Jet Physics with ALICE

Oliver Buschfor the ALICE collaboration

Oliver Busch – Tsukuba 2014 /03/13

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Outline• introduction• results from pp• jets in heavy-ion collisions• results from Pb-Pb collisions• jets in p-Pb collisions• outlook: LHC run 2

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Introduction

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Jets: seeing quarks and gluons• jet: collimated spray of hadrons• quasi-free parton scattering at high Q2:
the best available experimental equivalent to quarks and gluons
Oliver Busch – Tsukuba 2014 /03/13

4Jet fragmentation• initial hard scattering: high-pT partons• cascade of gluons: parton shower• at soft scale (O(ΛQCD)): hadronization

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LHC aerial view

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Jets at ALICE (LHC run 1)• charged particle tracking:- Inner Tracking System (ITS)- Time Projection Chamber- full azimuth, |η |< 0.9
pT > 150 MeV/c• EMCal :- neutral particles- Δφ = 107°, |η|<0.7
cluster ET > 300 MeV• jet trigger with EMCal and TRD• `charged’ (tracking) jets and `full’ jets
• full jets from charged particle tracking and EM energy:
conceptually different and complementary to traditional approach

7Results from pp collisions

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Full jets in pp at= 2.76 TeV• good agreement to NLO calculations for R = 0.2 and R = 0.4• reference for Pb-Pb at same energy
R = 0.4

R = 0.2

Phys. Lett. B 722 (2013) 262
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pp charged jet cross-sections• measured in minimum bias collisions at= 7 TeV• good agreement with ATLAS charged jet measurements(despite slightly different acceptance and track pT range)
nucl-ex/1411.4969

R = 0.4

R = 0.6

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pp jet fragmentation at•

distributionsof charged particles incharged jets

• scaling for z > 0.2• bulk production at low z:~ 5-10 charged particles per jet

nucl-ex/1411.4969= 7 TeVPID in jets : `TPC coherent fit’
• particle identification via specific ionization in TPC ('dE/dx'):• TPC coherent fit:use energy loss model parameterization as input,
adjust model
parameters and particle
fractions “on the fly”
during fit• regularization requiringcontinuity ofparticle fractions• complementary andconsistent:
multi-template fit
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Particle identified fragmentation• identified charged hadrons in charged jets at= 7 TeV• π, K, p, 5 < pTch jet < 20 GeV/c• scaling for zch > 0.2 for higher jet pT bins

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Particle ratios in jets
• strangeness content strongly enhanced for zch → 1• leading baryons suppressedOliver Busch – Tsukuba 2014 /03/13

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Event generator comparison• comparison to PYTHIA(pT ordered parton shower,Lund string fragmentation)• data reasonably welldescribed• best reproducedby Perugia tune without
color reconnections

Jets and Quark-Gluon Plasma

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QCD phase transition• in heavy-ion collisions at ultra-relativistic energies,
a quasi macroscopic fireball of hot, strongly interacting matterin local thermal equilibrium is created• lattice QCD predicts phase transition to deconfined,chirally symmetric matter• energy densityfrom the lattice:
rapid increase around TC,indicating increase ofdegrees of freedom(pion gas ->quarks and gluons)• TC = 154+/- 9 MeV

EC = 340 +/- 45 MeV/fm3

HotQCD, PRD 90, 094503

Partons in heavy-ion collisions• hard partons are produced early and traverse the hot and dense QGP• enhanced energy loss: `jet quenching’• ‘vacuum’ expectation calculable by pQCD : ‘calibrated probe of QGP’
• jets sensitive to properties ofthe medium (energydensity, , mfp, coupling ... )• ... but also jet/medium
interaction not trivial(strong / weak coupling,parton mass / type,
fireball dynamics ... )Hadrons in heavy-ion collisions• hard partons `proxy’ for jet• jet quenching for charged hadrons,
Pb-Pb collisions at √sNN = 2.76 TeV

PLB 720 (2013) 250• hadron observables biased towards
leading fragment→ study the effect for fully reconstructed jets

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Underlying event in heavy-ion collisions• jet reconstruction in heavy-ion collisions :difficult due to the high underlying event backgroundnot related to hard scattering• correct spectra for background fluctuations and detector effects
via unfolding• not possible down to 0 pT

jet area ~ 0.5 (R = 0.4)

centralperipheral

Jet nuclear modification factor• strong suppression observed, similar to hadron RAA-> parton energy not recovered inside jet cone
• increase of suppression
with centrality• mild pT
dependence• JEWEL andYaJEM
jet quenchingmodels reproduce
suppression	  nucl-ex/1502.01689
JEWEL: PLB 735 (2014)
YaJEM:PRC 88 (2013) 014905

