Hard QCD in pp Collisions at RHIC

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Hard QCD in pp Collisions at RHIC
ECT Workshop on Hard QCD with Antiprotons at GSI
FAIR

• Carl A. Gagliardi
• Texas AM University
• Outline

• Unpolarized pp collisions
• Longitudinally polarized pp collisions
• Transversely polarized pp collisions
• Looking ahead

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RHIC the Relativistic Heavy Ion Collider

• Search for and study the Quark-Gluon Plasma
• Explore the partonic structure of the proton
• Determine the partonic structure of nuclei

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Unpolarized pp collisions at RHIC

• Baseline physics!
• pp collisions provide an essential baseline to
  determine whats new in heavy-ion collisions
• Unpolarized pp collisions establish the
  applicability of pQCD to interpret results from
  polarized pp collisions
• Unpolarized pp collisions constrain the
  non-perturbative inputs for pQCD calculations

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Mid-rapidity p0 production at RHIC
PRL 91, 241803

• Data favor the KKP fragmentation function over
  Kretzer
• Mid-rapidity p0 cross section at 200 GeV is well
  described by pQCD over 8 orders of magnitude

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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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Forward pp ? p0 X cross sections at 200 GeV
PRL 97, 152302

• 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.

NLO pQCD calculations by Vogelsang, et al.
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Mid-rapidity protons and charged pions
PLB 637, 161

• pQCD calculations with AKK fragmentation
  functions give a reasonable description of pion
  and proton yields in elementary collisions
• Calculations with KKP significantly underestimate
  proton yields at high-pT
• Protons arise primarily from gluon fragmentation
  pions receive a large quark contribution at
  high-pT

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Forward rapidity p, K, p
PRL 98, 252001

• Charged pion and kaon yields at forward rapidity
  are described reasonably by a modified KKP
  fragmentation function
• AKK seriously misses the forward
  antiproton/proton ratio (expects 1, see 0.05
  above 2 GeV/c)
• KKP underestimates the ppbar yield by a factor
  of 10

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From tests to tools
de Florian et al, PRD 75, 114010

• RHIC data now provide important constraints for
  global analyses of pion fragmentation functions

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Kaon fragmentation functions
de Florian et al, PRD 75, 114010

• Also introducing important new constraints for
  kaon fragmentation functions

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Proton fragmentation functions
de Florian et al, arXiv0707.1506

• Mid-rapidity STAR data are the best constraint
  on the gluon fragmentation function into protons
  at large z
• Large BRAHMS forward proton to anti-proton excess
  remains an open question

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What about fundamental objects?
PRL 98, 012002

• The direct photon yield is well described by pQCD

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Jets
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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Partonic structure of the proton

• HERA data provide very strong constraints on
  unpolarized PDFs
• Much less polarized DIS data over a limited Q2
  region
• Gluon and sea-quark polarizations largely
  unconstrained by DIS

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Origin of the proton spin?
Polarized DIS 0.20.3
Poorly Constrained
Leader et al, hep-ph/0612360 ?G 0.13 0.16 ?G
0.006 ?G -0.20 0.41

• RHIC Spin program
• Longitudinal polarization Gluon polarization
  distribution
• Transverse polarization Parton orbital motion
  and transversity
• Down the road Anti-quark polarization

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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 flexibility for STAR and
  PHENIX measurements
• Billions of spin flips during a fill with
  little if any depolarization

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Inclusive ALL measurements (?0, ?, and jets)
For most RHIC kinematics, gg and qg dominate,
making ALL sensitive to gluon polarization.
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Predicted sensitivity for different ?G scenarios
Calculations by W. Vogelsang
Sampled x range for inclusive jets

• Jets (STAR) and p0 (PHENIX and STAR) easier
• ? and ALL(p) - ALL(p-) sensitive to the sign of
  ?G
• Inclusive measurements average over broad x ranges

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STAR jets from Runs 34PRL 97, 252001
Gluon polarization is not really big (GRSV-max
CL 0.02)
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Charged pions from Run 5
p
p-
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STAR neutral pions from Run 5
?z? ? Mean ratio of ?0 pT to Jet pT
p0

• ALL disfavors large (positive) gluon polarization
• Energetic p0 carry a significant fraction of the
  total transverse momentum of their associated jet

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PHENIX neutral pions from Run 5arXiv0704.3599

• ?2 from a comparison to the GRSV polarized parton
  distributions
• Uncertainties associated with GRSV functional
  form not included
• Large positive polarizations excluded large
  negative polarizations disfavored

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STAR jets from Run 5
?g g (max) ?g -g (min) ?g 0 GRSV-STD
2005 STAR preliminary

• CL from a comparison to the GRSV polarized parton
  distributions
• Uncertainties associated with GRSV functional
  form not included
• Large positive polarizations excluded large
  negative polarizations disfavored
• Uncertainties from Run 6 will be a factor of 3
  smaller at high pT

