Transverse%20Spin%20Studies%20with%20STAR%20at%20RHIC

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Transverse Spin Studies with STAR at RHIC

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

• Outline
• Previous results
• Separating the underlying effects
• Current status
• Future plans

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STAR detector layout
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Transverse spin runs at STAR

• RHIC Run 2 (01-02)
• First polarized pp run
• Used prototype Endcap EMC for forward p0
  detection
• 0.15 pb-1 with Pbeam 16
• AN measurement at lt?gt 3.8
• Run 3 (03)
• Forward p0 detectors (FPD) installed
• 0.25 pb-1 with Pbeam 27
• Measurements at lt?gt 3.7 and lt?gt 4.0
• Run 5 (05)
• 25 hours of transverse data during June, 05
• 0.07 pb-1 and Pbeam 46
• Run 6 (06)
• Started transverse data taking on April 7th
• FPD and full BarrelEndcap EMCs installed
• Measurements underway at both forward and
  mid-rapidities

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First AN Measurement at STAR
Similar to result from E704 experiment (vs20
GeV, 0.5 lt pT lt 2.0 GeV/c)
PRL 92, 171801 (2004)
Can be described by several models

• Sivers spin and k? correlation in parton
  distribution functions (initial state)
• Collins spin and k? correlation in fragmentation
  function (final state)
• Qiu and Sterman (initial state) / Koike (final
  state) twist-3 pQCD calculations, multi-parton
  correlations

vs200 GeV, lt?gt 3.8
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Forward p0 production at ISR energies
Bourrely and Soffer, EPJ C36, 371
NLO pQCD calculations underpredict the data
at low ?s from ISR Ratio appears to be a
function of angle and vs, in addition to pT
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pp ? p0 X cross sections at 200 GeV
nucl-ex/0602011

• The error bars are statistical plus
  point-to-point systematic
• Consistent with NLO pQCD calculations at 3.3 lt ?
  lt 4.0
• Data at low pT trend from KKP fragmentation
  functions toward Kretzer. PHENIX observed
  similar behavior at mid-rapidity.

NLO pQCD calculations by Vogelsang, et al.
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Inclusive p0 AN from runs 35
6s from 0 for xFgt0.4

• Sizable asymmetries for xF gt 0.4
• Back angle data consistent with AN 0

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Combined AN(pT) for xF gt 0.4 from runs 35
Online calibration of CNI polarimeter
Data are consistent with the 1/pT expectation of
pQCD
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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
Need to go beyond p0 detection to jets and direct
photons
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Event yield with FPD when N? gt 1

• 16 arises from jet-like events with 3
  observed photons
• Also expect ?/p0 gt 1 for single photons with xF gt
  0.5
• Need to separate these contributions reliably

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FPD ? FPD for run 6
Runs 3-5
West end of the STAR interaction region
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FPD ? FPD for run 6
Run 6
West end of the STAR interaction region
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FPD ? FPD for run 6
Run 6
West end of the STAR interaction region
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FPD enables jet and direct photon detection

• Trigger on EM energy in small cells
• Large enough to integrate over a jet cone
• Large enough for direct photon isolation cuts

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Projected sensitivity
L.C. Bland, hep-ex/0602012

• PYTHIA simulations
• Trigger on small cells sum over entire side
• Vector sum of photon momenta reproduces most
  forward hard-scattered parton
• Expect 45s effect for 5 pb-1 if observed p0 AN
  is due to Sivers effect

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Back-to-back di-jets access to gluon Sivers
function
Measurements near mid-rapidity with STAR search
for spin-dependent deviation from back-to-back
alignment
gt 7 GeV trigger jet gt 4 GeV away side jet
D. Boer and W. Vogelsang, Phys. Rev. D 69, 094025
(2004)
5 pb-1 measurement will be sensitive at level of
model predictions
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Level-2 trigger monitoring output after 0.84 pb-1
Occasional high-order bit problem in 1 of 5520
EMC towers

• Clear back-to-back di-jet signature
• 6-fold symmetry from Level-0 (single jet) trigger
  condition
• Level-2 filter is insensitive to the granularity

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Level-2 trigger monitoring output after 0.84 pb-1

• Clear back-to-back di-jet signature
• 6-fold symmetry from Level-0 (single jet) trigger
  condition
• Level-2 filter is insensitive to the granularity

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Run 7 and beyond
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Run 7 and beyond
STAR Forward Meson Spectrometer
Addition of FMS to STAR provides nearly
continuous EMC from -1lt?lt4
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Conclusions

• STAR finds sizable AN for inclusive p0 at forward
  rapidity
• Separating the Sivers contribution from the
  Collins contribution requires going beyond p0
  measurements
• Measurements are underway at both forward and
  mid-rapidity
• Stay tuned!

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The STAR Collaboration
U.S. Labs Argonne, Lawrence Berkeley, and
Brookhaven National Labs U.S. Universities
UC Berkeley, UC Davis, UCLA, Caltech,
Carnegie Mellon, Creighton, Indiana, Kent State,
MIT, MSU, CCNY, Ohio State, Penn State, Purdue,
Rice, Texas AM, UT Austin, Washington, Wayne
State, Valparaiso, Yale Brazil
Universidade de Sao Paolo China IHEP -
Beijing, IPP - Wuhan, USTC, Tsinghua, SINAP, IMP
Lanzhou Croatia Zagreb University Czech
Republic Nuclear Physics
Institute England University of Birmingham
France Institut de Recherches Subatomiques
Strasbourg, SUBATECH - Nantes Germany Max
Planck Institute Munich University of
Frankfurt India Bhubaneswar, Jammu, IIT-Mumbai,
Panjab, Rajasthan, VECC Netherlands NIKHEF/Utrec
ht Poland Warsaw University of
Technology Russia MEPHI Moscow, LPP/LHE
JINR Dubna, IHEP Protvino South
Korea Pusan National University
Switzerland University of Bern
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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

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Forward p0 production at a hadron collider
p0
Ep
qq
EN
qp
N
xgp
N
xqp
qg
EN
(collinear approx.)

• Large rapidity p production (?4) probes
  asymmetric partonic collisions
• Mostly high-x quark low-x gluon
• 0.3 lt xqlt 0.7
• 0.001lt xg lt 0.1
• ltzgt nearly constant and high 0.7-0.8
• A probe of low-x gluons

NLO pQCD S. Kretzer
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xF and pT range of existing FPD data
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Separated xF and pT dependence
Similar to ISR analysis J. Singh et al., Nucl.
Phys. B140, 189 (1978).
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Single Spin AsymmetryDefinitions

• Definition
• ds?(?) differential cross section of p0 when
  incoming proton has spin up(down)

• Two measurement techniques
• Single arm calorimeter
• R relative luminosity (by BBC)
• Pbeam beam polarization
• Two arms (left-right) calorimeter
• No relative luminosity needed

positive AN more p0 going left to polarized beam