Transcript Hard QCD in pp Collisions at RHIC

Hard QCD in pp Collisions at
RHIC
ECT* Workshop on
Hard QCD with Antiprotons at GSI FAIR
Carl A. Gagliardi
Texas A&M University
Outline
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Unpolarized pp collisions
Longitudinally polarized pp collisions
Transversely polarized pp collisions
Looking ahead
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Unpolarized pp collisions at RHIC
• “Baseline” physics!
• pp collisions provide an essential baseline to determine
what’s 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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Mid-rapidity π0 production at RHIC
PRL 91, 241803
• Data favor the KKP fragmentation
function over Kretzer
• Mid-rapidity π0 cross section at
200 GeV is well described by
pQCD over 8 orders of magnitude
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Forward π0 production at ISR energies
√s=23.3GeV
√s=52.8GeV
Data-pQCD
differences
at pT=1.5GeV
q=5o
q=10o
q=15o
NLO
calculations
with different
scales:
pT and pT/2
q=53o
q=22o
xF
xF
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 √s, in addition to pT
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Forward pp  π0 + X cross sections at 200 GeV
STAR PRL 97, 152302
The error bars are statistical
plus point-to-point systematic
Consistent with NLO pQCD
calculations at 3.3 < η < 4.0
Data at low pT trend from KKP
fragmentation functions toward
Kretzer.
NLO pQCD calculations by Vogelsang, et al.
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Mid-rapidity protons and charged pions
STAR PLB 637, 161
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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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Forward rapidity π, K, p
BRAHMS
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 p+pbar yield by a factor of ~10
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Kaon fragmentation functions
de Florian et al, PRD 75, 114010
• Also introducing important new constraints for kaon
fragmentation functions
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Proton fragmentation functions
de Florian et al,
arXiv:0707.1506
• 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”
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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What about “fundamental objects”?
PRL 98, 012002
• The direct photon yield is well described by pQCD
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Jets
STAR
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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Partonic structure of the proton
World Data on g1p as of 2005
World DIS
database
with
DGLAP
fits
All
fixedtarget
data
• 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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Origin of the proton spin?
Polarized DIS: 0.2~0.3
Poorly Constrained
1 1
 S  = =   G   Lqz    Lgz 
2 2
 = u  d  s  u  d  s
p
z
Leader et al, hepph/0612360
ΔG = 0.13 ± 0.16
ΔG ~ 0.006
ΔG = -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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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RHIC: the world’s first polarized hadron collider
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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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Inclusive ALL measurements (0, ±, and jets)
       f a f b
ALL =  

