Transcript Forward Particle Production and Transverse Single Spin Asymmetries OUTLINE

Forward Particle Production and
Transverse Single Spin Asymmetries
OUTLINE
• Transverse single spin effects in p+p collisions at s=200 GeV
• Towards understanding forward p0 cross sections
• Plans for the future
L.C. Bland
Brookhaven National Laboratory
RBRC Workshop on Parton Orbital
Angular Momentum
Albuquerque 25 February 2006
Installed and commissioned during run 4
To be commissioned
Installed/commissioned in run 5
Developments for runs 2 (1/02), 3 (3/03  5/03) and 4 (4/04  5/04)
• Helical dipole snake magnets
• CNI polarimeters in RHIC,AGS
 fast feedback
2/25/2006
• b*=1m operataion
• spin rotators  longitudinal polarization
• polarized atomic hydrogen jet target
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RHIC Spin Physics Program
• Direct measurement of polarized gluon distribution using
multiple probes
• Direct measurement of anti-quark polarization using
parity violating production of W
• Transverse spin: Transversity & transverse spin effects:
possible connections to orbital angular momentum?
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STAR detector layout
•
TPC: -1.0 <  < 1.0
•
FTPC: 2.8 <  < 3.8
•
BBC : 2.2 <  < 5.0
•
EEMC: 1 <  < 2
•
BEMC: -1 <  < 1
•
FPD: || ~ 4.0 & ~3.7
STAR characterized by azimuthally complete acceptance over
broad range of pseudorapidity.
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Single Spin Asymmetry
Definitions
• Definition:
d   d 
AN 
d   d 
• dσ↑(↓) – differential cross
section of p0 when incoming
proton has spin up(down)
Left
π0, xF<0
π0, xF>0
p
Two measurements:
• Single arm calorimeter:
1  N   RN  
L

AN 
  
R 
 
PBeam  N  RN 
L
R – relative luminosity (by BBC)
Pbeam – beam polarization
• Two arms (left-right) calorimeter:




1  N L  N R  N R  N L 
AN 

PBeam  N   N   N   N  
L
R
R
L 

p
No relative luminosity needed
Right
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positive AN: more p0 going
left to polarized beam
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First AN Measurement at STAR
prototype FPD results
STAR collaboration
Phys. Rev. Lett. 92 (2004) 171801
Similar to result from E704 experiment
(√s=20 GeV, 0.5 < pT < 2.0 GeV/c)
Can be described by several models
available as predictions:
Sivers: spin and k correlation in
parton distribution functions (initial
state)
Collins: spin and k correlation in
fragmentation function (final state)
Qiu and Sterman (initial state) /
Koike (final state): twist-3 pQCD
calculations, multi-parton correlations
√s=200 GeV, <η> = 3.8
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Dynamical Origins of Transverse SSA
p +p→p0+Х
• Sivers effect [Phys Rev D41 (1990) 83; 43 (1991) 261]:
Flavor dependent correlation between the proton spin (Sp),
momentum (Pp) and transverse momentum (kT) of the
unpolarized partons inside:

S

(
P

k
1
P
p
q )


N

ƒ q (x, k q , S P )  ƒ q (x, k q )  Δ q ƒ q (x, k q )
2
S P PP k q
• Collins effect [Nucl Phys B396 (1993) 161]:
Correlation between the quark spin (sq), momentum (pq) and
transverse momentum (kT) of the pion. The fragmentation
function of transversely polarized quark q takes the form:
s q  ( p q  k π )
1
N

