Particle detectors at LHC (II) - UCMteorica.fis.ucm.es/TAE2012/CHARLAS.DIR/CHAMIZO.DIR/TAE2012-lecture3.pdf · La reconstrucción está basada en técnicas de Kalman Filter (usadas

Particle detectors at LHC (II)

Maria Chamizo Llatas (CIEMAT) TAE2012 (18-19 July 2012)

•  Basic mechanism for calorimetry in particle physics: −  formation of electromagnetic

−  or hadronic showers

•  The energy is converted into ionization or excitation of the matter

•  Calorimetry is a “destructive” method. The energy and the particle get absorbed

•  Detector response ∝E

•  Calorimetry works both for charged (e± and hadrons) and neutral particles (n,γ)

Calorimetry

2 TAE July 2012

Charge Scintillator light

Cerenkov light

Maria Chamizo Llatas

•  Mean energy loss of a particle described by the Bethe-Bloch formula

Interaction of particles with matter

3 TAE July 2012

⎥⎥⎦

⎤

⎢⎢⎣

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22ln14 2

222

2222 δ

ββγ

βπ

Icm

AZzcmrN

dxdE e

eeA

Z=núm atómico material A=masa atómica material I=Potencial de ionización efectivo del átomo I≈ I0Z ( I0≈ 10 eV) δ= corrección del efecto de densidad

NA=Num. Avogadro βc= velocidad de la partícula z= carga de la partícula

•  dE/dx depends only on β •  Particles with higher charge losses more

energy per unit length •  Minimum Ionizing particle (MIP)

−  Particle that losses energy only via ionization and the energy loss corresponds to the minimum of the curve βγ ~3-4


•  The energy loss depends on the particle velocity and is independent of the mass

•  The energy loss as a function of particle momentum (p=mcβγ) is however depending on the particle’s mass

•  By measuring the particle momentum (deflection in the magnetic field) and measurement of the energy loss on can measure the particle mass

Particle identification

4 TAE July 2012 Maria Chamizo Llatas

•  Bethe-Bloch displays only the average energy loss −  Energy loss is a statistical process

−  Primary ionization •  Poisson distributed

•  Large fluctuations per reaction

−  Secondary ionization •  Created by high energy primary electrons

−  Total ionization: primary and secondary ionization

Ionization


•  Critical energy: energy at which the losses due to ionization are equal to the losses due to Bremsstrahlung

Energy loss for electrons

6 TAE July 2012

•  Bremsstrahlung is dominating at high energies (eàeγ)

•  At low energies: ionisation, additional scattering


•  Charged particles deviate from a strait track when traversing a medium due to the Coulomb field

Multiple scattering


•  Specifically designed to measure the energy and position of the particles that interact via electromagnetic interaction: −  e-, e+ & photons (γ)

•  At high energies −  γs interact with matter via pair productio: γ à e+e- −  An electron (or positron) looses energy via bremsstrahlung: e+àe+γ, e-à e- γ −  The two processes continue until the remaining particles have lowered their

energy reaching the critical energy: Ec −  Below the critical energy the electrons dissipate energy via ionization and

excitation of atoms until they are absorbed

•  An electromagnetic shower begins when a high energy e-, e+ or γ enters a material −  Pair production and bremsstrahlung generate more electrons and photons with

lower energy until they reach Ec

Electromagnetic calorimeter


•  Radiation length, X0, defines the amount of material a particle has to travel through until the energy of an electron is reduced by Bremsstrahlung to 1/e of its original

•  The scale variables t and x are commonly used in describing the electromagnetic shower behavior

•  t=x/X0 à distance measured in units of radiation length •  y=E/Ec à energy is measured in units of critical energy

•  Moliere radius: Radius of the cylinder containing on average 90% of the shower energy


9 TAE July 2012

!

X0 =716.4 A

Z(Z +1)ln 287Z

g.cm"2 Z atomic number A mass number

!

RM = 0.0265X0(Z +1.2)


•  Electromagnetic calorimeters are segmented in crystals or cells and surround the tracking systems

•  At CMS led tugnstate crystals (PbWO4) were chosen based on: −  PbWO4 has a short radiation length and small Moliere radius −  It is a fast scintillator & relatively easy to produce


10 TAE July 2012

PbWO4 characteristics

Barrel: 61200 crystals of 2.2x2.2x23 cm3 !End caps: 14648 crystals of 3x3x22 cm3

Energy resolution goal: 0.5% at high Energy

!Maria Chamizo Llatas

•  Typically a shower extends over several crystals −  Useful to reconstruct precisely the impact point from the “centre of gravity” of

the deposits in several cells •  At CMS the electron energy in the central crystal ~80%, in a 5x5 matrix

around it ~96% of the energy

Electromagnetic clusters


•  Example of cluster reconstruction −  Start from a seed crystal were the energy is greater than a threshold

−  Starting from this crystal add adjacent crystals if •  The crystal energy is above a noise level •  The crystal has not yet been assigned to another cluster •  The previous crystal added in the same direction has higher energy