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PHENIX p0 from Run 6

• Preliminary Run 6 ?2 comparison, including
  statistical uncertainties only (syst. are
  expected to be small)
• Its looking like ?G is quite small or negative

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Single-spin asymmetries at forward rapidity
PRL 92, 171801

• Large single-spin asymmetries at CM energies of
  20 and 200 GeV
• Werent supposed to be there in naïve pQCD
• May arise from the Sivers effect, Collins effect,
  or a combination

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Transverse momentum dependent distributions exist
SIDIS can distinguish transverse motion
preferences in PDFs (Sivers) vs. fragmentation
fcns. (Collins)
HERMES results ? both non-zero. ? vs. ?
differences suggest opposite signs for u and d
quarks.
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Sivers effect in di-jet production
J. Balewski (IUCF)
Sivers effect
Sivers ON
spin
?1

• Left/right asymmetry in the kT of the partons in
  a polarized proton
• Spin dependent sideways boost to di-jets
• Requires parton orbital angular momentum

? gt 180? for kTx gt 0
di-jet bisector
kTx
?2
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Sivers di-jet measurementarXiv0705.4629
Mostly z beam quark -z beam gluon

• Measure the di-jet opening angle as a function of
  proton spin
• Both beams polarized, xa ? xb ? pseudorapidity
  dependence can distinguish q vs. g Sivers effects.

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Sivers di-jet measurementarXiv0705.4629

• 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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BRAHMS forward rapidity pion measurements at 200
GeV
2.3 deg (?3.4)
4 deg (?3)

• Sign dependence of charged pion asymmetries seen
  in FNAL E704 persists to 200 GeV

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Additional BRAHMS forward rapidity results at 200
GeV
2.3 deg (?3.4)

• Charged kaon AN both positive slightly smaller
  or comparable to p
• Antiprotons show a sizable positive AN
• Protons show little asymmetry

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BRAHMS results at 62.4 GeV
Combined results from 2.3 and 3 deg

• Lower beam energy
• Larger xF
• Very large asymmetries!

Limitation of the BRAHMS measurements Very
strong correlation between xF and pT from small
acceptance
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Inclusive forward ?? asymmetry, AN
Data
Theory
Kouvaris et al, hep-ph/0609238
Decreasing ? Increasing pT
The data show exactly the opposite behavior
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AN(pT) in xF-bins

• Combined data from three runs
• at lt?gt3.3, 3.7 and 4.0
• In each xF bin, ltxFgt does not
• significantly changes with pT

• Measured AN is not a smooth
• decreasing function of pT
• as predicted by theoretical
• models

Kouvaris et al, hep-ph/0609238
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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 inclusive p0 to measurements of
  jets or direct ?
• Have some Run 6 data under analysis
• Will study in Run 8 with the STAR Forward Meson
  Spectrometer

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STAR Forward Meson Spectrometer
South half of FMS array during assembly

• Pb glass calorimeter covering 2.5 lt ? lt 4
• Detect direct photons, jets, di-jets, .. in
  addition to p0, for kinematics where p0
  single-spin asymmetries are known to be large

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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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The next few years ?G(x)
STAR (pre-)preliminary
2005 Data Simulation

• Di-jets access LO parton kinematics
• Involve a mixture of qq, qg, and gg scattering

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? jet events

• ? jet provides very good event-by-event
  determination of the parton kinematics
• 90 of the yield arises from qg scattering
• Can choose the kinematics to maximize the
  sensitivity to ?G(x)

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Down the road anti-quark polarization

• With two polarized beams and W and W-, can
  separate u, d, u, d polarizations
• These simulations are for the PHENIX muon arms
• STAR will do this with electrons
• Need 500 GeV collisions at high luminosity, and
  upgrades to both PHENIX and STAR

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Further future spin measurements in the RHIC II
era
Direct measurement of the ?s, ?s contributions in
charm-tagged W boson production
Sivers asymmetry AN for Drell-Yan di-muon and
di-electron production
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Conclusion

• pp collisions at RHIC are providing important new
  inputs for our understanding of fragmentation
  functions
• The worlds first polarized hadron collider is
  generating a wealth of new data regarding the
  spin structure of the proton
• Weve only barely started!

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Subtleties of Jet Analysis Trigger Bias

• High Tower and Jet Patch triggers require
  substantial fraction of jet energy in neutral
  hadrons
• Trigger efficiency turns on slowly above nominal
  threshold
• Efficiency differs for quark vs. gluon jets, due
  to different fragmentation features
• Simulations reproduce measured bias well, except
  for beam background at extreme EM energy fraction

• Conclude
• Cut out jets at very high or very low EMF
• Use simulations to estimate syst. errors from
  trigger bias