Study Jet Energy Loss with 2-particle Correlations

Hua Pei of ISU for PHENIX at QM08 1

Study Jet Energy Loss with 2-particle Correlations

How back-to-back jets lose energy

Different pT region of jet energy loss

Changing surface bias: 2+1 particle correlation

Hua Pei

Iowa State University

For PHENIX at RHIC


We know Jet Quenches in AA relative to p+p, and we measure per-trigger yield

0-20% Au+Au 20-40% 60-92%

Solid points are Au+Au, open points are p+p, red/blue colors show systematic errors.

More central arXiv:0801.4545v1[nucl-ex]

http://arxiv.org/abs/0801.4555v1


A Jet Function Can Be Divided into Regions

We define “HR”, the head region, as | – | < /6

This HR is where we expected the away-side jet appears.

For region “SR”, given in parallel talk M. McCumber

arXiv:0801.4545v1[nucl-ex]



We Measure the Central Au+Au Jet Spectrum at Head Region and Compare to p+p

Each panel shows a different pTa (trigger), and

plotted as function of pTb (assoc)




We use IAA to quantify the change of jet spectrum

IAA is defined as the modification of per-trigger yield Yjet_ind, of AA relative to p+p.


The Modification of per-trigger Yield (IAA) has strong assoc pT Dependence

At high pTa and pT

b, IAA ≈ RAA (inclusive) at head region.

Quantified suppression of far-side jet => further constraint on e-loss.




Compare Jet Spectrum (mean pT) of Head Region to Inclusive Spectrum

At most central collisions, the mean pT of Head Region approaches the inclusive spectrum (gray line near bottom of each panel)

“Punch-through” of jet at highest pT?




JAA is used to measure the change of particle pairs induced by di-jet

• JPY is the number of di-hadron pairs (both particles from jet) per event.

• JAA is the ratio of JPY of AA to p+p, normalized by Ncoll

• Thus by definition, JAA = IAA * RAA


JAA pair suppression

• JAA is suppressed at high pT at all centralities.

• At all centralities, JAA ≈ RAA2 (dashed line) at highest pT

a,b, indicating both trigger/assoc are suppressed.




Relative energy loss back-to-back jet

Ratio of MEAN parton transverse momentum

Be fitted from spectrum

measured

Relates ratio of particle pT

PHYSICAL REVIEW D 74, 072002 (2006)

Spectrum in the form of m (multiplicity), n (slope), xE, and xh


The spectrum is fitted to get xh^


Compare Au+Au x to p+p x

The Au+Au xh ~ 0.5-0.6 relative to p+p,

If no suppression happened to partons in medium, this ratio shall be 1

=> Larger energy loss on away-side due to medium

^

^

Shown by Michael J. Tannenbaum DNP07

^


Change the surface-bias of near-side?

Select events that have both a high-pt 0 and a back-to-back hadron (back-hemisphere of 0 )Trigger 0

“Conditional”

charged hadron

at high-pt

MediumAssoc h

Trigger 0

Assoc h

Shift distribution of hard scattering towards center of medium. Near-side parton travels through more medium

Removes some events where hard-scattering occurs near surface but not tangential (large difference between path lengths)

Path lengths comparable in dense medium.A.k.a., 2+1 correlations


This 2+1 changes near-side jets of both p+p and Cu+Cu

Per-trigger yield of p+p on near side increase with conditional particle pT.

Expected in p+p! Higher Q2 comes with higher pT away-side particle.

In Cu+Cu the yield also increases but not same slope as in p+p.


Centrality dependence of near-side yields

Cu+Cu yield increases from central (left) to peripheral (right) in each bin and approaches p+p (most right point in each bin)

The fact that Cu+Cu yield is reduced at central is possibly due to

1) weaker surface-bias, 2) more “+1” particles from underlying event


Another handle on path-length: Reaction plane dependence

In-Plane

Out-Plane

Evolution of “Head Region” is observed.

Need more work to quantify that


Conclusion

• Jets are affected by medium, on both near and far side.

• Medium effect on jets vary on centrality and pT.

• Thus, we quantify the medium effects as the suppression of jet, using per-trigger-yield, I_AA, J_AA. This suppression shows strong indication of jet particle sources at different kinetic region.

• 2+1 correlation brings another method of controlling jet source via the surface-bias, especially on exploring the near side jet suppression.

• PHENIX has the brand new 2007 Au+Au data and we are showing many more results in this QM08 and near future.


Back up


The Modification of per-trigger Yield (IAA) has strong assoc pT Dependence

At high pTa and pT

b, IAA ≈ RAA (inclusive) at head region.

Quantifies suppression of far-side jet => further constraint on e-loss.




The IAA of both “Head” and “Head + Shoulder”

At high pTa and pT

b, IAA ≈ RAA (inclusive) at head region




The mean pT as a function of pTB range




Source of formula on xh

PHYSICAL REVIEW D 74, 072002 (2006)

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Study Jet Energy Loss with 2-particle Correlations