Jets in pp and p-Pb Collisions
measured with ALICE

Oliver Busch
Tsukuba University
University of Heidelberg

for the ALICE collaboration
Oliver Busch – QCD challenges, 2016/01/19

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Outline• introduction• jet fragmentation in pp collisions• PID in jets in pp• jets in p-Pb collisions• jet QpPb
• strangeness in jets in p-Pb

Oliver Busch – QCD challenges, 2016/01/19

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Introduction

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Jet Fragmentation• initial hard (high Q2) scattering: high-pT, quasi-free partons• Q2 evolution: parton shower, gluon cascade• hadronization at ~ΛQCD• jets allow to study

QCD at various scales
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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

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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 :ZDC
ZDC- neutral particles- Δφ = 107°, |η|<0.7
cluster ET > 300 MeV• jet trigger with EMCal and TRD• `charged’ (tracking) jets and `full’ jets
• tracking jets: suitable especially under harsh pile-up conditions
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Results from pp Collisions

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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)
R = 0.4

R = 0.6

PRD 91 (2015)
112012

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pp Jet Fragmentation at 7 TeV•

distributionsof charged particles incharged jets at= 7 TeV• bulk production at low z:~ 5-10 charged particles per jet• for z > 0.2 distributions consistent
for all jet pT: ‘scaling’

ALICE, PRD 91, 112012

z Scaling in QCD• MLLA: analytic parton shower calculation, NLL resummation• hadronization not explicitly included• low-ξ (approximate) scaling afeature of parton shower(with some caveats about validityas ξ-> 0)• down to which jet pT will this work ?

CDF, PRD 68 (2003) 012003

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z Scaling: Low-pT Jets
• …. down to 10 GeV/c charged jet pT !
• deviations for 5-10 GeV/c jets
• low-pT jets and their properties
important in the context of
pp UE studies (high-multiplicity pp)• do we see increasing influence of
non-perturbative effects on essentially
free partons,
or non-perturbative objects(e.g. UE ‘fake jet’ clusters) ?

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Event Generator Comparison• low-pT jets rather well describedby PYTHIA(pT ordered parton shower,Lund string fragmentation)• Perugia2010NoCR OK,
no need to evoke MPI coherence• 5 GeV/c hadron clusters look ‘jetty’and can be described by single
hard scattering + fragmentation,
however non-perturbative effects
are important (even at high z ?!)Oliver Busch – QCD challenges, 2016/01/19

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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 modelparameters and particlefractions `on the fly’during fit• regularization requiringcontinuity ofparticle fractions

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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• complementary andconsistent with TCF

Oliver Busch – QCD 34
challenges, 2016/01/19

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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• z scaling for all species: no strong hadronization effects

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Particle Ratios in Jets
• strangeness content strongly enhanced for zch → 1• leading baryons suppressed

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Event Generator Comparison• comparison to PYTHIA6(pT ordered parton shower,Lund string fragmentation)• data reasonably welldescribed• well reproducedby Perugia0 NoCR
tune without
color reconnectionsOliver Busch – QCD challenges, 2016/01/19

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Results from p-Pb Collisions

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Jets in p-Pb Collisions• sensitive to cold-nuclear matter effects :• nuclear modification of the parton distribution• kT broadening and energy loss in cold nuclear matter• in p-Pb collisions at LHC, onset of collectivity observed inhigh-multiplicity events• hydro in small systems:
high initial Temperature ?
•~ T3• jet quenching in p-Pb ?
-> jet spectra a.f.o. centrality
E. Shuryak ,I. Zahed, Phys.Rev. C88 (2013) 4
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Centrality in p-Pb Collisions• ‘centrality’ related to collision geometry• in Pb-Pb:• Glauber MC + NBD fit tomeasured multiplicity distribution• strong correlation multiplicity - Npart - b• distance between 2 nuclei in Pb nucleus:(1 / 0.17 fm-3 )1/3 ~ 1.8 fm ~ 2 x rProton-> in p-Pb, correlation
multiplicity - b much worse• NPart ~ 7, small !
-> weak correlation multiplicity - Npart-> hard processes can easily bias
multiplicity selection
ALICE, Phys. Rev. C 91 (2015), 064905

