Jet Production in Polarized pp Collisions at RHIC

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Jet Production in Polarized pp Collisions at RHIC

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

• Introduction
• 2005 results
• 2006 results
• Looking ahead

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Origin of the proton spin?
Polarized DIS 0.20.3
Poorly Constrained

• 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

• A primary goal of the RHIC Spin program is to
  determine the gluon polarization distribution

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Polarized pp collisions at RHIC
Partonic fractions in jet production at 200 GeV
For most RHIC kinematics, gg and qg dominate,
making ALL for jets sensitive to gluon
polarization.
0
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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

Thousands of millions
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STAR detector
E-M Calorimeter
Projection           Chamber
Time of    Flight
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Jet reconstruction in STAR
Data jets MC
jets

• Midpoint cone algorithm
• (Adapted from Tevatron II - hep-ex/0005012)
• Seed energy 0.5 GeV
• Cone radius in ?-?
• R0.4 with 0.2 lt ? lt 0.8 (2005)
• R0.7 with -0.7 lt ? lt 0.9 (2006)
• Splitting/merging fraction f0.5

GEANT
Detector
Particle
PYTHIA
Use PYTHIA GEANT to quantify detector response
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Jet production at RHIC
PRL 97, 252001

• Jet structure at 200 GeV is well understood
• Mid-rapidity jet cross section is well described
  by pQCD over 7 orders of magnitude

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2005 inclusive jets ALL systematics
ALL systematics (x 10 -3)
Reconstruction Trigger Bias 2-5 (pT dep)
Non-longitudinal Polarization 0.1-0.8 (pT dep)
Relative Luminosity 0.94
Backgrounds 0.70
Neutral Energy Fraction
pT systematic -5.4,6.7

• Reconstruction bias combination of steeply
  falling spectrum and 25 jet ET resolution
• Trigger bias non-uniform sampling of gg, qg, and
  qq processes
• Will illustrate impact on ALL with 2006 data

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2005 inclusive jets ALL
Model calcs from Jager et al, PRD 70, 034010
hep-ex arXiv0710.2048
200 GeV 0.2 lt ? lt 0.8

• Comparison to predictions within the GRSV
  framework with various input values of ?G
  demonstrate sensitivity

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Limits on ?G from 2005 jet results
hep-ex arXiv0710.2048

• Significant new constraints on ?G when compared
  to predictions derived from one global fit to DIS
  data

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

• There are many other global analyses of the
  polarized DIS data

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Additional ALL predictions for 2005

• Many predictions for ALL vs. pT
• Is the sensitivity unique to GRSV?

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Comparison to other global analyses

• The STAR data exclude a broad range of models
  that have ?G larger than that in GRSV-std
• The counterexample is GS-C, which is negative at
  large x, has a node near x 0.1, and has a large
  first moment at small x

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2006 data

• Improved figure of merit
• Luminosity 2 ? 4.7 pb-1
• Polarization 50 ? 60 (online polarization)
• Barrel EMC ? coverage 0,1 ? -1,1
• In addition
• Jet cone radius 0.4 ? 0.7
• -0.7 lt ?jet axis lt 0.9
• Neutral energy fraction lt 0.85
• Increased trigger thresholds
• Inclusion of Endcap EMC towers
• Improved tracking at large ?
• Shielding from backgrounds

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Jet reconstruction and trigger bias for 2006 data
Step 1 Use PYTHIAGEANT to estimate the pT
shift from detector jets to particle jets
Step 2 Simulate the difference in ALL between
particle and detector jets for various gluon
polarization scenarios
Particle jets Shifted detector jets Detector jets

• Maximum deviation determines ALL systematic
• Confidence level calculations take account of any
  residual

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2006 inclusive jets ALL
Model curves calculated with cone radius 0.7 and
-0.7 lt ? lt 0.9
ALL systematics (x 10 -3)
Reconstruction Trigger Bias -1,3 (pT dep)
Non-longitudinal Polarization 0.03 (pT dep)
Relative Luminosity 0.94
Backgrounds 1st bin 0.5 else 0.1
200 GeV -0.7 lt ? lt 0.9
pT systematic ?6.7

• Statistical uncertainties are 3-4 times smaller
  than in 05 data for pT gt 13 GeV/c

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Limits on ?G from 2006 jet results

• Within the GRSV framework
• GRSV-std excluded with 99 CL
• ?G lt -0.7 excluded with 90 CL

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Looking beyond inclusive ALL measurements

• 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

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DSSV first global analysis with polarized jets
de Florian et al., arXiv0804.0422 hep-ph

• 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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The next few years 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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Projected di-jet sensitivity for the 2009 run
x1 0.33 x2 0.044
Rel FOM 70
GS-C
x1 0.45 x2 0.16
x1 0.27 x2 0.27

• Assumes 50 pb-1 of 200 GeV pp collisions with 60
  polarization

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Conclusions

• STAR inclusive jet ALL measurements have
  excluded extreme gluon polarization scenarios
• Over the next few years, STAR di-jet measurements
  will provide direct information about ?g(x)
• STAR will also obtain complementary information
  about ?g(x) from ? jet
• Stay tuned!

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Double longitudinal spin asymmetry, ALL
N Spin dependent yields ( of reconstructed
jets) P Beam polarization (measured by RHIC
Polarimeter) R Relative luminosities between
different spin states
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STAR detectors for jet measurements

• Time Projection Chamber
• ?lt1.4 tracking
• Barrel EM Calorimeter
• ?lt1 triggering calorimetry
• Endcap EM Calorimeter
• 1.09lt?lt2 triggering calorimetry
• Beam-Beam Counters
• 3.4lt?lt5 triggering, luminosity, local
  polarimetry

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Other global analyses
x?g(x) at Q2 10 GeV2
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Additional ALL predictions for 2005
ALL
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Estimating the non-jet background
Data
Monte Carlo
Charged Energy Fraction
Charged Energy Fraction

• Non-jet backgrounds stand out in the data as
  single-jet events that have a small charged
  energy fraction
• Isolate the background in the ratio of mono-jet
  to di-jet events vs. charged energy fraction

Data
Charged Energy Fraction
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ALL vs. run index for 2006 data
STAR Preliminary
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False asymmetries in 2006 data
STAR Preliminary
STAR Preliminary

• Longitudinal single-spin (parity-violating)
  asymmetries are consistent with zero, as expected
  at this precision

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Inclusive jet ALL for fixed values of ?G

• Calculations by Stratmann and Vogelsang within
  the GRSV framework