Probing the Spin Structure of the Proton at

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Probing the Spin Structure of the Proton at

• J. Sowinski
• Indiana University
• For the STAR Collaboration

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Collaboration

• RHIC and STAR
• STAR results both transverse and longitudinal
• Future measurements

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Many special devices in RHIC to generate,
preserve and measure polarization
Snakes
Development runs
First devoted physics run
Lum. recorded.at STAR
Long prod run 9
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Detector
Lum. Monitor Local Polarim.
Beam-Beam Counters
Special interest for spin
2lthlt 5
Triggering by all calorimeters BBC
h - ln(tan(q/2)
FMS EM Calorimeter
h0
h -1
h2.5
h2
h4
Forward Pion Detector
Endcap EM Calorimeter
1lthlt 2
-4.1lthlt -3.3
Time Projection Chamber -2lthlt 2
Solenoidal Magnetic Field 5kG
2003
2004
2005
2008
Tracking
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Next STAR Transverse Results and Prospects for
the Future

• Summary
• STARs large solid angle coverage allows
  detection of correlations between particles,
  jets, two jets etc
• Significant new data sets at 200 GeV taken in Run
  9
• First 500 GeV collisions in Run 9

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The First Spin Results from Transverse Program
Confirmed in following measurements

• Runs 2-3 with Forward Pion Detector (FPD)
• Transverse SSA are Large!
• Run 6
• Sivers effect, Collins and Twist 3 all nominally
  describe data
• SSA for h even larger
• Run 8
• First early data with FMS confirms previous
  results

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STAR p0 data pT dependence does not fall as pQCD
calcs predict
STAR BRAHMS data are very similar to E704 at
1/10 cm Energy
What is connection to low energy physics?
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Both Sivers and Collins effects promise access
to interesting physics
Sivers mechanism Transverse motion (orbital
angular momentum) wrt proton spin ISI and FSI
deflect jets
Collins mechanism Polarization of incident
quark (transversity) transferred to scattered
quark which self analyzes in spin dependent
fragmentation
Of interest in both forward and mid-rapidity
regions!
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Spin Transfer to Determine Transversity dq
pQCD predicts the transverse polarization of
quarks is preserved in forward scattering -
spin transfer parameter dTT

• How might we measure dq the polarization of
  scattered q?
• Fragmentation can be spin dependent!
• Collins functions
• Interference functions
• Leading hadron preference to one side of jet wrt
  s x pjet
• Preference for orientation of two leading hadrons
  wrt s x pjet

Collins, Heppelman et al.
Determined in ee-. experiment from Belle 10
effects seen
Measured jet asymmetry dq x dTT x e(z)
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Sivers with di-Jets at Mid-rapidity

• Initially thought effect could be large and give
  sensitivity to q and g orbital angular momentum

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Sivers with di-Jets at Mid-rapidity

• Initially thought could be large and give
  sensitivity to q and g orbital angular momentum

PRL 99 (2007) 142003 Observe small SSA Much
smaller than Sivers effects observed in SIDIS
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Non-Universality in Sivers Functions

• SSA requires interference between amplitudes
• FSI have one sign (SIDIS)
• ISI have opposite sign (Drell Yan)
• Di-jets have both - cancellation

Forward g-jet also has only ISI Important
upcoming FMS meas.
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Forward Correlation Studies Have Begun with 2008
Data
STAR Preliminary
First large xF J/? measurement at a collider
Correlation of particles in jet cone
Triple clusters to construct the w
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Including Correlations within and between jets
pp-gt p0hX
Forward mid rapidity correlations FMS p0 TPC
h-
Forward correlations Two FMS p0s
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• Transverse Summary
• Large inclusive p0 asymmetries at forward
  rapidity observed in first runs and now
  reproduced with higher precision and in h meson
• A Sivers effect would indicate orbital angular
  momentum
• A Collins effect would provide sensitivity to
  transversity
• Origin of large SSA to be illuminated via future
  correlation measurements with FMS and STARs
  large W coverage
• Investigate extension to 500 GeV (some run 9
  data)
• Sivers in g-jet and Drell-Yan should be opposite
  in sign to SIDIS future measurement with FMS

