Elena Bruna, for the STAR CollaborationYale University

Winter Workshop on Nuclear Dynamics, Big Sky Feb. 1-8 2009

Going from pp…Topological Jets in pp at RHIC energies

pT spectrum up to 50 GeVWhat we know:

Good agreement of jet spectrum with NLO/PYTHIA at RHIC energies

Jet spectra and fragmentation functions measured at Tevatron good agreement with theory

Perspectives in STAR:Measure jet Fragmentation Functions at RHIC energiesCompare conceptually different jet algorithms (kT, anti-kT,

cone)Energy resolution using PYTHIA simulationUnderstand the Trigger Bias

Goal: compare Fragmentation functions in pp and Au+Au 2Elena Bruna

...to Au+AuTopological jets measured in Au+Au in STAR

at RHIC energies!What we want to measure:

Jet pT spectrumJet Fragmentation Functions

What we expect with an unbiased jet population:pT spectrum Nbin scaling (jet production is a

hard process)Fragmentation functions modification

3Elena Bruna

Jets in high-energy collisionsHigh-pT partons produced in hard scatterings

a, xa b, xbσab

c, x

c

d, x

d

Full jet (spray of collimated hadrons) after parton fragmentation gives access to:

partonic kinematics jet cross sections

TOOL: Jet-Finding algorithms

4Elena Bruna

Jet Reconstruction

Seedless, not bound to a circular structure kT: starts from merging low pT particles close

in the phase-space Anti-kT: starts from merging high pT particles

close in the phase-space

Rcone

seed

tracks or towers

R=√(Δφ2+Δη2)

Seed Cone: ‘seed’ (E>Ethreshold) iterative approach

• Seedless Cone (SIS cone): all the particles used as

seeds Splitting/Merging

applied

Con

e

Alg

ori

thm

sR

ecom

bin

ati

on

A

lgori

thm

s

[Cacciari, Salam, Soyez, arXiv:0802.1189]

[Cacciari, Soyez, arXiv:0704.0292]

outgoing parton

fragmentation

seed

5Elena Bruna

• charged particle pT (TPC)

• neutral tower Et 0.05x0.05 (ηxϕ) (EMC)

• corrected for hadronic energy.

• Electron correction for double counting

• EMC provides fast trigger.

• Two trigger setups with the EMC:

• Jet Patch Trigger (JP):

• 1x1(ηxϕ), Et>8 GeV

• High Tower Trigger (HT):

•tower 0.05x0.05 (ηxϕ) Et> 5.4 GeV (pp)

• cluster 0.1x0.1 (ηxϕ) Et> 7.5 GeV (AuAu)

40 towers

: 120 towers in =0-2

6Elena Bruna

=-1

=0 =+1

EMC

Analyzed STAR data-sets:

• p+p (2006) High-Tower (HT) trigger (single tower Et>5.4 GeV)

• p+p (2006) Jet-Patch (JP) trigger (ηxϕ=1x1 with sum Et>8 GeV)

7Elena Bruna

Fragmentation functions for charged hadrons

Definition: =ln (Ejet/phadr)

We use: =ln (pT,jet/pT,hadr) : no assumptions on the particle mass ξξ ppT,hadr T,hadr (GeV/c)(GeV/c)

(p(pTjetTjet=30 GeV)=30 GeV)

11 1111

22 44

33 1.5 1.5

44 0.5 0.5

55 0.2 0.2

30< pT,jet<40 GeV

p+p √s=200 GeVJP trigger

Fragmentation function in qualitative agreement for all Jet-Finders•Statistical errors only

•Systematic studies ongoing 8Elena Bruna

STAR Preliminary

Jet-Patch vs High Tower triggers10< pT,jet<15 GeV

20< pT,jet<30 GeV

30< pT,jet<40 GeV

pT,jet>40 GeV

• Low pTjet: Multiplicity of charged particles influenced by Neutral Energy Fraction and high z fragmenting jets stronger trigger BIAS in the HT sample

