s -1 : (LHC) Inelastic proton-proton reactions: 10 9 / s bb pairs / s tt pairs 8 / s W e ν 150 / s Z e e 15 / s Higgs (150 GeV) 0.2 / s Gluino, Squarks (1 TeV) 0.03 / s LHC is a factory for: top-quarks, b-quarks, W, Z,. Higgs, The only problem: you have to detect them!

28 What experimental signatures can be used? Quark-quark scattering: q p q q q p No leptons / photons in the initial and final state If leptons with large transverse momentum are observed: interesting physics! Example: Higgs boson production and decay p q l q W ν W H l ν q q p Important signatures: Leptons und photons Missing transverse energy

29 Suppression of background: Reconstruction of objects with large transverse momentum Reconstructed tracks with pt > 25 GeV

30 Detector requirements from physics Good measurement of leptons and photons with large transverse momentum P T Good measurement of missing transverse energy (E T miss ) and energy measurements in the forward regions calorimeter coverage down to η ~ 5 Efficient b-tagging and τ identification (silicon strip and pixel detectors)

31 Detector requirements from the experimental environment (pile-up) LHC detectors must have fast response, otherwise integrate over many bunch crossings too large pile-up Typical response time : ns integrate over 1-2 bunch crossings pile-up of minimum bias events very challenging readout electronics High granularity to minimize probability that pile-up particles be in the same detector element as interesting object large number of electronic channels, high cost LHC detectors must be radiation resistant: high flux of particles from pp collisions high radiation environment e.g. in forward calorimeters: up to n / cm 2 in 10 years of LHC operation

32 How are the interesting events selected? Trigger : much more difficult than at e + e - machines TRIGGER: Interaction rate: ~ 10 9 events/s Can record ~ 100 events/s (event size 1 MB) trigger rejection ~ 10 7 Trigger decision µs larger than interaction rate of 25 ns detector store massive amount of data in pipelines while special trigger processors perform calculations PIPELINE trigger NO trash YES save 10 9 evts/s 10 2 evts/s

33 7.2.1 QCD processes at hadron colliders Hard scattering processes are dominated by QCD jet production originating from quark-quark, quark-gluon and gluon-gluon scattering Due to fragmentation of quarks and gluons in final state hadrons Jets with large transverse momentum P T in the detector Cross sections can be calculated in QCD (perturbation theory) Comparison between experimental data and theoretical predictions constitutes an important test of the theory. Deviations? Problem in the experiment? Problem in the theory (QCD)? New Physics, e.g. quark substructure?

34 A two jet event at the Tevatron (CDF) CDF Dijet Mass = 1364 GeV/c 2 E T = 666 GeV η = 0.43 CDF (φ-r view) E T = 633 GeV η = -0.19

35 The Jet Cross Section contributions of the various sub-processes to the inclusive jet cross section

36 Main experimental systematic uncertainty: Jet Energy Scale A Jet is NOT a well defined object (fragmentation, detector response) - one needs an algorithm to define a jet, to measure its energy (e.g., a cone around a local energy maximum in the calorimeter, cone size adapted such that a large fraction of jet energy is collected, typical values: R = Φ 2 + η 2 = 0.7 Cluster energy parton energy Main corrections: In general, calorimeters show different response to electrons/photons and hadrons (see lectures on detector physics) Subtraction of offset energy not originating from the hard scattering (inside the same collision or pile-up contributions, use minimum bias data to extract this) Correction for jet energy in/out of cone (corrected with jet data + Monte Carlo simulations)

37 Comparison with Theory Fully corrected inclusive jet cross section Red line - total systematic uncertainty (mainly JES) Additional 10% from luminosity uncertainty not plotted

38 Are quarks fundamental particles? REPETITION OF THE RUTHERFORD-EXPERIMENT p p Suche nach RESULT: Λ > 1600 GeV R < m