Carl Gagliardi – ECT* Hard QCD at GSI Workshop 1 Hard QCD in pp Collisions at RHIC Carl A. Gagliardi Texas A&M University Outline Unpolarized pp collisions Longitudinally polarized pp collisions Transversely polarized pp collisions Looking ahead ECT* Workshop on Hard QCD with Antiprotons at GSI FAIR

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 2 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 3 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 4 Mid-rapidity π 0 production at RHIC Data favor the KKP fragmentation function over Kretzer Mid-rapidity π 0 cross section at 200 GeV is well described by pQCD over 8 orders of magnitude PRL 91,

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 5 Forward π 0 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 p T √s=23.3GeV√s=52.8GeV xFxF xFxF           Ed 3  dp 3 [  b/GeV 3 ] NLO calculations with different scales: p T and p T /2 Data-pQCD differences at p T =1.5GeV

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 6 Forward pp  π 0 + X cross sections at 200 GeV NLO pQCD calculations by Vogelsang, et al. PRL 97, STAR The error bars are statistical plus point-to-point systematic Consistent with NLO pQCD calculations at 3.3 < η < 4.0 Data at low p T trend from KKP fragmentation functions toward Kretzer.

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 7 Mid-rapidity protons and charged pions PLB 637, 161 STAR 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-p T Protons arise primarily from gluon fragmentation; pions receive a large quark contribution at high-p T

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 8 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,

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 9 From “tests” to tools RHIC data now provide important constraints for global analyses of pion fragmentation functions de Florian et al, PRD 75,

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 10 Kaon fragmentation functions Also introducing important new constraints for kaon fragmentation functions de Florian et al, PRD 75,

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 11 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:

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 12 What about “fundamental objects”? The direct photon yield is well described by pQCD PRL 98,

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 13 Jets STAR PRL 97, 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 14 Partonic structure of the proton HERA data provide very strong constraints on unpolarized PDFs Much less polarized DIS data; over a limited Q 2 region Gluon and sea-quark polarizations largely unconstrained by DIS World DIS database with DGLAP fits All fixed- target data World Data on g 1 p as of 2005

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 15 Origin of the proton spin? Polarized DIS: 0.2~0.3 Poorly Constrained 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/ ΔG = 0.13 ± 0.16 ΔG ~ ΔG = ± 0.41

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 16 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

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 17 Inclusive A LL measurements (  0,  ±, and jets)  f: polarized parton distribution functions For most RHIC kinematics, gg and qg dominate, making A LL sensitive to gluon polarization pT(GeV)

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 18 Predicted sensitivity for different ΔG scenarios Jets (STAR) and π 0 (PHENIX and STAR) easier γ and A LL ( π + ) - A LL ( 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

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 19 STAR jets from Runs 3+4 PRL 97, Gluon polarization is not “really big” (GRSV-max: CL ~ 0.02) STAR

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 20 π+π+ Charged pions from Run 5 π-π- STAR

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 21 STAR neutral pions from Run 5  z   Mean ratio of  0 p T to Jet p T A LL disfavors large (positive) gluon polarization Energetic π 0 carry a significant fraction of the total transverse momentum of their associated jet π0π0 STAR

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 22 PHENIX neutral pions from Run 5 arXiv: χ 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

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 23 STAR jets from Run 5  g = g (max)  g = -g (min)  g = 0 GRSV-STD 2005 STAR preliminary STAR 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 p T

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 24 PHENIX π 0 from Run 6 Preliminary Run 6 χ 2 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 25 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, STAR

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 26 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

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 27 J. Balewski (IUCF) Sivers effect in di-jet production 11 22 spin di-jet bisector kTxkTx  > 180  for k T x > 0 Sivers effect: Left/right asymmetry in the k T 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 28 Sivers di-jet measurement arXiv: Measure the di-jet opening angle as a function of proton spin Both beams polarized, x a  x b  pseudorapidity dependence can distinguish q vs. g Sivers effects. STAR Mostly: +z beam quark −z beam gluon

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 29 Sivers di-jet measurement arXiv: 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? STAR

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 30 BRAHMS forward rapidity pion measurements at 200 GeV Sign dependence of charged pion asymmetries seen in FNAL E704 persists to 200 GeV BRAHMS 2.3 deg (  ~3.4)4 deg (  ~3)

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 31 Additional BRAHMS forward rapidity results at 200 GeV BRAHMS 2.3 deg (  ~3.4) Charged kaon A N both positive; slightly smaller or comparable to π + Antiprotons show a sizable positive A N Protons show little asymmetry

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 32 BRAHMS results at 62.4 GeV BRAHMS Combined results from 2.3 and 3 deg Lower beam energy Larger x F Very large asymmetries! Limitation of the BRAHMS measurements: Very strong correlation between x F and p T from small acceptance

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 33 Inclusive forward   asymmetry, A N STAR Kouvaris et al, hep-ph/ Decreasing η Increasing p T The data show exactly the opposite behavior Theory Data

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 34 A N (p T ) in x F -bins Combined data from three runs at =3.3, 3.7 and 4.0 In each x F bin, does not significantly changes with p T Measured A N is not a smooth decreasing function of p T as predicted by theoretical models Kouvaris et al, hep-ph/ STAR

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 35 Separating Sivers and Collins effects Collins mechanism: asymmetry in the forward jet fragmentation Sivers mechanism: asymmetry in the forward jet or γ production SPSP k T,q p p SPSP p p SqSq k T, π Sensitive to proton spin – parton transverse motion correlations Sensitive to transversity Need to go beyond inclusive π 0 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 36 STAR Forward Meson Spectrometer Pb glass calorimeter covering 2.5 < η < 4 Detect direct photons, jets, di-jets, ….. in addition to π 0, for kinematics where π 0 single-spin asymmetries are known to be large South half of FMS array during assembly

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 37 Looking beyond inclusive A LL measurements Inclusive A LL measurements at fixed p T 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 38 The next few years: ΔG(x) Di-jets access LO parton kinematics Involve a mixture of qq, qg, and gg scattering 2005 Data Simulation STAR (pre-)preliminary

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 39 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 40 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 41 Further future: spin measurements in the RHIC II era Direct measurement of the Δs, Δs contributions in charm-tagged W boson production Sivers asymmetry A N for Drell-Yan di-muon and di-electron production

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 42 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 43

Carl Gagliardi – ECT* Hard QCD at GSI Workshop 44 Subtleties of Jet Analysis: Trigger Bias EMF  Electromagnetic Energy Fraction   E T (EMC) / total jet p T 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