Spin Recent Highlights and Results

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SpinRecent Highlights and Results

• Carl A. Gagliardi
• Texas AM University
• for the Collaboration
• Outline

• Introduction
• STAR longitudinal spin program
• Recent results and future directions
• STAR transverse spin program
• - Recent results and future directions
• Conclusions

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What contributes to the proton spin?
Consider a proton moving toward the right
Proton spin ?
?
?
?q(x) ?g(x)
Polarized DIS 0.3
Both are poorly constrained
Spin sum rule
Proton spin ?
STAR spin program Exploring poorly determined
components of the proton
dq(x)
?
?
Transversity very little data
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RHIC the worlds first polarized hadron collider

• Spin varies from rf bucket to rf bucket (9.4 MHz)
• Spin pattern changes from fill to fill
• Spin rotators provide choice of spin orientation
• Billions of spin reversals during a fill with
  little if any depolarization

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STAR detector
E-M Calorimeter
Projection           Chamber
Time of    Flight
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Essential benchmark unpolarized cross sections
PRL 97, 252001
PRL 97, 152302

• Mid-rapidity jet cross section is consistent with
  NLO pQCD over 7 orders of magnitude
• Forward rapidity p0 cross section also consistent
  with NLO pQCD
• Many other examples
• pQCD works over a very broad kinematic range at
  RHIC energies

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STAR longitudinal spin program

• What is the polarization of the gluons in the
  proton?

• Three recent fits of equal quality
• ?G 0.13 0.16
• ?G 0.006
• ?G -0.20 0.41
• all at Q2 1 GeV2
• Leader et al, PRD 75, 074027

• Additional question for future 500 GeV runs
• What is the polarization of the anti-quarks?

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Longitudinally polarized pp collisions at RHIC
Partonic fractions in jet production at 200 GeV
For most RHIC kinematics, gg and qg dominate,
making ALL for inclusive jets and hadrons
sensitive to gluon polarization.
0
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STAR inclusive p0 ALL at various rapidities
? lt 0.95
1 lt ? lt 2
? 3.2, 3.7

• During Run 6, STAR measured ALL for inclusive p0
  for three different rapidity regions
• Mid-rapidity result excludes large gluon
  polarization scenarios
• Larger rapidity correlates to stronger dominance
  of qg scattering with larger x quarks and smaller
  x gluons
• Expect ALL to decrease as ? increases
• Forward rapidity baseline for future ? and
  ?-jet measurements

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STAR inclusive jet ALL from Run 6
200 GeV -0.7 lt ? lt 0.9

• Confidence level calculations from comparison to
  the GRSV polarized parton distributions
• Also compare to other polarized parton fits
• Large gluon polarization scenarios excluded,
  except for GS-C

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DSSV first global analysis with polarized jets
de Florian et al., PRL 101, 072001
STAR

• The first global NLO analysis to include
  inclusive DIS, SIDIS, and RHIC pp data on an
  equal footing
• Finds a node in the gluon distribution near x
  0.1, but with the opposite phase from GS-C

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Future inclusive jet ALL sensitivity
Projected sensitivities Run 9 future 500 GeV
running
Projected improvement in x?g from Run 9

• Goal for the current 200 GeV run
• 50 pb-1 _at_ 60 pol reduce ALL uncertainties a
  factor of 4
• Will provide much stronger constraints on gluon
  polarization
• Goal for future 500 GeV running
• 300 pb-1 _at_ 70 pol
• Extend precision determination to lower xg

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Looking beyond inclusive ALL measurements
PRL 100, 232003

• Inclusive ALL measurements at fixed pT average
  over a broad x range.
• Can hide considerable structure if ?g(x) has a
  node
• Correlation measurements can constrain the shape
  of ?g(x)

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Beyond inclusives charged pions opposite jets
ALL
ALL

• Trigger and reconstruct a jet, then look for a
  charged pion on the opposite side
• Correlation measurement significantly increases
  the sensitivity of ALL(p)

Full NLO calculations for this observable de
Florian, arXiv0904.4402
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Di-jets and ?g(x) projected sensitivity for Run 9

