Lecture 18 - Detectors

●

Detector systems

 Momentum of charged particles (trackers)  Energy of particles (calorimeters) ●

Detector at a collider experiment.

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The purpose of a detector

At, eg, a collider experiment the objective is to work out what happened in the primary collision between the particles. (i.e. the Feynman diagram level  distances

e -

jet

e -

g

,Z 0

jet jet jet Eg electron-proton DIS

e

  jet

e

 annihilation Measurements of properties of stable final state particles (   10  10 s) must be used to "reconstruct" the short-distance interactions. Take, as an e xample, electron-proton DIS.

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Electron-proton DIS

Some aims of a DIS experiment: (1) Measure the total neutral current DIS cross section:

d

2 

DIS

 as a function of two variables: scattered electron momentum

p

' From this, the structure function

F

2 and parton densities can be determined. (2) Measure the properties of the jet of hadrons produced by the scattered quark. Eg number of particles and energies of particles. This gives a more direct check of the scattered quark's properties.

g

,Z0

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DIS event at H1 detector at HERA

ep

collider

(4) Muon Tracker

m 

e (30 GeV)

e -

jet (3) Had. Calo (1) Tracker (2) EM Calo p (820 GeV)

Generic example of a collider detector system.

Four major components: (1) Inner tracking system (2) electromagnetic calorimeter (3) Hadronic Calorimeter (4) Muon tracking system.

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H1 Detector

Large detector based on the four major components listed earlier.

Investigate the precision and properties of each component.

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(1) Tracking system

m  The first detector component particles produced in the collision will encounter.

Tracking chambers typically exposed to magnetic field Purpose of trackers: (1) Measure the "tracks" of charged particles to reconstruct momenta. (2) Measure the energy loss of charged particles to allow identification of particles where possible.

z

(2) EM Calo (1) Tracker (3) Had. Calo

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x y

6

Tracker – Drift Chamber

One of the main technologies used for tracking systems.

A charged particle enters a cell of a drift chamber filled with a gas: eg Argon-ethane.

The particle ionises the gas as it passes. Electrons drift towards anode wires and produce a pulse "hit" upon arrival. From time difference of particle entering and leaving a drift time  particle trajectory can be measured. Typical resolution of "hit" positi FK7003 7

Measuring momentum

Drift chamber comprises many different drift cells.

In approximately circular track can be fitted from "hit" information.

F

 

mv p xy r

 2 

mv qvB

(16.01) 

qBr

(16.02) 

p x y p xy

.01

p xy

 0.015

  20 mrad   tan  =

p xy p z

  .

Degradation from: high

p xy

 Radius << tracker size (good resolution on momentum) Radius >> tracker size (poorer resolution on momentum) FK7003

x y

8

Measurements of ionisation energy loss From lecture 17.

Measure ionisation energy losses and through Bethe-Bloch formula, identify different particle species at low momentum (< 1 GeV).

e

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Tracking Chambers

H1

Different technologies/geometric arrangements available.

Drift chambers.

Time projection chambers Semi conductors trackers….

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Summary of trackers and some observations A tracker measures the momenta of charged particles.

For

p

 different particle species:  ,

K

 m  But .....

Resolution degrades at high momentum: 

p xy p xy

.01

p xy

 0.015

A tracker misses all of the neutral particles.

It can't identify muons over the full momentum range.

More detectors are needed.

z

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Calorimeters What is the purpose of the calorimeters ?

  To measure the electromagnetic and hadronic energy loss separately arising from a system of particles.

To measure the ’full energy’ arising from a collision (as much as possible). We’d like to be able to reconstruct what happened in the fundamental particle collision FK7003 12

Sampling Calorimeters

● ● We know that particles produce some type of shower when they go through material (lecture 17). Used for electromagnetic and hadronic calorimeters.

Sampling calorimeter – thin blocks of dense material interleaved with an active material which can measure the shower Active layer Dense material ● Many different possibilities  Dense blocks (eg lead) interleaved with scintillators (for light).

  Dense blocks with liquid argon (for ionisation) ….

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(2) Electromagnetic calorimeter

From lecture 17.

An electron or photon will initiate an electromagnetic shower. Number of electrons/positrons measured with scintillator light intensity as a function of distance (shower depth). Maximu m number of particles reached as shower approaches end.

Depth (in radiation length,

X

0 , units):

t

max  1 ln 2 ln(

E

0

E c

) (17.36) Maximum number of particles:

E 0 N

max 

E

0 (17.37)

E c

 Initial energy

E

0 can be d etermined! Resolution determined by number of particles (Poisson statistics) 

E E



E

0.1

 GeV  Depth of  0.01

10-30

X

0   reduces with increasing energy. absorption over full momentum range expected.

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(3) Hadronic Calorimeter

Similar principles as for sampling em calorimeter.

Lecture 17 - nuclear cascade is more complex. Depth of hadronic shower set by nuclear interaction length Eg iron : >>

X

0 .

X

0  13.8

 132 gc m  2 Hadronic calorimeters larger .

1-2m (of iron). Poorer resolution due to, eg missing energy from neutral particles, fluctuations of number of particles, nuclear binding and break-up uncertainties.



E

 0.5

E E

 0.02

EM Cascade Nuclear cascade FK7003 15

(4) Muon tracker

m  From lecture 17: Muon bremsstrahlung is rare and they don't interact hadronically (of course).

 muons penetrate through the calorimeters.

 to identify and measure muons an outer tracking chamber is plac ed around the calorimeters. This is based on the same tracking principles as described earlier.

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Summary so far

Did we achieve our aims ?

(1) Measure the total DIS cross section:

d

2 

DIS

 as a function of two variables: scattered electron momentum

p

' - tracker and em calo (2) Measure the properties of the jet of hadrons produced from the scattered quark. Eg number of particles and energies of particles (including muons). This gives a more direct check of the scattered quark's momentum.

- tracker, em and hadronic calo, and muon system. The detector is largely trackers and calorimeters. Tracker resolution worsens with increasing momentum as calorimeter resolution increases.

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Finding energetic weakly interacting neutral particles 

e



e

not measured

W -

Eg charged current:

e



p



e X

.

The neutrino is not be zero for NC DIS.

p T



p T

2     

particles p ix

   2     

particles

 0 if a neutrino escape d without detection.

p iy

   2  0 Use "missing" transverse momentum to detect high energy weakly interacting particles.

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Missing transverse momentum from charged current DIS data at HERA

W -



e

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ATLAS experiment at the LHC

Similar principles for ATLAS detector. No sense in describing each component. Take one example of what ATLAS can do.

Search for supersymmetry.

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Production of supersymmetric particles at the LHC

Very many possible production modes (if they exist).

One channel is squark pair production followed by the squark decay through a long decay chain ending up with two neutralinos  1 0 : WIMPs.

Predicted mas ses several hundred GeV.

 2 Missing transverse momentum (simulation, obviously) FK7003

SUSY SM Backgrounds

21

Summary

●

A typical collider experiment consists of tracking systems and calorimeters

●  Many different technologies.

 Reconstruction of momentum and energy  Particle identification.

We’ve reached the end.

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