d Au Collisions at STAR

1
dAu Collisions at STAR
Carl A. Gagliardi Texas AM University for the
Collaboration

• Outline
• dAu collisions and saturation physics at RHIC
• Recent STAR results
• STAR plans for the future

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STAR detector
E-M Calorimeter
Projection           Chamber
Time of    Flight
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Mid-rapidity pp at RHIC and NLO pQCD
PRL 91, 241803
PHENIX p0
STAR (hh-)/2 BRAHMS (hh-)/2
Calculations by W. Vogelsang
At 200 GeV, pQCD does a very good job describing
mid-rapidity yields
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Mid-rapidity dAu
PRL 91, 072304
Inclusive yields and back-to-back di-hadron
correlations are very similar in pp and dAu
collisions In contrast, AuAu collisions are
very different from pp and dAu but thats not
the subject of this talk
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Mid-rapidity vs. forward rapidity
Mid Rapidity
Forward Rapidity
CTEQ6M

• Gluon density cant grow forever.
• Saturation may set in at forward rapidity when
• gluons start to overlap.
• Can be explored by comparing p(d)A to pp.

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Forward particle production in dAu collisions
BRAHMS, PRL 93, 242303
Sizable suppression in charged hadron production
in dAu collisions relative to pp collisions at
forward rapidity
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Expectations for a color glass condensate
t related to rapidity of produced hadrons.
D. Kharzeev, hep-ph/0307037
As y grows
Iancu and Venugopalan, hep-ph/0303204
Are the BRAHMS data evidence for gluon saturation
at RHIC energies?
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Recent saturation model calculation
Very good description of the pT dependence of the
BRAHMS dAu ? h- X cross section at ?
3.2 (Dumitru, Hayashigaki, and Jalilian-Marian,
NP A765, 464)
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Is saturation really the explanation?
Difficult to explain BRAHMS results with standard
shadowing, but in NLO pQCD calculations ltxggt
0.02 is not that small (Guzey, Strikman, and
Vogelsang, PL B603, 173) In contrast, ltxggt lt
0.001 in CGC calculations (Dumitru, Hayashigaki,
and Jalilian-Marian, NP A765, 464 )
Basic difference pQCD 2 ? 2 CGC 2 ?
1
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Do we understand forward p0 production in p p?
Bourrely and Soffer, EPJ C36, 371
NLO pQCD calculations underpredict the data
at low ?s from ISR Ratio appears to be a
function of angle and vs, in addition to pT
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pp ? p0X at 200 GeV
nucl-ex/0602011

• The error bars are statistical plus
  point-to-point systematic
• Consistent with NLO pQCD calculations at 3.3 lt ?
  lt 4.0
• Data at low pT trend from KKP fragmentation
  functions toward Kretzer. PHENIX observed
  similar behavior at mid-rapidity.

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dAu ? p0X at 200 GeV
nucl-ex/0602011
pT dependence of dAu p0 cross section at lt?gt
4.0 is best described by a LO CGC
calculation. (Dumitru, Hayashigaki, and
Jalilian-Marian, NPA 765, 464)
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? dependence of RdAu
nucl-ex/0602011

• Observe significant rapidity dependence.
• pQCD calculations significantly over predict
  RdAu.

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Any difference between pp and dAu?
pp Di-jet
dAu Mono-jet?
Dilute parton system (deuteron)
PT is balanced by many gluons
Dense gluon field (Au)
Kharzeev, Levin, McLerran gives physics picture
(NPA748, 627)
Color glass condensate predicts that the
back-to-back correlation from pp should be
suppressed
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Back-to-back correlations with the color glass
The evolution between the jets makes the
correlations disappear.
(Kharzeev, Levin, and McLerran, NP A748, 627)
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Forward mid-rapidity di-hadron correlations

• HIJING predicts similar correlations in dAu as
  PYTHIA predicts for pp.
• Only significant difference is combinatorial
  background level.

25ltEplt35GeV
35ltEplt45GeV
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Forward mid-rapidity correlations in dAu
nucl-ex/0602011

• are suppressed at small ltxFgt and ltpT,pgt
• consistent with CGC picture
• are similar in dAu and pp at larger ltxFgt and
  ltpT,pgt
• as expected by HIJING

ltpT,pgt 1.0 GeV/c
25ltEplt35GeV
ltpT,pgt 1.3 GeV/c
p0 lt?gt 4.0 h ? lt 0.75 pT gt 0.5 GeV/c
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STAR Forward Meson Spectrometer

• FMS increases areal coverage of forward EMC from
  0.2 m2 to 4 m2
• Addition of FMS to STAR provides nearly
  continuous EMC from -1lt?lt4
• Available for Run 7