Next Results on DG
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STAR inclusive p0 ALL at various rapidities
? lt 0.95
1 lt ? lt 2
? 3.2, 3.7

• Run 6 we measured ALL for inclusive p0 for three
  different rapidity regions
• Mid-rapidity result excludes large gluon
  polarization scenarios
• While statistics similar, signal in calculations
  decreases with h
• Forward rapidity p0 prod. baseline for future ?
  and ?-jet measurements
• It remains important to confirm results in
  multiple channels

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Correlations at mid-rapidity used in DG program
Trigger on jet and analyze awayside charged pions
to avoid trigger bias
NLO calculations indicate that LO reconstructed z
here, and x1, x2 in di-jets, are good
approximations to NLO quantities
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To date inclusive jets have been the work horse
at
2003-4 Disfavor extreme PDFs offered as fix to
proton spin puzzle PRL 97, 252001 2005 data
add strong contraints on large positive DG for
0.02ltxlt0.3 PRL 100, 232003 2006 GRSV-std (DIS
data best fit) now upper limit. Constrains
large neg. values
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2006 Preliminary Results
STAR incl. jets
Existing data, STAR and others, have placed
strong constraints on DG
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Additional ALL predictions for 2005
ALL
Implications for many previous PDFs
Small range is allowed by current measurements
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Global fit D. de Florian, R. Sassot, M. Stratmann
and W. Vogelsang arXiv0804.0422 hep-ph
Significant constraints in RHIC range
0.05ltxlt0.2 Uncertainty at low x prevents a
constraint on the full integral DG Shape Dg(x)
and low x behavior clearly important
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Message for new measurements
x?g(x) at Q2 10 GeV2
nodes, large contributions at low x are not ruled
out
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• DG Results Summary
• Large contributions to DG for xgt0.05 unlikely
• Full integral not constrained due to uncertain
  low x contribution
• Shape Dg(x) important

Next Future Directions to constrain Dg(x)
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PRL 100, 232003
Increased statistics at high pT just begin to
separate large x behavior from wide integration
range at low pT, but also complicated with
subprocess mix
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Inclusive Jet Projections

• Accumulated FoM at 200 GeV from run 9 was 1/3
  that assumed in fig. below
• Substantial future running at 500 GeV gives
  sensitivity to lower x
• Higher vs at same pT gives lower xg, lower xq and
  hence less q polarization, but better statistical
  precision.

Projected statistics based on measured yields and
sys. error limits
xT 2 pT / vs
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Spin measurements, ALL Dg/g
Negative polarizations evolve toward 0 with incr.
pT
Need to measure small gluon polarization at low x
2 (GeV/c)2
4 (GeV/c)2
10 (GeV/c)2
40 (GeV/c)2
100 (GeV/c)2
Gehrmann and Stirling - C Small contribution at
large x Large contribution at low x Dg/g(x0.02)
0.04 for high scales
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Di Jets analyzed in 2005 data
M
Pythia based full detector MC vs. 2005 data

• Di-jets require large coincident solid angle
• High yields allow near triple differential
  distributions
• 
  dM, dh, dcos(?)
• Select kinematics for xg dependence
• Select kinematics for valence quarks and
  favorable aLL

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200 GeV projections 50 pb-1 P60
Two body kinematics Left two panels same ?1?2
(x1/x2) but different ?1-?2 (cos?) so smaller
ALL in upper
Same M X0.06 X0.16
Each panel gives an x dependence eg. X0.08,
0.10, 0.13 dDg/g0.02-0.03
Run 9 1/3rd FoM expect 2x error bars shown
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500 GeV projections 300 pb-1 P70

• Access to lower x
• Asymmetries smaller than at 200 GeV
• But uncertainties smaller as well
• No significant data so far