ˆ p/q (z, k p )   Dp/q (z, k p )
Dp/q (z, k , s q )  D
2
p q  k π

π
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
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Present Status
Uses online beam polarization values
Run 3 Preliminary Result:
-more Forward angles
-final FPD Detectors
Run 3 Preliminary Backward Angle Data:
-No significant asymmetry seen.
A. Ogawa, for STAR: [hep-ex/0502040]
Run 3 + Run 5 Preliminary
<>=3.7,4.0
D. Morozov, for STAR [hep-ex/0512013]
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STAR
xF and pT range of FPD data
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AN(pT) from run3+run5 at √s=200 GeV
Uses online beam polarization values
• Combined statistics
from run3 and run5
with xF>0.4
• There is evidence
that analyzing power
at xF>0.4 decreases
with increasing pT
• To do: systematics
study
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Forward p0 production in hadron collider
p0
Ep
p
E
d N
qq
xqp
qp
xgp
EN
qg
2E p
s
s  2E N
Ep
z

q
Eq
  ln(tan( ))
2
p 
xg  T e g
xq  xF / z
s
(collinear approx.)
Q 2 ~ pT2
p
Au
xF 
• Large rapidity p production (p>4) probes asymmetric partonic collisions

• Mostly high-x valence quark + low-x gluon
• 0.3 < xq< 0.7

p  p  p ,p  3.8, s  200GeV
0
<z>

• 0.001< xg < 0.1
<xq> NLO pQCD
Jaeger,Stratmann,Vogelsang,Kretzer
• <z> nearly constant and high 0.7 ~ 0.8
<xg>
• Large-x quark polarization is known to be large from DIS
• Directly couple to gluons  probe of low x gluons
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But, do we understand forward p0 production in p + p?
At s < 200 GeV, not really…
√s=23.3GeV
√s=52.8GeV
Data-pQCD
difference at
pT=1.5GeV
2 NLO
collinear
calculations
with different
scale:
q6o
q10o
q15o
pT and pT/2
q53o
q22o
xF
xF
Bourrely and Soffer (hep-ph/0311110, Data references therein):
NLO pQCD calculations underpredict the data at s < 200 GeV
(ISR and fixed target)
data/pQCD appears to be function of q, √s in addition to pT
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ppp0X cross sections at 200 GeV
The error bars are point-to-point
systematic and statistical errors added
in quadrature
The inclusive differential cross section
for p0 production is consistent with NLO
pQCD calculations at 3.3 < η < 4.0
The data at low pT are more consistent
with the Kretzer set of fragmentation
functions, similar to what was observed
by PHENIX for p0 production at
midrapidity.
D. Morozov (IHEP),
XXXXth Rencontres de Moriond - QCD,
March 12 - 19, 2005
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NLO pQCD calculations by Vogelsang, et al.
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STAR-FPD
Cross Sections
Similar to ISR analysis
J. Singh, et al Nucl. Phys.
B140 (1978) 189.
d 3
C
B
E 3  (1  xF ) pT
dp
C 5
B6
Expect QCD scaling of form:
d 3
C
E 3  xTa (1  xF ) pTn 
dp
(
)
s / 2 (1  xF ) pTna  B  n  a
a
C
 Require s dependence to disentangle pT and xT dependence
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PYTHIA: a guide to the physics
Forward Inclusive p0 Cross-Section:
Subprocesses involved:
q+g
g+g and
q+g  q+g+g
STAR FPD
Soft processes
• PYTHIA prediction agrees well with the inclusive p0 cross section at 3-4
• Dominant sources of large xF p0 production from:
●
q + g  q + g (22)  p0 + X
●
q + g  q + g + g (23)  p0 + X
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q
p0
g
q
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p0
g
g
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Plans for the Future
• STAR Forward Pion Detector upgrade (FPD++) planned
as an engineering test of the FMS during RHIC run 6
• STAR Forward Meson Spectrometer (FMS) planned for
installation by RHIC run 7
Disentangle the dynamical origins to transverse SSA in
p+p collisions via measurements of AN for
 jet-like events
 direct photon production
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FPD++ Physics for Run6
We intend to stage a large version of the FPD to prove our
ability to detect jet-like events, direct photons, etc.
Run-5 FPD
Run-6 FPD++
The center annulus of the run-6 FPD++ is similar to arrays used to measure
