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


Outlook
Motivation

STAR/FPD overview

Single Spin Asymmetry at FPD

0
Differential cross sections for forward π -Production

Separated xF and pT dependence of cross section

Conclusions


Motivation
p +p→π0+Х

Sivers effect:

T
Flavor dependent correlation between the proton spin (Sp), momentum (Pp) and transverse momentum (k ) of the unpolarized partons inside. The
T
unpolarized parton distribution function fq(x,k ) is modified to:

⊥
S
⋅
(
P
×
k
1
P
p
q)
⊥
⊥
N
⊥
Collins
ƒ qeffect:
(x,k q ,S P ) = ƒ q(x,kq ) + Δq ƒ q(x,kq )
⊥
2
T
S
P
k
Correlation between the quark spin (sq), momentum (pq) and transverse momentum (k ) of the pion. The
P fragmentation
P
qfunction of transversely
polarized quark q takes the form:

s q ⋅ (p q × k π⊥ )
1
N
⊥
ˆ π/q (z,kπ ) + ∆ Dπ/q (z,kπ )
Dπ/q (z,k ,s q ) = D
2
p q × k π⊥
⊥
π

⊥

∆qNfq – Sivers and ∆NDπ/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 < 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 PbGlass: 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:

AN =

1
PBeam

⋅

dσ − dσ
dσ ↑ + dσ ↓
↑

PBeam – beam polarization
↑(↓)
0
dσ
– differential cross section of π then
incoming proton has spin up(down)

Two measurements:
Single arm calorimeter:

 N ↑ − RN ↓ 

AN =
⋅  ↑
↓ 
R – relativePluminosity
(by BBC)
Beam  N + RN 
1

L↑
R= ↓
L

Two arms (left-right) calorimeter:

 N↑ ⋅N↓ − N↑ ⋅ N↓
1
L

R
R
L
No relative luminosity
AN =
⋅  needed
PBeam  N ↑ ⋅ N ↓ + N ↑ ⋅ N ↓
L
R
R
L


Left
π0,
xF<0

↓

π 0,
xF>0
0
p positive AN: more π going

p

left to polarized beam

Right







First AN Measurement at STAR
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:
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


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
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


pp→π 0X 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 π0 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.


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
Although

Data accumulated in different running years
with different calorimeters
with different readout electronics
taken at different angles


Conclusions
0
Large spin effects have been found at forward π 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 π
was observed at lower √s

0

production at √s = 200 GeV is consistent with NLO pQCD calculations, in contrast to what

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
Pb-Glass calorimeter analysis includes:
Clustering analysis
Fit to measured and parameterized shower shape

Applying cuts:

Number of photons ≥ 2
Etot > 20 GeV
Fiducial volume cut = 1/2 cell from the calorimeter edge
Energy sharing

zγ γ =

E1 − E2
(E + E )

0
Gains are determined from π peak position for each
1 tower2

< 0.7

(<0.3 for cross sections)

Energy dependent gain correction
Run/Luminosity dependent gain correction
MC to data comparison

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
2 photon cluster example

Energy

Try both
1γ Cluster

2γ Cluster

2nd moment of cluster (long axis)


Time/luminosity dependent
gain shift corrections

Luminosity
vs PMT gain

Gain stability (before correction)

Gain stability (after correction)