Particles and Universe: Particle detectors

Particles and Universe: Particle detectors Maria Krawczyk, Aleksander Filip Żarnecki April 12, 2016 M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 1 / 49

Lecture 5 1 Introduction 2 Ionization 3 Particle detectors Gas detectors Silicon detectors Scintillation detectors Cherenkov detectors Calorimeters 4 Collider experiments 5 Reconstructed events 6 Event selection 7 Simulation M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 2 / 49

Introduction Hypothesis of Luis de Broglie (1923): wave-particle duality Diffraction on hexagonal structures: Light Electrons M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 3 / 49

Introduction Classical Mechanics If we know initial positions and velocities off all constituents of the system (eg. Solar system objects), we can foresee the future state of the system (as well as determine its history). This reasoning fails on the subatomic scales! Wave-particle duality forces us to look for new methods to describe particle behavior... M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 4 / 49

Introduction Classical Mechanics If we know initial positions and velocities off all constituents of the system (eg. Solar system objects), we can foresee the future state of the system (as well as determine its history). Quantum mechanics Motion of a particle described as a propagation of probability wave (according to quantum wave equations) Probability of detecting particle at given point depends on the amplitude of its wave function. We can not be sure where the particle is until we make a measurement. Only the dedicated measurement can probe the wave function and give its actual position. Without the measurement, we can only guess... M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 4 / 49

Introduction Classical Mechanics If we know initial positions and velocities off all constituents of the system (eg. Solar system objects), we can foresee the future state of the system (as well as determine its history). Quantum mechanics Motion of a particle described as a propagation of probability wave (according to quantum wave equations) Probability of detecting particle at given point depends on the amplitude of its wave function. We can not be sure where the particle is until we make a measurement. Only the dedicated measurement can probe the wave function and give its actual position. Without the measurement, we can only guess... Also, we are not able to measure particle state with infinite precision - uncertainty principle M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 4 / 49

Introduction Particle detection In the macroscopic world, we are able to make observations which do not interfere with the process under study M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 5 / 49

Introduction Particle detection In the macroscopic world, we are able to make observations which do not interfere with the process under study In the particle world, each measurement is related to some interaction. We are not able to see particles without changing their state! We can not observe particles which do not interact M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 5 / 49

Introduction Particle detection In the macroscopic world, we are able to make observations which do not interfere with the process under study In the particle world, each measurement is related to some interaction. We are not able to see particles without changing their state! We can not observe particles which do not interact Main processes used for particle detection: ionization and scintillation photoelectric effect Cherenkov radiation M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 5 / 49

Ionization Structure of matter Properties of different materials depend on the strength of valence electrons bonding with atomic cores. Insulator All electrons tightly bonded with atoms Conductor Valence electrons can move freely M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 6 / 49

Ionization Ionization Is the phenomena used in most of the particle detectors Charged particle passing through the insulator interacts with valence electrons and passes part of its energy to them, sufficient to liberate them from their atoms. Free charge carriers are created M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 7 / 49

Gas detectors Ionization in gases is very small, of the order of 100 e/cm. Measurement of the corresponding current is not possible, unless we apply electric field strong enough to create an electron avalanche. In the uniform field we are not able to obtain large multiplication factors. M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 8 / 49

Gas detectors Ionization in gases is very small, of the order of 100 e/cm. Measurement of the corresponding current is not possible, unless we apply electric field strong enough to create an electron avalanche. In the uniform field we are not able Strong electric field resulting in large to obtain large multiplication factors. multiplication can be easily obtained around the thin anod wire. Detected charge is still very low, but can be measured with sensitive electronics. Charge multiplication is crucial... M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 8 / 49

Gas detectors Geiger-Müller counter Electrons accelerated in strong electric field (resulting from high voltage applied), can create secondary ionization when scattering off atoms. At highest voltages charge multiplication can lead to almost full ionization of the gas near the wire surface (Geiger-Müller mode). M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 9 / 49

