1. Measurements of Transverse Spin Effects with the Forward Pion Detector of STAR
Larisa Nogach
Institute of High Energy Physics, Protvino
for the STAR collaboration
Outline
Motivation and previous measurements
Detector layout
Status of the analysis of Run 6 data and results
Plans for Run 7 and beyond
Summary

2. Motivation AN for p0 production at RHIC at √s=200 GeV
hep-ph/
Phenomenological models expect that p0 analyzing power decreases with increasing pT
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3. Existing measurements at STAR
PRL 92, (2004)
nucl-ex/
√s=200 GeV, <η> = 3.8
Asymmetry revealed at lower energies
persists at √s=200 GeV
Cross-section is consistent
with NLO pQCD calculations
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4. Transverse spin runs at STAR with forward calorimetry: 2001→2006
detector
EEMC
and FPD
prototypes
6 matrices
of FPD
full FPD
(8 matrices)
East FPD
West FPD++
~15
~30
~45
~60
0.15
0.25
0.1
6.8
3.8
±3.3/±4.0
±3.7/±4.0
-3.7/3.3
sampled
FOM (P2L) in Run 6 is ~50 times larger than from all the previous STAR runs
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5. STAR detector layout TPC: -1.0 <  < 1.0
Run 6 – FPD++
TPC: <  < 1.0
FTPC: 2.8 <  < 3.8
BBC : 2.2 <  < 5.0
EEMC: 1 <  < 2
BEMC: -1 <  < 1
FPD++/FPD:
 ~ 3.3/-3.7
FPD++: engineering test
of the Forward Meson
Spectrometer
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6. Detector acceptance Strong correlation between xF
and pT in the individual detectors
Rapid change in number of
events for either increasing pT
at fixed xF or increasing xF
at fixed pT
Broader pT range in xF bins
when combining data at
<η>=3.3 and 3.7
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7. Details of data analysis
Calibration:
on hardware level
offline cell-by-cell
energy and time-dependent corrections
−> is known at the level of 2%
Event selection:
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8. Details of data analysis
<η>=3.3
<η>=3.7
Analyzing power is measured
by “cross-ratio” method with
two-arms (left-right) detector:
Run-by-run comparison with mean
is consistent with statistics, except
right near the threshold
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9. π0 AN at √s=200 GeV – xF-dependence
AN at positive xF grows with
increasing xF
AN at negative xF is consistent
with zero
Run 6 data at <η>=3.7 are
consistent with the existing
measurements
Small errors of the data points
allow quantitative comparison
with theory predictions
Theory expects the reverse
dependence on η
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10. AN(pT) at xF > 0.4 Run3+Run5 data (hep-ex/0512013): Run6 data:
Online calibration of CNI
polarimeter
Hint of AN decrease with
increasing pT at pT~1-2 GeV/c
Run6 data:
more precise measurements
consistent with the previous runs in the overlapping pT region
complicated dependence on pT
residual xF-dependence?
=> AN mapping in (xF,pT) plane
is required
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11. AN(pT) in xF-bins Combined data from three runs
at <η>=3.3, 3.7 and 4.0
In each xF bin, <xF> does not
significantly changes with pT
Measured AN is not a smooth
decreasing function of pT
as predicted by theoretical
models
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12. Jet-like events Expect 4-5σ effect from existing Run 6 data
Is the single spin asymmetry
observed for p0 also present
for the jet the p0 comes from?
Answer discriminates between
Sivers and Collins contributions
hep-ex/
PYTHIA simulations:
- trigger on small cells; sum over
entire side
- vector sum of photon momenta
reproduces most forward
hard-scattered parton
Expect 4-5σ effect
from existing Run 6 data
if observed π0 AN is due to
Sivers effect
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13. Forward Meson Spectrometer for Run 7
FMS will provide full azimuthal coverage for range 2.5  h  4.0
broad acceptance in xF-pT plane for inclusive g,p0,w,K0,… production in p+p and d(p)+Au
broad acceptance for g-p0 and p0-p0 from forward jet pairs to probe low-x gluon density in p+p and d(p)+Au collisions
spacers
Run-7 FMS as seen from STAR interaction point
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14. Summary In Run 6, 6.8 pb-1 of data with average beam polarization ~60%
has been acquired in p+p collisions at √s=200 GeV
(xF,pT)-mapping of the transverse single spin asymmetry
of inclusive p0 production at forward rapidity has been done
• AN at positive xF grows with increasing xF
• AN at negative xF is consistent with zero
• Measured AN at “fixed” xF up to xF~0.5 does not decrease with
increasing pT as expected by the theory
• High precision of the measurements allows for a quantitative
comparison with theoretical models and distinguishing between
different dynamics
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15. Outlook Analysis of the data at √s=62 GeV is underway Jet-like events:
• detector response simulation
has been started
• calibration of the FPD++ outer
matrices needs to be completed
Construction of the FMS is ongoing − expected to be
completed by Run 7
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16. Backup

