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B Physics Experiments

TASI June, 2000. B Physics Experiments. Sheldon Stone Syracuse University. Introduction: Objectives. Understand how a modern HEP B Physics experiment works Understand how the detector works Understand how the data is analyzed

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B Physics Experiments

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  1. TASI June, 2000 B Physics Experiments Sheldon Stone Syracuse University

  2. Introduction: Objectives • Understand how a modern HEP B Physics experiment works • Understand how the detector works • Understand how the data is analyzed • Understand what final states are most useful (i.e. easiest to deal with) • Understand what mistakes can be made

  3. Deconstructing The Detector

  4. Deconstructing The Detector 1 m What’s missing?

  5. Deconstructing The Detector • What do we want to measure? • To look for physics results we need to find specific decay modes of B’s • Sometimes we are interested in inclusive decays, i.e. BXe-n, often we are interested in exclusive decays, i.e. ByKS • We always need to find the 4-momenta of the particles at the origin of the decay • This includes measuring 3-momenta of charged tracks and identifying them • Also includes measuring energies & direction of g’s

  6. Outline of Required Measurements • How do we measure: • Charged particle positions & momenta? • Decay vertices? • Gamma rays? • Neutrinos? • How do we identify charged particles (e, m, p, K, p)?

  7. Trigger and Data Acquisition • How do we acquire the data? • Trigger – All interactions are not interesting. Even at e+e- colliders most of the collisions do not produce b-flavored hadrons. The largest rates are for • e+e- e+e- (g) • gg  hadrons So a decision needs to be made quickly on writing out the data or losing it. • DAQ – The data acquisition system – the hardware used for getting the data off the detector and on to tape, quickly. • These considerations must be part of the overall detector design

  8. Charged Particle Detection • Charged particles are detected because they ionize electrons in atoms • To calculate distribution of energy loss, consider elastic collisions of incident particle with atomic electrons • Binding energy of electrons must be taken into account • This is a difficult problem, first worked out by Landau [Journal of Physics, 201 (1944)] • We need to know • Energy distribution of ionized electrons (mean & width) • # of electrons freed per unit length

  9. The Landau Distribution • Energy loss distribution is not Gaussian, has long tail toward higher energy • Peak is called “most probable energy loss” • “Mean Energy loss” is less well defined • Many electrons involved usually Most probable energy loss, Emp Mean energy loss

  10. Number of Produced e- • N=(dE/dx)/W; where W depends on material (mean energy to produce an electron) • Empirically determined • ~30 for 1 cm of gas (Ar, CO2) • 25,000 for 300 mm of Silicon • So detect charged particles by applying an electric field to collect electrons or some other means of seeing the ionization

  11. Momentum Measurement • Bend tracks in a magnetic field • For q=1, pt = 0.3 r B, r in meters, B in Tesla, p in GeV • For B = 1T, and r = 1m, pt = 300 MeV

  12. Bubble Chambers • An old technology, no longer used • Very illustrative • Cold H2 liquid is both the target and the detector. • Liquid is superheated & boils due to ionization

  13. Most Famous Bubble Chamber Event

  14. Reasons bubble chambers are no longer used • They can’t cycle fast • There is no electronic readout. The film must be scanned and then digitized by hand • Particle identification is not good, nor is g detection (some experiments with lead plates)

  15. Modern Tracking Detectors • Gas detectors • Detect ionization by applying an electric field to a thin (~20 mm) wire • Multiplication- when electrons get close to the wire they have enough energy to ionize the gas; thus one ionization electron can turn into 10 –100 thousand detected electrons • Geometry • Many wires ~50,000 charged track e-“drift” towards the wire at “constant” drift velocity . E=1/r

  16. Drift Chambers • Position resolution • In any measuring device of cell size s the accuracy in position is , determined by the s of a rectangular distribution • Can do better by measuring the time from when the particle enters the system to when the first electrons hit the wire. This is called a "drift chamber." This works because the electron velocity in the gas is known • Note 2-fold ambiguity . t t

  17. A Modern Drift Chambers The KLOE Drift Chamber, 50,000 wires

  18. Limits to Precision • Magnetic Fields - inconvenient to get fields in excess of 1.8 T due to Fe saturation • Finite time resolutions translate to real drift chambers having resolutions of 100-200 mm. Due to ionization statistics, wire position errors, e- drift velocity calibrations, etc.. • Multiple scattering- due to material

  19. Stereo • Problem: How to measure two coordinates? • Planar geometry: crossed picket fences • Cylindrical geometry: small angle stereo, but this causes error to be much larger (10x) in one coordinate. Precision in r-f ~ 150 mm, and in z ~1.5 mm per layer stero + axial stero -

  20. Multiple Scattering • “It’s the material, stupid” • Due to Coulomb scattering from Nuclei. Well described by Moliere. Gaussian for small deflection angles, but with long tail due to Rutherford Scattering (This is a real pain!) • For 98% of the scatters where, Xo is the radiation length =

  21. How to Measure a Decay Vertex • c & b quarks are distinguished by their decay lengths. L=bgct, where t ~1 ps. • For bg = p/m = 1 & t =1 ps, L = 0.3 mm = 300 mm • Better if p is larger; note tB = 1.5 ps, tD+ = 1.1 ps • First done by Framm at CERN ~1982 • Made really good by E691 at Fermilab

