Particle Detectors

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PARTICLE DETECTORS

Mojtaba Mohammadi

IPM-CMPP- February 2008

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“seeing an object” = detecting light that has been reflected off the object's

surface light = electromagnetic wave; “visible light”= those electromagnetic waves that our eyes can

detect

generalize meaning of seeing:

seeing is to detect effect due to the presence of an object this method is used in electron microscope, as well as in

“scattering experiments” in nuclear and particle physics

PARTICLE PHYSICS EXPERIMENTS

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Introduction (Physics Motivations)

The Standard Model of particle physics describes the electroweakand strong interaction precisely and no significant deviation has been observed. BUT the SM leaves several unexplained questions.

-Find Higgs particle or exclude its existence in the region allowed by theory (< 1 TeV).

-Search for new particle in the mass region of ~50 GeV to ~5 TeV.

-Test the new theories like SUSY and the discovery of SUSY particles.

-Look for any deviation from Standard Model and precision measurement.

LHC as an accelerator and collider produces scattering events and DETECTORS are our electronic eyes which are used to record and identify the useful events to answer our questions about fundamental particles and their interactions.

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Introduction

Particles are detected via their interactions with matter.

A modern multi-purpose detector like CMS and ATLAS at the LHC typically consists of :

1- Tracker

2- Electromagnetic Calorimeter (ECAL)

3- Hadronic Calorimeter (HCAL)

4- Muon System

A simplified layout

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What do the elementary particles in SM look like in Detectors?The elementary particles in the SM consists ofquarks, leptons and gauge bosons.Some of them cannot be observed directly in thedetector.Heavy particles like top quark , W-boson and Z-bosonDecay promptly to lighter particles with a lifetime of10^{-25} seconds. But other quarks except for top quark will fragmentinto colour singlet hadrons due to QCD confinementwith a time scale of 10^{-24} seconds.

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What do the elementary particles in SM look like in Detectors?

Some particles are seen like neutrinosand LSP (no EM or HD int.).

LSP

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Detector Requirements

The fact that we do not know what we will find at the LHC means that our detectorsshould be ready anything. For Example, suppose that Higgs particle exists but we do not know exactly its mass. However, we know for any mass how it will decay:

► If Higgs is light (Higgs Mass < 150 GeV/c^{2}): One of the good ways to detect its present is through its decay to two photons (H +g g) The detector should have an excellent Electromagnetic Calorimeter (ECAL) to detect and extract this signal.

► If Higgs is heavy: One of the promising channels to detect Higgs is via its decay to two Z-bosons with the subsequent decay of Z-bosons to Muon-AntiMuon (H ZZ4µ). Therefore, the detector should be able to detect muons properties precisely and a Muon System with high quality is needed.

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► For SUSY , one of the best signatures is missing energy in the detector (LSP). Any imbalance in the momentum and energy of the all final particles is considered as Missing Energy which is coming from LSP or Neutrinos. So a full Coverage on space is needed to detect all particles (or hermetic detector is required) .

► To measure the momenta of Charged particles and reconstruct vertices and also to differentiate between Photons and Electrons a high quality tracker is needed. ► Excellent vertex position measurement.

► Fast Response ( around ns). Since the bunch crossing at the LHC occurs each 25 ns.

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Detector RequirementsExcellent Vertex Position Measurement is Necessary For B-jet and Tau Identification. e.g. for Top (BR(tWb)~0.99) study.

For a b-jet of 60 GeV, the decay length:

mml 2.6

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DETECTOR REQUIREMENTS: RADIATION LEVEL

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x

y

zq

p

protonproton

Some definitions

)2tan(lnln2

1,sin22

z

zyxT pE

pEpppp

Barrel

Endcap

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Reaction Rate

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Trigger

Trigger of interesting events at the LHC is much more complicated than at e+e- machines

interaction rate: ≈ 109 events/s max. record rate: ≈ 100 events/s event size ≈ 1 MByte 1000 TByte/year of data ~ 1.5 million CDs

Þ trigger rejection ≈ 107

collision rate is 25 ns trigger decision takes ≈ a few µs

store massive amount of data in front-end pipelines while special trigger processors perform calculations

Trigger = device making decision on whether to record an event

Trigger has to decide fast which events not to record, without rejecting the “good events”

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Trigger Design

There are several means to design a trigger such as: particle identification,Multiplicity, Kinematics, Event Topology ….

Modern detectors can trigger on: Muons by muon system, electron/photon asElectromagnetic objects, Jets and Missing Energy as Hadronic objects and aCombination of them.

Trigger Conditions are dependent on the Collider Phenomenology. according to collider phenomenology we know that what particle may be detected in what kinematical region.

Modern detectors like ATLAS and CMS at theLHC have three levels of trigger: Level-1 : Event rate 10^{9} Hz to ~ 10^{5} HzLevel-2 : ~10^{3} HzLevel-3 : 10^{2} Hz

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Momentum Measurement, Magnet

The momentum measurement of charged particles in the detector is based on the bending of their trajectories.

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Momentum Measurement

Consider a charged particle in solenoidal magnetic field, the radius of the curvature:

The curvature of the trajectory (s):

Tp

LBrrs

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3.0cos

2

The charge of particle is also measurable.

Resolution:

2LB

p

s

s

p

p T

T

T

Hence, the momentum resolution degrade linearly with increasing Pt.Improvement for higher magnetic field and L.The effect of multiple scattering should be considered.

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ATLAS and CMS

ATLAS CMS

length 46 m 22 m

diameter 25 m 15 m

weight 7000 t 12000 t

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CMS detector momentum resolution

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A little about Calorimetry

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Introduction

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The main principle of particle detection: Interaction with matter.

Introduction

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Introduction

When a high energy electron or photon strikes on a thick absorber (such as Lead), a cascade of secondary electrons and photons via Bremsstrahlung and pair production , respectively, is initiated.

With increasing the depth

The number of secondary particles is increased

The mean energy of particles decreased

This multiplication continues until the energy of particles fall below the critical energy, after this Photons and Electrons start the Ionization and Excitation processes.Need to be familiar with e/photon interactions with matter.

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Showering

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The dominant process of energy loss by an electronabove ~1 GeV when passing through matter is:Bremsstrahlung or Braking Radiation.(a free electron can not radiate a photon)

The energy loss per unit of distance:

Charged particle interaction

It is very small for Muons w.r.t Electrons

Below ~1 GeV: Excitation, Ionization, Vibration

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Transverse shower development:

RM

X0

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Study of these properties are helpful to choose the detector material.

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Photon interaction with matter

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Photon interaction with matter

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Summary of photon interaction with matter

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Z (Al) = 13 Z (Fe) = 26Z (Pb) = 82

For higher Z-material multiplication continues due to the smallercritical energy.

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A Simple Model for EM Shower

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Energy Resolution of a Calorimeter

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Energy Resolution of CMS ECAL

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No slide

Thanks for your attention

Particle Detectors

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