Electromagnetic clusters


•  Crystal transparency drops within a run (LHC beams colliding) and recovers during the interfill period (no beam) −  Inject fixed amount of light to monitor transparency loss −  Response loss up to 5% in barrel and 30%-50% in end cap (20% in the electron

acceptance region |η| < 2.5)

ECAL transparency


•  Weν E/p: Stable E scale during 2012 run after light monitoring (LM) corrections: −  ECAL Barrel (EB): RMS stability after corrections 0.19%

ECAL calibration, 2012 data

date (day/month)12/04 19/04 26/04 03/05 10/05 17/05 24/05 31/05 07/06

Rel

ativ

e E/

p sc

ale

0.930.940.950.960.970.980.99

11.011.021.03

with LM correctionwithout LM correction1 / LM correction

CMS 2012 Preliminary-1 = 8 TeV L = 3.95 fbs

ECAL Barrel: Prompt Reco

Mean 1RMS 0.0019

0 50 100

Mean 1RMS 0.0019Mean 0.97RMS 0.0063

Single electron energy scale (E/p) stability in barrel measured with W eν events


•  Zàee: Good resolution with preliminary energy calibration for 2012:

•  Instrumental resolution: 1.0 GeV in ECAL Barrel

ECAL calibration, 2012 data

Zee invariant mass distribution for electrons measured in the barrel

15

Electrons with low or no Bremsstrahlung

TAE July 2012 Maria Chamizo Llatas

•  Hadronic showers are more complicated than em showers: −  Besides the high energetic hadronic reaction a number of nuclear

processes are created by the impinging hadron: •  high energetic secondary hadrons taking a significant part of the momentum of the

primary particle [e.g. O(GeV)] •  A significan part of the total energy is transferred into nuclear processes (particles in

the MeV range)

•  Special case: Neutrale pions (1/3 of all pions), decay instantaneously into two photons è start of an em. shower

Hadronic cascades


•  Measures the quark, gluon and neutrino directions and energies −  By measuring the energy and direction of the particle jets −  And measuring the missing transverse energy

•  What is a jet? −  Quarks & gluons do not exist in free states −  Hadronization is the processus of the formation of hadrons out of quarks and

gluons

Hadronic calorimeter

17 TAE July 2012

u,u,d p

p u,u,d

q1

q1,q2

q’1

q’2,q’3

Hadrons: mesons or baryons


•  Need to have lateral segmentation to measure jet angles and to separate jets

•  The calorimeter must be hermetic (i.e. have as complete coverage as possible) to measure all energy emitted from the interaction point and thereby infer missing transverse energy

•  These measurements must be made over the maximum possible kinematic acceptance and as a result, calorimeters can be enormous −  The ATLAS calorimeter has a diameter of about 8500cm, and a total

length of about 12m.

Hadron calorimeters


CMS hadron calorimeter


•  Jets are the experimental signature of quarks and gluons, observed as highly collimated sprays of particles

•  A jet algorithm is a set of mathematical rules that reconstructs the properties of jets by combining the Pμ of their constituents −  Fixed cone algorithms

−  Successive recombination algorithms

•  Different inputs to the jet algorithm lead to different types of jets: −  Calorimeter energy depositions. −  Tracks.

−  Particle or energy flow objects.

Jets


•  Aim of the muon system: −  Identification of muons

−  Measurement of the momentum

The muon systems


•  Drift tubes in the barrel region •  Cathod Strip Chambers in the end cap regios

•  Resistive Plate Chambers in barrel and end caps for triggering on muons

The CMS muon system


•  Charged particle ionizes the gas and the electrons travel to the anode wire

•  The measurement of the time to travel to the anode give a measurement of the position of the particle

Drift Tubes

23 TAE July 2012

x

ánodo

Región E bajo deriva

Región E alto amplificación

dttvx D )(∫=


The CMS drift tubes

24 TAE July 2012

•  Installed in the barrel −  Cover an region 0 < |η| < 1.2

−  250 chambers

−  3 Super Layer/chamber: 2 SL to measure the r – φ coordinates and 1 superl layer to measure the r – z (except for the outermost chambers in the wheels)

−  Gas mixture: Ar + CO2 (85 % -15%)


25

Reconstrucción StandAlone

+ + + + + +

+ + + +

+ +

+

+ + +

+ +

+ +

Barrel

EndCap

ATLAS

Barrel

CMS

En la propagación se tiene en cuenta: - Las posibles pérdidas energéticas al atravesar . los materiales - La dispersión múltiple - El campo magnético (uniforme o no)

Los segmentos reconstruidos en las cámaras individuales se combinan para formar una traza “stand alone” en los detectores de muones

La reconstrucción está basada en técnicas de Kalman Filter (usadas también en la reconstrucción por los detectores de trazas).

-  Partiendo de una “semilla” de traza se realiza un ajuste y se extrapola en busca de más señales.

-  De forma recursiva se reajusta añadiendo las nuevas señales y se extrapola en busca de más.