  22. Technology: Use Silicon Detectors • Silicon is made into a p-n junction diode with appropriate doping. It is operated at a bias voltage that forms a sensitive region depleted of mobile charge and sets up an electric field that sweeps charge liberated by radiation to the electrodes. ~50 mm wide strips are placed on one side as a readout. Charged particles ionize the silicon and the charge is collected. • Sensitive electronics are required • Strips have relatively large Capacitance  noise • Many channels are required ~ several 100,000 • Position resolution of 50 mm/12 for binary readout, better for analog

  23. Silicon detector picture

  24. Problems with Silcon Strip Detectors • Ambiguity problem • Long strips are difficult because of large capacitance • Lots of material on edges due to electronic readout

  25. Pixel Detectors • Make a square or rectangular array of silicon • Put a small electronic circuit behind each one! (bump bonding) • Send signals out to periphery where only hit pixels are readout "sparsification" • Thicker than strip detectors, by a factor of ~3 • Useful for high track density • Useful for triggering on detached vertices Each cell or pixel is small, ex: 50 mm x 400 mm

  26. BTeV Pixel Test Results • Solid curve is a piece wise linear fit to a simulation based on a detailed Monte Carlo Track angle (mr)

  27. Photon Detection: Electromagnetic Calorimetry • Primary process is conversion of high energy g by pair production in Nuclear Coulomb field • Process leads to a "shower" atomic photoeffect Rayleigh scattering Pair production off Nucleus Compton scattering Pair production off atomic electrons Photonuclear absorption

  28. Sampling vs. Total Absorption Calorimeters • A sample device uses a heavy material such as lead to convert the g's and then a sampling material such as plastic scintillator to "sample" the energy. The energy absorbed in the Pb is lost. • Examples of sampling devices • Pb-liquid Argon • Pb-optical fiber (Shaslik) • Typical Energy resolutions

  29. Total Absorption Devices • Idea here is to convert all the energy in an active medium • Media can be transparent crystals: CsI, PbWO4 or cryogenic liquids such as Krypton or Xenon (too expensive)

  30. Calorimeter Readout • Crystals • Outside magnetic field: photomultiplier tubes; advantages: fast, quantum eff ~20%, can be rad hard, but not cheap • Inside magnetic field: photodiodes, avalanche photodiodes (they have gain of ~50), phototriodes (essential the first 3 stages of a pm tube); apd's are not cheap • Cryogenic liquids: collect charge on strips, so use analog electronics

  31. Calorimeter picture

  32. Neutrino Detection • We cannot detect the small number of low energy neutrinos produced in b decays! • n cross-section s(nN) ~ 6x1039 cm-2 E(GeV) • If anyone could figure out how to detect neutrinos it would make experiments much easier!

  33. Charged Lepton Identification • e± use an electromagnetic calorimeter; shower is almost identical to a photon • m±use the fact that muons don't have strong interactions. Use thick blocks of iron and see if the particles penetrate. Problem: p± and K± decay into muons, so can get fakes

  34. Charged Hadron Identification • We are interested in separating p/K/p • Technique depends on momentum range • P < ~900 MeV: Time-of-flight and dE/dx • TOF equations • dE/dx picture • Some poor dE/dx info ~ 1.5 – 3 GeV/c

  35. Ring Imaging Cherenkov Counters • Cherenkov radiation depends on particle velocity, sinqc=1/nb, n is index of refraction • Measure p using other devices so can derive m, the particle mass • Many recent developments

  36. Radiators • Choice depends on velocity (or p range). Need nb>1 • Require material to be transparent • Desire low chromatic dispersion, i.e. n(E) to not be too bad • In the few GeV/c range can choose liquids, ex. C5F12 or solids such as quartz or LiF

  37. Photodetectors • Must match radiator light output wavelength spectrum to that of photodetector • Some possibilities • TMAE gas 170-210 nm • TEA gas 135-165 nm • CsI (thin layer) 170-210 nm • Phototube 250-550 nm } Use with wire chamber

  38. Example of Comple System: CLEO III RICH Detector • Use CH4-TEA gas to detect single photons. Sensitive in VUV 135-165 nm • Use LiF radiators • Use N2 volume 15.7 cm thick to allow for Cherenkov cone to expand • Use MWPC with pad readout to measure  positions

  39. One Cherenkov g Detector

  40. Mating the Radiators to the Photon Detectors

  41. The End

  42. Data Acquisition • This arcane area is crucial in a modern experiment • Functions include • The trigger: Which particle interactions do we read-out the detector? At Y(4S) ¼ of the e+e- annihilations are b's, but there is a much larger rate of Bhabha and 2 photon collisions. In hadron colliders the b rate is much much lower, 1/500 at the Tevatron

  43. DAQ continued • Online monitoring: Reads out monitoring signals, for HV gas, temperature, etc…Samples the data during the run and histograms critical parameters • Data Acquisition: When a trigger occurs, the information from all the devices must be taken from the "front end" electronics and moved to "tape." • Definition "Dead Time" – The time that the experiment must be shut off to move the data onto tape. Modern readout systems try to eliminate this nasty feature.

  44. dE/dx (mean energy loss) • Note 1/b2 fall, and ln(bg) rise, called relativistic rise • This is limited in materials by so called density effect, d term • This information can be used to distinguish particles

  45. Silicon strip Detectors • Difference between • spatial resolution • impact parameter resolution • decay length resolution Impact parameter: minimum distance of track from a vertex b Spatial resolution: inherent to the detector. For 50 mm strip width & using binary (yes/no) readout, r.m.s. resolution, s=50/12 Decay length: distance Between primary & secondary vertices L

  46. Example of Complete System • Delphi & SLD: Use Liquid radiators enclosed in quartz AND gas radiators. TMAE based wire chambers with long drift in E field

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