Mateusz Ploskon For the STAR Collaboration

Inclusive jet cross-sections and correlations in Au+Au and p+p collisions at sqrt(sNN) = 200 GeV

Mateusz PloskonFor the STAR Collaboration

Mateusz Ploskon (LBNL), STAR, QM'09 2

Outline

• Motivation and strategy• Datasets, jet algorithms, correction schemes• Observables– Inclusive jet cross-section + Jet RAA

– Jet “radius” systematics– (di-)Hadron-jet recoil spectrum and Jet IAA

• Discussion: implications for jet quenching


Motivation and Strategy

1

Cross-section ratio AuAu/pp

JetAAR

€

pT

Physics of full jet reconstruction in heavy ion collisions

€

dσ JetdpT

€

pT

p+p

Au+Au

Energy shift?

Absorption?

R=0.4Measure energy flow into “cone” of radius R

o Total momentum is conserved even for strongly quenched jets o Unbiased jet reconstruction: recover the full jet energy within

cone radius R

o Compare Au+Au and p+p jet spectrao → Inclusive cross section: RAAjet~1o → p0+jet conditional yield: IAAjet~1

o Caveat: initial state nuclear effects

R

p0

Jet


Data setsEssential requirement: minimize trigger bias of jet population

• Inclusive spectrum data:– Au+Au (2007):

• online MinBias Trigger• offline select 10% most central events (8 x 106events)

– p+p (2006): • online “Jet Patch” trigger (int lumi 6.5 pb^-1)• offline correct bias at low ET

• h+jet coincidence data:– Both p+p (2006) and Au+Au (2007):

• Online “BEMC High Tower” trigger• Offline: hadron trigger energy from 3x3 tower cluster (ET>7 GeV)

• Track and Tower cuts:– BEMC towers energy > 0.2 GeV– TPC track momentum pT > 0.2 GeV/c


Jet algorithms

Algorithms: kt and anti-kt from FastJet*– Resolution parameter R = 0.4, 0.2– Jet acceptance: |hJET| < 1.-R– Recombination scheme: E-scheme with massless particles

*Cacciari, Salam and Soyez, JHEP 0804 (2008) 005 [arXiv:0802.1188]

anti-kT jetKT jet

Anti-kt expected to be less susceptible to background effects in heavy ion collisions

R

Hard scattering

Fragmentation

Sequentialrecombinationalgorithms

Cone basedalgorithms

http://www-spires.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+eprint+arxiv:0802.1188&FORMAT=www&SEQUENCE=





http://arxiv.org/abs/0802.1188


Systematic correctionsTrigger corrections

– p+p trigger bias correction

Particle level corrections:– Detector effects: efficiency and pT resolution– “Double* counting” of particle energies

• * electrons: - double; hadrons: - showering corrections• All towers matched to primary tracks are removed from the analysis

Jet level corrections:• Spectrum shift:

– Unobserved energy– TPC tracking efficiency

• BEMC calibration (dominant uncertainty in p+p)• Jet patch trigger efficiency (only in p+p)• Jet pT resolution• Underlying event (dominant uncertainty in Au+Au)

Full assessment of jet energy scale uncertainties

Data driven correction scheme• Weak model dependence: only for single-particle response, p+p trigger response• No dependence on quenching models


Heavy Ions and

background characterization


STAR Preliminary

Au+Au Central

Underlying event

Single di-jet event from a central Au+Au:- Two jet peaks on top of the HI background

Event background is characterized with median pT per unit area (r).<pT

BG> in R = 0.4 is ~45 GeV/c. S/B~0.5 at 20 GeV/c.

Central assumption:Signal and background can be factorized

with resolution:

True jet distribution smeared:sDATA ~ 6.8 GeV

Systematic studies indicate variation of sigma +/-1 is a conservative bracketing of systematic uncertainties.

Error bands indicate these limits.


Fake jet contamination

“Fake” jet rate estimation: • Central Au+Au dataset (real data)• Randomize azimuth of each

charged particle and calorimeter tower

• Run jet finder• Remove leading particle from each

found jet• Re-run jet finder

“Fake” jets: signal in excess of background model from random association of uncorrelated soft particles (i.e. not due to hard scattering)

STAR Preliminary


Fake jet contamination

“Fake” jet rate estimation: • Central Au+Au dataset (real data)• Randomize azimuth of each

charged particle and calorimeter tower

• Run jet finder• Remove leading particle from each

found jet• Re-run jet finder STAR Preliminary

“Fake” jets: signal in excess of background model from random association of uncorrelated soft particles (i.e. not due to hard scattering)


Spectrum unfolding: methodBackground non-uniformity (fluctuations) and energy resolution introduce pT-smearing

