Particle Identification in High Energy Physics Experiments Time-of-Flight techniques Matthew Jones June 22, 2004 Introduction • Subatomic particles observed as “tracks” using a Wilson Cloud Chamber • Easy to distinguish two types of tracks Introduction • Thick tracks: particles (He nucleus) – Example: 222Rn (radon) 218Po + • Thin tracks: particles – Example: 14C 14N + - (+ e) Particle Identification Particle Physics Experiments • Massive particles need energy to produce: E = m c2 • Typical experiments involve collisions: Beam particle Beam particle Target particle Interaction Beam particle Decay products FIXED TARGET COLLIDING BEAMS Particle Accelerators 2000 1931 Particle Detectors Particle Detectors • Most read out electronic signals • Event data is interpreted and visualized using computers: Tracks What kinds of particles make tracks in a detector? Particle Structure • Examples: – Protons and neutrons (make atomic nuclei) – Electrons (nucleus + electrons = atom) – Muons (found in cosmic rays) • The Standard Model of Particle Physics describes these particles and more… Particle Structure • The most fundamental “building blocks” of nature are quarks and leptons. Electric charge • Six types of quarks: up charm top Q = +2/3 down strange bottom Q = -1/3 • Six types of leptons: electron muon Q = -1 e Q=0 Increasing mass Particle Structure • Quarks have fractional charge: – up, charm, top have Q = +2/3 |e| – down, strange, bottom have Q = -1/3 |e| • We can’t observe quarks as free particles • We do observe particles that are made out of specific combinations of quarks: – Proton = (uud) has charge +1 – Neutron = (udd) is electrically neutral Particle Structure • Types of hadrons: – Baryons contain three quarks – Mesons contain quark anti-quark pairs – Glueballs and hybrids (maybe) • Examples: – Pions: – Kaons: • More examples: – D-mesons: – B-mesons: Particle Masses Light… easy to produce Particle Mass proton, neutron » 1 GeV pions 0.14 GeV Heavy… hard to produce kaons 0.5 GeV D mesons 1.8 GeV B mesons 5 GeV electron 0.0005 GeV muon 0.11 GeV tau 1.8 GeV neutrinos » 0 (!) Particle Decays • Most particles are unstable and decay to lighter particles: 100% 64% 21% 9% 0.02% Lifetimes • How long do these particles live? • How far would they travel in a detector? Non-relativistic, v << c True for all = v/c • We also measure momentum in GeV: Typical Lifetimes Particle Lifetime ct d when p=1 GeV Proton 1 1 1 Long Electron 1 1 1 Long + - , 2.2 s 659 m 6.2 km Long +, - 26 ns 7.8 m 56 m Long K+, K- 12 ns 3.7 m 7.5 m Long D+, D- 1.1 ps 315 m 169 m Short B+, B- 1.7 ps 496 m 94 m Short +, - 0.29 ps 87 m 49 m Short Tracks in the detector • Charged tracks are almost always: – Protons – Electrons – Muons – Kaons – Pions • All other particles decay to these before they can be detected directly • How can we tell them apart? • Why would we want to? Electrons and Muons • Electrons produce Bremsstrahlung: Heavy nucleus • High energy gamma rays produce e+e-: e+ e- • Electrons lose energy quickly in dense material. • Muons don’t interact much at all! The CDF Detector Muon chambers Calorimeter Tracking chamber Pions, kaons and protons • They all look alike: – A charged track in the tracking chamber – Energy spread out in the calorimeter – Nothing in the muon chambers • How can we tell them apart? but first… • Why? Why perform particle ID? • Example: studying heavy mesons • Start with a muon… Particle reconstruction This is the muon… These are the tracks… Which tracks make the D+s? Particle reconstruction • The tracking chamber measures momentum • We guess at the mass • Relativistic kinematics: Particle reconstruction “combinatorial background” (mostly pions) Real K+K- • There are a lot more pions than kaons. Pions, kaons and protons • How can we tell them apart? Their mass! • How do we measure mass? – The tracking chamber measures momentum: Radius of curvature (cm) Magnetic field (Tesla) Transverse momentum (GeV) • Relations between mass and momentum: (when v << c) • Measure p and v… calculate m. Measuring t Production point Detector • How well we can measure t determines how well we can distinguish between the two particles. Separation power Particle ID for CDF-II • Requirements: – Good K- separation below »1.5 GeV/c – Perform measurements every 132 ns – Must fit in the available space 5 cm space between tracking chamber and superconducting magnet Time-of-Flight Detectors • Components: – Charged particle detector • Plastic scintillator – Photomultiplier tubes • Change light pulse into pulse of electric current – Electronics • To measure arrival time of electric pulses • Requirements: – Precise enough for good K- separation – Do it fast. (every 132 ns!) Scintillation Detectors • Plastic scintillator: – Emits blue light when hit by charged particles – Light should be emitted fast (few ns) • Light propagates by internal reflection off the smooth surfaces – The attenuation length should be long (few m) • Many people have used Bicron 408 for TOF detectors. Bicron-408 data sheet Optics • Light propagates by internal reflection • Light with large angles comes out later: • We can select which light to accept: Photomultiplier Tubes • Convert light to electric pulse (106 gain) – Photocathode emits photoelectrons – Dynodes produce secondary electrons – Charge collected at the anode 0 volts 200 V 400 V 600 V Photon Current 700 voltspulse 300 V 500 V Photocathode Dynodes Anode Photomultiplier tubes • What about the magnetic field? Photoelectron curles up in B field Photon • Possible solutions: – Guide the light out of the B field – Fine mesh photomultiplier tubes Example: The CLEO-II Detector Light guides Photomultiplier tubes Fine Mesh Photomultiplier Tubes • Secondary electrons accelerated parallel to the B-field. • Gain with no field: 106 • With B=1.4 Tesla: 104 • We also added a preamplifier (gain of »15). • Attach directly to ends of scintillator bars. • Manufactured by Hamamatsu Photonics Electronics • Typical configuration: ADC • Time between START and STOP converted to voltage • Voltage digitized with ADC TOF Electronics in CDF How is it done in 132 ns? • Parallel processing: pipeline architecture. Electronics Offline Analysis • Several corrections need to be applied to the measured time: – Propagation time in the scintillator – PMT pulse height corrections Pions – Measuring the collision time Kaons Protons Offline Analysis • Reconstructing low momentum ! K+K-: Remaining Issues • Achieved goal of “100 ps” timing resolution • Small fraction of events are biased: • Currently a subject of investigation. Other Particle ID Techniques • Ionization energy loss in gas • Emission of Cerenkov light – RICH detectors – Threshold detectors • Transition radiation detectors – Especially for e- separation Summary • A rich variety of particles exist in nature – But only 5 quarks (+ the top quark) • Only observe 5 directly: – e§, §, §, K§, p • Particle ID an important part of many HEP experiments.
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