1. Neutral Pion Suppression at Forward Rapidities from d+Au Collisions at STAR
Greg Rakness
Penn State University/Brookhaven National Lab
for the STAR Collaboration
XIII International Workshop on Deep-Inelastic Scattering (DIS2005)
Madison, Wisconsin
28 April 2005

2. No collider data to constrain nuclear gluon distributions at low-x...
Low-x physics = gluons
x = fraction of proton momentum carried by parton
The proton:
Naïvely expect density of gluons in nucleus  A1/3...
The nucleus:
[Hirai, Kumano, Nagai, PRC 70 (2004) ]
Q2 (GeV2) 1 10
100
0.001
0.01
0.1 x
Accurate determinations of the proton's gluon structure have been extracted from scaling violations in inclusive Deep Inelastic Scattering (DIS)...
x G(x,Q2) x
Q = 100 GeV
Q = 2 GeV 1
100 10
10-1
10-2
10-3
10-4
No collider data to constrain nuclear gluon distributions at low-x...
Problem: as x continues to decrease,
the gluon density increases
cross sections must remain finite
[J. Pumplin, et al., JHEP 0207 (2002) 012]

3. The Relativistic Heavy Ion Collider
Au-Au
New state of matter
QGP
De-confinement
Deuteron-Au
Nuclear modification
Gluon saturation
Polarized
proton-proton
Nucleon Spin Structure
Spin Fragmentation
pQCD
RHIC is a QCD lab
PHENIX
STAR
BRAHMS
pp2pp
PHOBOS

4. Forward Hadron Production at RHIC
Nuclear modification factor:
Assume factorization to go from DIS to hadron collisions...
(xn,xm) nm fn(xn) fm(xm)
Suppression of inclusive h production of d+Au relative to p+p at forward rapidities… [I. Arsene, et al. (BRAHMS Coll.) PRL 93 (2004) ]
“Shadowing” in nuclear DIS emerging in hadronic collisions?

5. Many (recent) descriptions of low-x suppression...
A short list (i.e., probably incomplete):
Saturation (Color Glass Condensate)
Jalilian-Marian, NPA 748 (2005) 664.
Kharzeev, Kovchegov, and Tuchin, PLB 599 (2004) 23; PRD 68 (2003)
Multiple Scattering
Qiu and Vitev, PRL 93 (2004) ; hep-ph/
Shadowing
R. Vogt, PRC 70 (2004)
Armesto, Salgado, and Wiedemann, PRL 94 (2005)
Parton Recombination
Hwa, Yang, and Fries, PRC 71 (2005)
Factorization breaking
Kopeliovich, et al., hep-ph/
Nikolaev and Schaefer, PRD 71 (2005)
Others?
...
Some experimental issues that can be addressed at STAR:
Rapidity dependence?
Isospin?
p+p under control?
What x-values are probed?
Monojet?

6. STAR Detector TPC: -1.0 <  < 1.0 STAR Forward  Detector (FPD)
FPD: ||  3.8 (p+p)
||  4.0 (p+p,d+Au)  8 6 4 2
Integral Matter (Rad. Length)
<1 radiation length between interaction region and large rapidity region (2.2<<4.5)
No WN calorimeter for Run-3
Forward  Detector (FPD)
= Pb-glass electromagnetic calorimeter + Preshower

7. NLO pQCD compared with forward p + p  0 + X
STAR ]
E [GeV] gq gg qq qg
Process breakdown: Kretzer (hep-ph/ ) Dg Dq
Inclusive forward  production in p+p collisions consistent with NLO pQCD calculations at s = 200 GeV, in contrast to lower s [Bourrely and Soffer, EPJ C 36 (2004) 371]

8.  Dependence of RdAu y=0 As y grows STAR Example model (CGC):
Kharzeev, Kovchegov, and Tuchin, Phys. Rev. D 68 , (2003)
Example model (CGC):
 Observe significant rapidity dependence, similar to expectations from models which suppress gluon density in heavy nuclei
 RdAu for p0 systematically below linear extrapolation of h data to =4, consistent with expectations that p + p  h is isospin suppressed at large  [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)]

9. Measure two hadrons in final state
See L.C. Bland, et al., hep-ex/
Log10(xGluon)
Gluon
TPC
Barrel EMC
FTPC
FPD
For 22 processes
PYTHIA
Guzey, Strikman, and Vogelsang, Phys. Lett. B 603 (2004) 173.
pQCD
For 2  2 processes:
xBj correlated with h of away-side particle
strong azimuthal correlation expected (back-to-back peak)
 Analysis of di-hadron azimuthal and rapidity correlations can give insight on particle production mechanism...

