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Experimental Particle Physics

Experimental Particle Physics. Two types of Accelerator Experiments Accelerators Detectors An example – the BaBar Experiment. Two types of Accelerator Experiments. Fixed target Invariant mass of beam-target system is M where: Example: e - p with p e- = 20 GeV/c  s = 6.22 GeV.

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Experimental Particle Physics

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  1. Experimental Particle Physics • Two types of Accelerator Experiments • Accelerators • Detectors • An example – the BaBar Experiment Brian Meadows, U. Cincinnati.

  2. Two types of Accelerator Experiments • Fixed target • Invariant mass of beam-target system is M where: • Example: e- p with pe- = 20 GeV/c •  s = 6.22 GeV Particle detectors Target (stationary) Beam from accelerator Brian Meadows, U. Cincinnati

  3. Two types of Accelerator Experiments • Colliding beam (no target) • Invariant mass in beam-beam collision M is • Example: 10 GeV/c e- on 10 GeV/c e+  M = 20.03 GeV/c2 Particle detectors Beam from accelerator Beam from accelerator pb , mb pa , ma Brian Meadows, U. Cincinnati

  4. Main Accelerator Principal • Charged particles are accelerated by synchronous electric field in RF cavities. • Particles accelerated in “bunches” • Longitudinal stability • Late particle arrives a E-field peaks (is accelerated more) • Early particle arrives as E-fields still moves towards peak • Transverse stability from quadrupole magnets E E E Brian Meadows, U. Cincinnati

  5. Main Accelerator Principal • Accelerators are often circular: • Require bending magnets (dipoles) p = 0.3 B  (GeV/c) (T) (m) • Transverse focussing magnets (quadrupoles) • Transverse stability also comes from synchrotron radiation loss in -tron oscillations • Ramp up B ! p ramps up • Longitudinal stability tuned for v ¼ c • Relativistically, particle v ! c as p increases Brian Meadows, U. Cincinnati

  6. Accelerator Complex at CERN Brian Meadows, U. Cincinnati

  7. Two Types of Magnet From Perkins, p. 340 Dipole Quadrupole Brian Meadows, U. Cincinnati

  8. Detection of Charged Particles Brian Meadows, U. Cincinnati

  9. Interaction of Charged Particleswith Medium • Bethe-Bloch relationship describes energy loss • dE/dx does NOT depend on particle mass • DOES depend on velocity  • Is almost independent of medium (Z/A ~ 0.5 for all) • Typical value of (dE/dx)min approx 1–1.5 MeV cm2g-1 Brian Meadows, U. Cincinnati

  10. Interaction of Charged Particleswith Matter • Dominated at low p by 1/2 behaviour • Results mostly from Coulomb scattering of electron in medium by massive nucleus of charge z • Relativistic increase in transverse E-field of particle leads to “relativistic rise” • When E? spreads over several atoms, get polarization effect that results in plateau region “Polarization effect” “Relativistic rise” “Minimum ionization” From Perkins p. 352 Brian Meadows, U. Cincinnati

  11. Scintillation Counters • Organic “scintillator” material has molecules that are excited by passage of charged particle • Molecules de-excite, emitting mostly UV photons • Many of these photons pass through light-guide to a PMT PMT • Organic scintillator • Molecular excitations produce UV photons Brian Meadows, U. Cincinnati

  12. The Photo-multiplier (PMT)A Single Photon Detector  Brian Meadows, U. Cincinnati

  13. Array of Photo-Multipliersin BaBar’s “DIRC” Detector Brian Meadows, U. Cincinnati

  14. Bubble and Cloud Chambers • Charged particles lose energy dE/dx and release minute amounts of latent heat. This induces Local condensation of super-cooled vapour in cloud chambers Local boiling of super-heated liquid in bubble chambers g n e- e+ e+ e+ p- Brian Meadows, U. Cincinnati

  15. Bubble and Cloud Chambers • A magnetic field is usually present making it possible to determine particle momentum • Droplets (or bubbles) per unit track length is / dE/dx This determines  • Then p and  together determine particle identity m = p /  GeV/c2 GeV/c • p = 0.3 B  • (GeV/c) (T) (m) Brian Meadows, U. Cincinnati

  16. Electronic “ Wire Chambers” Drift chambers, multi-wire proportional and spark chambers • Consist of gases with electric fields (~104 V/m) • Can all be read out electronically – into computer • dE/dx energy loss produces e--ion pairs in gas • E-fields are strong (~104 V/m) at wire locations result in avalanche of electrons and detectable charges (~ 105 e-) Brian Meadows, U. Cincinnati

  17. Drift Chamber e- ’s • Consists of gas medium filled with wires • Carefully crafted E-field within “cells” throughout volume • E-fields very strong near “sense wires” • Charged particle passes through gas - creates e-ion pairs • Electrons (and ions) drift in E-field and are collected on sense wires • Sense wire positions and drift time determine particle position e- ’s Sense wires e- ’s e- ’s Brian Meadows, U. Cincinnati

  18. Example – Time Projection Chamber (TPC) Array of wires (MWPC) - measures x and y Uniform E z x e- drift time - measures z • The TPC uses the simplest form of drift field (uniform) • BUT – the field near the MWPC sense wires is complex Brian Meadows, U. Cincinnati

  19. “Gating” a TPC • It is possible to switch electron collection on or off • Use a “Gate” plane of wires z • This is useful when TPC runs in high radiation • Positive ions build up • Can trigger it ON only when interesting event occurs • See http://www.physics.uc.edu/~brian/Babar/Tpc/pictures/minitpc.gif Brian Meadows, U. Cincinnati

