Accelerators and Detectors An introduction Barbro Åsman

Why accelerate particles? Because: They should probe an other particles The high kinematic energy should be transformed into mass according to: E = mc2 e

The Discovery of the quarks 3 km lång accelerator

Linear Accelerator - + - + + + - + + - + + High voltage

Charged particles in a magnetic field Moving charged particles are affected by a magnetic field R = P B x q

Elektron i magnetfält High velocity low velocity

Cyklotron

Synkrotron SPS 0.045 - 1.800 Tesla

Why are the accelerators so big? The magnet fields can not be made too strong B = p / r x q Charged particles radiate synchrotron light when they accelerate. Energy Loss per particle and turn : DE =4pe2b3g4/(3xr) r = bending radius , b = v/c , g = (1 – b2) -1/2 = E / m0c2

High energy laboratories FNAL DESY CERN BNL KEK SLAC

See particles ? We can determine: The particles direction The type of particle The particles energy

Detecting Particles

Electromagnetic Interaction of Particles with Matter The incoming particle loses energy . The atoms are excited or ionized. The particle is deflected multiple scattering During scattering a photon can be emitted. Bremstrahlung The particle’s velocity larger than the velocity of light in the medium, -> Cherenkov Radiation. The particle crosses the boundary between two media -> Transition radiation.

Bethe - Bloch re = classical electron radius = e2/(4pe0mec2 ) me = electron mass NA = Avogadro's number I = mean excitation energy Z = atomic number A = atomic weight = density z = charge of incoming particle B = v/c of incoming particle Wm= the maximum transferable energy of the particle d = “density – effect” C = shell correction term

What is Wm ? Ek + mec2 = p´2c2 + me2c4 Homework -> Scattered particle with momentum p´´ q Scattered electron with momentum p´ Incoming particle with momentum p Ek + mec2 = p´2c2 + me2c4 Where Ek is the kinetic energy of the electron Homework ->

What is Wm ? Maximum energy transfer is when q = 0 Wm = 2mec2b2g2 1 + (me/m)2 + 2g(me/m) -1 me < m -> g >> m/me -> g <<m/me -> m = me -> Wm = 2mec2b2g2 2g(me/m) -1 Wm = mec2b2g2 / (1+g) Wm = 2mec2b2g2 1 + 2g(me/m) -1 Wm = 2mec2b2g2

-b2 comes from a quantum treatment of the energy loss by collisions of heavy spin 0 particles -d (g) is due to density effects, increases with energy -C/Z is due to non participation of inner electrons. Small and often ignored.

A B C D A rapid decrease following 1/b2 B minimum at about g = 3 C slow increase following ln(g ) D a plateau is reached due to density effects. 1) for low density materials like gases at g=1000 2) for solids g could be as low as 10

 A 1 GeV muon can traverse 1m of Iron 1/ dE/dx  1.4 MeV cm 2/g for ßγ  3 Thickness = 100 cm; ρ = 7.87 g/cm3 dE ≈ 1.4 * 100* 7.87 = 1102 MeV  A 1 GeV muon can traverse 1m of Iron 1/ This number must be multiplied with ρ [g/cm3] of the Material  dE/dx [MeV/cm]

Ionization in emulsion Kaon Pion Small energy loss  Fast particle Large energy loss  Slow particle

Range of particle in matter B C D Bragg Peak: For >3 the energy loss is  constant The energy of the particle falls below =3 the energy loss rises as 1/2

Energy Loss as a Function of the Momentum The energy loss as a function of particle momentum P= Mcβγ By measuring the particle momentum and measurement of the energy loss -> measure the particle mass  Particle Identification

Measure momentum by curvature of the particle track. Find dE/dx by measuring the deposited charge along the track. Particle ID

Energy Loss Fluctuations X X X X X XX X X X X X XXX X XX X XXX X E E -   The energy loss is a statistical process and will therefore fluctuate from event to event.

