experimental methods of particle physics -...

Experimental Methods ofExperimental Methods ofParticle PhysicsParticle Physics

(PHY461)(PHY461)Fall 2016Fall 2016

Olaf SteinkampOlaf Steinkamp

044 63 5576336-J-22 [email protected]

mailto:[email protected]

Introduction (2) O. SteinkampEMPP HS16

Overview1) Introduction / motivation

● measurement of particle momenta: magnetic field

● early detectors: cloud chamber, bubble chamber, spark chamber

2) Gaseous tracking detectors

● Multi Wire Proportional Chamber

● Drift Chambers

● Micro-Pattern Gaseous Detectors

3) Silicon detectors

● micro-strip detectors

● (pixel detectors)

4) Track reconstruction

● pattern recognition

● track fitting


LiteratureMain sources used in preparing this lecture:

● C. Niebuhr, Detektoren für die Teilchenphysik, Vorlesungen Uni Hamburghttp://www.desy.de/~niebuhr/Vorlesung.html

● D. Bortoletto, An Introduction to Semiconductor Detectors,lecture at 2004 Vienna Conference on Instrumentationhttp://vci.oeaw.ac.at/2004/presentations/monday/bortoletto.pdf

● R. Frühwirth et al., Data Analysis Techniques for High Energy PhysicsCambridge Monographs on Particle Physics, Nuclear Physics and Cosmology, 2000

Other useful resources:

● K. Kleinknecht, Detectors for Particle Radiation, Cambridge University Press

● J. Ferbel, Experimental Techniques in High Energy Physics

● Particle Data Group web page, http://pdg.lbl.gov/pdg.html

● also: R. K. Bock and A. Vasilescu, Particle Detector Briefbook, http://rkb.home.cern.ch/rkb/titleD.html


Particle Physics Experiments for Dummies

Accelerate a beam of (stable & charged) particles to high energies

● electrons/positrons, protons/antiprotons, heavy ions

Bring them into collision with

● another beam of particles (“collider experiment”)

● a target at rest (“fixed-target experiment”)

Measure properties of the particles created in the collision

● production & decay vertices

● flight paths

● momenta

● speed – Cherenkov detectors

● penetration power – muon detectors

● energy – calorimeters | charged and neutral

→ analyse and interpret data from many collisions (at LHCb: 109 / year)

chargedparticles

only

position-sensitive detectors(in magnetic field)


Detection based on interaction of particles in detector material

● energy deposition mostly due to excitation / ionisation (⇒ Bethe-Bloch)

● creation of free electric charge carriers

Electronic readout of detector signals:

● apply electric field across detector volume, collect charges on electrodes

● electronically integrate& amplify signal pulse

● digitize the signal:

● discriminator ⇒ binary information (hit / no hit)

● analog-to-digital converter (ADC) ⇒ encode pulse height

● time-to-digital converter (TDC) ⇒ encode signal arrival time

● transfer digital data to a huge computer farm for processing and storage

● need a “trigger” signal to decide when to read out the detector (→ Lea)

Particle Physics Detector for Dummies

or of scintillation light(not discussed in this lecture)


Tracking DetectorsObtain position information from finely segmented readout electrodes

● granularity determined by particle density and required spatial resolution

● close to interaction point: small tracking volume but high particle density

→ fine granularity and excellent position resolution

● further away: lower particle density but large tracking volume

→ coarser granularity, lower position resolution

Example: ATLAS experiment at the LHC

silicon pixels

silicon strips

drift tubes (TRT)

drift tubes (MDT)

5-12 cm

30-50 cm

56-107 cm

500-1000 cm

50 x 400 μm

80 μm x 13 cm

4 mm x 75 cm

3 cm x 6 m

1.8 m²

60 m²

( 680 m²)

5500 m²

radius frombeam axis

technology cell size area


ATLAS Detector


ATLAS Inner Tracker

silicon pixels

silicon strips (SCT)

straw drift tubes (TRT)

all: barrel + endcaps


● rate capability

● limited by charge collection time and dead-time of read-out electronics

● must match the expected rate of charged particles in the experiment

● material budget

● multiple scattering in detector material limits spatial resolution

● especially important if particle momenta are low

● radiation hardness

● degradation of detector material due to radiation damage

● detector must survive several (typically 10) years in the experiment

● cost !!!

