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Example 2: experiments at LHC Peter Krian University of Ljubljana - - PowerPoint PPT Presentation

Dec 14, 2022 490 likes •1.21k views

Example 2: experiments at LHC Peter Krian University of Ljubljana and J. Stefan Institute Joef Stefan University Institute of Ljubljana Peter Krian, Ljubljana Contents General purpose experiments: ATLAS and CMS Heavy ion

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Peter Križan, Ljubljana

Example 2: experiments at LHC

Peter Križan

University of Ljubljana and J. Stefan Institute

University

  • f Ljubljana

“Jožef Stefan” Institute

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Peter Križan, Ljubljana

Contents

General purpose experiments: ATLAS and CMS Heavy ion collisions: ALICE

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Peter Križan, Ljubljana

General purpose experiments: ATLAS and CMS

Goals:

  • Find Higgs
  • Search for new (heavy) particles
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Peter Križan, Ljubljana

Zakaj imajo delci maso: Higgsov bozon

Škotski fizik Peter Higgs in belgijski fizik Francois Englert, 1964: Maso delcev lahko pojasnimo, če predpostavimo, da je prostor napolnjen s poljem – Higgsovim poljem Elektromagnetno polje  nabit delec (e-) občuti silo

velikost sile odvisna od velikosti električnega naboja

Higgsovo polje  delci imajo maso

velikost mase odvisna od velikosti „Higgsovega naboja“

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Peter Križan, Ljubljana

Higgsov bozon

Škotski fizik Peter Higgs in belgijski fizik Francois Englert, 1964: Maso delcev lahko pojasnimo, če predpostavimo, da je prostor napolnjen s poljem, seveda – Higgsovim poljem Elektromagnetno polje  nabit delec (e-) občuti silo

velikost sile odvisna od velikosti električnega naboja

Higgsovo polje  delci imajo maso

velikost mase odvisna od velikosti „Higgsovega naboja“

elektromagnetno polje ima svoje delce – fotone Higgsovo polje ima svoje delce – Higgsove bozone

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Peter Križan, Ljubljana

Generic LHC Detector for all Particles

electron

muon hadron

Magnetic field: Bends charged particles enabling momentum measurement Low-mass tracker: Performs precision measurement of several hits along particle trajectory Electromagnetic calorimenter: Contains EM shower and measures its energy Hadronic calorimeter: Contains hadronic shower and measures its energy (with EM) Muon detector: Re-measures muon tracks

Only neutrinos escape detection

neutrino

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

Muon spectrum -ATLAS

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Peter Križan, Ljubljana eB = 0.3 (B/T) (1/m) GeV/c

4 720

2

  N eBL p p

T x T pT

 

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Peter Križan, Ljubljana eB = 0.3 (B/T) (1/m) GeV/c

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Peter Križan, Ljubljana

Tracking system of the inner detector

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Peter Križan, Ljubljana

What kind of momentum resolution do we need?

Reminder: example X    M2c4 = (E1 + E2)2 - (p1 + p2)2  M2c4 = 2 p1 p2 (1 - cos12) The X peak should be narrow to minimize the contribution of random coincidences (‘combinatorial background’) The required resolution in Mc2: about 1 GeV at 30 GeV. What is the corresponding momentum resolution? For simplicity assume X is at rest  12=1800, p1=p2=p=15 GeV/c, Mc2=2pc (Mc2) = 2 (pc) at p=15 GeV/c  (p)/p = 1 GeV/2/15GeV = 3%

CMS could-be-particle (porobably statistical fluctuation…)

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Peter Križan, Ljubljana

Momentum resolution

For B=2T, L = 1m, x = 0.1 mm For pT = 1 GeV:  pT /pT = 0.06% For pT = 10 GeV:  pT /pT = 0.6% For pT = 100 GeV:  pT /pT = 6% How to improve high momentum resolution?

  • Better resolution: wire chamber  silicon strip detector (full

CMS tracker, partly ATLAS)

  • Higher field: CMS B=4T
  • Longer lever arm for muons: additional tracking in the

magnetic muon system (ATLAS)

4 720

2

  N eBL p p

T x T pT

 

6 . 13 LX eB MeV pT

pT 

eB = 0.3 (B/T) (1/m) GeV/c

0006 . 54 720 1 2 ) / ( 3 . 10 1 .

