Correlations at Intermediate pT

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Correlations at Intermediate pT

• Rudolph C. Hwa
• University of Oregon

Correlations and Fluctuations in Relativistic
Nuclear Collisions MIT, April 2005
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Work done in collaboration with Chunbin Yang
(Hua-Zhong Normal University, Wuhan) Ziguang Tan
(Hua-Zhong Normal University, Wuhan) Charles Chiu
(University of Texas, Austin)
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Physics at Intermediate pT
intermediate
high
low
pT
0
2
4
6
8
10
hard
soft
semi-hard
thermal-thermal
thermal-shower
shower-shower
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Basic equations for pion production by
recombination
Shower parton distributions are determined from
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Thermal partons are determined from the final
state, not from the initial state.
(log scale)
Transverse plane
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No small parameter (rh/RA) in the problem.
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thermal
Pion distribution (log scale)
fragmentation
Transverse momentum
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? production in AuAu central collision at 200 GeV
Hwa CB Yang, PRC70, 024905 (2004)
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Compilation of Rp/? by R. Seto (UCR)
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Cronin Effect
q
p
A
Unchallenged for 30 years.
If the medium effect is before fragmentation,
then ? should be independent of h ? or p
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dAu collisions
pion
Hwa CB Yang, PRL 93, 082302 (2004)
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Forward production in dAu collisions
BRAHMS

Hwa, Yang, Fries, PRC 71, 024902 (2005)
Underlying physics for hadron production is not
changed from backward to forward rapidity.
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Correlations
1. Two-particle correlation with the two
particles treated on equal footing. (data
to be presented tomorrow)
2. Correlation in jets trigger, associated
particle, background subtraction, etc. (e.g.,
Fuqiang Wangs talk)
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Correlation function
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Correlation of partons in jets
A. Two shower partons in a jet in vacuum
k
Fixed hard parton momentum k (as in ee-
annihilation)
x1
x2
The two shower partons are correlated.
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B. Two shower partons in a jet in HIC
Hard parton momentum k is not fixed.
?fi(k)
?fi(k) ?fi(k)
?fi(k) is small for 0-10, smaller for 80-92
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Correlation of pions in jets
Two-particle distribution
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Correlation function of produced pions in HIC
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Hwa and Tan, nucl-th/0503052
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Physical reasons for the big dip
The dip occurs at low pT because at higher pT
power-law suppression of ?1(1) ?1(2) results in
C2(1,2) ?2(1,2) gt 0
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Porter Trainor, ISMD2004, APPB36, 353 (2005)
( pp collisions )
STAR
Transverse rapidity yt
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Hwa Tan, nucl-th/0503052
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Correlation with trigger particle
Study the associated particle distributions
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STAR has measured nucl-ex/0501016
Trigger 4 lt pT lt 6 GeV/c
Associated charged hadron distribution in pT
Background subtracted ?? and ?? distributions
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Associated particle pT distribution
p1 -- trigger
p2 -- associated
After background subtraction, consider only
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Reasonable agreement with data
Hwa Tan, nucl-th/0503052
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Hwa Tan, nucl-th/0503060
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?? and ?? distributions (from Fuqiang Wangs
talk)
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New issues to consider

• Angular distribution (1D -gt 3D)
• shower partons in jet cone

• Thermal distribution enhanced due to
• energy loss of hard parton

work done with C. Chiu
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Thermal partons
Events without jets
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For STST recombination
Sample with trigger particles and with background
subtracted
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Pedestal in ??
more reliable
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Assoc
p1 trigger
p2
?1
?
z
Exptl cut on ?trigger -0.7 lt ?1 lt 0.7
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shower parton
Shower parton angular distribution in jet cone
k
q2
hard parton
?
z
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Associated particle distribution in ??
Chiu Hwa (2005)
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Associated particle distribution in ??
Chiu Hwa (2005)
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We have not put in any (short- or long-range)
correlation by hand.
Correlation exists among the shower partons,
since they belong to the same jet.
The pedestal arises from the enhanced thermal
medium.
The peaks in ?? ?? arise from the recombination
of enhanced thermal partons with the shower
partons in jets with angular spread.
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Conclusion
Parton recombination provides a framework to
interpret the data on jet correlations.
There seems to be no evidence for any exotic
correlation outside of shower-shower correlation
in a jet.
For unbiased study without deciding on bkgd, we
suggest the measure, G2(1,2).
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Comments to stimulate discussion

• Fragmentation is not important until pT gt 9
  GeV/c.

• String model may be relevant for pp collisions,
  

• String/fragmentation has no phenomenological
  support in heavy-ion collisions.