Exploring the Spin Structure of the Proton with TwoBody Partonic Scattering at RHIC

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Exploring the Spin Structure of the Proton with
Two-Body Partonic Scattering at RHIC

• J. Sowinski

For the
Collaboration
Few Body 2006 8/24/06
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Where does the protons spin come from?
p is made of 2 u and 1d quark
S ½ S Sq
Explains magnetic moment of baryon octet
p
BUT partons have an x distribution and there are
sea quarks and gluons
Check via electron scattering and find quarks
carry only 1/3 of the protons spin!
Sz ½ ½ DS DG Lzq Lzg
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Parton Distribution Functions
Gluons carry 1/2 the momentum (mass)!
Maybe we shouldnt be surprised that quarks carry
only 1/3 of protons spin
DG is poorly constrained, even solutions with
zero crossing allowed
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DG via partonic scattering from a gluon
Know from DIS
Measure
g-jet coinc. rare

A P 3P 3a
g
part
LL
LL
pQCD
Jets and p0s
DG
Prefer

• Dominant reaction mechanism
• Experimentally clean reaction mechanism
• Large a
• But jet and p0 rates are sufficient to give
  significant DG const. in first RHIC pol. p data

Heavy flavor rare

LL
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The Relativistic Heavy Ion Collider
4 km circ. Collider
The first polarized p-p collider!
PHOBOS

• Heavy ions
• Au-Au
• Lighter ions
• Asymmetric d-Au

• 4 detectors
• STAR
• PHENIX
• PHOBOS
• Brahms
• pp2pp (p-p only)

Retired
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Dramatic Improvements in Polarized Beam
Performance
2003 ? 2006 ? gt 2 orders of magnitude
improvement in FOM P 4L relevant to 2-spin
asymmetries!
Factor 5--6 remains to reach enhanced design
goals
STAR ? s 200 GeV pp Sampled Luminosities
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The STAR Detector at RHIC
At the heart of STAR is the worlds largest Time
Projection Chamber

• STAR Detector
• Large solid angle
• Not hermetic
• Tracking in 5kG field
• EM Calorimetry
• Slow DAQ (100Hz)
• Sophisiticated triggers

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Detector
Lum. Monitor Local Polarim.
Triggering
Beam-Beam Counters
2lt?lt 5
h - ln(tan(q/2)
h0
h -1
h2
Triggering
Endcap EM Calorimeter
Forward Pion Detector
1lt?lt 2
-4.1lt?lt -3.3
Time Projection Chamber -2lt?lt 2
Solenoidal Magnetic Field 5kG
Tracking
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What is a jet?
Use Monte Carlo to correct data for comparison to
theory
(Resolution, trigger, efficiency, fragmentation )

• Midpoint Cone Algorithm
• Add 4 momenta of tracks and towers in cone around
  seed
• R 0.4 (h , f) year lt 2006
• Split and merge for stable groups

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2003 2004 Results
Jet Shape

• ?(?r) Fraction of jet pT in sub-cone ?r
• Study of trigger bias
• Study of data/MC agreement
• High Tower trigger
• Bias decreases with pT

Cross Section Correction Factors

• MinBias correction 1
• Corrections (1/c(pT) can be large for High Tower
  data

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First inclusive jet cross section result at RHIC
2004 pp run

• Sampled luminosity 0.16 pb-1
• Good agreement between minbias and high tower
  data
• Good agreement with NLO over 7 orders of
  magnitude slope
• Good agreement with NLO magnitude within
  systematic uncertainty
• Error bars Statistical uncertainty from data
• Systematic error band
• Leading systematic uncertainty
• 10 E-scale uncertainty ? 50 uncertainty on
  yield
• Out of cone hadronizaton and underlying event
  25 corr. not shown

hep-ex0608030
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First ALL Measurement for Inclusive Jet Production

• 2003 (pol.0.3) 2004 (pol. 0.4) total 0.4
  pb-1
• Total systematic uncertainty 0.01
• Backgrounds
• Relative Luminosity
• Residual transverse asymmetries
• Beam Polarization
• Trigger Bias

hep-ex0608030
Submitted for publication
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Current Constraints on ?G
Photon-gluon fusion results
COMPASS, HERMES, SMC photon-gluon fusion studies
? comparable ?G constraints to 20034 STAR jets
and 2005 PHENIX ? 0 ALL
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Projections from Collected Data

• 2005 Data
• Jet patch triggers
• Enhanced EM calorimeter coverage

• 2006 Data
• Software triggers
• Full EM calorimeter coverage -1lthlt2 including
  trigger
• DiJets
• Direct g-jet sample

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Next Step is to Explore Dg(x)

• Exploit 2 body kinematics
• Detect g and jet in coinc.
• Measure ujet, Eg and ug
• Extract x1, x2 and u
• Assume larger of x1 and x2 xquark
• Assume lesser xgluon
• Make cut that one x gt 0.2

• Large data sets at 200 and 500 GeV
• 500 GeV gt low x
• Overlap gives same x with different pT to check
  scaling
• Di-Jets
• Similar kinematics
• Less selective for gluons
• Lower sensitivity but larger cross section than
  g-jets

Large coincident solid angle is crucial
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Conclusions

• RHIC has made tremendous progress in delivering
  polarized protons over past few years
• Initial inclusive jet ALL results are providing
  significant constraints on DG
• Much better jet statistics are already in hand
  from 2005 and 2006 data
• Future studies with di-Jets and g-jet coinc. are
  expected to probe the shape, Dg(x)