2 3

     

 T T T p

p m m GeV m p p

T

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Peter Križan, Ljubljana

Momentum measurement for very high energy muons - example ATLAS

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Peter Križan, Ljubljana

Tipične številke

ATLAS B = 2T

 = - ln tg  /2

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Peter Križan, Ljubljana

Identification of charged particles

Particles are identified by their mass or by the way they interact. Determination of mass: from the relation between momentum and velocity, p=mv (p is known - radius of curvature in magnetic field) Measure velocity by:

  • time of flight
  • ionisation losses dE/dx
  • Cherenkov photon angle (and/or yield)
  • transition radiation

Mainly used for the identification of hadrons. Identification through interaction: electrons and muons calorimeters, muon systems

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Peter Križan, Ljubljana

Transition radiation

E.M. radiation emitted by a charged particle at the boundary of two media with different refractive indices Emission rate depends on  (Lorentz factor): becomes important at ~1000

  • Electrons at 0.5 GeV
  • Pions above 140 GeV

Emission probability per boundary ~ = 1/137 Emission angle ~1/ Typical photon energy: ~10 keV  X rays

~1/ TR photon

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Peter Križan, Ljubljana

Transition radiation - detection

Emission probability per boundary ~ = 1/137  Need many boundaries

  • Stacks of thin foils or
  • Porous materials – foam with many boundaries of individual ‘bubbles’

Typical photon energy: ~10 keV  X rays  Need a wire chamber with a high Z gas (Xe) in the gas mixture Emission angle ~1/  Hits from TR photons along the charged particle direction

  • Separation of X ray hits (high energy deposit on one place) against

ionisation losses (spread out along the track)

  • Two thresholds: lower for ionisation losses, higher for X ray detection
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Peter Križan, Ljubljana

Transition radiation - detection

 Hits from TR photons along the charged particle direction

  • Separation of X ray hits (high energy deposit on one place) against

ionisation losses (spread out along the track)

  • Two thresholds: lower for ionisation losses, higher for X ray detection
  • Small circles: low threshold

(ionisation)

  • Big circles: high threshold (X

ray detection)

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Peter Križan, Ljubljana

Transition radiation detectors

Example: Radiator: organic foam between the detector tubes (straws made of capton foil)

Performance: pion efficiency (fake prob.) vs electron efficiency

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Peter Križan, Ljubljana

Transition radiation detector in ATLAS: combination of a tracker and a transition radiation detector

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

ATLAS TRT

TRT module

Radiator: 3mm thick layers made of polypropylene-polyethylene fibers with ~19 micron diameter, density: 0.06 g/cm3 Straw tubes: 4mm diameter with 31 micron diameter anode wires, gas: 70% Xe, 27% CO2, 3% O2.

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Peter Križan, Ljubljana

TRT: pion-electron separation

Expected  fake probability at 90% e efficiency  JINST 3 (2008) S08003

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Peter Križan, Ljubljana

TRT performance in 2010 data

e/pion separation: high threshold hit probability per straw

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Peter Križan, Ljubljana

Additional feature of TRT: identification with a dE/dx measurement

dE/dx is a function of velocity  For particles with different mass the Bethe- Bloch curve gets displaced if plotted as a function of p For good separation: resolution should be ~5%

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Peter Križan, Ljubljana

Time-over-Threshold (ToT): dE/dx in ATLAS TRT

The relation between the track ToT measurement and the track ,

  • btained from MC studies.

2010 data: The track- averaged ToT distribution as a function of the track momentum.

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Peter Križan, Ljubljana

Identification of muons at LHC

  • example ATLAS
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Peter Križan, Ljubljana

Muon ID

Separate muons from hadrons (pions and kaons): exploit the fact that muons interact only electromag., while hadrons interact strongly  need a few interaction lengths to stop hadrons Interaction lengths = about 10x radiation length in iron, 20x in CsI. A particle is identified as a muon if it penetrates the material.

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Peter Križan, Ljubljana

Identification of muons in ATLAS

  • Identify muons
  • Measure their

momentum

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Peter Križan, Ljubljana

Muon spectrum

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Peter Križan, Ljubljana

Muon identification in ATLAS

Material in front of the muon system

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Peter Križan, Ljubljana

Muon identification efficiency

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Peter Križan, Ljubljana

Muon fake probability

Sources of fakes:

  • Hadrons: punch through negligible, >10 interaction legths of material in

front of the muon system (remain: muons from pion and kaon decays)

  • Electromagnetic showers triggered by energetic muons traversing the

calorimeters and support structures lead to low-momentum electron and positron tracks, an irreducible source of fake stand-alone muons. Most of them can be rejected by a cut on their transverse momentum (pT > 5 GeV reduces the fake rate to a few percent per triggered event); can be almost entirely rejected by requiring a match of the muon-spectrometer track with an inner-detector track.

  • Fake stand-alone muons from the background of thermal neutrons and

low energy -rays in the muon spectrometer ("cavern background"). Again: pT > 5 GeV reduces this below 2% per triggered event at 1033 cm-2 s-1. Can be reduced by almost an order of magnitude by requiring a match of the muon-spectrometer track with an inner-detector track.

