Forward and p(d) A Physics at RHIC

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Forward and p(d)APhysics at RHIC

• Conveners
• Carl Gagliardi, Mike Leitch, Kirill Tuchin
• http//www.phenix.bnl.gov/phenix/WWW/publish/leitc
  h/rhicii-forward/rhicii-forward.html

• Small-x physics and saturation
• Additional important measurements

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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.
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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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Gluon Shadowing and Saturation

• Leading twist gluon shadowing, e.g.
• Gerland, Frankfurt, Strikman,
• Stocker Greiner (hep-ph/9812322)
• phenomenological fit to DIS DY data, Eskola,
  Kolhinen, Vogt hep-ph/0104124
• and many others
• Amount of gluon shadowing differs by up to a
  factor of three between different models

• Saturation or Color Glass Condensate (CGC)
• At low x there are so many gluons, that the
  quantum occupation numbers get so large that the
  situation looks classical
• Nuclear amplification xGA(x) A1/3xG(x),
  i.e. gluon density is 6x higher in Gold than the
  nucleon

Iancu and Venugopalan hep-ph/0303204
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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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PHENIX and PHOBOS report similar effects
PRL 94, 082302
PRC 70, 061901(R)
Charged particles are suppressed in the forward
direction in dAu collsions
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? dependence of RdAu
Observe significant rapidity dependence, similar
to BRAHMS measurements and expectations from
saturation framework.
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Comparing dAu dN/d? to pemulsion
nucl-ex/0409021
PHOBOS attributes effects to limiting
fragmentation
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Heavy quarks at forward rapidity small-x
First forward rapidity prompt muon results (charm
beauty)

• J/? nuclear dependence does not scale with x2 so
  appears to NOT have a dominant effect from
  shadowing
• PHENIX, nucl-ex/0507032
• Apparent scaling with xF is similar to limiting
  fragmentation phenomena as reported by PHOBOS
• but other models involving Sudakov suppression
  (energy conservation)
• Kopeliovich et al, hep-ph/0501260 (2005)
• or (large) initial-state gluon energy loss could
  also explain this xF scaling

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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.

Parton recombination

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

Multiple scattering

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

Others?

• ...

Factorization breaking

• Kopeliovich, et al., hep-ph/0501260.
• Nikolaev and Schaefer, PRD 71 (2005) 014023.

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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, hep-ph/0506308)
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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)
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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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Correlations in dAu

• are suppressed at small ltxFgt and ltpT,pgt
• Spp-SdAu (9.0 1.5)
• consistent with CGC picture
• are consistent in dAu and pp at larger ltxFgt
  and ltpT,pgt
• as expected by HIJING

STAR Preliminary
STAR Preliminary
25ltEplt35GeV
STAR Preliminary
STAR Preliminary
35ltEplt45GeV
Statistical errors only
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Saturation physics at RHIC?

• Fundamental question regarding saturation Where
  does it set in?
• Forward hadron production at RHIC samples similar
  x values as mid-rapidity production at the LHC
• Complex interplay at the LHC
• May need pp, pPb, and PbPb at the same vs to
  unravel it fully

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CERN pNucleus Workshophttp//wwwth.cern.ch/pAatL
HC/pAworkshop2.html

• pA at the LHC is still officially an upgrade
• First year that LHC might run pPb 2010
• Possible target luminosity 1029 cm-2s-1
  (RHIC II 20x larger)
• Cant use the constant frequency solution that
  worked well at RHIC
• N-N center of mass not at rest in lab ?y 0.46
  for full energy pPb
• Company line no need for pp reference. Will
  come from interpolation between Tevatron and 14
  TeV
• Probably okay for really hard processes
• May be problematic for measurements focused on
  small-x saturation effects
• If the accelerator turn-on goes well, even
  getting the 14 TeV reference data may be a
  challenge

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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
coincidence structure. Sensitive to xg few x
10-3 in pQCD scenario few x 10-4 in CGC scenario.
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Upgrades

• STAR
• Forward Meson Spectrometer
• Forward tracker upgrade (SiGEM)
• PHENIX
• Nosecone calorimeter (W-Si)
• forward muon trigger (RPC)
• Forward Silicon Vertex detector (mini-strips)