10. Back-to-back Azimuthal Correlations with large 
Beam View
Top View
Fit LCP normalized distributions and with Gaussian+constant
Trigger by forward   ]
Coicidence Probability
[1/radian]
E > 25 GeV
  4 ]
Midrapidity h tracks in TPC
-0.75 < < +0.75
Leading Charged Particle(LCP)
pT > 0.5 GeV/c
LCP
S = Probability of “correlated” event under Gaussian
B = Probability of “un-correlated” event under constant
s = Width of Gaussian

11. PYTHIA: a guide to the physics
Forward Inclusive  Cross-Section:
g+g and
q+g  q+g+g
Soft processes
Subprocesses involved:
STAR FPD
q+g
PYTHIA prediction agrees well with the inclusive 0 cross section at 3-4
Dominant sources of large xF  production from:
q + g  q + g (22)   + X
q + g  q + g + g (23)   + X q g  q  g

12. Statistical errors only
STAR
45<Ep<55 GeV
25<Ep<35 GeV
STAR preliminary
PYTHIA=LO pQCD with parton showers (including detector effects), predicts
S grows with <xF> (<pT,p>)
sS decreases with <xF> (<pT,p>)
PYTHIA prediction agrees with p+p data
Larger intrinsic kT required to fit data (see Abazov, et al., hep- ex/ )
25<E<35GeV
45<E<55GeV
 Partonic scattering good language to discuss forward p0 production from p+p collisions at s = 200 GeV...

13. Expectation from HIJING (PYTHIA+shadowing +nuclear effects)
X.N.Wang and M Gyulassy, PR D44(1991) 3501
with detector effects
HIJING predicts clear correlation in d+Au
Small difference in “S” and “s” between p+p and d+Au
“B” is bigger in d+Au due to increased particle multiplicity at midrapidity
25<Ep<35GeV
35<Ep<45GeV

14. Exploratory d+Au  p0 + h + X Correlations Preliminary Data
STAR
Statistical errors only
25<Ep<35GeV
35<Ep<45GeV
STAR preliminary
PT is balanced by many gluons
“Mono-jet”
Dilute parton system (deuteron)
Dense gluon field (Au) 
E > 25 GeV
  4  
Beam View
Top View

15. More d+Au data needed...
STAR
Large  0+h± correlations:
Statistical errors only
25<Ep<35GeV
35<Ep<45GeV
STAR preliminary
Suppressed at small <xF> , <pT,>
Consistent with CGC picture
Consistent in d+Au and p+p at larger <xF> and <pT,>
More data needed to measure dependence on pT, h, flavor...
Fixed as
E & pT grows

16. Outlook at STAR: Forward Meson Spectrometer
L. C. Bland, et al., hep-ex/
See talk by S. Heppelmann in Spin parallel session, 29 April
STAR
Physics Motivation:
probing gluon saturation in p(d)+A collisions via…
large rapidity particle production (,,,’,,K0,D0) detected through all  decays
forward  probes gluons with smallest x in Au nucleus
di-jets with large rapidity interval (Mueller-Navelet jets): full EM calorimetry coverage from 4 < h < -1
disentangling dynamical origins of large xF analyzing power in p+p collisions
2 in 
2.2<<4
 To be built from existing calorimetry from FNAL E831 (Colorado)

17. New FMS Calorimeter
Loaded On a Rental Truck
for Trip To BNL
Lead Glass From FNAL E831

18. Conclusions
Forward p0 production at s = 200 GeV:
... is consistent with partonic scattering calculations in p+p collisions
 Inclusive cross section agrees with NLO pQCD and PYTHIA
 Large Dh correlations agree with PYTHIA
 Selects collisions of high-x quarks with low-x gluons
... is different in exploratory d+Au collisions (d-side):
 Inclusive yield normalized to p+p is suppressed
 Trend expected in models that suppress gluon density in nuclei
 Shows evidence of isospin effects
 Large Dh correlations are suppressed relative to p+p
 Direction of suppression qualitatively consistent with CGC
... More data and quantitative theoretical understanding are needed to make definitive physics conclusions…
The tools are coming into place to study low- x physics at RHIC