Landau Distribution P() P(): Probability for energy loss  in matter of thickness D. Landau distribution is very asymmetric. Average and most probable energy loss must be distinguished ! Energy loss (arbitrary scale)

Electrons in Matter Ionization: Electrons compared to heavier particles they are always relativistic they can lose all energy in one interaction interaction of two identical particles has to be allowed according to quantum mechanics

Electrons ionization in Matter dE = 2Cmc2 Zr ln pg3/2mc2 – a/2 – d/2 – C/Z dx A I C = 2pNAre2 b = 1 z = 1 a = 2.9 for electron a = 3.6 for positron Heavy particle dE = 2Cmc2 Zr ln 2g2mc2 – 1 – d/2 – C/Z dx A I

Electromagnetic Interaction of Particles with Matter The incoming particle loses energy . The atoms are excited or ionized. The particle is deflected multiple scattering During scattering a photon can be emitted. Bremstrahlung The particle’s velocity larger than the velocity of light in the medium, -> Cherenkov Radiation. The particle crosses the boundary between two media -> Transition radiation.

Bremsstrahlung Electron are accelerated in the field of the nuclei and radiate. Passage through matter results in emission of high energy photons.

Bremsstrahlung small g large g X0(cm): Distance where the Energy E0 of the incoming particle decreases E0 e-1

Electron in Matter

Critical Energy Muon Momentum Electron Momentum 5 50 500 MeV/c Critical Energy: If dE/dx (Ionization) = dE/dx (Bremsstrahlung) Myon in Copper: p  400GeV Electron in Copper: p  20MeV

Electromagnetic Interaction of Particles with Matter The incoming particle loses energy . The atoms are excited or ionized. The particle is deflected multiple scattering During scattering a photon can be emitted. Bremstrahlung The particle’s velocity larger than the velocity of light in the medium, -> Cherenkov Radiation. The particle crosses the boundary between two media -> Transition radiation.

Multiple scattering XFe = 2 cm XBe = 33 cm Statistical analysis of multiple collisions gives: The root mean square of the projected scattering angle distribution is given by: x = the thickness of the material X0 = radiation length of the material Z1 . = charge of the incoming particle bc = velocity of the incoming particle p = momentum of the incoming particle XFe = 2 cm XBe = 33 cm

LEP Experimental Chambers All 4 LEP experiments had beryllium vacuum chambers around the IP Longest LEP chamber was OPAL Phase II Each LEP chamber was removed and re-fitted ~5 times LEP chambers were baked-out in the lab, not in-situ as for ALICE Each LEP chamber was mechanically modified at least once Comparison of OPAL and ALICE Dimensions Inner diameter (mm) Wall thickness (mm) Be length (mm) Total Length (mm) OPAL Phase II 106 1.1 4500 6000 ALICE 58 0.8 3950 4750

Electromagnetic Interaction of Particles with Matter The incoming particle loses energy . The atoms are excited or ionized. The particle is deflected multiple scattering During scattering a photon can be emitted. Bremstrahlung The particle’s velocity larger than the velocity of light in the medium, -> Cherenkov Radiation. The particle crosses the boundary between two media -> Transition radiation.

Cherenkov radiation Charge particles polarizes atoms along its track -> they become electric dipoles. The time variation of the dipole field -> emission of electromagnetic radiation v < c/n -> the dipoles are symmetric along the track -> integration over all dipoles -> no field v > c/n -> the dipoles are asymmetric along the track -> cherenkov radiation Cherenkov radiation contributes little to the energy loss . For gases Z>7 less than 1% ionization For He, H about 7% less than ionization

Cherenkov Light . c/n qc bc Threshold: Cherenkov radiates on if b >bt = 1/n ; Max is reached when b=1 Max is small for gases 1,40 in air and 450 in glass. For a fix energy the threshold depends on the of the mass of the particle -> particle identification The major problem of Cherenkov radiation is the modest light output:

Cherenkov light Threshold Cherenkov detectors Only particles with velocity above the threshold are detected. Differential Cherenkov detectors Only particles in a range of velocities are detected Ring-Imaging Cherenkov detectors Spherical mirrors are used to focus the cone of Cherenkov light on a position sensitive light detector. There are only ‘a few’ photons per event  one needs highly sensitive photon detectors to measure the rings !

Delphi particle identification Protons from L decay p from K decay K+- from D0 decay

Transition Radiation Radiation emitted by ultra relativistic particles when they transverse the boarder of two materials of different dielectric permittivity. The intensity of transition radiation is roughly proportional to the particle energy, I = mg -> Particle Identification at highly relativistic energies The typical emission angle is 1/γ.