Correct via “unfolding”: inversion of full bin-migration matrix

Check numerical stability of procedure using jet spectrum shape from PYTHIA

Procedure is numerically stableCorrection depends critically on background model

→ main systematic uncertainty for Au+Au

unfolding

Pythia

Pythia smeared

Pythia unfolded


Spectrum unfolding

• Corrections for smearing of jet pT due to HI backround non-uniformities

• 1) raw spectrum

STAR Preliminary


Spectrum unfolding


• 1) raw spectrum• 2) removal of “fake”-

correlationsSTAR Preliminary


Spectrum unfolding



correlations• 3) unfolding STAR Preliminary


Spectrum unfolding



correlations• 3) unfolding• 4) correction for pT

resolution

STAR Preliminary


Jet yields in p+p at sqrt(sNN) = 200 GeV


Inclusive jet cross-section in p+p at sqrt(sNN) = 200 GeV

o Fully corrected jet cross-section reconstructed with kt algorithm

o Very good agreement between the algorithms

STAR PreliminaryUncertainty due to BEMC calibration


Inclusive jet cross-section in p+p at sqrt(sNN) = 200 GeV

• Comparison to published STAR data– run 2003/2004

• Note: – published data

reconstructed with different jet algorithm:• Mid-point cone (R=0.4)

STAR Preliminary

Phys. Rev. Lett. 97 (2006) 252001


Inclusive jet yields in 10% most central Au+Au at sqrt(sNN) = 200 GeV


Inclusive jet yields in 10% most central Au+Au at sqrt(sNN) = 200 GeV

o Fully corrected jet spectrum

o Exactly the same algorithms and jet definitions used as compared to p+p

o Bands on data points represent estimation of systematic uncertainties due to background subtraction

STAR Preliminary

Uncertainty due to BEMC calibration


Inclusive jet spectrum: p+p and central Au+Au (R=0.4 and R=0.2)

p+p Au+Au central

STAR Preliminary

STAR Preliminary


Cross-section ratios in p+p and Au+ Au with R=0.2/R=0.4

p+p: “Narrowing” of the jet structure with increasing jet energy

Au+Au: Strong broadening of the jet energy profile

STAR Preliminary

p+p

Au+Au

Many systematics effects cancel in the ratio


Nuclear modification factor for jets


R = 0.4

5% uncertainty on BEMC calibration

o Significant energy recovered as compared to RAA~0.2 for hadrons

o Visible trends: o different sensitivity of

the algorithmso Central values drop as a

function of jet pT

STAR Preliminary

RAA Jets


RAA Jets and Energy flow in smaller “cone” radii

Significant drop of RAA as a function of jet pT for R=0.2 as compared to R=0.4Jet energy not fully recovered in small “cones” – shift towards lower pT

STAR Preliminary

R=0.4

R=0.2


Hadron-jet coincidences


Di-hadron – jet correlations

Yiel

d pe

r trig

ger

STAR, PRL 97, 162301 (2006)

Df

STAR high pT dihadrons: bias towards non-interacting jet population

A

B

Recoil


Hadron+jet coincidence

1

Cond. yield ratio AuAu/pp

JetAAI

€

pT

€

pT

p+p

Au+Au

Energy shift?

Absorption?

R=0.4

o Trigger on hard, leading p0 (pT>6 GeV/c)o 3x3 tower cluster in BEMC

o Construct spectrum of recoil jetso normalized per di-hadron trigger

This event selection will maximize the recoil path length distribution in matter

Cond

ition

al y

ield

R

Jet

p0


H – recoil jet coincidences

A

B pT > 0.5 GeV/c

Recoil jet

A

B pT > 6 GeV/c

Recoil jet

Use jet fragmentation bias to vary jet path length distribution?



A

B

STAR Preliminary

(Au+Au: 10% central)Anti-kt R=0.4



STAR Preliminary


A

B



STAR Preliminary


A

B



STAR Preliminary


A

B

Significant suppression of the bias free recoil jet spectrum


STAR Preliminary

Summaryo Qualitatively new measurement of jet

quenching in terms of energy flow (rather than hadronic observables) has been established

o What we have shown:o Minimum bias dataset: RAA~0.5 or largero Significant broadening of jet energy profile

R=0.2 -> R=0.4o Strong suppression of recoil jet rate at maximum

path-length

o Consistent interpretation: o quenching induces jet broadening o R=0.4 (with the presented jet definitions) is

insufficient for unbiased reconstruction

STAR Preliminary

STAR Preliminary

STAR Preliminary

Au+Au10% central

RAu+Au

10% central


Outlook

• Rich new set of observables to confront calculations

• New MC jet quenching models (qPythia, JEWEL, T. Renk)

• New jet algorithms for HI collisions?– Post-processing – integration of significant energy

flow outside the “initial” jet areas (however probably hard to calculate/resolve theoretically


Extra slides


Spectrum unfolding- correction for smearing of jet pt due to bg nonuniformities

R=0.4

R=0.2

Documents

Mateusz Ploskon For the STAR Collaboration