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