Nuclear and Particle Physics Franz Muheim 1
Accelerators and Accelerators and DetectorsDetectors
AcceleratorsLinear AcceleratorsCyclotrons and SynchrotronsStorage Rings and CollidersParticle Physics Laboratories
Interactions of Particles with MatterCharged ParticlesNeutral Particles, Photons
Detectors in Particle PhysicsPosition sensitive devicesCalorimetersParticle Identification
ExperimentsMainly at colliders
OutlineOutline
SVTSVT1.5-T Solenoid
Nuclear and Particle Physics Franz Muheim 2
Particle AcceleratorsParticle Accelerators
Accelerator PrincipleCharged particles are accelerated to high energies using electromagnetic fieldse-, e+, p, anti-p, ionised nuclei, muons
Why are Accelerators used?Higher energies or momentaallow to probe shorter distancesde Broglie wavelength
e.g. 20 GeV/c probes 0.010 fmCockroft-Walton Accelerator
High DC voltage accelerates particles through steps created by a voltage dividerLimited to ~ 1 MV
Fermilab InjectorIonised hydrogen, H2
- sourceaccelerated to 750 kV
Van de Graaff Acceleratorcharge transported by belt Limited to ~ 10 MV
[GeV/c] fmMeV 197
ppcc ⋅==
hλ
Nuclear and Particle Physics Franz Muheim 3
Linear Accelerators Linear Accelerators -- LinacLinac
Working Principle - LinacCharged particles in vacuum tubesaccelerated by Radio Frequency (RF) waves
RF tubes increase in length size aparticle speed increases (protons, for e- v≅c)
Radio Frequency AccelerationRadio Frequency fields O(few 100 MHz) Field strengths – few MV/m - klystronstransported by RF cavities Oscillating RF polarities producesuccessive accelerating kicksto charged particles when RF is deceleratingparticles shielded in RF tubesParticles in phase with RF
Fermilab Injector400 MeV protons, 150 m long
Stanford Linear Accelerator (SLAC)Largest Linac - 3 km long, 50 GeV e- and e+
Nuclear and Particle Physics Franz Muheim 4
Circular AcceleratorsCircular Accelerators
Cyclotron
Charged particles are deflected in magnetic field B Lorentz Force
radius of curvature ρ
Particle accelerated by RFin magnet with E perp. BProtons, limited to ~ 10 MeVrelativistic effects
SynchrotronB-field and RF synchronised SpS at CERNwith particle speedradius ρ stays constantSuperconducting dipole magnetsB-fields up to 8 TeslaQuadrupole magnets focus beamalternate focusing and defocusingin horizontal and vertical plane
Synchrotron RadiationAccelerated particles radiateEnergy loss per turnmost important for e-
CyclotronCyclotron
SynchrotronSynchrotron
BveFL
rrr∧=
[ ] [ ] [ ]mTBcGeVp ρ3.0/ =
ργβπ 432
34 eE =∆
Nuclear and Particle Physics Franz Muheim 5
Storage Ring Storage Ring -- CollidersColliders
Beams from synchrotron or linachave bunch structure
Secondary BeamsAccelerated beam from synchrotron or linacon target → e, µ, π, p, K, n, ν, ZAX beamsMany different types of experiments
Storage RingsParticle beams accelerated in synchrotron and stored for extended periods of time
CollidersTwo counter-rotating beams collide at several interaction points around a ring Luminosity
FermilabChicago, http://www.fnal.govTevatronCurrent highest ECoMenergy collider1 TeV p on 1 TeV anti-pmaximum 1012 anti-p
Ni # of particles in beam InB # of bunches/beamrx,y beam dimension in x,yf revolution frequency
Byx
fnrrNNL 21=
Nuclear and Particle Physics Franz Muheim 6
Storage Ring Storage Ring -- CollidersColliders
CERNGeneva, Switzerland, http://www.cern.chPS -- 29 GeV, injector for SPSSPS -- 450 GeV p onto 450 GeV anti-p
injector for LEP/LHCLEP -- 100 GeV e- onto 100 GeV e+
LHC -- will start in 20077 TeV p onto 7 TeV p4 experimentsATLAS, CMS, LHCb, Alice
Nuclear and Particle Physics Franz Muheim 7
Particle Physics Particle Physics LaboratoriesLaboratories
CERN, FermilabSee previous slides
SLACCalifornia, http://www.slac.stanford.eduSLC -- 50 GeV e- onto 50 GeV e+PEP-II -- 9.0 GeV e- onto 3.1 GeV e+
B-factory
DESYHamburg, http://www.desy.deHERA -- 920 GeV p onto 30 GeV e-
KEKJapan, http://www.kek.jpKEKB -- 8.0 GeV e- onto 3.5 GeV e+
B-factoryJPARC -- 50 GeV synchroton (in construction)
CornellIthaca, NY, http://www.lns.cornell.eduCESR -- 3… 6 GeV e- onto 3 … 6 GeV e+
Nuclear and Particle Physics Franz Muheim 8
Interactions with MatterInteractions with Matter
Basic PrinciplesMainly electromagnetic interactions, ionization and excitation of matter“Applied QED”, lots of other interesting physics
Charged ParticlesElectrons, positronsHeavier particles: µ, π, K, p, ...