● often dominated by number of electronic readout channels

● detector granularity as fine as needed but not much finer than that

Additional Requirements

⇒ different detector technologies to match different conditions


Momentum Measurement

Measure bending radius ρ of the particle trajectory in a magnetic field B

● gives momentum component transverse to magnetic field lines:

B-field⊙

Δθ

B-field

tracking detectors

⊙

● direction of bending gives sign of the particle charge

pT = q⋅B⋅ρ pT [GeV ] = 0.3⋅B [T ]⋅ρ [m ]

Typical collider experiment:● solenoid magnet

● field lines parallel to beam axis

● barrel and endcap detectorsinside magnetic field

Typical fixed-target experiment:● dipole magnet

● field lines orthogonal to beam axis

● planar detection layers before and after the magnet


Momentum Resolution (I)

Determine sagitta of trajectory from three position measurements

● from geometry:

s = ρ⋅(1−cosϕ

2 ) ≈ ρ⋅[1−( 1−12 (ϕ2 )

2

)] = ρ⋅ϕ2

8

● deflection in magnetic field (q = 1):

● position measurements with resolution σx:

ρ =pT

0.3⋅B⇒ ϕ =

Lρ =

0.3⋅B⋅LpT

⇒ s =0.38

⋅L2

⋅BpT

s = x 2 −x 1+x 3

2⇒ σ s

2 =32

σ x2

L /2ρ = sin

ϕ

2≈

ϕ

2(for ϕ not too large)

σ (pT )

pT=

σ s

s= √3

2σ x⋅

8 pT0.3 B L2

⊙B-field

x1

x2

x3

L

ρ

s


Momentum Resolution (II)

Relative momentum resolution

● deteriorates linearly with transverse momentum

● improves linearly with the strength of B-field

● improves quadratically with the length of the measured track segment

⇒ large size of high-energy particle physics experiments

e.g. ATLAS

● magnetic field: 2T

● overall diameter: 25 m

● overall length: 46 m

For N equidistant measurements (N ≥ 10):

σ (pT )

pT=

σ κ

κ = √ 720N+4

⋅σ x⋅pT

0.3 B L2

with κ = 1/ρ = curvature

[Gluckstern, NIM 24 (1963) 381]


Momentum Resolution (III)

Additional uncertainty from multiple scattering

● average deflection angle in bending plane

● X0 = radiation length of detector material

● L' = L / sin θ = passlength through detector material

Total uncertainty on momentum measurement

θB L' L

spatial resolution

multiple scattering

θ resolution

(σ p

p )2

= (√ 720N+4

⋅σ x⋅p⋅sinθ

0.3⋅B⋅L2 )2

+ ( 52.3×10−3

β⋅B⋅√L⋅sinθ⋅X 0)2

+ (σθ⋅cotθ )2

ϑ rms =13.6×10−3

β⋅p [GeV ]⋅z ⋅√ L 'X 0

⋅(1 + 0.038 ln( L 'X 0)) ( → Katharina )


Early Tracking Detectors (I)

Cloud Chamber (Wilson, 1912)

● vessel filled with supersaturated water vapour(created by rapid adiabatic expansion)

● charged particle creates ionisation clusters

● ionisation clusters act as condensation nuclei

● trail of water droplets along particle trajectory

● photograph trails through windows in the vessel

● spatial resolution ~ 100 μm

● estimate particle energy from density of droplets

● most important experimental tool until 1950s

● main disadvantages:

● large dead time of the order of seconds

● need to compress/expand vessel to remove droplets after each exposure

● photographs need to be analysed manuallydiscovery of positron

(Anderson, 1932)


Bubble Chamber (Glaser, 1952)

● vessel filled with superheated transparent liquid(created by rapid adiabatic expansion)

● energy deposition brings liquid to boil

● trail of bubbles along particle trajectory

● photograph trails through windows in the vessel

● spatial resolution ~ 100 μm

● estimate particle energy from density of bubbles

● advantage compared to Cloud Chamber: higher density of detection medium

● detection medium can serve as target materialfor fixed-target experiments at particle beams

● gives higher sensitivity to rare processes

● disadvantages: same as for Cloud Chamberdiscovery of neutral currents(Gargamelle, CERN 1973)

Early Tracking Detectors (II)


Spark Chamber (Fukui/Myamoto, 1959)

● stack of thin metal plates filled with He/Ne gas

● apply high voltage between alternate layers

● just below the break-down voltage

● charged particle ionizes gas molecules in gap

● causes discharge in between adjacent plates

● creates trail of sparks along particle trajectory

● reduce high voltage to stop discharges

● readout mostly optical (also: acoustic/electronic)

● advantage: detector dead-time only ~ ms

● factor 100 faster than Bubble chambers

● disadvantage: spatial resolution only ~ mm

● factor 10 worse than Cloud and Bubble chambers discovery of muonneutrino (BNL, 1962)

Early Tracking Detectors (III)

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