forward p0 SSA. The FPD++ annulus is surrounded by additional calorimetry
to increase the acceptance for jet-like events and direct g events.
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STAR Configuration for Run 6
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Basic physics Goals
Ideas to be tested using FPD++ in RHIC run 6
• Prototype for FMS (planned completion for RHIC run 7)
• Discriminate dynamical origin of the forward AN
– Measurement of jetlike events and AN for those
• Similar to FPD (left/right symmetric) but with larger active area
• Measure shape of forward jet
– Measure direct photons cross section, possibly AN,
requiring separation of p0 and direct gamma
• Continue the study of p0 asymmetry in pp
• other
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New FMS Calorimeter
Lead Glass From FNAL E831
Loaded On a Rental Truck for Trip To BNL
804 cells of 5.8cm5.8cm60cm
Schott F2 lead glass
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Students prepare cells
at test Lab at BNL
Individual lead glass detectors are prepared and tested prior to
installation in the calorimeter. In total, 13 students have been
involved in this work since May, 2005.
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Status report
• Calorimeter cells for
free thanks to FNAL /
U.Col. and Protvino
• Cells were
refurbished and
tested at BNL
• South calorimeter in
place on new FMS
platform, readout
electronics in place
and tested
• In situ cell-by-cell
tests followed
installation
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Completed FPD++
Provides left/right symmetric calorimeters for detection of jet-like events
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Jet spin asymmetry
• Is the single spin
asymmetry observed for
p0 also present for the
jet the p0 comes from?
• Answer discriminates
between Sivers and
Collins contributions
• Trigger on energy in
small cells, reconstruct
p0 and measure the
energy in the entire
FPD++
• Average over the Collins
angle and define a new
xF for the event, then
measure analyzing
power versus xF
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Expect that jet-like events are
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~15% of p0 events
Planned readout
• Trigger on summed energy
Etrig is energy sum from only the
small cells of one calorimeter
• Determine total energy for event
Esum is the energy sum from all
cells of one calorimeter
• Photon and p0 finding will be
based on existing FPD software
 Reconstruct photon multiplicity
(Ng); p0,… invariant mass; etc.
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Jet-Like Events
L.C. Bland (hep-ex/0602012)
• Ng>3 requirement should allow p0p0 analysis
• (upper left) for each event, examine PYTHIA
record for final-state hard scattered partons
 event selection chooses jet-like events.
• (upper right) event-averaged correlation
between photon energy and distance in ,f
space from thrust axis
 events are expected to exhibit similar jet
characteristics as found at 0
• (middle) multi-photon final states enable
reconstruction of parent parton kinematics via
momentum sum of observed photons.
• (bottom) projected statistical accuracy for
data sample having 5 pb-1 and 50% beam
polarization.
Azimuthal symmetry of FPD++ around thrust axis, selected by Etrig condition, enables
• integration over the Collins angle  isolating the Sivers effect, or
Partonthe
OAM
•2/25/2006
dependence on the CollinsL.C.Bland,
angle RBRC
isolating
Collins/Heppelmann effect
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How do we detect direct
photons?
Isolate photons by having sensitivity to partner in decay of known particles:
π0gg
M=0.135 GeV BR=98.8%
K0  π0π0 gg gg 0.497
31%
 gg
0.547
39%
 π0 g gg g
0.782
8.9%
Detailed simulations underway
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Where do decay partners go?
di-photon parameters
zgg = |E1-E2|/(E1+E2)
fgg = opening angle
Mm = 0.135 GeV/c2 (p0)
Mm=0.548 GeV/c2 ()
for candidate photon with E1  Eg ,
E2 
1  zgg
1  zgg
Eg , gives the energy of second photon
fggmax
fggmin
M m c 2 1  zgg
M mc2
1
sin