• High pTjet: JP and HT do not show difference due to a smaller bias of different trigger selections JP and HT fragmentation functions similar for higher jet energy

STAR preliminary

STAR preliminary

STAR preliminary

STAR preliminary

kT

R=0.7Uncorrecte

d

10GeV jet

pThadr

(GeV/c)

20GeV jet pThadr

(GeV/c)

1 3.7 7.4

3 0.5 1

5 0.07 0.1

30GeV jet pThadr

(GeV/c)

40GeV jet pThadr

(GeV/c)

1 11 14.7

3 1.5 2

5 0.2 0.26

9

STAR Preliminary

STAR Preliminary

STAR Preliminary

STAR Preliminary

p+p √s=200 GeV

p+p √s=200 GeV

p+p √s=200 GeV

p+p √s=200 GeV

for different jet energies (1)

Elena Bruna

JP trigger

R=0.4R=0.4

10< pT,jet<15 GeV

20< pT,jet<30 GeV

30< pT,jet<40 GeV

pT,jet>40 GeV

10

• Uncorrected spectra • Different Jet Finders show similar performance for a given R

p+p √s=200 GeVJP trigger

p+p √s=200 GeVJP trigger

p+p √s=200 GeVJP trigger

p+p √s=200 GeVJP trigger

pT>=1 GeV/c

pT>=1 GeV/c

for different jet energies (2)10< pT,jet<15 GeV

20< pT,jet<30 GeV

30< pT,jet<40 GeV

pT,jet>40 GeVJP JP

triggertriggerR=0.7R=0.7

p+p √s=200 GeVJP trigger

p+p √s=200 GeVJP trigger

p+p √s=200 GeVJP trigger

p+p √s=200 GeVJP trigger

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pT>=1 GeV/c

pT>=1 GeV/c

pT>=1 GeV/c

pT>=1 GeV/c

pT>=1 GeV/c

pT>=1 GeV/c

• Conceptually different Jet Finders Similar performance for different cone radii and for different jet pT suggests no significant NLO effects at RHIC energies

p+p Fragmentation function vs. PYTHIA

IncreasingCone R

IncreasingJet EnergyIncreasingJet Energy

20<Ereco<30 GeV 30<Ereco<40 GeV 40<Ereco<50

R<0.4

R<0.7

Good agreement with PYTHIA especially at low R

JP triggerR=0.7 |ηjet|<0.3

12Elena Bruna

pp: reference for Au+AuJets in pp are a safe

baseline for Au+AuGood agreement with

NLO/PYTHIAFree choice of jet algorithms

in ppSystematic studies ongoing (e-

identification, hadronic shower, trigger bias)

Elena Bruna 13

Analyzed STAR data-sets:

• Au+Au (2007) High-Tower (HT) trigger (cluster ET>7.5 GeV)

• Au+Au (2007) Minimum-Bias (MB) trigger

Jet Finding in Heavy-Ion collisions

GOAL: Fully reconstruct the jet in high-multiplicity environment

How to suppress background:Reduce the jet areaApply a pTcut on tracks and towers

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Jet energy fraction outside cone R=0.3

CDF preliminary

€

R = Δη 2 + Δφ2

• Background estimation:•Mean energy in out-of-cone areas

ϕη

pt p

er g

rid

cel

l [G

eV]

STAR preliminary~ 21 GeV

Reconstructed Jet

Out-of-cone area

Background in Au+Au 0-20%

<p t

,Bkg

> [

Gev

]

Reference multiplicity (~centrality)

Au+Au 0-20%Rc=0.4, no pt cut, out-of-cone area

Bac

kgro

und

flu

ctua

tions

[G

ev]

Rc

• Event-by-event basis:

• <pT,Bkg>=mean pT in out-of-cone area • Bkg for Fragmentation Functions = mean FF in out-of-cone area for a given pTjet

•Background energy in R=0.4 ~ 45 GeV (no pT cut)

