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 olafs@physik.uzh.ch

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

Introduction (3) O. SteinkampEMPP HS16

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

Introduction (4) O. SteinkampEMPP HS16

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)

Introduction (5) O. SteinkampEMPP HS16

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)

Introduction (6) O. SteinkampEMPP HS16

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

Introduction (7) O. SteinkampEMPP HS16

ATLAS Detector

Introduction (8) O. SteinkampEMPP HS16

ATLAS Inner Tracker

silicon pixels

silicon strips (SCT)

straw drift tubes (TRT)

all: barrel + endcaps

Introduction (9) O. SteinkampEMPP HS16

● 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

Introduction (10) O. SteinkampEMPP HS16

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

Introduction (11) O. SteinkampEMPP HS16

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

Introduction (12) O. SteinkampEMPP HS16

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]

Introduction (13) O. SteinkampEMPP HS16

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 )

Introduction (14) O. SteinkampEMPP HS16

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)

Introduction (15) O. SteinkampEMPP HS16

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)

Introduction (16) O. SteinkampEMPP HS16

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)