  20. Silicon Strip Detector ~250 mm • Similar to Drift chamber/TPC • Much better spatial resolution • High purity Silicon used. Brian Meadows, U. Cincinnati

  21. Cherenkov Detectors • When charged particles traverse a medium they produce Cherenkov photons if their speed v > c / n • Photons are in a forward cone with half-angle  cos  = c / (v n) • Number of photons Ng with energy Eg is d2Ng / dxdE = a z2/(~c) sin2 ¼ 364 sin2q(cm-1¢ eV-1 ) • Spectrum of Cherenkov photons is flat in energy E=hn This provides two ways to identify charged particles: • Threshold mode (binary) OR • Measure q to obtain v - then m = p/(cbg) Brian Meadows, U. Cincinnati

  22. Detection of Neutral Particles Brian Meadows, U. Cincinnati

  23. Interaction of Electrons and Photonswith Matter • Hierarchy of photon interactions with electrons in medium: • Absorption by atoms in medium - Below “K” X-ray cutoff • Compton Scattering - Below ~ 1.02 MeV • Pair production – Below ~ 1.02 MeV Threshold for pair-production is at ~ 1.02 MeV • Electron interactions • Electron-electron scattering (Moller scattering) • Bremstrahlung radiation (resulting from scatters) Brian Meadows, U. Cincinnati

  24. e- g x Interaction of Electrons and Photonswith Matter • Together these lead to electromagnetic showers • The energy loss –dE/dx: • Peaks at depth xp/ ln(E0) (E0 is energy of initial electron or photon) • The number of shower products is / E0 • The total integrated track length is also / E0 • Total energy deposited fluctuates statistically with standard deviation / E0-1/2 dE dx x Xp Brian Meadows, U. Cincinnati

  25. Interaction of Electrons and Photonswith Matter • These electromagnetic processes result from charge of nuclei in the medium • Greatest effect in materials with high Z • Radiation length (energy of photons reduced by factor two) is given by • Some examples: • Material X0 (g/cm2) • ---------------------------------------- • H2 63.1 • He 94.3 • C 42.7 • Al 24.0 • Fe 13.8 • Pb 6.4 Brian Meadows, U. Cincinnati

  26. Neutral Particle DetectionCalorimeters “Calorimeters “ can detect neutral (or charged) particles • They are designed to stop particles, using dense media • Energy deposited measures neutral energy (approximately) • Two kinds of calorimeter: Electromagnetic – stop photons and electrons • Require many radiation lengths (high Z material) • Not many interaction lengths so that hadrons are not stopped Hadronic – stop hadrons • Require many interaction lengths (but low Z) • Usual experimental arrangement is to have E/M calorimeter in front of hadron calorimeter. Brian Meadows, U. Cincinnati

  27. Segmented Detector n Dense medium Neutral Particle DetectionCalorimeters • Photons and electrons, produce electromagnetic showers most effectively in high Z materials (e.g. Pb, W, …) • Neutrons and K0 mesons produce charged hadrons • Use layers of dense material with position-sensitive (segmented) charged detectors between Brian Meadows, U. Cincinnati

  28. The BaBar Detector At Stanford Linear Accelerator Center (SLAC) Brian Meadows, U. Cincinnati

  29. The SLAC B-Factory (PEP2) • Asymmetric e+e- collisions at (4S). •  = 0.56 (3.1 GeV e+, 9.0 GeV e-) • Principal purpose • study CPV in B decays Brian Meadows, U. Cincinnati

  30. The BaBar Detector at SLAC Brian Meadows, U. Cincinnati

  31. Silicon Vertex Tracker (SVT) • 5 Layers double sided AC-coupled Silicon • Rad-hard readout IC (2 MRad – replace ~2005) • Low mass design • Stand alone tracking for slow particles • Point resolution z» 20 m • Radius 32-140 mm Brian Meadows, U. Cincinnati

  32. Drift Chamber 40 layer small cell design 7104 cells He-Isobutane for low multiple scattering dE/dx Resolution »7.5% Mean position Resolution 125 m Brian Meadows, U. Cincinnati

  33. Drift Chamber Layout Brian Meadows, U. Cincinnati

  34. Drift Chamber Cell ( from BaBar) Equi-potential lines determine time-to-distance relationship Brian Meadows, U. Cincinnati

  35. 144 quartz bars Particle ID - DIRC • Measures Cherenkov angle in 144 quartz bars arranged as a “barrel”. • Photons transported by internal reflection • Along the bars themselves. • Detected at end by ~ 10,000 PMT’s Detector of Internally Reflected Cherenkov light PMT’s Brian Meadows, U. Cincinnati

  36. The DIRC Principle Brian Meadows, U. Cincinnati

  37. Particle ID - DIRC It Works Beautifully! 10 8 6 4 2 0 BABAR K/ separation () Provides excellent K/ separation over the whole kinematic range • 2.5 3 3.5 4 • Momentum (GeV/c) Brian Meadows, U. Cincinnati

  38. Electromagnetic Calorimeter • CsI (doped with Tl) crystals • Arranged in 48()£120() • » 2.5% gaps in . • Forward endcap with 8 more  rings (820 crystals). g g  BABAR  0!gg 0!gg Brian Meadows, U. Cincinnati

  39. The Muon System Resistive Plate Chambers (RPC’s) - upgrade with Limited Streamer Tubes (LST’s) in progress Brian Meadows, U. Cincinnati

  40. “A Typical Event”  clusters Brian Meadows, U. Cincinnati

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