PARTICLE DETECTORS Günther Dissertori CERN-EP CERN Teachers Seminar July 2001

Outlook zIntroduction zWhat to measure, why? zBasic Principles yTracking yCalorimetry yParticle Identification zLarge detector systems zConclusions

Introduction zHE physics experiments study interaction of particles yby scattering of particles on other particles zResults of these interactions are ychange in flight direction/energy/momentum of original particles yproduction of new particles

Introduction... zThese interactions are produced in 1 2 p 2 = 0 zGoal zGoal : measure as many as possible of the resulting particles from the interaction yput detector “around” the interaction point p 1 = -p 2 12 Detector elements

What to measure, why? zIf we have an “ideal” detector, we can reconstruct the 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 z“ Ideal detector” measures yall produced particles ytheir energy, momentum ytype (mass, charge, life time, Spin, decays) z“ Ideal detector” measures yall produced particles ytheir energy, momentum ytype (mass, charge, life time, Spin, decays)

Measured quantities zThe creation/passage of a particle ( --> type) Electronic equipment eg. Geiger counter zIts four-momentum Energy momentum in x-dir momentum in y-dir momentum in z-dir EpEp =  Its velocity  = v/c p x p = p y p z

Derived properties zMass  in principle, if E and p measured: E 2 = m 2 c 4 + p 2 c 2  if v and p measured: p = m v /  (1 -  2 )  from E and p of decay products:  m 2 c 4 = (E 1 +E 2 ) 2 - (cp 1 + cp 2 ) 2 m E1,p1E1,p1 E2,p2E2,p2

Further properties... zThe charge (at least the sign…) yfrom curvature in a magnetic field  The lifetime  yfrom flight path before decay Magnetic field, pointing out of the plane Negative charge positive charge length

So, how measure the four-momentum? calorimeter zEnergy : from “calorimeter” (see later) zMomentum : magnetic spectrometer+tracking detector yfrom “magnetic spectrometer+tracking detector” Magnetic field, pointing out of the plane Negative charge positive charge R1R1 R2R2 p2p2 p1p1 p 1 <p 2  R 1 < R 2 q v B = m v 2 /R q B R = m v = p Lorentz-force zvelocity : ytime of flight or Cherenkov radiation (see later) L v t = L

Principles of a measurement zMeasurement occurs via the interaction (again…) of a particle with the detector(material) ycreation of a measureable signal xIonisation xExcitation/Scintillation xChange of the particle trajectory curving in a magnetic field, energy loss scattering, change of direction, absorption p e-e- p e-e- p p 

Detected Particles zCharged particles  e -, e +, p (protons),  , K  (mesons),   (muons) zNeutral particles   (photons), n (neutrons), K 0 (mesons),   neutrinos, very difficult) zDifferent particle types interact differently with matter (detector) (eg. photons do not feel a magnetic field) yneed different types of detectors to measure different types of particles

Typical detector concept zCombine different detector types/technologies into one large detector system Interaction point Precision vertex detector tracking detector Magnetic spectrometer Electromagnetic calorimeter Hadronic calorimeter Muon detectors

Tracking system Electromagnetic calorimeter Hadronic calorimeter Muon detector system Electron e - Photon  Hadron, eg. proton p Muon  - Meson K 0

Tracking Detectors zBasic goal zBasic goal: ymake the passage of particles through matter visible --> measure the tracks yReconstruct yReconstruct from the measured space points the flight path momentum yExtract information on the momentum (see previous transparencies) No dense materials yNOTE: the particle should not be too much affected by the detector: No dense materials !

This is achieved by zDetectors where yIyIonisation signals are recorded GGeiger-Müller counter MMWPC ( Multi-Wire Proportional Chambers ) TTPC ( Time Projection Chamber ) xsxsilicon detectors xBxBubble chambers (see separate lecture) yScintillation light is produced xeg. scintillating fibers

Principle of gaseous counters + HV signal cathode Anode Wire Gas-filled tube t = t = t 1  Track ionises gas atoms F electrons drift towards anode, ions towards cathode F around anode electrons are accelerated (increasing field strength)  further ionisation --> signal enhancement --> signal induced on wire

Principle of gaseous counters... gas filling

Now : Tracking zBasic idea : put many counters close to each other Realization: wire chamber (MWPC) Nobel prize: G.Charpak, 1992 Anode wires Cathode: pads or wires x y

