J' Sowinski

1
Run 8 Spin Plan for STAR

• J. Sowinski
• Indiana University
• for the Spin PWG

Many thanks to Carl Gagliardi for his slides!
2
Primary goals for Run 6 and Status

• Longitudinally polarized protons
• Use ALL for inclusive jet production to
  distinguish between GRSV-std and ?g 0 scenarios
  at the 45 sigma level
• Preliminary result announced recently at DNP
• First statistically significant ALL measurement
  with di-jets
• Di-jet analysis is well underway
• Transversely polarized protons
• AN for forward rapidity jet-like events
  distinguish between Collins and Sivers effects
• ?0 AN found a surprise! Paper moving thru
  process in STAR
• Measure gluon Sivers effect with mid-rapidity
  di-jets
• First publication from RHIC Run 6
• Unpolarized protons
• Significant sample of gammajet events to
  optimize algorithms for future measurements
• Work in progress

3
Transverse polarization Run 6
Preliminary

• Forward ?0 AN measurements during Run 6
• Fit to Sivers effect in semi-inclusive DIS
  provides qualitative description of the xF
  dependence
• Relates the observed asymmetry to parton orbital
  motion
• Data do not show the 1/pT behavior expected by
  theory

4
Transverse polarization Run 8

• Acceptance of FMS and projected RHIC performance
  will enable
• Further reach for inclusive p0 and heavy mesons
• Spin-dependent near-side correlations (p0-p0) ?
  separation of Sivers and Collins effects
• Spin-dependent away-side correlations (p0-jet) ?
  isolation of Sivers effect
• Embark on spin-dependent inclusive g and gjet

Projections for run-8
5
Transverse spin direct g
Theory expects repulsive color charge
interactions to result in an opposite sign to
spin-correlated momentum imbalance for gjet.
Magnitude of effect requires gt105 events to see
significant effect.
Bacchetta et al., Phys. Rev. Lett. 99, 212002
(2007)
Comparison of run-6 data to simulation provides
indication that prompt g can be extracted. The
large acceptance and dynamic range of the FMS is
needed to veto daughter g from p0,h,
decays. Expect gt0.5M g events into small fiducial
volume in 30 pb-1 sample with Eggt25 GeV.
6
Longitudinal polarization Run 6

• ALL for inclusive jets from Run 6
• ?G is small or negative at least within the
  GRSV framework

7
Longitudinal polarization Run 8

• There is still room for further improvement in
  the inclusive jet ALL determination, but need to
  watch for diminishing returns
• Will focus on pT gt 15 GeV/c in Run 8
• This is the sweet spot to distinguish negative
  ?G scenarios with inclusive jets

8
Looking beyond inclusive ALL measurements
Highest bin in Run 5 paper
Lowest bin in Run 5 paper

• Inclusive ALL measurements at fixed pT average
  over a broad x range.
• Need a global analysis to determine the
  implications
• Can hide considerable structure if ?G(x) has a
  node

9
Next few years Investigate x dependence of DG
Leader et al, hep-ph/0612360 ?G 0.13 0.16 ?G
0.006 ?G -0.20 0.41
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The next few years ?G(x)

• Di-jets access LO parton kinematics
• Involve a mixture of qq, qg, and gg scattering
• Choose large x1, small x2 to maximize qg
  scattering with highly polarized quarks
• g-jets isolate qg and offer better pT resol.

11
Estimates of di-jet sensitivity for 2008
20 pb-1 recorded and 65 polarization Uniform
pTgt5GeV (should be optimized by region) Separated
by calo region coinc. to emphasize xmin
ranges M-gtx1x2 (h1h2)-gtx1/x2 (h1-h2)-gtcosq
Will also have FMSBEMC and FMSEEMC to sample
the low x region
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Each combination of detectors covers different x
regions
Trigger on dijets with graded thresholds by
region to emphasize kinematics with at least one
parton with xgt0.2
Endcap-West Barrel
Endcap-East Barrel
Endcap-Endcap
FMS- endcap
Barrel-Barrel
13
Projected future sensitivity with ? jet events
Goal is 80 pb-1 recorded over 2008-9 1/4 1/3 in
2008

• ? jet provides very good event-by-event
  determination of the parton kinematics
• 90 of the yield arises from qg scattering
• Can choose the kinematics to maximize the
  sensitivity to ?G(x)
• Outstanding question How low in photon pT can
  we go?

14
Planning for Run 8

• Need 90 pb-1 delivered with 65 polarization to
  reach our physics goals
• Plan a 1/3-2/3 transverse-longitudinal split,
  with transverse first
• Need to carefully balance triggers and use of
  fast detectors to optimize use of available
  bandwidth
• Emphasis on forward program in transverse
• Emphasis on 2-body final states over full
  calorimeter acceptance in longitudinal