-  Con el conjunto final se realiza el ajuste final


TAE July 2012 26

Reconstrucción global Las trayectorias de los muones se reconstruyen independientemente en los detectores de trazas

Se buscan la compatibilidad de las trayectorias reconstruidas independientemente por los detectores de trazas y los de muones se combinan y se ajustan las señales para determinar la trayectoria global

+ +++ + +

+ +

+ + µ + +

+

+ + +

+ +

+ +



The trigger system

•  Needs to reduce the data rate from ~O(10)MHz range to ~O(100)Hz •  Only ~300Hz will be written to tape for further analysis •  Filters out “interesting” events

TAE July 2012 28 Maria Chamizo Llatas

•  Level 1 trigger (made by custom electronic): −  Reduces the input rate from 15 MHz (40Mhz for LHC in nominal

conditions) to 100 kHz −  Decision made in 3.2μs

•  High Level trigger (made by a processor farm): −  Reduces the rate to ~300 Hz −  Decision made in 1 second

The CMS trigger system


The Level 1 trigger

30 TAE July 2012

Signal observed in the calorimeters

Signal observed in the muon systems

Maximum output rate of 100 kHz


Efficiency of triggers

31 TAE July 2012

•  Trigger should have a fast turn on curve to be able to detect particles efficiently in all the momentum range


The High Level Trigger

2011: average pile up ~7events/crossing 2012: 17 events/crossing

è Code improved to reduce pile up dependence

32

Muon cross section at HLT with pT> 40 GeV Lower and flat in 2012 thanks to better track quality cuts

Trigger rate/instantaneous luminosity should be flat


Pile up dependency


Reconstruction

•  Optimal combination of information from all subdetectors •  Returns a list of reconstructed particles

−  e,µ,γ, charged and neutral hadrons •  Used in the analysis as if it came from a list of generated particles •  Used as building blocks for jets, taus , missing transverse energy , isolation

and PU particle identification 35

Global Event Description (Pflow) Made possible by CMS granularity and high magnetic field


Electron and photon reconstruction

36 TAE July 2012

Electrons ECAL Barrel (EB)

•  Energy scale and resolution −  Extensive control with Z and J/ψèee for both electrons and

photons


Muon reconstruction and identification

•  Start with particle flow muons •  Efficiency above 96% down to pT = 5 GeV

−  Above 99% efficiency for pT > 10 GeV

−  Efficiency in data using J/Ψ and Z peak

J/Ψ→µµ Tag&Prob

e

Z→µµ Tag&Prob

e

Tighter quality criteria applied in some analyses

37


Muon reconstruction with pile up

•  Control of efficiency versus the number of reconstructed vertices (pile up)

Efficiency is stable in high PU environment


Particle-based isolation

Sum energy of particles in ΔR cone around the lepton

• Global event description eliminates double counting

MUONS

Efficiency is stable in high PU environment

39

Detector Isolation


Tau ID + Isolation efficiency

§  Tau identification: §  Reconstruct individual decay modes §  Charged hadrons + electromagnetic obj

§  EM strips account for material effects

§  Tau isolation: §  Multivariate discriminator using sum of

energy deposits in dR rings around the tau

Real taus Fake taus

1 Prong 3 Prong 1 Prong + Strip

Tau Identification

τ-> πν τ-> a1ν τ-> ρν

40 TAE July 2012 40 Maria Chamizo Llatas

Typical jet Pileup jet

§  Pileup jets structure differs wrt regular jets: §  Pileup jets originate from several overlapping jets which merge together §  Likelihood grows rapidly with high pileup

§  Discriminant exploits shape and tracking variables §  discrimination both inside and outside tracker acceptance

Jet Identification

41

•  Jet reconstruction •  Reconstruction with particle flow objects


Standard Model: Precision Jets, W, and γ*/Z

Differential Drell-Yan cross section: 2.5M µµ pairs tests NNLO cross sections and PDFs

42 42

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§  Fabulous agreement §  Lots of data … on to the Higgs…

43 43

ATLAS cross section measurements

44

Inner error: statistical Outer error: total

q  Important on their own and as foundation for Higgs searches q Most of these processes are reducible or irreducible backgrounds to Higgs q  Reconstruction and measurement of challenging processes (e.g. fully hadronic tt, single top, ..) are good training for some complex Higgs final states


Back up

•  Cluster reconstruction in the electromagnetic calorimeter −  Common for both electrons and photons (Electrons also reconstructed as photons) −  Designed to collect bremsstrahlung and conversions in extended phi region

•  Dedicated track reconstruction for electrons −  Gaussian Sum Filter allows for tracks w/large bremsstrahlung

•  Photon identification specific to Hèϒϒ

Electron/Photon reconstruction


•  Minimal supersymmetric model −  For each ½ integer spin particle (fermion) there is an integer spin

(boson) partner and vice-versa −  The spins are different by half a unit

−  They are heavier (else we would have already seen them)

MSSM


Documents

Particle detectors at LHC (II) - UCMteorica.fis.ucm.es/TAE2012/CHARLAS.DIR/CHAMIZO.DIR/TAE2012-lecture3.pdf · La reconstrucción está basada en técnicas de Kalman Filter (usadas