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Double parton correlations
CDF, PRL 79, 584
PRL 88, 031801
A-dependence of 4-jet yields in pA collisions
can be used to measure x1 x2 momentum
correlations within the proton. Would require pA
collisions.
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Tagged Drell-Yan production at RHIC? (from
Jen-Chieh Peng)
One can tag on forward-going proton, neutron, ?,
? in coincidence with lepton-pair
Assuming factorization, then dsDY/dy dm dxF
(pp?nµ µ- x) dsDY/dm dxF
(pp?µµ- x) fMB(y) and fMB(y) is the
probability for p ? p n, where n carries a
fraction y of the proton momentum
Tagged Drell-Yan production could provide
information on the antiquark distribution in the
mesons of the nucleon sea and on fMB(y)
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Tagged DY with forward neutrons
Or with tagged protons
(from Jen-Chieh Peng)
ZDC
Roman pots
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Physics with tagged forward protons at
RHIC-II from Wlodek Guryn

• processes with forward tagged protons select
  exchanges mediated by gluon-rich objects
• color-singlet objects with vacuum quantum
  numbers Pomerons
• double Pomeron exchange can produce massive
  systems glueballs and other gluon-rich states

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Ultra-peripheral Collisions (UPCs)

• Can ultra-peripheral reactions (UPCs) at forward
  rapidity probe small-x gluon shadowing or
  saturation?
• e.g. J/? production probes gluon distributions
  of nuclei (private comm. Mark Strikman)

QM05
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Conclusions

• dAu results provide hints that saturation
  effects are becoming important
• Both STAR and PHENIX have upgrade plans that will
  dramatically improve their forward capabilities
• RHIC may be the ideal accelerator to explore the
  onset of saturation

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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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Another recent calculation
Very good description of the pT dependence of the
BRAHMS dAu ? h- X cross section at ?
3.2 (Dumitru, Hayashigaki, and Jalilian-Marian,
hep-ph/0506308)
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dAu ? p0X at 200 GeV
dAu p0 cross section at ? 4.0 is well
described by a LO CGC calculation with a K-factor
of 0.8 (Dumitru, Hayashigaki, and
Jalilian-Marian, hep-ph/0506308)
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Other possible tagged Drell-Yan measurements
(from Jen-Chieh Peng)

• p p ? ? l l - x
• Probe the p ? ? K component, and the strange
  quark contents in the proton
• p? p? ? n l l - x
• Measure helicity asymmetry ALL due to the meson
  cloud (expect ALL 0)
• p A ? A l l - x
• probe pion excess in nuclei via the
  A-dependence measurement
• Doubly-tagged Drell-Yan p p ? p p l l -
  x
• meson-meson annihilation (or pomeron
  interactions)?
• Other hard-diffractive processes at RHIC?
• tagged J/? production?
• tagged jet production?
• (gluon content of the meson cloud)

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Gluon saturation at small x shadowing in nuclei
Also important for AA initial state
RHIC allows study of transition. LHC always
saturated (except very high pT and ylt-3)
Especially for LHC where always saturated at
midrapidity

• Models
• gluon saturation, CGC
• leading twist shadowing (coherence)
• mass renorm. (Vitev)
• Sudakov suppr. (Kopeliovich)
• limiting fragmentation

LHC only 1-month/yr shared between pp, pA and AA
earliest pA 2010?

• How to distinguish??
• correlations
• energy,rapidity dependence
• universality

polarized and diffractive pA are useful - need
theoretical calculations.
need hard processes sensitive to gluons

• Forward hadrons
• need forw. p0 ? STAR FMS
• hadron PID for ygt1

compare open closed
Heavy-quarks (c,b) bound states
Direct photons for ygt1 ? PHENIX NCC
need cleaner way to get open-c, -b
wide kinematic range to understand physics
differentiate models
vertex detectors!

• Onia rare processes
• PHENIX onia
• Need STAR forward J/? ? ee

• separating D B in single-lepton spectra?
• use B ? J/? X measurement
• very high statistics could allow seeing 2 c?s

D ? Kp nice but difficult!
High Luminosities needed!
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Antiquark distribution in nucleon and nuclei
Also important for understanding gluon
distribution in nuclei
Flavor structure of parton distribution in nuclei
is practically unknown.

• Models
• Enhancement of antiquark in nuclei due to meson
  cloud
• Leading twist shadowing of antiquark at low-x
• Meson cloud explains the d-bar, u-bar asymmetry
  in nucleon

Spin structure of the nucleon is closely
connected to the flavor structure of the nucleon
Nuclear shadowing of antiquark sea can be
measured at RHIC in pA
need hard process sensitive to antiquarks
Polarized Drell-Yan can probe seaquark
polarization complementary to W-production
Forward-tagging in p-p Drell-Yan can probe the
pion cloud directly
Drell-Yan process in pA and pp is ideal for
probing antiquarks
Roman pot and forward neutron tagging are
required
High Luminosities needed!