• Substantial region-to-region background fluctuations

significantly reduced by applying a ⇒ pT cut

15Elena Bruna

STAR Preliminary

STAR Preliminary

Simulation1) Jet Finder on PYTHIA events

2) a. PYTHIA event embedded in Au+Au real event

b. Jet Finder on PYTHIA + AuAu

3) Compare (1) and (2)detector effects neglected 16Elena Bruna

STAR preliminary

17

SimulationSimulation

STAR Preliminary

Simulation: ξ distribution for 30 GeV (pt,cut>2 GeV)

Charged particle FF:Rc(FF)=0.7 and pt>0 GeV

• ξ background subtraction method and jet energy resolution in Au+Au 0-20% causes deviations < 10-20% for ξ<2-2.5 using using Pythia fragmentationPythia fragmentation

• Systematic deviations in the ξ shape ratio at low ξ are caused by jet-energy resolution

• Only statistical errors

STAR preliminary

30 GeV mono-jet embedded in 0-20% central Au+Au STAR event

18

STAR preliminarySTAR Preliminary

Data: ξ distribution for jet energies > 30 GeV in Au+Au

Cone and kt algorithm give similar fragmentation-function measurements

for ptcut > 2 GeV and reconstructed jet pt above 30 GeV in Au+Au 0-

10%

LOCone

FastJet kt

STAR preliminarystat. errors only

pthadron~10 GeV

Au+Au HT Et>7.5 GeV

19Elena Bruna

Fragmentation Function in Au+Au 0-20% and p+p for 30 GeV jets

1) pt,jetrec.(pp) > 30 GeV

2) pt,jetrec.(Au+Au)>31 GeV for pt

cut>2 GeV3) pt,jet

rec.(Au+Au)>35 GeV for ptcut>1 GeV

No apparent modification in the fragmentation

function with respect to p+p

20Elena Bruna

STAR Preliminary

dominated by uncertaintiesdue to backgroundsubtraction for pt

hadron<2 GeV

STAR Preliminary

AuAu jet energies should correspond to 30 GeV pp jets

Statistical errors only

Au+Au results:Simulation jet Finding works in high multiplicity

environment! background reasonably under controlData:

SPECTRA (MinBias data): Binary scaling Binary scaling observed with no pT cut (S. Salur talk)

PYTHIA fragmentation assumed

FRAGMENTATION FUNCTIONS (HT trigger data):

-- no apparent modification w.r.t. pp Corrections for energy resolution (bkg fluctuations) Corrections based on PYTHIA fragmentation

-- we would expect a modification (high-pT hadron suppression)

Measurements consistent: why? 21

What’s happening!

22

Effect A:Effect A: Biased sample of jets due to the High-Tower Trigger: the HT trigger favors “surface” jets that are not modified by the

medium Ejet (AuAu) = Ejet (pp) FF unmodified If this is true HT jets should not binary scale even without pTcut!

Effect BEffect B: Biased sample of jets due to energy loss and pTcut

The jet softens in the medium Its energy is not recovered with pTcut AND

assuming PYTHIA fragmentation Its energy is UNDERESTIMATED ξ=ln(pt

jet/pt) should be larger If this is true Quenching models could address this issue

dN/d

ξ

ξ

Au+Aup+p

dN/d

ξ

ξ

Au+Aup+p

ConclusionsFull jet reconstruction feasible at RHICJet Fragmentation Functions:

Measured and under control in ppPowerful tool to study medium effects in

Au+Au

Elena Bruna 23

We do not see modification of the Fragmentation Functions

Surface effect Surface effect on HT events or SofteningSoftening of jets in the medium or both both?

What’s nextHow to address this issue:

High-Tower jet-pT spectrumdi-jets in Min Bias and HT events Quenching models will helpSystematic studies (detector effect,…) to have more

control on our understanding of jet finder in Au+Au

Jets are a precious tool to explore the medium: exciting physics is coming!

Elena Bruna 24

Thank you !