spin effects in forward π 0 -production in polarized proton-proton collisions at star

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


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


The STAR Collaboration

522 collaborators

51 institutions

12 countries

Solenoid Tracker At RHIC


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


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


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


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


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


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


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


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!


BACK-UP SLIDES


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


MC & Data comparison


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)


Cluster categorizationE

ner

gy

2nd moment of cluster (long axis)

2Cluster

1Cluster

2 photon cluster example

Try both


Time/luminosity dependent gain shift corrections

Luminosity vs PMT gain

Gain stability (before correction)

Gain stability (after correction)

spin effects in forward π 0 -production in polarized proton-proton collisions at star

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