Heavy-Ion Physics with the CMS experiment at the Large Hadron Collider

Philip Allfrey

University of Auckland,

for the CMS Collaboration

Outline

• Large Hadron Collider and Compact Muon Solenoid Experiment

• Quark-Gluon Plasma

• Jet Quenching

• Upsilon suppression

• Grid computing

Large Hadron Collider

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MUON

(Barrel)

TRACKER

(Pixels and Strips)

EM Calorimeter (ECAL) Hadron Calorimeter (HCAL)

MUON

(Endcaps)

Forward Calorimeter

(HF)

Beam Scintillator Counters (BSC)

CMS Detector

Quarks and Gluons

• Protons and neutrons made up of quarks and gluons

• Quarks and gluons normally bound within a nucleon, can’t exist in isolation

• If enough matter with enough energy compressed into small enough volume, get a ‘soup’ of unbound quarks and gluons

• This is known as a quark-gluon plasma (QGP) 5

Heavy Ion Physics

• Goal of high-energy heavy-ion physics to produce and study a quark-gluon plasma

• Allows experimental study of QCD in extreme temperature/energy density

• Various probes/signatures – jet quenching, heavy quark production, elliptic flow, correlations…

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Heavy Ion Collisions

Lorentz-contracted nuclei approach

Nuclei collide. Binary collisions between quarks and gluons can produce jets or heavy quarks

Quarks and gluons freed, plasma formed

Plasma expands and cools, quarks and gluons form bound states (particles)

Jet quenching

• Pair of jets must be formed back-to-back to conserve momentum

• Jets not necessarily formed in centre of colliding nuclei, so have different path lengths in the medium

• Therefore back-to-back jets with different energies imply energy loss

• Signature of QGP

Examples of Jets in CMS

Balanced Energy

Unbalanced Energy

Dijet energy imbalance

Central Semi-Central Semi-Peripheral

Pb Pb Pb Pb Pb Pb

21

21

j

T

j

T

j

T

j

TJ

EE

EEA

Jet energy

asymmetry Phys Rev C 84 (2011) 024906

Quarkonia

• Bound state of quark-antiquark pair is called quarkonium

• Quarks and gluons carry colour charge, can move around in QGP => screening

• If screening radius drops below binding radius, quarkonia should melt

• Screening radius decreases with increasing temperature, therefore suppression of quarkonia acts as thermometer of QGP

• Look for suppression of Upsilon (ϒ, b anti-b pair) in both heavy-ion and proton-proton collisions

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ϒ→μ+μ- mass spectrum

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Pb-Pb proton-proton

Phys. Rev. Lett., 107 (2011) 052302

ϒ(1S) ϒ(2S)

ϒ(3S)

Double Ratio

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• Compare ratios of ϒ(2S+3S) relative to ϒ(1S) in PbPb & pp

• Benefits from cancellation of possible acceptance and efficiency differences

(sys) 0.02±(stat)0.24(1S)3S)+(2S +0.13

0.12PbPb

(sys) 0.03±(stat)0.31(1S)3S)+(2S

(1S)3S)+(2S0.19+

0.15

pp

PbPb

(sys) 0.02±(stat)0.78(1S)3S)+(2S +0.16

0.14pp

PbPb

pp

First observation of suppression of excited ϒ states

Grid Overview

• Computing and data storage requirements of LHC experiments too large for single site to handle

• Distributed among computing centres worldwide

• Classed as Tier-0,1,2 or 3 depending on resources and responsibilities

• Based on OSG or gLite middleware

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CMS Grid Sites by Tier

15 Tier 0 Tier 1 Tier 2 Tier 3

Architecture

16 Non-trivial to set up, even if documented!

Architecture

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Monitoring

Computing

Storage

Interface

CMS-Specific

Performance

• Certified by Asia-Pacific ROC

• 50k CPU-hours over past 12 months

• Users from 15 different countries

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Summary

• Jet quenching and suppression of excited upsilon states observed at CMS

• Made possible by greater energy at LHC

• Interpreted as signatures of QGP formation

• NZ contributing to analysis of LHC data with Tier-3 Grid site