Slajd 1

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???????????? ?????????? ? ??????????? p0-???????
? ??????? ???????????? ??????????????? ??????????
????? ?? ????????? ???? ? ???????????
?.?????, ????, ????????the STAR Collaboration
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Motivation

• Contrary to simple pQCD predictions, first
  measurements of ? production in p?p collisions
  found large transverse single-spin asymmetries.
  Similar large effects have recently been found in
  ee- and semi-inclusive deep-inelastic
  scattering.
• Significant developments in theory in the past
  few years suggest common origins for these
  effects, but large transverse spin asymmetries in
  p?p ? ? X production are not yet fully
  understood.

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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

According to the latest theoretical developments,
both mechanisms contribute to ?? AN
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RHIC Polarized Collider
RHIC pC Polarimeters
Absolute Polarimeter (H? jet)
BRAHMS PP2PP
PHOBOS
Siberian Snakes
Siberian Snakes
PHENIX
STAR
Spin Rotators (longitudinal polarization)
Spin Rotators (longitudinal polarization)
Pol. H- Source
LINAC
BOOSTER
Helical Partial Siberian Snake
AGS
200 MeV Polarimeter
AGS pC Polarimeter
Strong AGS Snake
1 MHz collision rate Polarization 0.5-0.6
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STAR detector layout
Forward Meson Spectrometer

• TPC -1.0 lt ? lt 1.0
• FTPC 2.8 lt ??? lt 3.8
• BBC 2.2 lt ??? lt 5.0
• EEMC 1 lt ? lt 2
• BEMC -1 lt ? lt 1

FPD/FPD ? -3.7/3.3
FPD/FMS ? -4.1/2.5-4.0
1/2 of the FMS
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Transverse spin runs at STAR with forward
calorimetry 2001?2008
sampled
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Single Spin Asymmetry

• Definition
• ds?(?) 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

positive AN more p0 going left to polarized beam
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Detectors from FPD to FMS

• Run8 and beyond FMS
• FMS provides full azimuthal coverage for range
  2.5 ? h ? 4.0
• broad acceptance in xF-pT plane for inclusive
  g,p0,w,K0, production in pp and d(p)Au
• broad acceptance for g-p0 and p0-p0 from
  forward jet pairs

• Runs 3-6 FPD, FPD
• Inclusive p0 cross sections
• AN for inclusive p0 production

three different positions of modular detectors
20x more acceptance then previous detectors
single monolithic detector
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Published measurements with FPD
PRL 92, 171801 (2004)

• PRL 101, 222001 (2008)
• Large transverse single-spin asymmetries at large
  xF
• xF dependence matches Sivers effect expectations
  qualitatively
• pT dependence at fixed xF is not consistent with
  1/pT expectation of theoretical model calculations

vs200 GeV, lt?gt 3.8
Asymmetry revealed at lower energies persists at
vs200 GeV
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Run8 data analysis -calibration

• the calibration methodologies employed for the
  FPD have been successfully adapted to the FMS
• offline cell-by-cell calibration using p0 peak
  position in the sorted by high tower mass
  distributions

• Event selection done with

minbias condition hightower ADC
threshold (400/200 cts. for small/large cells)
(for small cells only) fiducial volume cut (0.5
cell)
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LED monitoring system
Minimal time dependence in p0 peak position is
observed
LED system critical calibration tool
Calorimeter stable at the level of a few
percents
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Data to simulations comparison

• Full PYTHIA/GEANT simulations have adequate
  statistics to reach moderate xF
• Mass resolution in data is reasonable, given
  Run6 FPD performance
• Simulations have somewhat better resolution
  than data

DATA
SIMULATION
Present understanding sufficient further
investigations to be done
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First look at p0 AN from Run8 data
STAR preliminary
stat.errors only
75 of Run8 data
Octant subdivision of FMS for inclusive p0 spin
sorting

• AN comparable to prior measurements
• Azimuthal variation appears to be as expected

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Summary

• High precision AN measurements with the FPD
  allows for a quantitative comparison with
  theoretical models.
• FMS was commissioned and fully operated in Run8.
  It has 20x the acceptance of FPD (1264 channels
  compared to 98 for the pair of FPD modules).
• Reconstruction and calibration procedures
  successfully ported from FPD to FMS.
• Calibration is completed and data show good
  agreement with the simulated sample of events.
• Inclusive p0 AN(xF) from the FMS is in agreement
  with the FPD measurements.

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Outlook

• Complete analysis of p0 asymmetry
• Going beyond p0 detection
• - AN for a state that contains p0 pairs
• - AN for states with heavier mesons
• (?, ?, J/?)
• - direct photons, jet-like events can
  help
• to discriminate between Sivers and
• Collins effects

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BACKUP
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p0 cross section measurements
PRL 97, 152302 (2006)
Cross-section is consistent with NLO pQCD
calculations it was used in a recent global
analysis of ee-, SIDIS and pp data to determine
fragmentation functions PR D76, 074033 (2007)
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Separating Sivers and Collins effects
Collins mechanism asymmetry in the forward jet
fragmentation
Sivers mechanism asymmetry in the forward jet or
? production
SP
SP
kT,q
p
p
p
p
Sq
kT,p
Sensitive to proton spin parton transverse
motion correlations
Sensitive to transversity
To discriminate between the two effects we need
to go beyond p0 detection to direct photons or
jet-like events
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Energy dependent corrections

• p0 peak position depends on the energy
• linear correction extracted from p0 peak
  position and being applied to photon energies
  works for both p0s and ?s, and significantly
  decreases shift from zero in dE?? Esimu -
  Ereco.

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Association analysis

• comparison of generated quantities to GEANT
  simulations
• we consider

CORRECTED
UNCORRECTED
Eliminating energy dependence in p0 mass peak
gives the correct average neutral pion energy
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Simulation and search algorithm for p0p0 in FMS
and its engineering prototype

• it is possible to find events where the p0 pair
  originated at a significant distance from the
  vertex
• one source of such events are decays KS?p0p0
  (31 branching fraction)
• plot shows the mass distribution for displaced
  vertices above 100 cm from the BBC vertex a
  pronounced KS mass bump is visible