P ARTICLE D ETECTORS Mojtaba Mohammadi IPM-CMPP- February

2 “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 P ARTICLE PHYSICS EXPERIMENTS

3 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. -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. DETECTORS are our electronic eyes which 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.

4 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 

5 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.

6 What do the elementary particles in SM look like in Detectors? Some particles are seen like neutrinos and LSP (no EM or HD int.). LSP

7 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 H   detect its present is through its decay to two photons (H   )  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.

8 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.

9 Detector Requirements

10 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:

D ETECTOR REQUIREMENTS : RADIATION LEVEL 11

12 x y z q p proton Some definitions Barrel Endcap

13 Reaction Rate

14 Trigger Trigger of interesting events at the LHC is much more complicated than at e + e - machines  interaction rate: ≈ 10 9 events/s  max. record rate: ≈ 100 events/s  event size ≈ 1 MByte  1000 TByte/year of data ~ 1.5 million CDs  trigger rejection ≈ 10 7  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”

15 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

16 Momentum Measurement, Magnet The momentum measurement of charged particles in the detector is based on the bending of their trajectories.

17 Momentum Measurement Consider a charged particle in solenoidal magnetic field, the radius of the curvature: The curvature of the trajectory (s): The charge of particle is also measurable. Resolution : Hence, the momentum resolution degrade linearly with increasing Pt. Improvement for higher magnetic field and L. The effect of multiple scattering should be considered.

18 ATLAS and CMS ATLASCMS length  46 m  22 m diameter  25 m  15 m weight  7000 t  t

19 CMS detector momentum resolution

20 A little about Calorimetry

21 Introduction

22 The main principle of particle detection: Interaction with matter. Introduction

23 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.

24 Showering

25 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: Charged particle interaction It is very small for Muons w.r.t Electrons Below ~1 GeV: Excitation, Ionization, Vibration

26 Charged particle interaction

27 Charged particle interaction Transverse shower development:

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

29 Photon interaction with matter

30 Photon interaction with matter

31 Summary of photon interaction with matter

32 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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34 A Simple Model for EM Shower

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36 Energy Resolution of a Calorimeter

37 Energy Resolution of CMS ECAL

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39 No slide Thanks for your attention