De Florian, Frixione, Signer and Vogelsang NPB
539 (1999) 455 and PC for present calc.
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Direct photon jet from qg Compton 90 from qg
process
g/p0 ratio at mid-rapidity -challenging
backgrounds from inclusive hadronic channels
LO estimate of statistical errors for 50 pb-1 at
200 GeV with 100 eff. and no background and pT gt
10 GeV. More stats at lower pT but more BG
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• DG Future measurements
• Significant step in statistics at 200 GeV with
  run 9
• Enhanced statistics for inclusives
• Shape Dg(x) to be constrained by di-jet and g-jet
  correlations
• Lower x from 500 GeV and forward detectors

Next The Polarized Sea
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Flavor Asymmetry of the Sea
NMC and

• Gottfried Sum Rule in DIS
• DIS on nuclei
• SIDIS
• Drell-Yan
• Quantitative calculations of Pauli blocking not
  conclusive
• Non-perturbative processes seem to be needed in
  generating the sea

Phys.Rev.Lett. 80 (1998) 3715
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E866 Results
Polarized q Flavor Asymmetry

• pQCD motivated models predict
• Du(x)-Dd(x)dx d(x)-u(x)dx
• Chiral motivated models tend to disagree

B. Dressler et al., Chiral Quark Soliton Model
Predictions
D. De Florian et al. Phys.Rev.D80
034030,2009 Global fit
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W(-) Production in p-p at ? s 500 GeV/c2

• V-A coupling
• only LH u and RH d couple to W
• Likewise LH d and RH u to W -
• Only LH Ws produced
• Neutrino helicity gives preferential
  directionality in decay

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Parity violating single spin asymmetry
AL (Helicity flip in one beam while averaging
over other)
_

Allows kinematic separation especially for W- in
EEMC
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Ws at STAR (mid-rapidity)
Run9 W Algorithm Simulation Results
In preparation for analysis of the 500 GeV data
from run 9, STAR has been studying the
reconstruction of the W.
The simulations use full detector response and
realistic QCD background.
The main source of background is hadrons so good
e/h separation is necessary.
The current analysis uses a combination of
tracking, shower shape, near side isolation cuts,
far side isolation cuts, and event shape cuts.
A goal of run 9 is to observe this Jacobian peak.
Analysis is ongoing.
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W Detection at STAR

• Need to detect e e- and measure pT
• Need to find in huge hadronic background
• Need to separate e and e- charge

Large solid angle of STAR
h
EM Calorimetry
e
Forward GEM Tracker
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Projections Sensitivity 500 GeV 300 pb-1

• Realistic BG subtraction
• Recent PDFs represent current allowed Du/Dd
  range
• Dd and Du (W -) isolated in forward region
• Dd and Du (W)sensitivity spread over h

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• Conclusions
• RHIC run 9
• New level of statistics at 200 GeV
• First 500 GeV collisions
• STAR transverse results
• Surprisingly large SSA for forward incl. p0 and h
• Led to predictions for new measurements based on
  QCD color interactions
• Sivers and Collins effects provide windows to
  orbital angular momentum and transversity
  (respectively)
• STAR has made important constraints on the gluon
  spin contribution to that of the proton
• Importance of x dependence shape and low x for DG
• 2009 and future Di-jets and g-jets to address
  these
• W production offers opportunities to constrain
  the flavor asymmetry of the anti-quarks

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Backup slides
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Forward g jet at mid-rapidity gives access to
few 10-3 lt x lt few 10-2
Narrow fiducial volume and Eg gt 35 GeV
gives sufficient rates
Rest of FMS as isol. veto gives S/B 21
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Model for high pT scattering in pp collisions
p
p
Jets and p0s
Our tool for determining the spin of partons in
the proton But a possibly bigger question How
well and when does this all work as a precise
quantitative tool?
Assumptions Asymptotic freedom Factorization Univ
ersality Evolution
Partonic Process Spin Dependence Calc. NLO pQCD
Fragmentation Function - measured
Parton Distribution Functions
g(x,Q2),q(x,Q2) - measured
Partonic Process Cross Section Calc. NLO pQCD