17. Possible mechanisms
Sivers effect [Phys. Rev. D 41, 83 (1990); 43, 261 (1991)]:
Flavor dependent correlation between the proton spin (Sp), proton momentum (Pp) and transverse momentum (kT) of the unpolarized partons inside. The unpolarized parton distribution function fq(x,kT) is modified to:
Collins effect [Nucl. Phys. B396, 161 (1993)]:
Correlation between the quark spin (sq), quark momentum (pq) and transverse momentum (kT) of the pion. The fragmentation function of transversely polarized quark q takes the form:
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18. Separating Sivers and Collins effects
Sivers mechanism: asymmetry in the forward jet or γ production
Collins mechanism: asymmetry in the forward jet fragmentation SP SP
kT,q p p p p Sq
kT,π
Sensitive to proton spin – parton transverse motion correlations
Sensitive to transversity
Need to go beyond π0 detection to
jets and direct photons
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19. Single Spin Asymmetry No relative luminosity needed Left Right p  p
Definition:
dσ↑(↓) – differential cross section of p0 when incoming proton has spin up(down)
Two methods of measurements:
Single arm calorimeter:
R – relative luminosity (by BBC)
Pbeam – beam polarization
Two arm (left-right) calorimeter:
No relative luminosity needed
π0, xF<0
π0, xF>0
Left
Right
p  p
positive AN: more p0 going
left to polarized beam
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20. Run6 − FPD++
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21. Cell-by-cell calibration
gains are determined from
π0 peak position in 2γ invariant
mass distributions sorted by
high towers
accuracy of the calibration is
at the level of ~2%
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22. Energy-dependent corrections
π0 peak position depends on the energy – due to energy leakages and ADC granularity
(dedicated MC study has been done)
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23. Run-dependent corrections
The FPD response vary with time/beam conditions:
used for spin sorting
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24. Correlation between FPD gains and BBC rates

25. (<xF>,<pT>)
0.25<xF< <xF<0.35
pT bin
<pT>, GeV/c
<xF>
0.95
0.273
1.26
0.282
1.61
0.278
2.10
2.53
0.284
pT bin
<pT>, GeV/c
<xF>
1.06
0.321
1.47
0.322
1.87
0.330
2.41
0.323
2.85
0.35<xF< <xF<0.47
1.29
0.368
1.70
0.370
2.08
0.374
2.64
0.373
3.18
0.377
1.45
0.423
1.94
0.426
2.38
0.433
2.94
0.430
3.54
0.435
0.47<xF<0.56
1.70
0.496
2.16
2.52
0.507
3.10
0.508
3.79
0.506

26. distributions in pT bins

27. Cross section − separated xF and pT dependence
Similar to ISR analysis:
J. Singh et al., Nucl. Phys.
B140, 189 (1978)
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28. Jet-like events Expect that jet-like events are ~15% of p0 events
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