Low-x Physics at RHIC

1
Low-x Physics at RHIC
Akio Ogawa

• Introduction
• RHIC Experiments
• Run3 Results
• Inclusive Hadron
• Back-to-back Correlations
• Run8 and Detector Upgrades
• Summary

2
Gluon density at low x
Gluon density cant grow forever.
Mid Rapidity
Forward Rapidity

• Nuclear pdf in animatiron?

Saturation must set in at forward rapidity when
gluons start to overlap and when recombination
becomes important Can we see gluon saturation?
Before LHC?
3
Why is forward rapidity _at_ hadron collider?
Ep
p0
qq
PN
qp
N
xgpN
N
xqpN
PN
qg

• Forward scattering probes asymmetric partonic
  collisions
• Mostly scattering of
• high-x valence quarks (with known large
  polarization)
• 0.25 lt xq lt 0.7
• on
• low-x gluons
• 0.001 lt xg lt 0.1

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The Relativistic Heavy Ion Collider
AuAu Polarized pp dAu RHIC is a QCD lab
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RHIC Experiments with forward detectors

• Run3 dAu
• Run8 dAu pp

Forward spectrometer PIDed charged hadrons h
2.2 - 3.2
Forward Pion Detector (FPD) pi0 h 3.5
4.2 Forward Meson Spectrometer (FMS) pi0 h 2.5
4.2
Muon Arm (penetrated) Hadrons h 1.2 2.2 Muon
Piston Calorimeter pi0 h 3.1 3.7
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(No Transcript)
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The STAR Detector (run3)
TPC -1.0 lt ? lt 1.0 FTPC 2.8 lt ??? lt 3.8 FPD
? ? 3.8 (pp) ? ? 4.0 (pp, dAu)

• Forward ?? Detector (FPD)
• Pb-glass EM calorimeter
• Shower-Maximum Detector (SMD)
• Preshower

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The PHENIX detector

• Central Arms
• ? lt 0.35.
• hadrons, electrons, photons
• Muon arms
• 1.2 lt ? lt 2.2
• muons, also light mesons
• Muon Piston Calorimeter
• 3.1 lt ? lt 3.7
• pi0

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Mid-rapidity pp
PRL??????
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 hadron yields
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Forward rapidity pp
nucl-ex/??????
nucl-ex/0602011
At 200 GeV, pQCD does a very good job describing
mid-rapidity hadron yields
both mid and forward rapidity (0lthlt4)
11
Mid-rapidity dAu
PRL 91, 072304
pp and dAu very similar in both inclusive
yields back-to-back di-hadron correlations In
contrast, AuAu collisions are very different
from pp and dAu
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Expectations from Color Glass Condensate
Forward rapidity dAu?
t related to rapidity of produced hadrons.
D. Kharzeev hep-ph/0307037
As y grows
Iancu and Venugopalan, hep-ph/0303204
CGC expects suppression of forward hadron
production
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PHENIX dAu hadron ratios
-2.2lt?lt-1.4 and 1.4lt?lt2.2
PRL.94082302,2005
Hadrons in Muon Arm Panch Through Hadron Hadron
Decay Muon

• Backward
• (Au direction)
• ? Forward
• (d direction)

No suppression at backward rapidity Sizable
suppression at forward rapidity
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BRAHMS dAu charged hadron ratios
? 3.2
PRL 93, 242303
Taken from dEnterria
Sizable suppression at forward rapidity pQCDShado
wing expects suppression, but not enough CGC
gives best description
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STAR dAu forward p0
PRL 97, 152302
? 4.0
(NPA 765, 464)
Sizable suppression pQCDShadowing expects
suppression, but not enough CGC gives best
description on pT dependence
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RdAu rapidity dependence
? 0?4
PRL 97, 152302
PRL 93, 242303
? 4 p0

• Observe significant rapidity dependence
• similar to expectations from the CGC framework