19. Backup transparencies

20. Inclusive 0 cross section vs. pT at fixed xF
Similar analyses performed prior:
CERN ISR [J. Singh, et al., (CHLM Collab.) NP B140, 189 (1978)]
0 production at s=45 GeV, 0.55<pT<1.05 GeV/c, xF>0.3
BNL RHIC [S. S. Adler, et al., (PHENIX Collab.) PRL 91, (2003)]
0 production at s=200 GeV, 1<pT<14 GeV/c, xF=0
CONCLUSIONS:
For 0.3<xF<0.5,  production:
~ (1/pT)6
independent of xF
At pT=2 GeV/c, 0 production:
~ (1-xF)5
ISR at pT=1.05 GeV/c, xF>0.3:
+: 0.12 (1-xF) (1-xF)2.93
: 0.55 (1-xF) (1-xF)3.02
At pT=2GeV/c:

21. Near-Term Future Plans
reconstruction of
HIJING + GEANT simulations
Simulations suggest that forward detection is feasible in centrality- averaged Cu+Cu collisions at s=200 GeV. In addition to establishing RCuCu at large rapidity, the FPD can trigger full STAR readout to examine particle correlations with large- rapidity . This can be useful to study flavor dependence of recoil jets at midrapidity.
L.C. Bland, HardProbes

22. xBj at RHIC and LHC
For a few cases…:
Collinear partons with momentum fractions x+, x elastically scatter to 
 For 
New kinematic regimes will soon be explored in nuclei both at RHIC and at the LHC…

23. NLO pQCD compared with midrapidity p+p
Analysis of h: KKP, NP B597, 337 (2001)
: S. S. Adler, et al., PRL 91, (2003)
Process breakdown: Kretzer (hep-ph/ )
qg+gq qq gg
pT(GeV/c)
Partonic scattering good model to describe p+p collisions at s = 200 GeV
pT(GeV/c)
P? [GeV]

24. p+p midrapidity production cross sections in comparison to NLO pQCD
charged hadrons
Direct photons
PHENIX, hep-ex/
Fixed order pQCD calculations agree with data for several different reactions...

25. Forward 0 Production in p + p collisions at s << 200 GeV
Data-pQCD difference at pT=1.5GeV/c
√s=23.3GeV
√s=52.8GeV
Ed3dp3[b/GeV3]
Ed3dp3[b/GeV3]
2 NLO calculation with different scale:
pT and pT/2




 xF xF
Bourelly and Soffer (hep-ph/ , Data references therein):
NLO pQCD calculations underpredict the data at low s from ISR
data/pQCD appears to be function of , √s in addition to pT

26. Large Analyzing Powers at RHIC at  = 3.8
azimuthal asymmetry in particle yields from a transversely polarized beam on an unpolarized target =
STAR
STAR collab., PRL 92, (2004)
Similar to FNAL E704 result at s = 20 GeV
In agreement with several models including different dynamics:
Sivers: spin and k correlation in initial state (related to orbital angular momentum?)
Collins: Transversity distribution function & spin-dependent fragmentation function
Qiu and Sterman (initial-state) / Koike (final-state) twist-3 pQCD calculations
First measurement of AN for forward 0 production at s=200GeV

27. Partonic Correlations from PYTHIA
Large energy deposited at =3.8
one parton in hard scattering with peak in forward direction + broad  range
other parton spread over broad  range q g  + q  g

28. Some advantages of  mesons/experimental details...
p + p  0 + X
25<E<35
[GeV]
35<E<45
45<E<55
E
MGeV/c2]
Identified  allows absolute energy calibration to ~1%...
Efficiency driven by geometrical acceptance of calorimeter...
<z>
<xq>
 carries most of the energy of the fragmenting parton…
…and pions are well described by NLO pQCD calculations over a broad rapidity window at large s…
<xg>