Tracking: MWPC ITC (ALEPH) Inner Tracking Chamber

Further development: Time Projection Chambers (TPC) Gas-filled cylinder Anode Wires MWPC gives r,  MWPC gives r,  E B z = v drift t

TPC

Limitations zPrecision limited by wire distance Error on space point  d cannot be reduced arbitrarily! Uncertainties on space pointsUncertainties on track origin and momentum

Step forward: Silicon Microstrip Detectors Now precision limited by strip distance  m Now precision limited by strip distance  m Creation of electron-hole pairs by ionising particle Creation of electron-hole pairs by ionising particle Same principle as gas counters Silicon wafers, doped mm

Silicon microstrip detectors...

Silicon Microstrip detectors... ALEPH VDET OPAL VDET Future ATLAS tracking detector

Increase in precision 0 1cm x =Beam crossing point

Mean Lifetime of tau  =290 x sec !! --> c  = 87  m !?

Scintillating fibers zCertain materials emit scintillation light after particle passage (plastic scintillators, aromatic polymers, silicate glass hosts….) Photomultiplier: converts light into electronic signal Scintillating material Scintillating material PM Total reflection Put many fibers close to each other --> make track visible

Scintillating fibers...

Calorimetry zBasic principle: secondary particles and/or heat yIn the interaction of a particle with dense material all/most of its energy is converted into secondary particles and/or heat. yThese secondary particles are recorded xeg. Number, energy, density of secondaries proportional to xthis is proportional to the initial energy zNOTE: last year calorimetry was discussed in detail in talks prepared by teachers

Electromagnetic showers zInteractions of electrons and photons with matter: Matter block, eg. lead Lead atom zShower partially or completely absorbed

How to measure the secondaries? sampling calorimeters z1. With sampling calorimeters: Dense blocks, such as lead Detectors, such as wire chambers, or scintillators Sandwich structure ! Total amount of signals registered is proportional to incident energy. But has to be calibrated with beams of known energy! Sandwich structure ! Total amount of signals registered is proportional to incident energy. But has to be calibrated with beams of known energy!

Sampling Calorimeters

e+e+ e-e-

ALEPH ECAL pions electron

muons photons

How to measure the secondaries? homogenous calorimeters, such as crystal calorimeters z2. With homogenous calorimeters, such as crystal calorimeters: signal photons Note : these crystals are also used in other fields (eg. Medical imaging, PET) Photo diode Crystal (BGO, PbWO 4,…)

CMS

L3

Hadronic calorimeters zHadronic particles (protons, neutrons, pions) can traverse the electromagnetic calorimeters. They can also interact via nuclear reactions ! zUsually: Put again a sampling calorimeter after the ECAL Dense blocks, such as iron, uranium Detectors, such as wire chambers, or scintillators 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! 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!

ALEPH  iron

Particle Identification zBzBasic principles: yvyvia different interaction with matter (see previous transparencies) ybyby measuring the mass from the decay products ybyby measuring the velocity and i ii independently (!) the momentum yOyObservables sensitive to velocity are xmxmean energy loss xCxCherenkov radiation

Mean energy loss by ionization zParticles which traverse a gas loose energy, eg. by ionization  E lost  / path length = func( particle-velocity v/c ) Bethe-Bloch formula   E lost   amount 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! Note : if plotted as a function of v and not p all the bands would lie on top of each other!

Cherenkov radiation faster than the speed of light in that medium Cherenkov radiation zParticles which in a medium travel faster than the speed of light in that medium emit a radiation --> Cherenkov radiation c 0 = speed of light in vacuum Cherenkovlight wavefront Compare : shock wave of supersonic airplanes See for a nice illustration

Large detector systems zAll these concepts have been put together and realized in large detector systems zExamples at LEP yALEPH yALEPH, OPAL, L3, DELPHI zFixed Target yNA48 zFuture experiments at LHC yATLAS, yATLAS, CMS, LHCb, ALICE

ATLAS See

See

Summary zI have tried to explain ywhat ywhat are the things we want to measure in HEP experiments yhow yhow we do it ( tracking, calorimetry, particle identification ) zThis is an enormously large field, of course many things have been left out yDAQ (data acquisition) yother detector technologies yapplications in particle astrophysics (cosmic rays, neutrinos,…) yapplications outside HEP zI invite you to study these points