Energy loss Inelastic collisions with the atomic electrons
DeflectionElastic scattering from nuclei
BremsstrahlungPhoton emission (Scintillation, Cherenkov)
Neutral ParticlesPhotons
Photo electric effectCompton scatteringPair Production
Neutrons, Neutral hadronsNuclear reactions
NeutrinosWeak reactions
Nuclear and Particle Physics Franz Muheim 9
Energy LossEnergy Loss
Coulomb ScatteringTraversing charged particle scatters off atomic electronsof medium, causes ionisation
Energy loss of charged particlesby ionisation
N0 - Avogadro’s numberZ, A - atomic and mass number of medium x - path length in medium in g/cm2
x = ρt mass density ρ and thickness t in cmdE/dx measured in [MeV g-1 cm2]α - fine structure constant re = e2/4πε0mec2 = 2.82 fm (classical e radius)β, γ - speed and Lorentz boost of charged particle Maximum energy transfer TmaxMean excitation energy I
Valid for “heavy” particles (m≥mµ)
e- and e+ (mproj = mtarget) → BremsstrahlungdE/dx ~ 1/mtarget → scattering off nuclei very small
⎥⎦
⎤⎢⎣
⎡−−−=
22ln14 2
2
max222
21
2222 δβ
γββ
πI
TcmAZzcmrN
dxdE e
eeA
( ) eV10 with //21
2002
222max =≈
++= IZII
MmMmcmT
ee
e
γγβ
e-
θ
khh ,ω
0,mvr
Bethe-Bloch
Nuclear and Particle Physics Franz Muheim 10
BetheBethe--BlochBloch
⎥⎦
⎤⎢⎣
⎡−−−=
22ln14 2
2
max222
21
2222 δβ
γββ
πI
TcmAZzcmrN
dxdE e
eeA
dE/dx Energy Dependenceonly on βγ independent of mprojFor small β dE/dx ∝ 1/β2
Minimum Ionising ParticlesFor βγ ≈ 4 or β ≈ 0.97 (MIP)
Relativistic riseFor βγ >> 1 ln γ2 termRelativistic expansion of transverse E-field larger for gases than dense media
Density effect δ termCancels relativistic rise cancelled at very high γpolarization of medium screens more distant atoms
dE/dx rather independent of Zexcept hydrogen
212.1 cm MeV g..dxdE −≈
Nuclear and Particle Physics Franz Muheim 11
Interaction of PhotonsInteraction of Photons
How do Photons interact?Photons are neutralPhotons can create charged particlesor transfer energy to charged particles
Photoelectric effectCompton scatteringe+e- Pair Production
in Coulomb field of nucleuswhich absorbs recoil, requires
γ energy does not degrade, intensity is attenuatedAbsorption of Photons in Matter
µ: Mass attenuationcoefficient
[ ]gcmA
Ni
Ai /2σµ =
I: Intensityx: Target thickness
22 cmE e≥γ
nucleuseenucleus +→+ −+γ
Z
e +
e -
( )...