, sin


give max and min opening angle
2
2 Eg 1  zgg
2
E1  E2 g m
• Gain sensitivity to direct photons by ensuring we have high probability to catch decay partners
• This means we need dynamic range, because photon energies get low (~0.25 GeV), and
sufficient
area (typical opening angles
are onlyRBRC
a fewParton
degrees
at our  ranges).
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OAM
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Sample decays on FPD++
With FPD++ module size and electronic dynamic range, have
>95% probability of detecting second photon from p0 decay.
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STAR Forward Meson Spectrometer
[hep-ex/0502040]
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Forward Meson Spectrometer for Run 7
• FMS will provide full azimuthal
coverage for range 2.5    4.0
• broad acceptance in xF-pT plane
0
0
for inclusive g,p ,,K ,… production
in p+p and d(p)+Au collisions
0
• broad acceptance for gp and
0
0
p p from forward jet pairs to probe
low-x gluon density in p+p and
d(p)+Au collisions
Run-7 FMS as seen from STAR
interaction point
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STAR detector layout with FMS
TPC: -1.0 <  < 1.0
FTPC: 2.8 <  < 3.8
BBC : 2.2 <  < 5.0
EEMC:1 <  < 2
BEMC:-1 <  < 1
FPD:
~ 4.04.0
& ~3.7
FMS: ||
2.5<<
With FMS addition, STAR
will have nearly contiguous
electromagnetic calorimetry
for 1<<4
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Three Highlighted Objectives In FMS Proposal
(not exclusive)
1.
A d(p)+Aup0p0+X measurement of the
parton model gluon density distributions xg(x)
in gold nuclei for 0.001< x <0.1. For 0.01<x<0.1,
this measurement tests the universality of the
gluon distribution.
2.
Characterization of correlated pion cross sections
as a function of Q2 (pT2) to search for the onset of
gluon saturation effects associated with
macroscopic gluon fields. (again d-Au)
3.
Measurements with transversely polarized
protons that are expected to resolve the origin of
the large transverse spin asymmetries in
reactions for forward p0 production.
(polarized pp)
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Frankfurt, Guzey and Strikman,
J. Phys. G27 (2001) R23 [hep-ph/0010248].
• constrain x value of gluon probed by high-x quark
by detection of second hadron serving as jet surrogate.
• span broad pseudorapidity range (-1<<+4) for
second hadron  span broad range of xgluon
• provide sensitivity to higher pT for forward p0 
reduce 23 (inelastic) parton process contributions
thereby reducing uncorrelated background in f
correlation.
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Pythia Simulation
Timeline for the Baseline RHIC Spin Program
Ongoing progress on developing luminosity and polarization
Research Plan for
Spin Physics at RHIC
(2/05)
Program divides into 2 phases:
s=200 GeV with present detectors for gluon polarization (g) at
higher x & transverse asymmetries;
s=500 GeV with detector upgrades for g at lower x & W production
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Summary / Outlook
• Large transverse single spin asymmetries are observed for large rapidity p0
production for polarized p+p collisions at s = 200 GeV
 AN grows with increasing xF for xF>0.35
 AN is zero for negative xF
• Large rapidity p0 cross sections for p+p collisions at s = 200 GeV is in
agreement with NLO pQCD, unlike at lower s. Particle correlations are
consistent with expectations of LO pQCD (+ parton showers).
• Plan partial mapping of AN in xFpT plane for p0 and measurement of AN for
jet-like events in RHIC run-6
• Propose increase in forward calorimetry in STAR to probe low-x gluon
densities and further studies of transverse SSA (complete upgrade by 11/06).
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Backups
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Time/luminosity
dependent
PMT
Gain
Matching
Di-photon gain
Massshift
Reconstruction
and calibration
corrections
Pb-glass reconstruction (no SMD)
FTPC-FPD matching
p0 reconstruction
• Clustering
and
moment analysis
Cluster
categorization
Photon conversion
in
beam pipe
efficiency
Luminosity
•+Fitting
shape
p vs
pPMT
with
p0 parametrized
(+ X)  g shower
(+ g) 
e+ egain
MC
& Data comparison
2 photon
• Number of photons found
>= 2 cluster example
Beam pipe
Mass resolution ~ 20MeV
• Fiducial volume > 1/2 cell width from edge