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Identified Particle RdAu
? 3
RdAu for p,(p0 from STAR), K,p all shows
RdA lt1 for pTlt3 GeV/c The protons may
exhibit less suppression
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Back-to-back Angular Correlations
pQCD 2?2 process back-to back Di-jet (Works
well for pp)
Forward jet
p
p
Mid-rapidity jet
Kharzeev, Levin, McLerran (NPA748, 627)
CGC predicts suppression of back-to-back
correlation
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pp Forward-Midrapidity Correlations
Forward pi0 (FPD) Mid-rapidity Leading
Charged Particle (TPC)

• PYTHIA detector predict
• As ltxFgt and ltpT,pgt grows
• S grows with
• ss decrease
• PYTHIA prediction agrees with data

25ltEplt35GeV
?trig 4.0 ?asso 0.0
45ltEplt55GeV
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PYTHIA (pp) vs HIJING (dAu)

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

25ltEplt35GeV
?trig 4.0 ?asso 0.0
35ltEplt45GeV
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An initial glimpse correlations in dAu (Run3)

• 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

PRL 97, 152302 (2006)
ltpT,pgt 1.0 GeV/c
25ltEplt35GeV
ltpT,pgt 1.3 GeV/c
?trig 4.0 ?asso 0.0
p0 lt?gt 4.0 h ? lt 0.75 pT gt 0.5
GeV/c
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Back-to-back Angular Correlations
Phys. Rev. Lett. 96, 222301
-2.2lt?trig lt2.2 ?asso 0.0
Conditional yields and width are the same for
(triggers in 3 spectrometers) x (pp and dAu)
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Run8 dAu
Max expectation
PHENIX STAR
Min expectation
Run3
x30 luminosity Great success! (polarizaed) pp
run ? reference data
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Muon Piston Calorimeter (MPC)
2.2?2.2 ?18 cm3
192 PbWO4 crystals with APD readout
37.4cm
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Muon Piston Calorimeter (MPC)
Foreground ?? Pairs Mixed Events

• E gt 8 GeV
• Asymmetry zlt 0.6

Eta peak above bkg
Both north and south side working in Run8
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PHENIX Run8
Plans, measurements, etc
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STAR Setup 2008

• Particle ID
• MRPC ToF (parts)
• Calorimetry
• Photon Multiplicity Detector (PMD)
• Barrel EMC
• Endcap EMC
• Forward Meson Spectrometer

• Charged Particle Tracking
• Main TPC
• 1/24 with DAQ1000
• Forward TPC (FTPC)

• Event Characterization Trigger
• Beam-Beam Counter (BBC)
• Zero Degree Calorimeter (ZDC)
• Forward Pion Detectors (FPD)

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FPD to FMS
Run3-5 FPD Inclusive p0 cross sections AN for
inclusive p0 production
RUN3 dAu only one module (South) At deuteron
side (west) Inclusive p0 cross sections in dAu
and RdA Forward-mid rapidity particle
correlations
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FPD to FMS

• Run8 and beyond FMS
• FMS will provide full azimuthal coverage for
  range 2.5 ? h ? 4.0
• broad acceptance in xF-pT plane for inclusive
  g,p0,w,K0, production in pp and d(p)Au
• broad acceptance for g-p0 and p0-p0 from
  forward jet pairs to probe low-x gluon density in
  pp and d(p)Au collisions

47 x more area gt Order of magnitude more
luminosity
d
Au
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Forward Meson Spectrometer for Run8
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Need multiple iteration through the data since
pion and photon energy get spread over several
towers.
Run 6 resolution of d(Mp)/Mp10 should
be possible.
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Is saturation really the explanation?
Difficult to explain suppression with standard
shadowing But in NLO pQCD calculations ltxggt
0.02 is not that small (Guzey, Strikman,
Vogelsang, PL B603, 173) In contrast, ltxggt lt
0.001 in CGC calculations (Dumitru, Hayashigaki,
Jalilian-Marian, NP A765, 464 )
Basic difference pQCD 2 ? 2 CGC 2 ?
1
Forward-forward di-hadron correlation
ltxggt10-3 with CGC ltxggt10-4


