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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? - PowerPoint PPT Presentation
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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
05/27/2008 RHIC & AGS User Meeting 2
Outline
• Why do we use two particle correlations?
• What do we learn from two particle correlations?
• What do we learn from two particle - correlations?
05/27/2008 RHIC & AGS User Meeting 3
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
05/27/2008 RHIC & AGS User Meeting 4
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.
05/27/2008 RHIC & AGS User Meeting 5
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
05/27/2008 RHIC & AGS User Meeting 6
The jet is modified!
Phys. Rev. Lett. 98, 232302 (2007)
The awayside peak moved to ~ +/- 1.1 in central collisions!
05/27/2008 RHIC & AGS User Meeting 7
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
05/27/2008 RHIC & AGS User Meeting 8
2-D correlations
shoulder ridge
rad
Peripheral Au+Au Central Au+Au
Both near and away sideare modified!
05/27/2008 RHIC & AGS User Meeting 9
Yield in slices: peripheral
Au+Au ~ p+p
0<||<0.1 0.1<||<0.3
0.3<||<0.5 0.5<||<0.7
05/27/2008 RHIC & AGS User Meeting 10
Yield in slices: 0-20% central
ridge
0<||<0.1 0.1<||<0.3
0.3<||<0.5 0.5<||<0.7
05/27/2008 RHIC & AGS User Meeting 11
Jet induced medium response
• Nearside: ridge• Awayside: shoulder• Do they correlate with each other?
05/27/2008 RHIC & AGS User Meeting 12
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
05/27/2008 RHIC & AGS User Meeting 14
shoulder & ridge increase with centrality
Yields are very similar!
05/27/2008 RHIC & AGS User Meeting 15
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
05/27/2008 RHIC & AGS User Meeting 16
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)
05/27/2008 RHIC & AGS User Meeting 17
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
05/27/2008 RHIC & AGS User Meeting 18
Near & away increase with centrality
Number of particles pT Weighted yield
05/27/2008 RHIC & AGS User Meeting 19
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
05/27/2008 RHIC & AGS User Meeting 20
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
05/27/2008 RHIC & AGS User Meeting 22
SpassoT,away/ SpassoT,near
When we extend the associated particle pT range, the result still holds!
05/27/2008 RHIC & AGS User Meeting 23
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
05/27/2008 RHIC & AGS User Meeting 24
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