We
understand
~2% level
• Energy
sharing zggE1Egain
2/(E1E2) < 0.7
• Absolute gain determined from p0 peak
Limit
with
<0.5
cutiscorrection)
Gain
stability
(before
Efficiencies
almost purely
position
forzggeach
tower
Try both
• Energy
dependent gain correction
geometrically
determined
Energy
Run/luminosity dependent gain correction
from• reconstruction
f

of MC(PYTHIA+GEANT)
• Checking1gCluster
with MC (PYTHIA+GEANT)
Gain
stability (after
correction)
FPD position known
Geometrical
limit
f
relative to STAR
2gCluster
High tower sorted mass distributions
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2nd moment of cluster (long axis)
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Why Consider Forward Physics at a Collider?
Deep inelastic scattering
Kinematics
Hard scattering hadroproduction
Can Bjorken x values be selected in hard scattering?
Assume:
1. Initial partons are collinear
2. Partonic interaction is elastic


pT,1  pT,2
Studying pseudorapidity, =-ln(tanq/2), dependence of particle production
probes parton distributions at different Bjorken x values and involves different
admixtures of gg, qg and qq’ subprocesses.
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Simple Kinematic Limits
NLO pQCD (Vogelsang)
Mid-rapidity particle detection:
1.0
p+p  p0+X, s = 200 GeV, =0
10 and <2>0
 xq  xg  xT = 2 pT / s
fraction
0.8
Large-rapidity particle detection:
qq
0.6
qg
0.4
0.2
gg
0.0
1>>2
0
 xq  xT e1  xF (Feynman x), and
10
20
30
pT,p (GeV/c)
xg  xF e(12)
 Large rapidity particle production and correlations involving large
rapidity particle probes low-x parton distributions using valence quarks
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Constraining the x-values probed in hadronic scattering
Guzey, Strikman, and Vogelsang,
Phys. Lett. B 603, 173 (2004).
Log10(xGluon)
For 22 processes
TPC
FTPC
FPD
FTPC
Barrel EMC
FPD
Log10(xGluon)
Collinear partons:
+
+1 + e+2)
● x = p /s (e
T
1 + e2)

● x = p /s (e
T
CONCLUSION: Measure two
particles in the final state to constrain
the x-values probed
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Gluon
• FPD: ||  4.0
• TPC and Barrel EMC: || < 1.0
• Endcap EMC: 1.0 <  < 2.0
• FTPC: 2.8 <  < 3.8
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How can one infer the dynamics of particle production?
Particle production and correlations near 0 in p+p collisions at s = 200 GeV
Inclusive p0 cross section
Two particle correlations (h)
STAR
STAR, Phys. Rev. Lett. 90 (2003), nucl-ex/0210033
At √s = 200GeV and mid-rapidity, both
NLO pQCD and PYTHIA explains p+p
data well, down to pT~1GeV/c, consistent
with partonic origin
Phys. Rev. Lett. 91, 241803 (2003)
hep-ex/0304038
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Do they work for
forward rapidity?
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Back-to-back Azimuthal Correlations
with large 
Top View
Fit ffpfLCP normalized
distributions and with
Gaussian+constant
Trigger by
] forward p0
f
• Ep > 25 GeV
• p  4
]
Midrapidity h tracks in TPC
• -0.75 < < +0.75
Leading Charged Particle(LCP)
Coicidence Probability
[1/radian]
Beam View
ffpfLCP
• pT > 0.5 GeV/c
S = Probability of “correlated” event under Gaussian
B = Probability of “un-correlated” event under constant
s = Width of Gaussian
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STAR
PYTHIA (with detector
effects) predicts
• “S” grows with <xF>
and <pT,p>
• “s” decrease with
<xF> and <pT,p>
25<Ep<35GeV
PYTHIA
prediction agrees
with p+p data
Larger intrinsic kT
required to fit data
45<Ep<55GeV
Statistical errors only
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New Physics at high gluon density
1. Shadowing. Gluons hiding
behind other gluons. Modification
of g(x) in nuclei. Modified distributions
needed by codes that hope to calculate
energy density after heavy ion collision.
2. Saturation Physics. New phenomena
associated with large gluon density.
• Coherent gluon contributions.
• Macroscopic gluon fields.
• Higher twist effects.
• “Color Glass Condensate”
Figure 3 Diagram showing the boundary
between possible “phase” regions in the
t=ln(1/x) vs ln Q2 plane
Edmond Iancu and Raju Venugopalan, review for Quark Gluon Plasma 3,
.
R.C. Hwa and X.-N. Wang (eds.), World Scientific, 2003 [hep-ph/0303204].
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 Dependence of RdAu
Ed 3
 inelastic
dp 3 dAu
1  dAu
pp
RdAu 