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pp and dAu ? p0p0X correlations with forward
p0
dAu in HIJING
hep-ex/0502040
pp in PYTHIA
FMS
FTPC
EEMC
TPC Barrel EMC
FTPC
FTPC
FPD
Conventional shadowing will change yield, but not
angular correlation CGC evolution will change
yield and the angular correlation Sensitive to xg
10-3 in pQCD scenario few x 10-4 in CGC
scenario Forward-forward di-hadron correlation to
reach lowest x
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Summary and Outlook

• Forward rapidity at RHIC access to low-x physics
• pp mid-rapidity, pp forward and dAu mid-rapidity
  under control
• Run3 dAu
• Hadron production
• No suppression at backward
• Suppression in forward hadron production
• Di-hadron back-to-back correlation
• No suppression at etalt2.2
• Suppression at eta4 low pt
• Run8 dAu
• x30 more integrated luminosity
• STAR FMS ( x50 acceptance)
• PHENIX Muon Arm?

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Possible addition
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Calibration is ongoing
Minimal run-by-run dependence in mass peak
observed
Calorimeter stable at level of 1.
LED system critical calibration tool

• MIT (LED optics)
• UC Berkeley/SSL (flasher boards)
• Texas / Protovino / BNL (assembly)
• SULI program (Stony Brook students) / BNL
• (control electronics)

Entire Run 8 data set should become quickly
available with final calibration.
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IdAu with combined statistics
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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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B.Kopeliovich (PRC72(2005)054606)

• Sudakov Suppression, not low-x phenomena.
• Reproduce pt trend and centrality dependence.

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backup
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Forward single-spin asymmetries in STAR
arXivhep-ex/0801.2990

• Large transverse single-spin asymmetries at large
  xF
• xF dependence matches Sivers effect expectations
  qualitatively (but not quantitatively)
• pT dependence at fixed xF not consistent with
  1/pT expectation of pQCD-based calculations

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Access to range of Q2 and x
The x-Q2 region accessible is illustrated in the
following. Note the region reachable at RHIC. In
p(d)Aa the saturation will decrease the
effectivex-range by A1/3 At RHIC at y3 can
reach into x2 10-3
Saturation
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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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Foward Rapidity pp
nucl-ex/0602011
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A newly posted paper uses these BRAHMS data in a
global fit to ee- SIDIS, and pp data to extract
fragmentation functions and explicitly check
factorization. An example of their fit to our K-
data are shown. The RHIC pp data are important
to obtain flavor separated fragmentation
functions.
D.Florian, R.Sassot and M.Stratman, Hep-ph/0703242
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Nuclear Modification on Jet Correlation
Nuclear Modification on Jet Correlation is
measured by comparing the jet strength in pp
collisions and the jet strength in specified
event classes of dAu collisions. It is defined
as
It is noted that there are two factors convolute
into this quantity. The modification on jet
production, such as jet multiplicity. The
modification on single particle( trigger)
production.
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Summary and discussions

• We do not see any strong nuclear modification on
  per trigger jet yield. Especially we do not see a
  depletion in per trigger jet yield when
  triggering at forward. This contradicts with the
  predictions from CGC and power corrections.
• In fact, at forward rapidity, our data seems to
  suggest a slight enhancement on per trigger jet
  yield in dAu relative to pp
• We noted that per trigger yield are determined by
  singled particle production and jet production
  together. From single measurement, we know there
  is a strong suppression at forward rapidities in
  dAu. Thus a slight enhancement on per trigger
  jet yield may suggest

1. Jet production is not modified.
2. Jet production is also suppressed, but is less
   suppressed than single is.

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RdAu rapidity dependence
PRL 93, 242303
Sizable suppression as rapidity goes forward
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Back-to-back Angular Correlations
Phys. Rev. Lett. 96, 222301
htrig ?trig 1.2 2.4 2.0 lt pTtrig lt
5 GeV/c h,asso ?asso lt 0.35 1.0 lt
pT assolt 1.5 GeV/c
Ratio of conditional yields
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Compare with power correction (Dr. Ivan Vitev)
Back-to-back Angular Correlations
Phys. Rev. Lett. 96, 222301
No change in dAu relative to pp at
1.4lthtriglt2.0 Contradicts with the predictions
from CGC and power corrections STAR results at
lthtriggt4.0