Hybrid Model• determine event classes from energy in zero-degree calorimeter(115 m from interaction point - no multiplicity bias)• calculate Ncoll from signal in V0A forward-scintillator• Ncoll, Pb-side: assume charged
particle multiplicity is proportional
to Npart - 1 = Ncoll

ALICE, Phys. Rev. C 91 (2015), 064905
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Jet Background Subtraction• jet reconstruction in p-Pb: background from underlying event not
related to hard parton fragmentation• estimate average background density from kT clusters
and subtract jet-by-jet

Ai : jet area
C: occupancy correction• background fluctuations
and detector effects
corrected
through unfolding
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Charged Jet Cross Section• 4 centrality bins
R = 0.4

R = 0.2

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Nuclear Modification Factor•• <TpPb> : nuclear overlap function from Glauber Calculation
using Ncoll, Pb side
• no significant nuclear modification observed

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Cross Section Ratio• spectral ratio R = 0.2 / R = 0.4 sensitive to radial jet structure• no significant centrality dependence

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Strangeness Production in p-Pb
• inclusive strangeness production in central Pb-Pb collisions:strong enhancement of Λ/K0S ratio at intermediate pT• involving several phenomena:• flow• hadronization through recombination ?
• jet fragmentation• qualitatively similar
observation in p-Pb
• strangeness in jets in p-Pb:
separate soft and hardprocesses

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Strangeness in Jets in p-Pb
• K0S and Λ identication via decay topology (‘V0’)• reconstruct V0s and jets independently• match V0s to jet cone: R(V0, jet) < Rjet
• signal extraction via invariant mass,
subtraction of combinatorial background

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pT Spectra• transverse momentum spectra of K0S and Λ in charged jetswith pT,jet > 10 GeV/c• normalized per event and unit acceptance (∆η∆φ)28Λ / K0S Ratio• Λ / K0S ratio in jets in Minimum Bias collisions• significantly lower than inclusive ratio

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Event Multiplicity Dependence• results for 2 V0A multiplicity classes• no significant dependence on multiplicity

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PYTHIA Comparison• PYTHIA8 pp : Λ / K0S ratio in jets similar to inclusive• simulations describe strangeness in jets, but not inclusive production

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Summary• charged jet fragmentation:dN/dz scaling in low-pT jets
• identified kaons, protons and charged pions in jets
well described by PYTHIA 6 simulations without CR• charged jets in p-Pb collisions:no nuclear modification observed for central collisions• strangeness in p-Pb:
strange baryon/meson ratio smaller than inclusive production, noevent multiplicity dependence

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- Backup -Oliver Busch – QCD challenges, 2016/01/1933

ALICE in run 2: DCal

• run 2: DCal upgrade- significantly extended jet acceptance- back-to-back in azimuth (di-jet topology)
Oliver Busch – QCD challenges, 2016/01/19

PHENIX jet RdAu
• PHENIX d-Au:• suppression in central events• enhancement in peripheral events• can not be explained by trivial multiplicity bias

arXiv: 1509.04657 [nucl-ex]

PHENIX and ATLAS
ATLAS, 1412.4092 [nucl-ex]• similar observation by ATLAS• initial state effect scaling with x ?

A. Hanks, QM 15

Jet finder comparison''nucl-ex/1411.4969
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kT: sequentialrecombination
SISCone: cone algorithm

Method comparison• uncorrected hadron fractions from Multi-Template Fit andTPC Coherent Fit• 2 complementary methods
obtain consistent resultsHadrons in heavy-ion collisions
PLB 720 (2013) 250• high- pT hadrons `proxy’ for jet• jet quenching for charged hadrons,

Pb-Pb collisions at √sNN = 2.76 TeV• hadron observables biased towards
leading fragment→ study the effect for fully reconstructed jets

Oliver Busch – QCD challenges, 2016/01/19

Jet nuclear modification factor• strong suppression observed, similar to hadron RAA→ parton energy not recovered inside jet cone• increase of suppression
with centrality• weak pT
dependence• JEWEL andYaJEM
jet quenchingmodels reproduce
suppression

Phys.Lett. B746 (2015) 1
JEWEL: PLB 735 (2014)
YaJEM:PRC 88 (2013) 014905