inelastic
2  197  pp
N binary  dAu
Ed 3 3
dp pp
y=0
As y grows
G. Rakness (Penn1 State/BNL),
 dAu
RdAu 
XXXXth Rencontres
2 197de
 ppMoriond - QCD,
March 12 - 19, 2005
Kharzeev, Kovchegov, and Tuchin,
Phys. Rev. D 68 , 094013 (2003)
See also J. Jalilian-Marian,
Nucl. Phys. A739, 319 (2004)
isospin considerations, p + p  h is expected to be suppressed relative to d
+ nucleon  h at large  [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)]
• From
• Observe significant rapidity dependence similar to expectations from a “toy
model” of RpA within the Color Glass Condensate framework.
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Towards establishing consistency between
FPD (p0)/BRAHMS(h)
Extrapolate xF dependence at pT=2.5 GeV/c to
compare with BRAHMS h data. Issues to
consider:
• <> of BRAHMS data for 2.3<pT<2.9 GeV/c
bin. From Fig. 1 of PRL 94 (2005) 032301
take <>=3.07  <xF>=0.27
• p/h ratio?
Results appear consistent but have
insufficient accuracy to establish p+pp/p0
isospin effects
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Systematics
Measurements utilizing
independent calorimeters
consistent within uncertainties
Systematics:
Normalization uncertainty = 16%:
position uncertainty (dominant)
Energy dependent uncertainty = 13% - 27%:
energy calibration to 1% (dominant)
background/bin migration correction
kinematical constraints
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FPD Detector and pº reconstruction
• robust di-photon
reconstructions with FPD
in d+Au collisions on
deuteron beam side.
• average number of
photons reconstructed
increases by 0.5
compared to p+p data.
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d+Au  p0+p0+X, pseudorapidity correlations with forward p0
HIJIING 1.381 Simulations
• increased pT for forward p0 over run-3 results is
expected to reduce the background in f correlation
• detection of p0 in interval -1<<+1 correlated with
forward p0 (3<<4) is expected to probe
0.01<xgluon<0.1  provides a universality test of
nuclear gluon distribution determined from DIS
• detection of p0 in interval 1<<4 correlated with
forward p0 (3<<4) is expected to probe
0.001<xgluon<0.01  smallest x range until eRHIC
• at d+Au interaction rates achieved at the end of
run-3 (Rint~30 kHz), expect 9,700200 (5,600140)
p0p0 coincident events that probe 0.001<xgluon<0.01
for “no shadowing” (“shadowing”) scenarios.
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STAR Forward Calorimetry
Recent History and Plans
•
Prototype FPD proposal Dec 2000
–
–
•
Approved March 2001
Run 2 polarized proton data (published
2004 spin asymmetry and cross section)
FPD proposal June 2002
–
–
•
Review July 2002
Run 3 data pp dAu (Preliminary An
Results)
FMS Proposal: Complete Forward
EM Coverage
(hep-ex/0502040).
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Disentangling Dynamics of Single Spin
Asymmetries
Spin-dependent particle correlations
Collins/Hepplemann mechanism
requires transversity and spindependent fragmentation
Sivers mechanism asymmetry is
present for forward jet or g
Large acceptance of FMS will enable disentangling dynamics of spin asymmetries
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