Particle Detectors

Iain Bertram

Lancaster University

June 2002

Outline

Basic Principles Bubble Chamber Pictures Modern Detectors

What to Measure? How to identify Particles? Detector Types? Different Experiments

Basic Principles

How and what do we see?

All perception of the material world is via Electromagnetic Interactions

Detectors To first order can only see charged objects. Energy deposition in the detector via

ionization processes. (Exception is photons which “see” electric

charge)

Bubble Chamber Pictures

Birmingham Package

Bubble Chamber – super heated liquid. Small addition of energy boils the liquid creating small bubble

Easy to visualize, too slow for modern experiments

Interpretation

Vertex – neutral particle decaying

Scatter off of an electron – light particle loops

Scatter off of a proton.

Bubble Chamber Information

Bubble Chamber in Magnetic Field Momentum of Particle Charge of Particle Energy Loss as function of distance (dE/dx)

Type of interaction V – Neutral decay Tree – Charged Decay Kink – decay with a neutral Momentum Balance on Tree gives presence of

neutral ?

What do we want to Measure?

Particle Properties Particle Type, electron, pion, kaon, proton,

muon, etc. Charge of the particle Location of Particles Momentum of Particles Lifetime

Need to combine all the above information into an event. For example XttppZ o , e e -

Detection – Rough Particle ID

EM hadronicB

InteractionPoint

Scintillating FiberSilicon Tracking Calorimeter (dense)

Wire Chambers

Abs

orbe

r M

ater

ial

electron

photon

jet

muon

neutrino -- or any non-interacting particle missing transverse momentum

Charged Particle Tracks Energy Muon Tracks

We know x,y starting momenta is zero, butalong the z axis it is not, so many of our measurements are in the xy plane, or transverse

Tracking Detectors

Measure x-y-z location of all charged particles as the pass through predetermined parts of the detector Series of dots Get position of tracks Connect lines to find decay vertices

How do we get momenta Detector in magnetic field Tracks bend in a magnetic field

How to Measure Momentum?

Most Particles are Charged. If a charged particle moves through in a magnetic field it experiences a field

So the acceleration is proportional to the magnetic field and perpendicular to the direction of motion.

Momentum given by:

How can we measure Momentum

BvqF

mavqBF

Magnetic Field into Page

Velocity in direction of arrow.

Dashed Arrow: Force

Charged Particle in a Magnetic Field

In the Program the momentum is given by the following equation: P = momentum (in GeV/c) B strength of magnetic field in Tesla R = radius of curve in metres

So we can calculate the momentum if we can measure the radius of the curve made by the particle in the magnetic field

Simple Mathematics: x2 + y2 = r2

Pick three points on a circle and you get a radius

Brp 3.0

Tracking

Vertex – decaying particle

Curved track – charge and momentum

Particle Lifetimes

Most Particles are unstable – I.e they decay. Important property is the lifetime of the particle Quantum Mechanical Effect – Decay is random We have to measure many decays and take the

average to determine a real lifetime (in fact we need to fit the data)

Relativistic Effects are important: Need to take account time dilation etc.

12

2

c

L

cm

E

K

beam

Particle Identification

Want to identify what type of particle in many cases

Simple Classification Muon, electron, jet, photon, …… Straight forward differentiation in most

detectors

Exact type of particle Pion, kaon, proton, etc. Energy Deposition as function of momentum

Detection – Rough Particle ID

EM hadronicB

InteractionPoint

Scintillating FiberSilicon Tracking Calorimeter (dense)

Wire Chambers

Abs

orbe

r M

ater

ial

electron

photon

jet

muon

neutrino -- or any non-interacting particle missing transverse momentum

Charged Particle Tracks Energy Muon Tracks

We know x,y starting momenta is zero, butalong the z axis it is not, so many of our measurements are in the xy plane, or transverse

Energy Deposition

Example of energy deposition for different particles

Note: measure momentum and dE/dx and get particle ID.

Only works for fixed momentum range: Not good at higher momenta

How else can we ID particles?

Masses -

The kaon decays to two pions:

By measuring the momentum and the angle between the two pions we can calculate the mass of the kaon if we know the mass of the pion:

Hence we can identify a kaon via calculating its invariant mass

2121422 cos22 EEppcmcmK

0K

Energy Measurement

1. Measure Momentum – id particle – finished

2. Measure energy without magnetic field Sample energy deposition many times in

dense material Calorimeter Calibrate with known energy depositions Depends on particle type

DØ Detector

CalorimeterMuons

Tracking Goals:

Searches –higgs, SUSY particles, etc.

Top physics,W physics,QCD,

General Purpose detector

DØ Detector

ForwardPreshower

Silicon Microstrip Tracker Fibre Tracker

Solenoid Central Preshower

“Typical DØ Dijet Event”

ET,1 = 475 GeV, 1 = -0.69, x1=0.66ET,2 = 472 GeV, 2 = 0.69, x2=0.66

MJJ = 1.18 TeVQ2 = ET,1×ET,2=2.2x105 GeV2

DØ Event: W e

DØ top event: bqqWtbeWt

BaBar Detector

Opal Detector

Opal Basic Muon Event

Track

Calorimeter

MuonMuons penetrate: Therefore go all the way through the detector

Opal Electron

Track

Calorimeter

Electrons dump energy quickly

Photon: No track

Opal Jets

Jets

Jets are evidence of quarks or gluons.

Collection of hadrons, penetrate further than electrons and usually > 1 particle

Belle Event: B → +– :

http://teachers.web.cern.ch/teachers/archiv/HST2000/teaching/resource/bubble/bubble.htm#An%20Introduction.

Bubble Chambers – Birmingham http://www.ep.ph.bham.ac.uk/user/watkins/seeweb/BubbleChamber.htm

J/KS