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 Slide 2 SPIN2006,
Kyoto2 Motivation Phenomenological models expect that 0 analyzing
power decreases with increasing p T hep-ph/0609238 A N for
production at RHIC at s=200 GeV Slide 3 SPIN2006, Kyoto3 Existing
measurements at STAR PRL 92, 171801 (2004) s=200 GeV, = 3.8
nucl-ex/0602011 Cross-section is consistent with NLO pQCD
calculations Asymmetry revealed at lower energies persists at s=200
GeV Slide 4 SPIN2006, Kyoto4 Transverse spin runs at STAR with
forward calorimetry: 20012006 Run2Run3Run5Run6 detectorEEMC and FPD
prototypes 6 matrices of FPD full FPD (8 matrices) East FPD West
FPD++ ~15~30~45~60 0.150.250.16.8 3.83.3/4.03.7/4.0-3.7/3.3 sampled
FOM (P 2 L) in Run 6 is ~50 times larger than from all the previous
STAR runs Slide 5 SPIN2006, Kyoto5 STAR detector layout TPC: -1.0
< < 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 Run 6 FPD++
Slide 6 SPIN2006, Kyoto6 Detector acceptance Strong correlation
between x F and p T in the individual detectors Rapid change in
number of events for either increasing p T at fixed x F or
increasing x F at fixed p T Broader p T range in x F bins when
combining data at =3.3 and 3.7 Slide 7 SPIN2006, Kyoto7 Details of
data analysis Event selection: Calibration: on hardware level
offline cell-by-cell energy and time-dependent corrections > is
known at the level of 2% Slide 8 SPIN2006, Kyoto8 Details of data
analysis 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 =3.7
=3.3 Slide 9 SPIN2006, Kyoto9 0 A N at s=200 GeV x F -dependence A
N at positive x F grows with increasing x F A N at negative x F is
consistent with zero Run 6 data at =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 Slide 10 SPIN2006, Kyoto10 A N (p T ) at x F
> 0.4 Run3+Run5 data (hep-ex/0512013): Run6 data: more precise
measurements consistent with the previous runs in the overlapping p
T region complicated dependence on p T Online calibration of CNI
polarimeter Hint of A N decrease with increasing p T at p T ~1-2
GeV/c residual x F -dependence? => A N mapping in (x F,p T )
plane is required Slide 11 SPIN2006, Kyoto11 A N (p T ) in x F
-bins Combined data from three runs at =3.3, 3.7 and 4.0 In each x
F bin, does not significantly changes with p T Measured A N is not
a smooth decreasing function of p T as predicted by theoretical
models Slide 12 SPIN2006, Kyoto12 Jet-like events PYTHIA
simulations: - trigger on small cells; sum over entire side -
vector sum of photon momenta reproduces most forward hard-scattered
parton Expect 4-5 effect from existing Run 6 data if observed 0 A N
is due to Sivers effect hep-ex/0602012 Is the single spin asymmetry
observed for 0 also present for the jet the 0 comes from? Answer
discriminates between Sivers and Collins contributions Slide 13
SPIN2006, Kyoto13 Run-7 FMS as seen from STAR interaction point
Forward Meson Spectrometer for Run 7 FMS will provide full
azimuthal coverage for range 2.5 4.0 broad acceptance in x F -p T
plane for inclusive , , ,K , production in p+p and d(p)+Au broad
acceptance for and from forward jet pairs to probe low- x gluon
density in p+p and d(p)+Au collisions spacers Slide 14 SPIN2006,
Kyoto14 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
(x F,p T )-mapping of the transverse single spin asymmetry of
inclusive 0 production at forward rapidity has been done A N at
positive x F grows with increasing x F A N at negative x F is
consistent with zero Measured A N at fixed x F up to x F ~0.5 does
not decrease with increasing p T as expected by the theory High
precision of the measurements allows for a quantitative comparison
with theoretical models and distinguishing between different
dynamics Slide 15 SPIN2006, Kyoto15 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 Slide 16 Backup Slide 17 SPIN2006, Kyoto17
Possible mechanisms Sivers effect [Phys. Rev. D 41, 83 (1990); 43,
261 (1991)]: Flavor dependent correlation between the proton spin
(S p ), proton momentum (P p ) and transverse momentum (k T ) of
the unpolarized partons inside. The unpolarized parton distribution
function f q (x,k T ) is modified to: Collins effect [Nucl. Phys.
B396, 161 (1993)]: Correlation between the quark spin (s q ), quark
momentum (p q ) and transverse momentum (k T ) of the pion. The
fragmentation function of transversely polarized quark q takes the
form: Slide 18 SPIN2006, Kyoto18 Separating Sivers and Collins
effects Collins mechanism: asymmetry in the forward jet
fragmentation Sivers mechanism: asymmetry in the forward jet or
production SPSP k T,q p p SPSP p p SqSq k T, Need to go beyond 0
detection to jets and direct photons Sensitive to proton spin
parton transverse motion correlations Sensitive to transversity
Slide 19 SPIN2006, Kyoto19 Single Spin Asymmetry Definition: d ()
differential cross section of when incoming proton has spin
up(down) Two methods of measurements: Single arm calorimeter: R
relative luminosity (by BBC) P beam beam polarization Two arm
(left-right) calorimeter: No relative luminosity needed 0, x F 0
Left Right p p positive A N : more 0 going left to polarized beam
Slide 20 SPIN2006, Kyoto20 Run6 FPD++ Slide 21 SPIN2006, Kyoto21
Cell-by-cell calibration gains are determined from 0 peak position
in 2 invariant mass distributions sorted by high towers accuracy of
the calibration is at the level of ~2% Slide 22 SPIN2006, Kyoto22
Energy-dependent corrections 0 peak position depends on the energy
due to energy leakages and ADC granularity (dedicated MC study has
been done) Slide 23 SPIN2006, Kyoto23 Run-dependent corrections The
FPD response vary with time/beam conditions: used for spin sorting
Slide 24 Correlation between FPD gains and BBC rates Slide 25 (, )
p T bin, GeV/c 0.5-1.10.950.273 1.1-1.41.260.282 1.4-1.91.610.278
1.9-2.42.100.273 2.4-5.02.530.284 p T bin, GeV/c 0.5-1.21.060.321
1.2-1.71.470.322 1.7-2.21.870.330 2.2-2.72.410.323 2.7-5.02.850.330
0.25
