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

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Backup slides

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SpassoT,away/ SpassoT,near

When we extend the associated particle pT range, the result still holds!

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Weight the integrated yield with pTasso

(along trigger particle direction)

• both nearside and awayside pTasso is increased

• Awayside pTasso increases less than AS yields

• Crossover of “head” and “shoulder” at Npart at 150-220

• Charged associated particles only• Not include the trigger particle

contribution

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Ratio of Away pTasso / near pT

asso

• Ratio constant with centrality• Consistent with p+p value, also with PYTHIA• Even though the total vector sum of nearside/awayside both increase, the ratio of the

two still hold constant• The ratio of the total contribution from “head” and “shoulder” part is hold, but the cont

ribution from the two is changing.

head

shoulder

Head+shoulder

PYTHIA

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Extend to Lower pTasso

• The ratio still holds when we include softer particles!

pTasso =[1.0-5.0] GeV/c pT

asso =[0.5-5.0] GeV/c