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Particle Detectors Iain Bertram Lancaster University June 2002 Outline Basic Principles Bubble Chamber Pictures Modern Detectors What to Measure? How to identify Particles? Detector Types? Different Experiments Basic Principles How and what do we see? All perception of the material world is via Electromagnetic Interactions Detectors To first order can only see charged objects. Energy deposition in the detector via ionization processes. (Exception is photons which “see” electric charge) Bubble Chamber Pictures Birmingham Package Bubble Chamber – super heated liquid. Small addition of energy boils the liquid creating small bubble Easy to visualize, too slow for modern experiments Interpretation Scatter off of an electron – light particle loops Vertex – neutral particle decaying Scatter off of a proton. Bubble Chamber Information Bubble Chamber in Magnetic Field Momentum of Particle Charge of Particle Energy Loss as function of distance (dE/dx) Type of interaction V – Neutral decay Tree – Charged Decay Kink – decay with a neutral Momentum Balance on Tree gives presence of neutral ? What do we want to Measure? Particle Properties Particle Type, electron, pion, kaon, proton, muon, etc. Charge of the particle Location of Particles Momentum of Particles Lifetime Need to combine all the above information into an event. For example e e- Z o , p p t t X Detection – Rough Particle ID Charged Particle Tracks B Calorimeter (dense) EM Absorber Material Interaction Point Scintillating Fiber Silicon Tracking Muon Tracks Energy hadronic electron photon Wire Chambers jet muon neutrino -- or any non-interacting particle missing transverse momentum We know x,y starting momenta is zero, but along the z axis it is not, so many of our measurements are in the xy plane, or transverse Tracking Detectors Measure x-y-z location of all charged particles as the pass through predetermined parts of the detector Series of dots Get position of tracks Connect lines to find decay vertices How do we get momenta Detector in magnetic field Tracks bend in a magnetic field How to Measure Momentum? Most Particles are Charged. If a charged particle moves through in a magnetic field it experiences a field F vqB ma F qv B So the acceleration is proportional to the magnetic field and perpendicular to the direction of motion. Momentum given by: Magnetic Field into Page Velocity in direction of arrow. How can we measure Momentum Dashed Arrow: Force Charged Particle in a Magnetic Field In the Program the momentum is given by the following equation: p 0.3Br P = momentum (in GeV/c) B strength of magnetic field in Tesla R = radius of curve in metres So we can calculate the momentum if we can measure the radius of the curve made by the particle in the magnetic field Simple Mathematics: x2 + y2 = r2 Pick three points on a circle and you get a radius Tracking Curved track – charge and momentum Vertex – decaying particle Particle Lifetimes Most Particles are unstable – I.e they decay. Important property is the lifetime of the particle Quantum Mechanical Effect – Decay is random We have to measure many decays and take the average to determine a real lifetime (in fact we need to fit the data) Ebeam Relativistic Effects are important: 2 m c K Need to take account time dilation etc. L c 2 1 Particle Identification Want to identify what type of particle in many cases Simple Classification Muon, electron, jet, photon, …… Straight forward differentiation in most detectors Exact type of particle Pion, kaon, proton, etc. Energy Deposition as function of momentum Detection – Rough Particle ID Charged Particle Tracks B Calorimeter (dense) EM Absorber Material Interaction Point Scintillating Fiber Silicon Tracking Muon Tracks Energy hadronic electron photon Wire Chambers jet muon neutrino -- or any non-interacting particle missing transverse momentum We know x,y starting momenta is zero, but along the z axis it is not, so many of our measurements are in the xy plane, or transverse Energy Deposition Example of energy deposition for different particles Note: measure momentum and dE/dx and get particle ID. Only works for fixed momentum range: Not good at higher momenta How else can we ID particles? Masses The kaon decays to two pions: K 0 By measuring the momentum and the angle between the two pions we can calculate the mass of the kaon if we know the mass of the pion: mK c 2 2m2 c 4 2 p1 p2 cos E1E2 Hence we can identify a kaon via calculating its invariant mass Energy Measurement 1. Measure Momentum – id particle – finished 2. Measure energy without magnetic field Sample energy deposition many times in dense material Calorimeter Calibrate with known energy depositions Depends on particle type DØ Detector Tracking Goals: Searches – higgs, SUSY particles, etc. Top physics, W physics, QCD, General Purpose detector Calorimeter Muons DØ Detector Silicon Microstrip Tracker Fibre Tracker Forward Preshower Solenoid Central Preshower “Typical DØ Dijet Event” ET,1 = 475 GeV, h1 = -0.69, x1=0.66 ET,2 = 472 GeV, h2 = 0.69, x2=0.66 MJJ = 1.18 TeV Q2 = ET,1×ET,2=2.2x105 GeV2 DØ Event: W e DØ top event: t W e b t W qq b BaBar Detector Opal Detector Opal Basic Muon Event Calorimeter Track Muon Muons penetrate: Therefore go all the way through the detector Opal Electron Track Calorimeter Electrons dump energy quickly Photon: No track Opal Jets Jets Jets are evidence of quarks or gluons. Collection of hadrons, penetrate further than electrons and usually > 1 particle Belle Event: B → +– : http://teachers.web.cern.ch/teachers/archiv /HST2000/teaching/resource/bubble/bubble .htm#An%20Introduction. Bubble Chambers – Birmingham http://www.ep.ph.bham.ac.uk/user/watkins/seeweb/BubbleChamber.htm B0 J/y KS