aˆ LL

 
f a fb
f: polarized parton distribution functions
cosq 
For most RHIC kinematics, gg and qg
dominate, making ALL sensitive to
gluon polarization.
10
20
30 pT(GeV)
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Predicted sensitivity for different ΔG scenarios
-1<<2 Inclusive o
-1<<2 Inclusive 
Calculations by
W. Vogelsang
Sampled x range
for inclusive jets
-1<<2 Inclusive jet
-1<<2 Inclusive 
• Jets (STAR) and π0 (PHENIX and STAR) easier
• γ and ALL(π+) - ALL(π-) sensitive to the sign of ΔG
• Inclusive measurements average over broad x ranges
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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STAR jets from Runs 3+4
PRL 97, 252001
STAR
Gluon polarization is not “really big”
(GRSV-max: CL ~ 0.02)
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Charged pions from Run 5
π+
πSTAR
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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STAR neutral pions from Run 5
STAR
z  Mean ratio of 0 pT to Jet pT
π0
• ALL disfavors large (positive) gluon polarization
• Energetic π0 carry a significant fraction of the total
transverse momentum of their associated jet
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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PHENIX neutral pions from Run 5
arXiv:0704.3599
• χ2 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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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STAR
g = g (max)
g = -g (min)
g = 0
GRSV-STD
STAR jets from Run 5
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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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PHENIX π0 from Run 6
• Preliminary Run 6 χ2 comparison, including statistical
uncertainties only (syst. are expected to be small)
• Its looking like ΔG is quite small or negative
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Single-spin asymmetries at forward rapidity
STAR PRL 92, 171801
• Large single-spin asymmetries at CM energies of 20 and 200 GeV
• Weren’t 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 PDF’s (Sivers) vs.
fragmentation fcns. (Collins)
HERMES results  both non-zero.
+ vs. – differences suggest opposite
signs for u and d quarks.
Collins
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
Sivers
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Sivers effect in di-jet production
J. Balewski (IUCF)
• 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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
spin
Sivers effect:
1
open 180
kTx
di-jet
bisector
2
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Sivers di-jet measurement
STAR
arXiv:0705.4629
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.
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Sivers di-jet measurement
STAR
arXiv:0705.4629
• 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?
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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BRAHMS forward rapidity pion measurements
at 200 GeV
BRAHMS
4 deg (~3)
2.3 deg (~3.4)
• Sign dependence of charged pion asymmetries seen in FNAL E704
persists to 200 GeV
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Additional BRAHMS forward rapidity results
at 200 GeV
BRAHMS
2.3 deg (~3.4)
• Charged kaon AN both positive; slightly smaller or comparable to π+
• Antiprotons show a sizable positive AN
• Protons show little asymmetry
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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BRAHMS results at 62.4 GeV
BRAHMS
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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Inclusive forward o asymmetry, AN
Data
Theory
Kouvaris et al, hep-ph/0609238
Decreasing η
Increasing pT
STAR
The data show exactly the opposite behavior
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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STAR
AN(pT) in xF-bins
• Combined data from three runs
at <η>=3.3, 3.7 and 4.0
• In each xF bin, <xF> 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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Separating Sivers and Collins effects
Sivers mechanism: asymmetry
in the forward jet or γ production
SP
Collins mechanism: asymmetry
in the forward jet fragmentation
SP
kT,q
p
p
p
Sensitive to proton spin –
parton transverse motion
correlations
p
Sensitive to
transversity
Sq
kT,π
• Need to go beyond inclusive π0 to measurements of jets or direct γ
• Have some Run 6 data under analysis
• Will study in Run 8 with the STAR Forward Meson Spectrometer
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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STAR Forward Meson Spectrometer
South half of FMS
array during assembly
• Pb glass calorimeter covering 2.5 < η < 4
• Detect direct photons, jets, di-jets, ….. in addition to π0, for
0 single-spin asymmetries are known to be large
kinematics
where
π
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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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 ΔG(x) has a node
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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The next few years: ΔG(x)
STAR (pre-)preliminary


1
p3e3  p4e4
s
1
x2 =
p3e 3  p4e 4
s
x1 =

2005 Data
Simulation

M = x1 x2 s
y=
1 x1 3  4
ln =
2 x2
2
|   |
| cosq * |= tanh 3 4
2
• Di-jets access LO parton kinematics
• Involve a mixture of qq, qg, and gg scattering
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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γ + jet events
• γ + 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 ΔG(x)
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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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
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Further future: spin measurements in the RHIC II era
Sivers asymmetry AN
for Drell-Yan di-muon and
di-electron production
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
Direct measurement of the
Δs, Δs contributions in
charm-tagged W boson
production
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Conclusion
• pp collisions at RHIC are providing important new inputs
for our understanding of fragmentation functions
• The world’s first polarized hadron collider is generating a
wealth of new data regarding the spin structure of the
proton
• We’ve only barely started!
Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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Carl Gagliardi – ECT* Hard QCD at GSI Workshop
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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:
Shaded bands
= simulations
Fake jets from
upstream
beam bkgd.
 Electromagnetic
Energy
Fraction   ET(EMC) / total jet pT
Carl GagliardiEMF
– ECT*
Hard QCD at GSI
Workshop
 Cut out jets at
very high or very
low EMF
 Use simulations
to estimate syst.
errors from trigger
bias
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