hard qcd in pp collisions at rhic

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 1

Hard QCD in pp Collisions at RHIC

Carl A. GagliardiTexas A&M University

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

ECT* Workshop onHard QCD with Antiprotons at GSI FAIR


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


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


Mid-rapidity π0 production at RHIC

• 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

PRL 91, 241803


Forward π0 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 √s, in addition to pT

√s=23.3GeV √s=52.8GeV

xF xF

Ed3 d

p3 [b/

Ge V

3 ]

Ed3 d

p3 [ b/

GeV

3 ]

NLO calculations with different

scales:

pT and pT/2

Data-pQCD differences

at pT=1.5GeV


Forward pp π0 + X cross sections at 200 GeV

NLO pQCD calculations by Vogelsang, et al.

PRL 97, 152302STARSTAR

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.


Mid-rapidity protons and charged pionsPLB 637, 161STARSTAR

• 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


Forward rapidity π, K, p

• 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

BRAHMS

PRL 98, 252001


From “tests” to tools

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

de Florian et al, PRD 75, 114010


Kaon fragmentation functions

• Also introducing important new constraints for kaon fragmentation functions

de Florian et al, PRD 75, 114010


Proton fragmentation functions

• 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”

de Florian et al,arXiv:0707.1506


What about “fundamental objects”?

• The direct photon yield is well described by pQCD

PRL 98, 012002


Jets

STARSTAR 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


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

World DIS

database with

DGLAP fits

All fixed-target data

World Data on g1p as of 2005


Origin of the proton spin?Polarized DIS: 0.2~0.3 Poorly Constrained

sdusdu

gz

qz

pz LLGS

21

21

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

Leader et al, hep-ph/0612360ΔG = 0.13 ± 0.16ΔG ~ 0.006ΔG = -0.20 ± 0.41


RHIC: the world’s 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


Inclusive ALL measurements (0, ±, and jets)

LLba

baLL a

ffffA ˆ

f: polarized parton distribution functions

cos

For most RHIC kinematics, gg and qg dominate, making ALL sensitive to gluon polarization.

10 20 30 pT(GeV)


Predicted sensitivity for different ΔG scenarios

• 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

-1<<2 Inclusive o -1<<2 Inclusive

-1<<2 Inclusive jet -1<<2 Inclusive

Sampled x range for inclusive jets

Calculations by W. Vogelsang


STAR jets from Runs 3+4PRL 97, 252001

Gluon polarization is not “really big”(GRSV-max: CL ~ 0.02)

STARSTAR


π+

Charged pions from Run 5

π-

STARSTAR


STAR neutral pions from Run 5

z Mean ratio of 0 pT to Jet pT

• ALL disfavors large (positive) gluon polarization• Energetic π0 carry a significant fraction of the total

transverse momentum of their associated jet

π0

STARSTAR


PHENIX neutral pions from Run 5arXiv: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


STAR jets from Run 5

g = g (max)g = -g (min) g = 0GRSV-STD

2005 STAR preliminary

STARSTAR

• 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


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


Single-spin asymmetries at forward rapidity

• 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

PRL 92, 171801STARSTAR


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 Sivers

Transverse momentum dependent distributions exist


J. Balewski (IUCF)Sivers effect in di-jet production

180open

1

2

spin

di-jetbisector

kTx

> 1

80 f

or

k Tx >

0

Sivers effect:

• 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


Sivers di-jet measurementarXiv:0705.4629

• 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.

STARSTAR

Mostly: +z beam quark −z beam gluon


Sivers di-jet measurementarXiv: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?

STARSTAR


BRAHMS forward rapidity pion measurements at 200 GeV

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

BRAHMS2.3 deg (~3.4)4 deg (~3)


Additional BRAHMS forward rapidity results at 200 GeV

BRAHMS2.3 deg (~3.4)

• Charged kaon AN both positive; slightly smaller or comparable to π+

• Antiprotons show a sizable positive AN

• Protons show little asymmetry


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


Inclusive forward asymmetry, AN

STARSTAR

Kouvaris et al, hep-ph/0609238

Decreasing ηIncreasing pT

The data show exactly the opposite behavior

Theory Data


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

STARSTAR


Separating Sivers and Collins effects

Collins mechanism: asymmetry in the forward jet fragmentation

Sivers mechanism: asymmetry in the forward jet or γ production

SP kT,q

p

p

SP

p

p

Sq kT,πSensitive to proton spin – parton transverse motion correlations

Sensitive to transversity

• 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


STAR Forward Meson Spectrometer

• Pb glass calorimeter covering 2.5 < η < 4• Detect direct photons, jets, di-jets, ….. in addition to π0, for

kinematics where π0 single-spin asymmetries are known to be large

South half of FMS array during assembly


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


The next few years: ΔG(x)

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

2||tanh|cos|

2ln

21

1

1

43*

43

2

1

21

432

431

43

43

xxy

sxxM

epeps

x

epeps

x 2005 Data Simulation

STAR (pre-)preliminary


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)


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


Further future: spin measurements in the RHIC II era

Direct measurement of the Δs, Δs contributions in charm-tagged W boson production

Sivers asymmetry AN

for Drell-Yan di-muon and di-electron production


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!


Subtleties of Jet Analysis: Trigger Bias

EMF Electromagnetic Energy Fraction ET(EMC) / total jet pT

Shaded bands = simulations

Fake jets from upstream

beam bkgd.

Conclude:

Cut out jets at very high or very low EMF

Use simulations to estimate syst. errors from 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

hard qcd in pp collisions at rhic

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