Spin Effects in Forward π0-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... Motivation STAR/FPD overview Single Spin

Spin Effects in Forward π0-Production

in Polarized Proton-Proton Collisions at STAR

Dmitry Morozov, IHEP (Protvino)

for the STAR Collaboration

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

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:

⊥

⊥

⊥

⊥

+

=

q P

P

q p

P q

q

N q q

q P

q q

)

( )

k (x, ƒ

Δ 2

1 )

k (x, ƒ

) ,

(x,

ƒ

k P S

k P

S S

k

⊥

⊥

⊥

⊥

⊥

×

×

⋅

∆ +

=

π q

π q

q N

q π

k p

k p

s s

) k (z, D

) k (z, D

) , (z,

2

1 ˆ

p +p→π0+Х

azimuthal anisotropy

The STAR Collaboration

522 collaborators

51 institutions

12 countries

STAR detector layout

TPC: -1.0 < η < 1.0

FTPC: 2.8 < | η| < 3.8

BBC : 2.2 < | η| < 5.0

EEMC:1 < η < 2

BEMC:0 < η < 1

FPD: |η| ~ 4.0 & ~3.7

Forward Pion Detector

FPD module (7×7 matrix of

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 polarization

dσ↑(↓) – 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

↓

↑

↓

↑

+

−

⋅

=

σ σ

σ

σ

d d

d

d P

A

Beam N

1

↓

↑

↓

↑

↓

↑

=









+

−

⋅

=

L

L R RN

N

RN

N P

A

Beam N

1

















⋅ +

⋅

⋅

−

⋅

⋅

=

↓

↑

↓

↑

↓

↑

↓

↑

L R

R L

L R

R L

Beam

N

N N

N N

N N

N

N P

π 0 ,

0 ,

x F >0

Left

Right

First AN Measurement at STAR

STAR collaboration

Phys Rev Lett 92 (2004)

171801

Sivers: spin and k⊥ correlation in parton distribution functions (initial state)

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 STAR

Shown at SPIN 2004, Trieste, Italy

The asymmetry is found to be zero for negative -0.6 < xF < -0.2

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 π 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.

pp →π0X 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

Large spin effects have been found at forward π0 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 π0 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

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 analysis

Fit to measured and parameterized shower shape

Applying cuts:

Number of photons ≥ 2

E tot > 20 GeV Fiducial 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 tower

Energy dependent gain correction

Run/Luminosity dependent gain correction

MC to data comparison

7

0 )

2

+

−

=

E E

E

E

zγ γ

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 categorization

2γ Cluster

1γ Cluster

2 photon cluster example

Try both

Time/luminosity dependent

gain shift corrections

Luminosity

vs PMT gain

Gain stability (before correction)

Gain stability (after correction)