Transition Radiation Radiator Detector The number of photon can be increased by placing many foils with low Z The Photons would otherwise not be able to escape the radiator

Electromagnetic Interaction of Particles with Matter The incoming particle loses energy . The atoms are excited or ionized. The particle is deflected multiple scattering During scattering a photon can be emitted. Bremstrahlung The particle’s velocity larger than the velocity of light in the medium, -> Cherenkov Radiation. The particle crosses the boundary between two media -> Transition radiation.

Detecting Particles

Photons in Matter Photoelectric effect Compton Scattering Pair Production

For E>>mec2=0.5MeV :  = 9/7X0 the attenuation length due to pair production is 9/7 times the radiation length. σp.e. = Atomic photoelectric effect σRayleigh = Rayleigh (coherent) scattering–atom neither ionized nor excited σCompton = Incoherent scattering (Compton scattering off an electron) knuc = Pair production, nuclear field ke = Pair production, electron field

Bremsstrahlung + Pair Production = Electromagnitic Shower

Detecting Particles

Strong Interaction Hadrons interact with nuclei through the strong interaction and built up showers in matter. The total cross section is given by The average interaction length is given by The interaction range can be calculated from the inelastic part of the hadronic cross section and one gets: A = mass of the material (g/mol) NA =Avogadros number (mol-1) r = density (g/cm3) rl gives the area density in g/cm2

Electromagnetic shower versus Hadronic shower For the hadron interactions: Many different final states Up to 30% of incident energy may be lost The cross section is a function of both energy and particle type

Particle identification Time of Flight -> measure of velocity Cherenkov radiation detectors Transition radiation Ionization Development of showers separate electron and hadrons

Detecting Particles

Detecting Neutrino

If neutrinos have mass: CNGS If neutrinos have mass: ne nt nm Muon neutrinos produced at CERN. See if tau neutrinos arrive in Italy.

Neutrinos at CNGS: Some Numbers For 1 day of CNGS operation, one expect: protons on target 2 x 1017 pions / kaons at entrance to decay tunnel 3 x 1017 nm in direction of Gran Sasso 1017 nm in 100 m2 at Gran Sasso 3 x 1012 nm events per day in OPERA  25 per day nt events (from oscillation)  2 per year

Opera Experiment at Gran Sasso Basic unit: brick 56 Pb sheets + 56 photographic films (emulsion sheets) Lead plates: massive target Emulsions: micrometric precision 8.3kg brick Brick Pb Couche de gélatine photographique 40 mm n t 1 mm 10.2 x 12.7 x 7.5 cm3

Opera Experiment at Gran Sasso 31 target planes / supermodule In total: 206336 bricks, 1766 tons SM1 SM2  Targets Magnetic Spectrometers First observation of CNGS beam neutrinos : August 18th, 2006 W. Riegler/CERN

Electromagnetic Interaction of Particles with Matter The incoming particle loses energy . The atoms are excited or ionized. The particle is deflected multiple scattering During scattering a photon can be emitted. Bremstrahlung The particle’s velocity larger than the velocity of light in the medium, -> Cherenkov Radiation. The particle crosses the boundary between two media -> Transition radiation.

Detectors Based on Excitation Emission of photons of by excited atoms, typically UV to visible light. Scintillation Counters Used for: Triggers Time of Flight … 1 cm thick Inorganic for example NaI 104 3 - 4 ns Organic for example Plexiglass 3x104 250 ns

Scintillation Counters Photons are being reflected towards the ends of the scintillator. A light guide brings the photons to the Photomultipliers where the photons are converted to an electrical signal.

Rutherford’s experiment

Detectors based on Ionization Gas detectors: Wire Chambers Drift Chambers Time Projection Chambers Transport of Electrons and Ions in Gases Solid State Detectors Transport of Electrons and Holes in Solids Si- Detectors Diamond Detectors Detectors based on Ionization

Some General Features Resolution time Dead time 2ns 100 ns 100-150 ps 10 ns Spatial resolution 1 mm 2 mm – 2 mm Drift chambers Scintillators Photo emulsion Silicon pixel

High energy physics detectors Tracking with momentum measurement Electron and hadron separation Particle identification Energy determination Triggering Data acquisition

Two jet event

Why Four LEP Detectors? DELPHI OPAL L3 ALEPH