exp0
+++=
−=
pairComptonphoto
xII
µµµµ
µγ
−+ +→+ eatomatomγ
e-
X+X
Nuclear and Particle Physics Franz Muheim 12
Particle DetectionParticle Detection
Experimental MeasurementsMomentum, energy, and mass identificationof charged and neutral long-lived particlese, µ, π, K, p, γ, n, ν
Hadrons versus QuarksExperiments/detectors measure hadronsTheory predicts quark and parton distributionsHadronisation by Monte Carlo methods
Nuclear and Particle Physics Franz Muheim 13
Charged Particle TrackingCharged Particle Tracking
Momentum MeasurementCharged particle trajectories are curvedin magnetic fieldsmeasure transverse momentum
Tracking Detectors (before 1970)mostly optical tracking devices - cloud chamber,bubble chamber, spark chamber, emulsionsSlow for data taking (triggering) and analysis
Geiger-Mueller CounterIonisation in gasO(100 e- ion-pairs/cm)Avalanche multiplicationnear wire with gain up to 106
Multi-Wire-Proportional Chamber MWPC
Many wires in a planeact as individual countersTypical dimensions: L = 5 mmd = 1 mm, a = 20 µmsignal ∝ ionisationFast, high rate capabilitySpatial resolution limitedby wire spacing
Anode wire
Signal
Cathode
+V
Gas
CharpakInventor of MWPC
[ ] [ ] [ ]mTBcGeVp ρ3.0/ =⊥
Multi-Wire-Proportional Chamber MWPCMulti-Wire-Proportional Chamber MWPC
Nuclear and Particle Physics Franz Muheim 14
Tracking DetectorsTracking Detectors
Drift chamber
Measure also drift timeDrift velocities 5 … 50 mm/µsImproved spatial resolution100 … 200 µmFewer wires, less materialVolumes up to 20 m3
Best for e+e- detectorsBaBar experiment (SLAC)
~29000 wires~7000 signal wires
Silicon detectorsMetal strips, pads, pixels evaporated on silicon waferSemi-conductor deviceReverse bias modeTypically 50µm spacing and 10 µm resolution
CMS experiment (LHC)250 m2 silicon detectors~ 10 million channels
Drift Chamber Drift Chamber
anode
TDCStartStop
DELAYscintillator
drift
low field region drift
high field region gas amplification
Silicon Detectors Silicon Detectors
300µm
Nuclear and Particle Physics Franz Muheim 15
CalorimetersCalorimeters
Shower CascadesElectron lose energy by Bremsstrahlung
High energy e- (e+) or γ-> electromagnetic showersπ, K, p, n, … produce hadronic showers
Sampling CalorimeterAlternate betweenabsorber materials (Fe, Pb, U) andactive layers (e.g. plastic scintillators)
Homogeneous CalorimeterMeasures all deposited energyExamples: scintillating crystals (NaJ, CsI, BGO, …)or cryogenic liquids (argon, krypton, xenon)
Energy measurementBetter resolution forelectromagnetic shower
BaBar experiment6580 CsI (Tl) crystals
2
2
2
2
0
22
31
183ln4
14mE
ZE
mcez
AZN
dxdE
A ∝⎟⎟⎠
⎞⎜⎜⎝
⎛=−
πεα
Nuclear and Particle Physics Franz Muheim 16
Particle IdentificationParticle Identification
How do we measure Particle type?Uniquely identified by its mass mParticles have different interactionsMomentum p = mγβc of charged particles measured with tracking detectors
Electrons, PhotonsElectromagnetic Calorimeter (crystal)Comparison with momentum (electron)Shower shape (electrons, photons)
Charged Particle IdentificationHave momentum p = mγβc γ2 =1/(1-β2)Need to measure particle velocity v = βcCharged particles radiate Cherenkov photons in medium with speed v larger than c/n (refr. index n)
Cherenkov anglemeasures v = βc
Ring Imaging Cherenkov Detector
MuonsMost penetrating particle little Bremsstrahlunga few metres of iron and only muons are left
1)(with1cos ≥== λβ
θ nnnC