Gas detectors Multiwire proportional chamber (MWPC) Georges Charpak 1970 (Nobel 1992) Cheap! Electronic readout possible! electronics+computers revolution M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 10 / 49

Gas detectors TPC Time Projection Chamber Heavy Ion collision event STAR detector at RHIC (BNL) M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 11 / 49

Semiconductor detectors Silicon is much denser than gas. About 100 electron-hole pairs are created in 1µm. Charge multiplication not needed, direct charge measurement possible. Large semiconductor crystals can be used for energy measurement M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 12 / 49

Semiconductor detectors Silicon is much denser than gas. About 100 electron-hole pairs are created in 1µm. Charge multiplication not needed, direct charge measurement possible. Large semiconductor crystals can be used for energy measurement Very precise position measurement possible with proper sensor segmentation M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 12 / 49

Silicon detectors Pixel detectors most precise position measurement Many different technologies used, including CCD sensors (as used for digital photography) Each CCD camera is a particle detector! Image from astronomic CCD camera: Enlarged section: It s not UFO. It s a particle... M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 13 / 49

Silicon detectors Rapid development Silicon detectors give very high measurement precision. They are still relatively expensive, but prices decrease fast with technology development. more and more widely used Single pixel sensor M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 14 / 49

Scintillation detectors Scintillation Charged particle passing the medium can ionize or excite the atom. Photons Photons can also interact with electrons in atom. They can transfer all their energy to single electron (photoelectric effect) or only part of it (Compton effect) Return of the atom to its ground state can be accompanied by the photon emission - scintillation In both cases, electron is released from atom. M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 15 / 49

Scintillation detectors Scintillation In some materials, atoms excited by ionizing particle emit photons. Light produced in scintillator can be measured with photomultiplier. Classical set-up: No position measuremen Very good time measurement M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 16 / 49

Scintillation detectors Photomultiplier Single electrons can release single electrons from photocatode. We multiply this charge by applying high voltage between subsequent dynodes. When hitting dynodes accelerated electrons produce secondary electrons - charge avalanche. Single photon, if it produces the first electron (photoelectric effect) results in macroscopic charge. M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 17 / 49

Scintillation detectors Pixelized Photon Detector (PPD) Also called the Silicon Photomultiplier (SiPM). Large number ( 10 3 ) of avalanche photo-diodes on small ( 1mm 2 ) surface - possibility of counting single photons Parameters similar to PMT: 10 5 10 6 gain, response time 1ns Much smaller! Low voltage operation! more and more popular M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 18 / 49

Scintillation detectors Modern detectors Classical scintillation detectors not used frequently New solution: scintillating fibres, pixelized photon detectors M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 19 / 49

Cherenkov detectors Cherenkov radiation Emitted by a charged particle traveling with speed larger than the speed of light in given medium. Light emitted in a cone. When passing a thin medium layer distinctive annulus shape is obtained on the screen. Observed in water, ice, air... Very cheap technology for very large detectors! M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 20 / 49

Cherenkov detectors Cherenkov radiation If the particle path in medium is longer, we can use special mirrors to focus produced Cherenkov light Scheme Image in the detector M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 21 / 49

Calorimeters All detectors described so far were designed to measure position of the charged particle tracking detectors To measure the particle energy, we have to force it to transfer its full energy to the detector calorimeters. Electromagnetic cascade High energy electrons loose energy in bremsstrahlung High energy photons convert to e + e pairs e γ γ e e + M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 22 / 49

Calorimeters Electromagnetic calorimeters High energy electron or photon entering the detector creates a cascade of secondaries, with N E particles By counting secondaries or measuring their total track length in the detector (total ionization) we can determine the energy of primary particle. Uniform calorimeter Sampling calorimeter eg. scintillating cristal detector layers interleaved with dense absorber M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 23 / 49

Calorimeters Hadronic calorimeters Simulation of the hadronic cascade (proton energy measurement) Hadronic cascade much longer than the electromagnetic one M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 24 / 49

