05/27/2008 RHIC & AGS User Meeting 1

Studying the Medium Response by Two Particle Correlations

John Chin-Hao Chen (for PHENIX Collaboration)Department of Physics and Astronomy

Stony Brook University

RHIC & AGS Annual Users' Meeting05/27/2008

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Outline

• Why do we use two particle correlations?

• What do we learn from two particle correlations?

• What do we learn from two particle - correlations?

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Is there a jet?

• In heavy ion collisions, due to huge background of soft particles, it is difficult to find the jet.

• So we turn to two particle correlations

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What is two particle correlation?

• We select a high pT particle as the trigger particle.

• Select another lower pT particle, calculate the angular distribution.

• Use ZYAM (Zero Yield At Minimum) method to remove the flow background.

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The jet has disappeared!

• In p+p and d+Au collisions, we see a clear awayside peak

• In central Au+Au collision, the awayside peak is disappeared.

Phys. Rev. Lett. 91, 072304

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The jet is modified!

Phys. Rev. Lett. 98, 232302 (2007)

The awayside peak moved to ~ +/- 1.1 in central collisions!

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The nearside is also modified!

• The nearside width is wider than pp

• The yield at low partner p

T is also larger than pp  arXiv:0801.4545

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2-D correlations

shoulder ridge

rad

Peripheral Au+Au Central Au+Au

Both near and away sideare modified!

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Yield in slices: peripheral

Au+Au ~ p+p

0<||<0.1 0.1<||<0.3

0.3<||<0.5 0.5<||<0.7

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Yield in slices: 0-20% central

ridge

0<||<0.1 0.1<||<0.3

0.3<||<0.5 0.5<||<0.7

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Jet induced medium response

• Nearside: ridge• Awayside: shoulder• Do they correlate with each other?

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Decomposition method

• Fit away side jet with sum of three Gaussians to decompose components:

• Head: punch through jet• Shoulder: new peak

either side of (medium response ?!?!)

• Treat all components as Gaussian in shape

• Use ZYAM method to fix background level

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nearside enhancement vs centrality

Enhancement!

pp

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shoulder & ridge increase with centrality

Yields are very similar!

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Shoulder & ridge pT spectra vs. p+p

• Both are softer than hard scattering.

• Ridge harder than shoulder?

• Shoulder not quite as soft as inclusive hadrons

RIDGE

SHOULDER

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Where does the momentum go?

• Compare to pp, the awayside per trigger yield at high pT is suppressed.

• At lower pT, the awayside yield is enhanced.• During the collision, the total transverse momentum is co

nserved– How does the jet momentum redistribute into the medium?

Phys. Rev. C 77, 011901(R) (2008)

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Momentum flow• Weight the per trigger yield of each partner pT bins with <pT

as

sociated> Ensemble averaged vector sum of associated particles.

Vector sum is along the trigger direction

• e.g.

projected on trigger pT direction

partner pT

trigger pT

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Near & away increase with centrality

Number of particles pT Weighted yield

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away/near to ~ cancel acceptance

pT is transferred from head shoulder

shoulder to ridge ratio is roughly constant

Near = ridge

0.5< <0.7

Near = jet

0 < < 0.1

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Summary

• In heavy ion collisions, both the nearside and awayside of the jet are modified– Nearside->ridge– Awayside->shoulder

• Both ridge and shoulder are softer than hard scattering and slightly harder than inclusive hadron

• The momentum sum of head and shoulder scales with nearside in central region