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pp and dAu ? p0p0X correlations with forward
p0
hep-ex/0502040
pp in PYTHIA
dAu in HIJING
Conventional shadowing will change yield, but not
angular correlation. Saturation will change
yield and modify the angular correlation. Sensitiv
e down to xg 10-3 in pQCD scenario few x 10-4
in CGC scenario.
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Conclusions

• Forward rapidity inclusive p0 production at RHIC
  is well described by pQCD calculations
• dAu results at RHIC provide hints that
  saturation effects are becoming important
• Future STAR measurements will elucidate the
  dynamics underlying forward inclusive particle
  suppression at RHIC
• RHIC may be the ideal accelerator to explore the
  onset of saturation

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The STAR Collaboration
U.S. Labs Argonne, Lawrence Berkeley, and
Brookhaven National Labs U.S. Universities
UC Berkeley, UC Davis, UCLA, Caltech,
Carnegie Mellon, Creighton, Indiana, Kent State,
MIT, MSU, CCNY, Ohio State, Penn State, Purdue,
Rice, Texas AM, UT Austin, Washington, Wayne
State, Valparaiso, Yale Brazil
Universidade de Sao Paolo China IHEP -
Beijing, IPP - Wuhan, USTC, Tsinghua, SINAP, IMP
Lanzhou Croatia Zagreb University Czech
Republic Nuclear Physics
Institute England University of Birmingham
France Institut de Recherches Subatomiques
Strasbourg, SUBATECH - Nantes Germany Max
Planck Institute Munich University of
Frankfurt India Bhubaneswar, Jammu, IIT-Mumbai,
Panjab, Rajasthan, VECC Netherlands NIKHEF/Utrec
ht Poland Warsaw University of
Technology Russia MEPHI Moscow, LPP/LHE
JINR Dubna, IHEP Protvino South
Korea Pusan National University
Switzerland University of Bern
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Pseudo-rapidity yield asymmetry vs pT
Au direction / d direction
PRC 70, 064907
Back/front asymmetry in 200 GeV dAu consistent
with general expectations of saturation or
coalescence doesnt match pQCD prediction.
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Nuclear Gluon Density
e.g., see M. Hirai, S. Kumano, T.-H. Nagai, Phys.
Rev. C70 (2004) 044905 and data references
therein
World data on nuclear DIS constrains nuclear
modifications to gluon density only for xgluon gt
0.02
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One calculation within the saturation picture
RdAu
RCP
Saturation model calculation with additional
valence quark contribution (Kharzeev, Kovchegov,
and Tuchin, PL B599, 23)
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x values in saturation calculations
In CGC calculations, the BRAHMS kinematics
corresponds to ltxggt lt 0.001 (Dumitru,
Hayashigaki, and Jalilian-Marian, NP A765, 464 )
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Many recent descriptions of low-x suppression
A short list (probably incomplete)
Saturation (color glass condensate)
Shadowing

• R. Vogt, PRC 70 (2004) 064902.
• Guzey, Strikman, and Vogelsang, PLB 603 (2004)
  173.

• Jalilian-Marian, NPA 748 (2005) 664.
• Kharzeev, Kovchegov, and Tuchin, PLB 599 (2004)
  23 PRD 68 (2003) 094013.
• Armesto, Salgado, and Wiedemann, PRL 94 (2005)
  022002.
• Dumitru, Hayashigaki, and Jalilian-Marian, NPA
  765 (2006) 464

Parton recombination

• Hwa, Yang, and Fries, PRC 71 (2005) 024902.

Others?
Multiple scattering

• ...

• Qiu and Vitev, PRL 93 (2004) 262301
  hep-ph/0410218.

Factorization breaking

• Kopeliovich et al., PRC 72 (2005) 054606.
• Nikolaev and Schaefer, PRD 71 (2005) 014023.

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Forward p0 production at a hadron collider
p0
Ep
qq
EN
qp
N
xgp
N
xqp
qg
EN
(collinear approx.)

• Large rapidity p production (?4) probes
  asymmetric partonic collisions
• Mostly high-x quark low-x gluon
• 0.3 lt xqlt 0.7
• 0.001lt xg lt 0.1
• ltzgt nearly constant and high 0.7-0.8
• A probe of low-x gluons

NLO pQCD S. Kretzer
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Constraining the x-values probed in hadronic
scattering
Guzey, Strikman, and Vogelsang, Phys. Lett. B
603, 173
Log10(xGluon)

• Collinear partons
• x pT/?s (eh1 eh2)
• x? pT/?s (e?h1 e?h2)

• FPD ? ? 4.0
• TPC and Barrel EMC ? lt 1.0
• Endcap EMC 1.0 lt ? lt 2.0
• FTPC 2.8 lt ??? lt 3.8

Measure two particles in the final state to
constrain the x-values probed