Collider experiments Layer structure Modern universal detectors at particle colliders are build from many different sub-components. We arrange detectors in such a way as to obtain best measurement for all possible particles, as well as their (partial) identification. detectors which interact least with produced particles are placed closest to the interaction point - gas detectors, thin silicon sensors detectors which absorb particles are placed at largest distances from interaction point - calorimeters, muon detectors M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 25 / 49

Collider experiments Layer structure Modern universal detectors at particle colliders are build from many different sub-components. M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 25 / 49

Collider experiments Universal detector Layout describing most of recent and present-day experiments at colliders (LEP, HERA, Tevatron, LHC, ILC): Starting from the center of the detector: vertex detector as close to the beam line as possible, used to measure the exact position of the interaction point, allows for identification of short-lived particles (secondary vertexes) silicon pixel detectors M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 26 / 49

Collider experiments Universal detector Layout describing most of recent and present-day experiments at colliders (LEP, HERA, Tevatron, LHC, ILC): Starting from the center of the detector: vertex detector as close to the beam line as possible, used to measure the exact position of the interaction point, allows for identification of short-lived particles (secondary vertexes) silicon pixel detectors tracking detectors measure tracks of charged particles, allows to determine their momentum from bending in magnetic field gas detectors or silicon strip detectors M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 26 / 49

Collider experiments Universal detector electromagnetic calorimeter electron and photon energy measurement dense material absorbing EM cascade (copper, lead, tungsten) M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 27 / 49

Collider experiments Universal detector electromagnetic calorimeter electron and photon energy measurement dense material absorbing EM cascade (copper, lead, tungsten) hadron calorimeter hadron energy measurement (protons, neutrons, pions, kaons) dense material absorbing hadronic cascade; hadronic cascade is many times longer than EM one M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 27 / 49

Collider experiments Universal detector electromagnetic calorimeter electron and photon energy measurement dense material absorbing EM cascade (copper, lead, tungsten) hadron calorimeter hadron energy measurement (protons, neutrons, pions, kaons) dense material absorbing hadronic cascade; hadronic cascade is many times longer than EM one muon detectors identify muons - only charged particles which can pass both calorimeters with small energy losses M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 27 / 49

Collider experiments tracking det. calorimeters VTX TPC e. m. hadronic muon det. e γ µ ν + + π, p... + ο n, K... M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 28 / 49

ATLAS experiment M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 29 / 49

CMS experiment M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 30 / 49

Collider experiments Compact Muon Solenoid - CMS M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 31 / 49

Reconstructed events CMS Schematic detector view https://www.i2u2.org/elab/cms/event-display/ M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 32 / 49

Reconstructed events CMS Single muon event M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 32 / 49

Reconstructed events CMS Event with two muons M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 33 / 49

Reconstructed events CMS Event with electron production M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 34 / 49

Reconstructed events CMS Event with two electrons M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 35 / 49

Reconstructed events CMS Event with two photon production (H γγ candidate) M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 36 / 49

Reconstructed events CMS Four lepton event (H ZZ e + e µ + µ candidate) M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 37 / 49

Reconstructed events CMS How do we interpret events like this? M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 38 / 49

Hadronization When a pair of quarks is produced in the collision: gg q q M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 39 / 49

Hadronization When a pair of quarks is produced in the collision: gg q q Color field increases between quarks moving apart M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 39 / 49

Hadronization When a pair of quarks is produced in the collision: gg q q Color field increases between quarks moving apart Gluons are emitted M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 39 / 49

Hadronization When a pair of quarks is produced in the collision: gg q q Color field increases between quarks moving apart Gluons are emitted Gluons convert to quark-antiquark pairs M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 39 / 49

Hadronization When a pair of quarks is produced in the collision: gg q q Color field increases between quarks moving apart Gluons are emitted Gluons convert to quark-antiquark pairs Quarks and antiquarks form white hadrons M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 39 / 49

