Particle Detectors

1. Particle Detectors Iain Bertram Lancaster University June 2002

2. Outline • Basic Principles • Bubble Chamber Pictures • Modern Detectors • What to Measure? • How to identify Particles? • Detector Types? • Different Experiments

3. 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)

4. 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 Bubble Chamber Pictures

5. Interpretation Scatter off of an electron – light particle loops Vertex – neutral particle decaying Scatter off of a proton.

6. 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 ?

7. 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

8. Detection – Rough Particle ID Muon Tracks Charged Particle Tracks Energy Scintillating Fiber Silicon Tracking Calorimeter (dense) Interaction Point Absorber Material Ä B EM 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

9. 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

10. How to Measure Momentum? • Most Particles are Charged. • If a charged particle moves through in a magnetic field it experiences a field • So the acceleration is proportional to the magnetic field and perpendicular to the direction of motion. • Momentum given by: • How can we measure Momentum Magnetic Field into Page Velocity in direction of arrow. Dashed Arrow: Force

11. Charged Particle in a Magnetic Field • In the Program the momentum is given by the following equation: • 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

12. Curved track – charge and momentum Vertex – decaying particle Tracking

13. 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) • Relativistic Effects are important: Need to take account time dilation etc.

14. 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

15. Detection – Rough Particle ID Muon Tracks Charged Particle Tracks Energy Scintillating Fiber Silicon Tracking Calorimeter (dense) Interaction Point Absorber Material Ä B EM 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

16. 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? Energy Deposition

17. Masses - • The kaon decays to two pions: • 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: • Hence we can identify a kaon via calculating its invariant mass

18. Energy Measurement • Measure Momentum – id particle – finished • Measure energy without magnetic field • Sample energy deposition many times in dense material • Calorimeter • Calibrate with known energy depositions • Depends on particle type

19. Tracking Calorimeter Muons DØ Detector Goals: Searches –higgs, SUSY particles, etc. Top physics,W physics,QCD, General Purpose detector

20. Fibre Tracker Silicon Microstrip Tracker Forward Preshower Solenoid Central Preshower DØ Detector

21. “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

22. DØ Event: W e

23. DØ top event:

24. BaBar Detector

25. Opal Detector

26. Opal Basic Muon Event Calorimeter Track Muons penetrate: Therefore go all the way through the detector Muon

27. Opal Electron Track Calorimeter Electrons dump energy quickly Photon: No track

28. Jets Opal Jets Jets are evidence of quarks or gluons. Collection of hadrons, penetrate further than electrons and usually > 1 particle

29. p+ p+ p- p- Belle Event: B → p+p– :

30. http://teachers.web.cern.ch/teachers/archiv/HST2000/teaching/resource/bubble/bubble.htm#An%20Introduction.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

33. B0  J/y KS