• Di-jets provide direct access to initial parton
  kinematics at LO
• Di-jets in different regions of the STAR detector
• Sample different mixtures of qq, qg, gg
• Sample different ranges of xg
• Goal for the current run
• 50 pb-1 _at_ 60 pol

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500 GeV pp quark and anti-quark polarizations

• Projected sensitivity for parity-violating W
  asymmetries
• With two polarized beams and W and W-, can
  separate u, d, u, d polarizations
• Explores the origin of the sea quarks

Will constrain u
Will constrain d
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STAR transverse spin program
PRL 92, 171801

• Large rapidity transverse single-spin asymmetries
  at 200 GeV are BIG
• May arise from the Sivers effect, Collins effect,
  or a combination

Parton orbital motion
Transversity
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Run 6 inclusive p0 AN at forward rapidity
PRL 101, 222001

• Large transverse single-spin asymmetries at large
  xF
• xF dependence matches Sivers effect expectations
  qualitatively (but not quantitatively)
• pT dependence at fixed xF does not follow 1/pT
  expectation of pQCD-based calculations

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Run 6 inclusive ? AN at large xF
? 3.66

• To date, the ? meson has looked like a
  high-mass, low-yield p0 in all measurements at
  RHIC
• AN for the ? mass region is much larger at high
  xF

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Separating Sivers and Collins effects in pp
collisions
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

• Need to go beyond inclusive hadrons to
  measurements of jets or direct ?

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FMS expanding STARs forward acceptance
STAR Forward Meson Spectrometer 2.5 lt ? lt 4.0

• Expanded pT range for inclusive p0 AN during Run 8

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Run 8 forward p0 hadron correlation measurements
pp ? p0 (FMS) h (TPC) X
pp ? p0 (FMS) p0 (FMS) X
Uncorrected Coincidence Probability (radian-1)

• Trigger on a p0 in the FMS with pT gt 2.5 GeV/c
• Correlate with hadrons with pT gt 1.5 GeV/c
• FMS-TPC and FMS-FMS back-to-back correlations
  enable
• di-hadron / di-jet Sivers effect
  measurements
• FMS-FMS near-side correlations sensitive to
  transversity
• FMS-TPC key step toward future transverse spin
  ? jet study

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First look at jet-like events with the STAR FMS

• Comparisons of the jet profile and effective mass
  in data vs. PYTHIA GEANT simulations
• If we find
• AN for jets must arise from Sivers effect
• Azimuthal variation in jet shape must arise from
  Collins effect

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Future transverse spin forward g mid-rapidity
jet
Bacchetta et al., PRL 99, 212002

• Conventional calculations predict the asymmetry
  to have the same sign in SIDIS and ?jet
• Calculations that account for the repulsive
  interactions between like color charges predict
  opposite sign
• Critical test of our basic theoretical
  understanding

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Conclusions

• STAR is making significant contributions to three
  poorly constrained pieces of the spin puzzle
• Gluon polarization
• Flavor-separated quark and anti-quark
  polarizations
• Parton orbital motion and transversity
• STAR in embarking on precision explorations of
  these phenomena through correlation measurements
• STAR is generating a wealth of new data regarding
  the spin structure of the proton

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Other global analyses
x?g(x) at Q2 10 GeV2

• There have been many global analyses of the
  polarized DIS data

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Jethadron correlations at NLOfrom de Florian,
arXiv0904.4402

• NLO calculations show strong correlation between
  the real x and z values and LO estimates
• NLO calculations verify the increased sensitivity
  of ALL(p)
• Typical scale dependence 20 of calculated ALL
  value

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Di-jets and ?g(x)
2005 preliminary di-jet distributions
Mass
Rapidity
cos(?)
Ratio

• Di-jets provide direct access to parton
  kinematics at LO

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Full set of DSSV polarized distributions
de Florian et al, PRL 101, 072001
and arXiv0904.3821
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Sivers di-jet measurementPRL 99, 142003

• Observed asymmetries are an order of magnitude
  smaller than seen in semi-inclusive DIS by HERMES
• Detailed cancellations of initial vs. final state
  effects and u vs. d quark effects?

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Current status on transversity
Anselmino et al, arXiv0812.4366

• Global analysis combining Collins effect
  measurements in SIDIS from HERMES and COMPASS
  with measurements of the Collins fragmentation
  function by BELLE