Reconstructed events CMS Event with six jet production M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 40 / 49

Collider experiments CMS Unbalanced transverse momentum - signature of missing particle M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 41 / 49

Reconstructed events ATLAS Event with W + boson production (W + e + ν e ) M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 42 / 49

Event selection Up to 50 proton pairs collide at LHC at each bunch crossing New particles are produced at almost each collision About billion of interactions per second! We have no possibilities to store more than about 100 events per second! How to select the interesting ones? M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 43 / 49

Event selection Trigger system Signals from the detector components are checked after each bunch crossing by the dedicated electronics, so called trigger. Only interesting signals are read from the detector. These events are then passed through subsequent filters - dedicated programs rejecting all rubbish Only events looking interesting are stored to disk! σ LHC s=14tev L=10 34 cm -2 s -1 rate ev/year barn mb µb nb pb fb W Z W lν Z l + l - qq qq H SM H SM γγ σ inelastic bb tt gg H SM Z SM 3γ h γγ tanβ=2-50 H SM ZZ ( * ) 4l LV1 input max LV2 input max LV1 output max LV2 output SUSY q ~ q ~ +q ~ g ~ +g ~ g ~ tanβ=2, µ=m g ~ =m q ~ tanβ=2, µ=m g ~ =m q ~ /2 scalar LQ Z ARL l + l - 50 100 200 500 1000 2000 5000 particle mass (GeV) M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 44 / 49 Z η l + l - GHz MHz khz Hz mhz µhz 10 16 10 15 10 14 10 13 10 12 10 11 10 10 10 9 10 8 10 7 10 6 10 5 10 4 10 3 10 2 10 1

Event selection Trigger system Final event selection require their very detailed analysis. But no computer could process 40 milion events per second! Solution: multilevel trigger system! Level 1: very fast (dedicated electronics), rejects 99.9% of rubbish Level 2: checks basic event parameters, selects 1% for final analysis Level 3: full even analysis and final decision M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 45 / 49

Event selection Trigger system scheme M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 46 / 49

Simulation We use very complicated detector systems to measure particle interactions. We use trigger systems to select events of interest. We use dedicated algorithms to reconstruct the particle properties. How do we know how to interpret the measurements? How to determine the efficiency of event selection? How to estimate precision of energy or mass measurement? How to verify that our results are consistent with expectations? Monte Carlo method We use simulation methods to model all aspects of the experiment: beam particle collision, interactions inside the detector, detector response, trigger system decision. We produce event samples which are equivalent to the actual data. We can reconstruct them in the same way and compare verify our knowledge about physics and detector performance M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 47 / 49

Simulation Monte Carlo method Simple example: how to calculate the area of an irregular figure? Defined eg. by a complicated set of mathematical formula There are two approaches: We can divide the space into a large number of unit elements and count the elements belonging to the figure M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 48 / 49

Simulation Monte Carlo method Simple example: how to calculate the area of an irregular figure? Defined eg. by a complicated set of mathematical formula There are two approaches: We can divide the space into a large number of unit elements and count the elements belonging to the figure We can generate random points and count those inside the figure much more efficient in large number of dimensions M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 48 / 49

Simulation Monte Carlo simulation is just an efficient way to integrate all our knowledge on the observed phenomena. It allows to predict the experimental result with arbitrary precision. But it is not a magic box - predictions can be calculated only for the processes which are fully understood! Examples of Monte Carlo simulation results compared with data: Events/5 GeV 40 35 30 Data 2011+ 2012 SM Higgs Boson m H =124.3 GeV (fit) Background Z, ZZ* Background Z+jets, tt Syst.Unc. ATLAS H ZZ* 4l 1 s = 7 TeV Ldt = 4.6 fb 1 s = 8 TeV Ldt = 20.7 fb 25 20 15 10 5 0 100 150 200 250 m 4l [GeV] M.Krawczyk, A.F.Żarnecki Particles and Universe 5 April 12, 2016 49 / 49