Transcript Slide 1

PARTICLE DETECTORS
Mojtaba Mohammadi
IPM-CMPP- February 2008
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PARTICLE PHYSICS EXPERIMENTS
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“seeing an object”
 = detecting light that has been reflected off the object's surface
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light = electromagnetic wave;
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“visible light”= those electromagnetic waves that our eyes can detect
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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
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Introduction
(Physics Motivations)
The Standard Model of particle physics describes the electroweak
and strong interaction precisely and no significant deviation has been
observed. BUT the SM leaves several unexplained questions.
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.
-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.
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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 of
quarks, leptons and gauge bosons.
Some of them cannot be observed directly in the
detector.
Heavy particles like top quark , W-boson and Z-boson
Decay promptly to lighter particles with a lifetime of
10^{-25} seconds.
But other quarks except for top quark will fragment
into colour singlet hadrons due to QCD confinement
with a time scale of 10^{-24} seconds.
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What do the elementary particles in SM look like in Detectors?
LSP
Some particles are seen like neutrinos
and LSP (no EM or HD int.).
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Detector Requirements
The fact that we do not know what we will find at the LHC means that our detectors
should 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 ZZ4µ).
Therefore, the detector should be able to detect muons properties precisely
and a Muon System with high quality is needed.
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Detector Requirements
► 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 Requirements
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Detector Requirements
Excellent Vertex Position Measurement is Necessary For B-jet and Tau
Identification. e.g. for Top (BR(tWb)~0.99) study.
For a b-jet of 60 GeV, the
decay length:
l  6.2 mm
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DETECTOR REQUIREMENTS: RADIATION LEVEL
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Some definitions
x
Barrel
p
Endcap
q
z
proton
proton
y
pT 


1  E + pz 
   ln tan( )
p + p  p sin  ,   ln
2
2  E  pz 
2
x
2
y

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Reaction Rate
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Trigger
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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”
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
 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
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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 as
Electromagnetic objects, Jets and Missing Energy as Hadronic objects and a
Combination 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 the
LHC have three levels of trigger:
Level-1 : Event rate  10^{9} Hz to ~ 10^{5} Hz
Level-2 : ~10^{3} Hz
Level-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):
0.3B  L2
s  r  r cos 
8 pT
The charge of particle is also measurable.
Resolution:
pT s
pT


pT
s
B  L2
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
 7000 t
 12000 t
weight
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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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Introduction
The main principle of particle detection: Interaction with matter.
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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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Charged particle interaction
The dominant process of energy loss by an electron
above ~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:
It is very small for
Muons w.r.t Electrons
Below ~1 GeV: Excitation, Ionization, Vibration
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Charged particle interaction
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Charged particle interaction
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) = 26
Z (Pb) = 82
For higher Z-material multiplication continues due to the smaller
critical 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
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