Spin Effects in Forward π0-Production in Polarized
Proton-Proton Collisions at STAR
Dmitry Morozov, IHEP (Protvino)
for the STAR Collaboration
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 2
Outlook
Motivation
STAR/FPD overview
Single Spin Asymmetry at FPD
Differential cross sections for forward π0-Production
Separated xF and pT dependence of cross section
Conclusions
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 3
Motivation
Sivers effect: Flavor dependent correlation between the proton spin (Sp), momentum (Pp) and
transverse momentum (kT) of the unpolarized partons inside. The unpolarized parton distribution function fq(x,kT) is modified to:
Collins effect:
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:
qPP
qpPqq
NqqqPqq
)()k(x,ƒΔ
21
)k(x,ƒ),(x,ƒkPS
kPSSk
πq
πqqNqπ
kp
kpssk
)()k(z,D)k(z,D),(z,D /q/q/q 2
1ˆ
p +p→+Х
∆qNfq – Sivers and ∆ND/q- Collins Functions may produce
azimuthal anisotropy
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 4
The STAR Collaboration
522 collaborators
51 institutions
12 countries
Solenoid Tracker At RHIC
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 5
STAR detector layout
TPC: -1.0 < < 1.0
FTPC: 2.8 < < 3.8
BBC : 2.2 < < 5.0
EEMC:1 < < 2
BEMC:0 < < 1
FPD: || ~ 4.0 & ~3.7
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 6
Forward Pion Detector
FPD module (7×7 matrix of Pb-Glass: 3.8 × 3.8 × 45 cm3)
7 Pb-Glass active preshower detectors
Two 48-strip scintillator SMD
Pb plate in front ~2.5 radiation lengths
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 7
Single Spin Asymmetry
Definition:
PBeam – beam polarizationdσ↑(↓) – differential cross section of π0 then incoming proton has spin up(down)
Two measurements:Single arm calorimeter:
R – relative luminosity (by BBC)Two arms (left-right) calorimeter:
No relative luminosity needed
dd
dd
PA
BeamN
1
L
LR
RNN
RNN
PA
BeamN
1
LRRL
LRRL
BeamN
NNNN
NNNN
PA
1
π0, xF<0
π0, xF>0
Left
Right
p p positive AN: more 0 going
left to polarized beam
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 8
First AN Measurement at STAR
STAR collaboration Phys. Rev. Lett. 92 (2004) 171801
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
Can be described by several models:
Similar to result from E704 experiment (√s=20 GeV, 0.5 < pT < 2.0 GeV/c)
√s=200 GeV, <η> = 3.8
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 9
AN for Forward π0 at STARShown at SPIN 2004, Trieste, Italy
The asymmetry is found to be zero for negative -0.6 < xF < -0.2
AN for positive xF is consistent with zero up to xF ~ 0.35, then increases with increasing xF
Run2 (2002): <PBeam>(online) = 20%, integrated luminosity ~ 0.15 pb-1
Run3 (2003): <PBeam>(online) = 30%, integrated luminosity ~ 0.5 pb-1
more precise measurements
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 10
The error bars are point-to-point systematic and statistical errors added in quadrature
The inclusive differential cross section for 0 production is consistent with NLO pQCD calculations at 3.3 < η < 4.0
As η increases, systematics regarding the comparison with NLO pQCD calculations begin to emerge. The data at low pT are more consistent with the Kretzer set of fragmentation functions. Similar to what was observed by PHENIX.
ppX cross sections at 200 GeV
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 11
Separated xF and pT dependence
The data is represented in the way similar to J. Singh, et al Nucl. Phys. B140
(1978) 189 - ISR experiment at √s=45 GeV
Cross sections fall with pT at fixed xF with exponent (~ 6) independent of xF
Data show exponential dependence on xF with fixed pT = 2 GeV/c. The value of the fitted exponent (~5) may be sensitive to the interplay between hard and soft scattering processes
Data accumulated in different running years with different calorimeters
with different readout electronics
taken at different angles
Although
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 12
Conclusions
Large spin effects have been found at forward 0 production in p p reaction at highest energy √s = 200 GeV
The single spin asymmetry for positive xF is consistent with zero up to xF~0.35, then increases with increasing xF
The asymmetry is found to be zero for negative xF
The inclusive differential cross section for forward 0 production at √s = 200 GeV is consistent with NLO pQCD calculations, in contrast to what was observed at lower √s
Mapping of the cross section in xF pT plane has begun … coming soon with analyzing power!
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 13
BACK-UP SLIDES
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 14
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
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 15
MC & Data comparison
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 16
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 17
Di-Photon Mass Reconstruction
Clustering analysisFit to measured and parameterized shower shapeApplying cuts:
Number of photons ≥ 2Etot > 20 GeVFiducial volume cut = 1/2 cell from the calorimeter edge
Energy sharing (<0.3 for cross sections)
Gains are determined from π0 peak position for each towerEnergy dependent gain correctionRun/Luminosity dependent gain correctionMC to data comparison
7.0)( 21
21
EE
EEz
Pb-Glass calorimeter analysis includes:
Mass resolution ~ 20 MeV
The calibration is known at 2% level
Efficiencies is geometrically determined (dominated by the geometrical acceptance of the calorimeter)
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 18
Cluster categorizationE
ner
gy
2nd moment of cluster (long axis)
2Cluster
1Cluster
2 photon cluster example
Try both
XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Dmitry Morozov (IHEP, Russia) 19
Time/luminosity dependent gain shift corrections
Luminosity vs PMT gain
Gain stability (before correction)
Gain stability (after correction)