High pT Physics in Heavy Ion Collisions

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High pT Physics in Heavy Ion Collisions

• Rudolph C. Hwa
• University of Oregon

CIAE, Beijing June 13, 2005
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High pT Physics of Nuclear Collisions at High
Energy
Well studied for 20 years ---- pQCD
What was a discovery yesterday is now used for
calibration today.
Instead of being concerned with 5 discrepancy in
pp collisions, there are problems involving
factors of 10 differences to understand in
nuclear collisions.
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Work done in separate collaborations with
Chunbin Yang (HZNU, Wuhan UO) Rainer Fries
(Univ. of Minnesota) Zhiquang Tan (HZNU, Wuhan
UO) Charles Chiu (Univ. of Texas, Austin)
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Outline
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Conventional approach to hadron production at
high pT
Hard scattering near the surface because of
energy loss in medium --- jet quenching.
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If hard parton fragments in vacuum, then the
fragmentation products should be independent of
the medium.
h
Particle ratio should depend on the FF D(z) only.
D(z)
q
The observed data reveal several anomalies
according to that picture.
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Anomaly 1 Rp/p ? 1
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cm energy
cm energy
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Anomaly 2 in pA or dA collisions
Unchallenged for 30 years.
If the medium effect is before fragmentation,
then ? should be independent of h ? or p
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RHIC data from dAu collisions at 200 GeV per NN
pair
Ratio of central to peripheral collisions RCP
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STAR
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Anomaly 3 Azimuthal anisotropy
v2 coeff. of 2nd harmonic of ? distribution
PHENIX, PRL 91 (2003)
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Anomaly 4 Forward-backward asymmetry at
intermed. pT
in dAu collisions (STAR)
B/F
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Forward-backward asymmetry in dAu collisions
If initial transverse broadening of parton gives
hadrons at high pT, then
Expects more forward particles at high pT than
backward particles
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Rapidity dependence of RCP in dAu collisions
BRAHMS
PRL 93, 242303(2004)
Central more suppressed than peripheral collisions
RCP lt 1 at ?3.2
Interpreted as possible signature of Color Glass
Condensate.
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Anomaly 5 Jet structure
Hard parton ? jet ?(p1) ?(p2) ?(p3)

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Fuqiang Wang (STAR) nucl-ex/0404010
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How can recombination solve all those puzzles?
hadron momentum
Parton distribution (log scale)
p
p
q
p1p2
(recombine)
(fragment)
higher yield
heavy penalty
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The black box of fragmentation
q
p
1
z
A QCD process from quark to pion, not calculable
in pQCD
Momentum fraction z lt 1
Dp/q
Phenomenological fragmentation function
z
1
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Lets look inside the black box of fragmentation.
q
p
1
z
fragmentation
gluon radiation
quark pair creation
Although not calculable in pQCD (especially when
Q2 gets low), gluon radiation and quark-pair
creation and subsequent hadronization
nevertheless take place to form pions and other
hadrons.
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Description of fragmentation by recombination
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Shower Parton Distributions
Hwa CB Yang, PRC 70, 024904 (04)
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BKK fragmentation functions
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Once the shower parton distributions are known,
they can be applied to heavy-ion collisions.
The recombination of thermal partons with shower
partons becomes conceptually unavoidable.
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Once the shower parton distributions are known,
they can be applied to heavy-ion collisions.
The recombination of thermal partons with shower
partons becomes conceptually unavoidable.
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hard parton (u quark)
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Inclusive distribution of pions in any direction
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Pion formation
distribution
thermal
shower
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Thermal distribution
Contains hydrodynamical properties, not included
in our model.
Fit low-pT data to determine C T.
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thermal
Pion distribution (log scale)
fragmentation
Transverse momentum
Now, we go to REAL DATA, and real theoretical
results.
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? production in AuAu central collision at 200 GeV
Hwa CB Yang, PRC70, 024905 (2004)
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Proton production in AuAu collisions
TTSTSS
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Anomaly 1 Proton/pion ratio
resolved
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Compilation of Rp/? by R. Seto (UCR)
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Anomaly 2 dAu collisions (to study the
Cronin Effect)
peripheral
central
d
d
more ?T ? more TS
less ?T ? less TS
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dAu collisions
Pions
Hwa CB Yang, PRL 93, 082302 (2004)
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Proton
Hwa Yang, PRC 70, 037901 (2004)
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Nuclear Modification Factor
This is the most important result that validates
parton recombination.
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Anomaly 3 Azimuthal anisotropy
Molnar and Voloshin, PRL 91, 092301
(2003). Parton coalescence implies that v2(pT)
scales with the number of constituents
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Anomaly 4 Forward-backward asymmetry
Less soft partons in forward (d) direction than
backward (Au) direction.
Less TS recombination in forward than in backward
direction.
It is natural for parton recombination to result
in forward-backward asymmetry
More interesting behavior found in large pT and
large pL region.
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Forward production in dAu collisions
BRAHMS data
Hwa, Yang, Fries, PRC 71, 024902 (2005)
Underlying physics for hadron production is not
changed from backward to forward rapidity.
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Jet Structure
Since TS recombination is more important in
AuAu than in pp collisions, we expect jets in
AuAu to be different from those in pp.
Consider dihadron correlation in the same jet on
the near side.
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Correlations

1. Correlation in jets trigger, associated
particle, background subtraction, etc.
2. Two-particle correlation with the two
particles treated on equal footing.
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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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Hwa Tan, nucl-th/0503052
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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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Hwa Tan, nucl-th/0503052
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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 Tan, nucl-th/0503052
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Hwa and Tan, nucl-th/0503052
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Trigger at 4 lt pT lt 6 GeV/c
Correlation studied with triggers
pp mainly SS fragmentation AuAu
mainly TS
Associated particle
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Correlation of pions in jets
Two-particle distribution
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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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?? and ?? distributions
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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

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later
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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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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 ??
P1
0.15 lt p2 lt 4 GeV/c, P1 0.4 2 lt p2 lt 4
GeV/c, P2 0.04
P2
more reliable
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Chiu Hwa, nucl-th/0505014
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Chiu Hwa, nucl-th/0505014
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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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Summary
Traditional classification by scattering
pQCD FF
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Conclusion
Recombination is the hadronization process ----
at all pT.
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.
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shower parton
Shower parton angular distribution in jet cone
k
q2
hard parton
?
z
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Correlation without triggers
Normalized correlation function
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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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