HEP Experiments Detectors and their Technologies Sascha Marc Schmeling CERN

Overview  Introduction and Concepts  Properties of Particles –Are they measurable? –If yes, how?  HEP –Main Sub-Detectors –Infrastructure

Studying Interactions  by scattering  by annihilation  and the production of new particles  all interactions are produced in –Colliding Beam Experiments or –Fixed Target Experiments

Ideal Detectors?  In an ideal detector, one could record the full interaction, capture and measure all properties of all emerging particles, and by this reconstruct the complete event.  This would give us the power to compare the interaction directly to theoretical predictions without most uncertainties.

Particle Properties  Which properties does a particle have? –energy –momentum –charge –mass –life time –spin –decay modes  And which of those are measurable?

Particle Properties  Which properties can we derive?

Particle Properties  charge  lifetime

Measuring Particle Properties  momentum  velocity time of flight  energy calorimeter

Measurement Principles  Measurement occurs via the interaction (again…) of a particle with the detector (material) –creation of a measureable signal  Ionisation  Excitation/Scintillation  Change of the particle trajectory –curving in a magnetic field, energy loss –scattering, change of direction, absorption p e-e- p e-e- p p 

Which particles can be detected?  Charged Particles  Neutral Particles  Different particle types interact very differently with the detector material.

A Typical Detector Concept Interaction point Precision vertex detector tracking detector Magnetic spectrometer Electromagnetic calorimeter Hadronic calorimeter Muon detectors

Ingredients Tracking Subsystem Electromagnetic Calorimeter Hadronic Calorimeter Muon System

Passage of Particles  Electrons  Photons  Hadrons  Muons  Mesons

Tracking Detectors  measure the tracks of emerging particles  determine –charge and –momentum in connection with a magnetic field  tracks are reconstructed from measured space-points do not use dense material!

How do tracking detectors work?  two main flavors –ionization detectors  Geiger-Müller counter  MWPC  TPC  silicon detectors –scintillation detectors Multi-Wire Proportional Chamber Time Projection Chamber

t = 0 Ionization Counters + HV signal cathode Anode Wire Gas-filled tube t = t 1

Tracking Realization: wire chamber (MWPC) Nobel prize: G.Charpak, 1992 Anode wires Cathode: pads or wires x y

MWPC ITC (ALEPH) Inner Tracking Chamber

Time Projection Chamber Gas-filled cylinder Anode Wires MWPC gives r,  MWPC gives r,  E B z = v drift t

TPC ALICE TPC sector detail

ALEPH TPC

Limitations  Precision 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... 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 !?

Scintillation Detectors Photomultiplier: converts light into electronic signal Scintillating material Scintillating material PM Total reflection Put many fibers close to each other --> make track visible

Calorimeters  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

Electromagnetic Showers Block of Matter, e.g. lead Lead atom

How to measure the secondary particles?  1. 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

ALEPH ECAL pions electron

muons photons

How to measure the secondary particles?  2. With homogenous calorimeters, such as crystal calorimeters: signal photons Photo diode Crystal (BGO, PbWO 4,…)

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 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  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

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

Cherenkov Radiation  Particles which in a given medium travel faster than the speed of light in that medium emit radiation: Cherenkov radiation c 0 = speed of light in vacuum Cherenkovlight wavefront

HEP  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

ATLAS See

See

Infrastructure  experiments are not only detectors  you need –possibilities to control the detectors –possibilities to take the data out and record it –possibilities to analyze the recorded data –…