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Peter Križan, UL FMF + IJS

Razpad Higgsovega delca v dva visokoenrgijska žarka gamma, H  v detektorju ATLAS

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Peter Križan, Ljubljana

Calorimetry

Energy measurement by total absorption, combined with spatial reconstruction. Calorimetry is a “destructive” method Detector response  E Calorimetry works both for

  • charged (e± and hadrons) and
  • neutral particles (n,)

Basic mechanism: formation of electromagnetic or hadronic showers. Finally, the energy is converted into ionization or excitation of the matter.

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

Electrons: fractional energy loss, 1/E dE/dx

Critical energy Ec

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

Interaction of photons with matter

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

Hadronic showers are much longer and broader than electromagnetic ones!

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Peter Križan, Ljubljana

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Peter Križan, Ljubljana

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Peter Križan, UL FMF + IJS

Detektor ATLAS med gradnjo

Viden delež slovenske raziskovalne skupine (IJS in FMF UL) Marko Mikuž

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Peter Križan, UL FMF + IJS

Kontrolna soba med meritvami...

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Peter Križan, UL FMF + IJS

Iskanje Higgsove delca z detektorjema ATLAS in CMS ob LHC

  • Trkalnik in oba velika detektorja, ATLAS in CMS odlično delujejo od

konca leta 2009

  • Julij 2012: ATLAS in CMS objavita odkritje Higgsovega bozona –

pravzaprav delca, za katerega zaenkrat vse kaže, da ima take lastnosti, kot jih pričakujemo od Higgsovega delca (‘Higgs-like particle’).

  • Na dokončno potrditev je bilo treba počakati do 2013, ko so nabrali

dovolj velik vzorec podatkov, da so lahko opravili dodatne meritve.

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Peter Križan, UL FMF + IJS

Rezultat meritve: iskanje razpada Higgsovega bozona v dva žarka gamma, H  

Masa vsake zabeležene kombinacije dveh visokoenergijskih žarkov gama: – veliko večino predstavljajo naključne kombinacije

  • vrh pri energiji 126 GeV

ustreza razpadom H   Izmerjena porazdelitev minus ozadje  signal!

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Peter Križan, UL FMF + IJS

Rezultat meritve: iskanje razpada Higgsovega bozona v dva žarka gamma, H  

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Peter Križan, UL FMF + IJS

Odkritje Higgsovega delca

Na dokončno potrditev je bilo treba počakati do 2013, ko so nabrali dovolj velik vzorec podatkov, da so lahko opravili dodatne meritve.

  • Primerjava števila razpadov Higgsovega bozona v različnih razpadnih

kanalih

  • Kotne porazdelitve delcev v končnem stanju – določanje lastnosti tega

delca (spin – vrtilna količina).  Novi delec ima take lastnosti, kot jih predvideva Standardni model

Nobelova nagrada 2013!

Francois Englert in Peter W. Higgs

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Peter Križan, UL FMF + IJS

Rezultat meritve: iskanje razpada Higgsovega bozona v štiri leptone, H     

Masa vsake zabeležene kombinacije štirih mionov – večinoma kombinacije drugih procesov - ozadja (rdeče in vijolično). Modro: signal, kot bi ga pričakovali za Higgsov delec

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Peter Križan, Ljubljana

Heavy ion collisions: ALICE

Goals:

  • Search for a new state of matter: quark-gluon plasma

Challenge: several thousand particles produced in a collision of two Pb nuclei.

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Peter Križan, Ljubljana

Tracking in ALICE: a time-projection chamber (TPC)

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Peter Križan, Ljubljana

Identification with the dE/dx measurement

dE/dx is a function of velocity  For particles with different mass the Bethe- Bloch curve gets displaced if plotted as a function of p For good separation: resolution should be ~5%

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Peter Križan, Ljubljana

dE/dx in ALICE

relativistic rise region

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Peter Križan, Ljubljana

CsI based RICH counters: HADES, COMPASS, ALICE

ALICE:

  • liquid radiator
  • proximity focusing

HADES and COMPASS RICH: gas radiator + CsI photocathode – long term experience in operation

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Peter Križan, Ljubljana Thickness monitor PC Remote controlled enclosure box

Photocathode produced with a well defined, several step procedure, with CsI vaccum deposition and subsequent heat conditioning

4 CsI sources + shutters

protectiv e box pcb substr ate

CERN CsI deposition plant

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Peter Križan, Ljubljana

ALICE RICH = HMPID

The largest scale (11 m2) application

  • f CsI photo-cathodes in HEP!

CsI photo-cathode is segmented in 0.8x0.84 cm pads

Six photo-cathodes per module

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Peter Križan, Ljubljana

ALICE HMPID performance

Pb-Pb @ 2.76 TeV/nucleon

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Peter Križan, Ljubljana

Back-up slides

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Peter Križan, Ljubljana

TRT performance

at 90% electron efficiency

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Peter Križan, Ljubljana

TRT performance in 2010 data 2

dE/dx performance: time-over-threshold Additional e/pion separation in time-over-threshold