Jet structure•	  ‘jet structure ratio’ R=0.2 / R=0.3 for charged jets
• sensitive to potential broadening of jet shape• consistent with PYTHIA pp:
no modification observed
within small radii(jet core)	  	  JHEP 03 (2014) 013

Results from p-Pb collisions

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Charged jets in p-Pb collisions	  • p + Pb collisions at √sNN = 5.02 TeV• check potential initial state effects(nuclear modification of PDF)
and effects of cold nuclear matter

nucl-ex/1503.00681• measured spectrum consistent withPOWHEG NLO calculations( = NLO + PYTHIA fragmentation)

Oliver Busch – Tsukuba 2014 /03/1324	  Charged jet Rp-Pb	  • Rp-Pb using scaled charged jet spectrum measured at 7 TeV as reference• no jet suppression observed in p-Pb-> suppression in Pb-Pb is final state effectnucl-ex/1503.00681

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25	  Jet structure in p-Pb	  • `jet structure ratio’ R=0.2 / R=0.4 for different centrality classes• no modification, even for most central p-Pb

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Perspectives for LHC run 2

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LHC run 2• LHC run 2: 2015 - 2017
• increased CMS energy for Pb-Pb collisions from 2.76 → 5.1 TeV• quenching strength ~ s ~ ε3/4
• expect (modest) increase in ε, T→ measure energy density
dependence of jet quenching• note: also a dependence onparton ‘input spectrum’(increased RAA ??? )ALICE, PRL 105, 252301

ALICE in run 2: DCal

• run 2: DCal upgrade- significantly extended jet acceptance- back-to-back in azimuth (di-jet topology)
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Summary• jet cross sections and properties in pp• identified jet fragmentation in pp• strong jet suppression observed in Pb-Pb collisions• p-Pb results: initial-state and cold-nuclear-matter effects negligible• looking forward to LHC run 2 !

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Jet reconstruction• Establish correspondence between
detector measurements /final state particles / partons• two types of jet finder:- iterative cone- sequential recombination(e.g. anti-kT)• resolution parameter R

hep-ph/0802.1189

Jet finder comparison●●kT: sequential
recombination
SISCone: cone algorithm

nucl-ex/1411.4969
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Multi Template Fit• TPC multi-template fit- best possible description of dE/dx from external reference- parametrize dependences on η, TPC nClusters- templates in transverse momentum (z, xi) slices• dE/dx in one z slice (0.6 < z < 0.65), 10-15 GeV/c fitted with
4 templates

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Method comparison• uncorrected hadron fractions from Multi-Template Fit andTPC Coherent Fit• 2 complementary methods
obtain consistent results

pp jet structure at= 7 TeV• transverse structure:pT sum in radial slices• increasing collimation
for higher pT jets
• fragmentation and jetstructure reasonably
described by event
generators

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QCD matter at LHC• direct photons: thermal radiation from the early stage of the fireball• indicates initial temperature way above TC

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Reaction plane dependence• different medium thickness in- and out-of plane• sensitive to path length dependence of jet quenchingpQCD radiative E-loss : ~L2collisional E-loss : ~Lstrong coupling (ADS/CFT) : ~L337

Local background subtraction• prerequisite: take event plane dependence of background density
into account• event-by-event
local ρ fit38

Jet v2 : results• charged jets, R = 0.2• quantify azimuthal asymmetry via 2nd Fourier harmonic v2• strongly hints to non-zero jet v2

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Recoil	  jets	  uncorrected	  jet	  • semi – inclusive recoil jet spectra :- reduce fake jet background through ‘dijet’ topology- residual combinatorics subtracted in Δrecoil observable• trigger hadron surface biased → maximize path length for jet

Δrecoil	  • fully corrected Δrecoil : R=0.2 and R=0.4• measurement down to constituent pT of 150 MeV/c,
for large radii (up to R = 0.5) and low jet pT,
without biasing leading constituent

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ΔIAA	  • ΔIAA : compare to PYTHIA pp reference• suppression observed42Jet structure ratio	  • Δrecoil Ratio R=0.2 / R=0.5 :sensitive to potential broadening of jet structure• consistent with PYTHIA within large experimental uncertainties
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