Studying the medium response by two particle correlations
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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' Meeting 05/27/2008. Outline. Why do we use two particle correlations?

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Studying the Medium Response by Two Particle Correlations

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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' Meeting


RHIC & AGS User Meeting


  • Why do we use two particle correlations?

  • What do we learn from two particle Df correlations?

  • What do we learn from two particle Dh-Df correlations?

RHIC & AGS User Meeting

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

RHIC & AGS User Meeting

What is two particle correlation?

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

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

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

RHIC & AGS User Meeting

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

RHIC & AGS User Meeting

The jet is modified!

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

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

RHIC & AGS User Meeting

The nearside is also modified!

  • The nearside width is wider than pp

  • The yield at low partner pT is also larger than pp


RHIC & AGS User Meeting

2-D Dh-Df correlations

Peripheral Au+Au

Central Au+Au





Df rad

Both near and away side

are modified!

RHIC & AGS User Meeting

Yield in Dh slices: peripheral



Au+Au ~ p+p



RHIC & AGS User Meeting


Yield in Dh slices: 0-20% central





RHIC & AGS User Meeting

Jet induced medium response

  • Nearside: ridge

  • Awayside: shoulder

  • Do they correlate with each other?

RHIC & AGS User Meeting

Decomposition method

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

  • Head: punch through jet

  • Shoulder: new peak either side of p (medium response ?!?!)

  • Treat all components as Gaussian in shape

  • Use ZYAM method to fix background level

RHIC & AGS User Meeting

nearside enhancement vs centrality



RHIC & AGS User Meeting

shoulder & ridge increase with centrality

Yields are very similar!

RHIC & AGS User Meeting

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



RHIC & AGS User Meeting

Where does the momentum go?

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

  • 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 conserved

    • How does the jet momentum redistribute into the medium?

RHIC & AGS User Meeting

Momentum flow

  • Weight the per trigger yield of each partner pT bins with <pTassociated>

  •  Ensemble averaged vector sum of associated particles.

    Vector sum is along the trigger direction

  • e.g.

partner pT

trigger pT

projected on trigger pT direction

RHIC & AGS User Meeting

Near & away increase with centrality

Number of particles

pT Weighted yield

RHIC & AGS User Meeting

Near = ridge

0.5< Dh <0.7

Near = jet

0 < Dh < 0.1

away/near to ~ cancel acceptance

pT is transferred from head  shoulder

shoulder to ridge ratio is roughly constant

RHIC & AGS User Meeting


  • 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 Dh region

RHIC & AGS User Meeting

Backup slides

RHIC & AGS User Meeting

SpassoT,away/ SpassoT,near

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

RHIC & AGS User Meeting

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

RHIC & AGS User Meeting

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 contribution from the two is changing.

Ratio of Away pTasso / near pTasso





RHIC & AGS User Meeting

Extend to Lower pTasso

  • pTasso =[1.0-5.0] GeV/c

  • pTasso =[0.5-5.0] GeV/c

  • The ratio still holds when we include softer particles!

RHIC & AGS User Meeting

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