PARTICLE DETECTORS Günther Dissertori CERN-EP CERN Teachers Seminar July 2001
Outlook • Introduction • What to measure, why? • Basic Principles • Tracking • Calorimetry • Particle Identification • Large detector systems • Conclusions
Introduction • HE physics experiments study interaction of particles • by scattering of particles on other particles • Results of these interactions are • change in flight direction/energy/momentum of original particles • production of new particles
Detector elements p1 = -p2 1 2 p2 = 0 1 2 Introduction... • These interactions are produced in • Goal : measure as many as possible of the resulting particles from the interaction • put detector “around” the interaction point
What to measure, why? • If we have an “ideal” detector, we can reconstructthe interaction, ie. obtain all possible information on it. This is then compared to theoretical predictions and ultimately leads to a better understanding of the interaction/properties of particles • “Ideal detector” measures • all produced particles • their energy, momentum • type (mass, charge, life time, Spin, decays)
Electronic equipment eg. Geiger counter • Its four-momentum E p Energy momentum in x-dir momentum in y-dir momentum in z-dir = px p = py pz Measured quantities • The creation/passage of a particle ( --> type) • Its velocity b = v/c
E1,p1 m E2,p2 Derived properties • Mass • in principle, if E and p measured:E2 = m2c4 + p2c2 • if v and p measured:p = m v / (1 - b2) • from E and p of decay products: • m2 c4 = (E1+E2)2 - (cp1+ cp2)2
Negative charge Magnetic field, pointing out of the plane positive charge length Further properties... • The charge (at least the sign…) • from curvature in a magnetic field • The lifetime t • from flight path before decay
Negative charge Magnetic field, pointing out of the plane Lorentz-force q v B = m v2/R p2 positive charge R1 p1 R2 q B R = m v= p p1<p2 R1 < R2 v t = L L So, how measure the four-momentum? • Energy : from “calorimeter” (see later) • Momentum : • from “magnetic spectrometer+tracking detector” • velocity : • time of flight or Cherenkov radiation (see later)
e- p p p g e- p • Change of the particle trajectory • curving in a magnetic field, energy loss • scattering, change of direction, absorption Principles of a measurement • Measurement occurs via the interaction (again…) of a particle with the detector(material) • creation of a measureable signal • Ionisation • Excitation/Scintillation
Detected Particles • Charged particles • e-, e+, p (protons), p, K (mesons), m (muons) • Neutral particles • g (photons), n (neutrons), K0 (mesons), • n (neutrinos, very difficult) • Different particle types interact differently with matter (detector) (eg. photons do not feel a magnetic field) • need different types of detectors to measure different types of particles
Interaction point Magnetic spectrometer tracking detector Hadronic calorimeter Muon detectors Electromagnetic calorimeter Precision vertex detector Typical detector concept • Combine different detector types/technologies into one large detector system
Electromagnetic calorimeter Muon detector system Tracking system Hadronic calorimeter Electron e- Photon g Hadron, eg. proton p Muon m- Meson K0
Tracking Detectors • Basic goal: • make the passage of particles through matter visible --> measure the tracks • Reconstruct from the measured space points the flight path • Extract information on the momentum(see previous transparencies) • NOTE: the particle should not be too much affected by the detector: No dense materials !
This is achieved by • Detectors where • Ionisation signals are recorded • Geiger-Müller counter • MWPC(Multi-Wire Proportional Chambers) • TPC (Time Projection Chamber) • silicon detectors • Bubble chambers (see separate lecture) • Scintillation light is produced • eg. scintillating fibers
- + cathode - + - + - + - + t = 0 signal Anode Wire + Gas-filled tube + + HV + + - + - - - - t = t1 Principle of gaseous counters • Track ionises gas atoms • electrons drift towards anode, ions towards cathode • around anode electrons are accelerated (increasing field strength) • further ionisation --> signal enhancement --> signal induced on wire
gas filling Principle of gaseous counters...
Anode wires Cathode: pads or wires Realization: wire chamber (MWPC) Nobel prize: G.Charpak, 1992 y x Now : Tracking • Basic idea : put many counters close to each other
ITC (ALEPH) Inner Tracking Chamber Tracking: MWPC
MWPC gives r,f E B - - + - + + - + - Anode Wires - + + Gas-filled cylinder z = vdrift t Further development:Time Projection Chambers (TPC) - - - - - - -
Uncertainties on space points Uncertainties on track origin and momentum Limitations • Precision limited by wire distance Error on space point d cannot be reduced arbitrarily!
0.2 - 0.3 mm Now precision limited by strip distance 10 - 100 mm Silicon wafers, doped Creation of electron-hole pairs by ionising particle Same principle as gas counters Step forward:Silicon Microstrip Detectors
OPAL VDET Future ATLAS tracking detector ALEPH VDET Silicon Microstrip detectors...
Increase in precision =Beam crossing point 0 1cm x
Total reflection Photomultiplier: converts light into electronic signal PM Scintillating material Put many fibers close to each other --> make track visible Scintillating fibers • Certain materials emit scintillation light after particle passage (plastic scintillators, aromatic polymers, silicate glass hosts….)
Calorimetry • Basic principle: • In the interaction of a particle with dense material all/most of its energy is converted into secondary particles and/or heat. • These secondary particles are recorded • eg. Number, energy, density of secondaries • this is proportional to the initial energy • NOTE: last year calorimetry was discussed in detail in talks prepared by teachers
Electromagnetic showers • Interactions of electrons and photons with matter: Lead atom Matter block, eg. lead • Shower partially or completely absorbed
Sandwich structure ! Total amount of signals registered is proportional to incident energy. But has to be calibrated with beams of known energy! Detectors, such as wire chambers, or scintillators Dense blocks, such as lead How to measure the secondaries? • 1. With sampling calorimeters:
ALEPH ECAL pions electron
signal Photo diode photons Crystal (BGO, PbWO4,…) How to measure the secondaries? • 2. With homogenous calorimeters, such as crystal calorimeters: Note : these crystals are also used in other fields (eg. Medical imaging, PET)
Sandwich structure ! Total amount of signals registered is proportional to incident energy. Same energy lost in nuclear excitations! Has to be calibrated with beams of known energy! Detectors, such as wire chambers, or scintillators Dense blocks, such as iron, uranium Hadronic calorimeters • Hadronic particles (protons, neutrons, pions) can traverse the electromagnetic calorimeters. They can also interact via nuclear reactions ! • Usually: Put again a sampling calorimeter after the ECAL
Particle Identification • Basic principles: • via different interaction with matter (see previous transparencies) • by measuring the mass from the decay products • by measuring the velocity and independently (!) the momentum • Observables sensitive to velocity are • mean energy loss • Cherenkov radiation
Elost / path length = func( particle-velocity v/c ) Bethe-Bloch formula Mean energy loss • Particles which traverse a gas loose energy, eg. by ionization • Elostamount of ionization size of signals on wires Note : if plotted as a function of v and not p all the bands would lie on top of each other!
Cherenkovlight wavefront Compare : shock wave of supersonic airplanes c0 = speed of light in vacuum Cherenkov radiation • Particles which in a medium travel faster than the speed of light in that medium emit a radiation --> Cherenkov radiation See http://webphysics.davidson.edu/applets/applets.html for a nice illustration
Large detector systems • All these concepts have been put together and realized in large detector systems • Examples at LEP • ALEPH , OPAL , L3 , DELPHI • Fixed Target • NA48 • Future experiments at LHC • ATLAS, CMS, LHCb, ALICE