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STAR Forward Physics at RHIC Transverse Spin Effects and Probing Low-x Gluons OUTLINE • Transverse single spin effects in p+p collisions at s=200 GeV • Towards understanding forward p0 cross sections • Probing low-x gluon densities • Plans for the future L.C. Bland Brookhaven National Laboratory The Partonic Structure of Hadrons ECT*, Trento 9 May 2005 Transverse Spin Effects 9 May 2005 L.C.Bland, ECT* Workshop 2 A Brief History… p p p X • At leading twist and with collinear factorization, the chiral properties of QCD predict small analyzing s=20 GeV, pT=0.5-2.0 GeV/c powers for particle production with transversely polarized protons colliding at high energies. • The FermiLab E-704 experiment found strikingly large transverse single-spin effects in p+p fixed-target collisions with 200 GeV polarized proton beam. • Theoretical models were developed to explain these effects using spin and transverse-momentum dependent distribution or fragmentation functions or higher-twist effects. • Large transverse single-spin effects were observed p0 – E704, PLB261 (1991) 201. in semi-inclusive electroproduction experiments. • p+/- - E704, PLB264 (1991) 462. • 9 May 2005 L.C.Bland, ECT* Workshop 3 Two Models for Transverse Single-Spin Effects p +p→p0+Х • Sivers effect [Phys Rev D41 (1990) 83; 43 (1991) 261]: Flavor dependent correlation between the proton spin (Sp), momentum (Pp) and transverse momentum (k) of the unpolarized partons inside. The unpolarized parton distribution function fq(x,k) is modified to: 1 N S P ( Pp k q ) ƒ q (x, k q , S P ) ƒ q (x, k q ) Δ q ƒ q (x, k q ) 2 S P PP k q • Collins effect [Nucl Phys B396 (1993) 161]: Correlation between the quark spin (sq), momentum (pq) and transverse momentum (k) of the pion. The fragmentation function of transversely polarized quark q takes the form: 1 N sq (pq k π ) ˆ Dp/q (z, k , s q ) Dp/q (z, k p ) Dp/q (z, k p ) π 2 pq k π 9 May 2005 L.C.Bland, ECT* Workshop 4 Questions • Do transverse single spin effects persist to RHIC energies (200<s<500 GeV)? • Do we understand the unpolarized cross section where transverse single spin effects are large? • Can we disentangle the dynamics? 9 May 2005 L.C.Bland, ECT* Workshop 5 Installed and commissioned during run 4 Planned to be commissioned during run 5 Installed in run 5 and to be commissioned in run 5 Developments for runs 2 (1/02), 3 (3/03 5/03) and 4 (4/04 5/03) • Helical dipole snake magnets • b*=1m operataion • CNI polarimeters in RHIC,AGS • spin rotators longitudinal polarization fast feedback • polarized atomic hydrogen jet target 9 May 2005 L.C.Bland, ECT* Workshop 6 Run-5 Status Longitudinal Polarization at STAR/PHENIX Transverse Polarization at BRAHMS L dt = 0.8 pb1 Scheduled to run until 6/25/05 Original STAR goals: Pbeam > 0.4, L dt = 14 pb1 (long) / 4 pb1 (trans) 9 May 2005 L.C.Bland, ECT* Workshop 7 STAR detector layout • TPC: -1.0 < h < 1.0 • FTPC: 2.8 < h < 3.8 • BBC : 2.2 < h < 5.0 • EEMC:1 < h < 2 • BEMC:0 < h < 1 • FPD: |h| ~ 4.0 & ~3.7 9 May 2005 L.C.Bland, ECT* Workshop 8 STAR Forward Calorimetry Recent History and Plans • Prototype FPD proposal Dec 2000 – Approved March 2001 – Run 2 polarized proton data (published 2004 spin asymmetry and cross section) • FPD proposal June 2002 – Review July 2002 – Run 3 data pp dAu (Preliminary An Results) • FMS Proposal Submitted Jan 2005. Near full Forward EM Coverage. (hep-ex/0502040). 9 May 2005 L.C.Bland, ECT* Workshop 9 First AN Measurement at STAR prototype FPD results STAR collaboration Similar to result from E704 experiment Phys. Rev. Lett. 92 (2004) 171801 (√s=20 GeV, 0.5 < pT < 2.0 GeV/c) Can be described by several models available as predictions: Sivers: spin and k correlation in parton distribution functions (initial state) Collins: spin and k correlation in fragmentation function (final state) Qiu and Sterman (initial state) / Koike (final state): twist-3 pQCD calculations, multi-parton correlations √s=200 GeV, <η> = 3.8 9 May 2005 L.C.Bland, ECT* Workshop 10 Single Spin Asymmetry Definitions d d Two measurements: • Definition: AN d d • Single arm calorimeter: 1 N RN L • dσ↑(↓) – differential cross AN N RN R section of p0 then incoming PBeam L proton has spin up(down) R – relative luminosity (by BBC) Pbeam – beam polarization Left • Two arms (left-right) calorimeter: π0, xF<0 π0, xF>0 1 NL NR NR NL AN PBeam N N N N p p L R R L No relative luminosity needed Right positive AN: more p0 going left to polarized beam 9 May 2005 L.C.Bland, ECT* Workshop 11 Caveats: -RHIC CNI Absolute polarization still preliminary. -Result Averaged over azimuthal acceptance of detectors. -Positive XF (small angle scattering of the polarized proton). Run 2 Published Result. Run 3 Preliminary Result. -More Forward angles. -FPD Detectors. Run 3 Preliminary Backward Angle Data. -No significant Asymmetry seen. (Presented at Spin 2004: hep-ex/0502040) 9 May 2005 L.C.Bland, ECT* Workshop 12 STAR xF and pT range of FPD data 9 May 2005 L.C.Bland, ECT* Workshop 13 Forward p0 Cross Sections at RHIC 9 May 2005 L.C.Bland, ECT* Workshop 14 Hard Scattering Hard scattering hadroproduction p Factorization theorems state that the inclusive cross section for p+p p +X can be computed in perturbative QCD using universal PDF and fragmentation p functions, Dc (z ) and perturbatively calculated hard-scattering cross sections, d ab , for partonic process a+bc. All ˆc such processes are summed over to yield the inclusive p production cross section. d p dxa dxb dzc p f a ( xa ) f b ( xb ) Dc ( zc )dˆ c ab a ,b , c 9 May 2005 L.C.Bland, ECT* Workshop 15 Why Consider Forward Physics at a Collider? Kinematics Deep inelastic scattering Hard scattering hadroproduction Can Bjorken x values be selected in hard scattering? Assume: 1. Initial partons are collinear 2. Partonic interaction is elastic pT,1 pT,2 Studying pseudorapidity, h=-ln(tanq/2), dependence of particle production probes parton distributions at different Bjorken x values and involves different admixtures of gg, qg and qq’ subprocesses. 9 May 2005 L.C.Bland, ECT* Workshop 16 Simple Kinematic Limits Mid-rapidity particle detection: NLO pQCD (Vogelsang) 1.0 p+p p0+X, s = 200 GeV, h=0 h10 and <h2>0 0.8 qq xq xg xT = 2 pT / s fraction 0.6 0.4 qg 0.2 gg Large-rapidity particle detection: 0.0 h1>>h2 0 10 20 30 pT,p(GeV/c) xq xT eh1 xF (Feynman x), and xg xF e(h1h2) Large rapidity particle production and correlations involving large rapidity particle probes low-x parton distributions using valence quarks 9 May 2005 L.C.Bland, ECT* Workshop 17 How can one infer the dynamics of particle production? Particle production and correlations near h0 in p+p collisions at s = 200 GeV Inclusive p0 cross section Two particle correlations (h) STAR STAR, Phys. Rev. Lett. 90 (2003), nucl-ex/0210033 At √s = 200GeV and mid-rapidity, both NLO pQCD and PYTHIA explains p+p data well, down to pT~1GeV/c, consistent with partonic origin Do they work for Phys. Rev. Lett. 91, 241803 (2003) hep-ex/0304038 forward rapidity? 9 May 2005 L.C.Bland, ECT* Workshop 18 Forward p0 production in hadron collider Q 2 ~ pT 2 2E p xF Ep p0 s 2E N s p E E d N qq qp q z p h ln(tan( )) Eq xgp p 2 xqp p h Au xq xF / z qg EN xg T e g (collinear approx.) s • Large rapidity p production (hp~4) probes asymmetric partonic collisions p p p 0,hp 3.8, s 200GeV • Mostly high-x valence quark + low-x gluon <z> • 0.3 < xq< 0.7 <xq> NLO pQCD • 0.001< xg < 0.1 Jaeger,Stratmann,Vogelsang,Kretzer • <z> nearly constant and high 0.7 ~ 0.8 <xg> • Large-x quark polarization is known to be large from DIS • Directly couple to gluons = A probe of low x gluons 9 May 2005 L.C.Bland, ECT* Workshop 19 But, do we understand forward p0 production in p + p? At s << 200 GeV, not really…. √s=23.3GeV √s=52.8GeV Data-pQCD difference at pT=1.5GeV 2 NLO collinear calculations q6o with different scale: q15o q10o pT and pT/2 q22o q53o xF xF Bourrely and Soffer (hep-ph/0311110, Data references therein): NLO pQCD calculations underpredict the data at low s from ISR data/pQCD appears to be function of q, √s in addition to pT 9 May 2005 L.C.Bland, ECT* Workshop 20 Time/luminosity dependent PMT Gain Matching Di-photon Mass Reconstruction and calibration gain shift corrections Pb-glass reconstruction with STAR FPD FTPC-FPD p0 reconstruction matching Track in FTPC Cluster categorization • Clusteringconversionanalysis Photon and moment in beam pipe efficiency Luminosity vs p (+ X) g MC e e (+ g) p + p PMTwith parametrized shower shape 0 • Fitting gain + - & Data comparison 2 photon cluster example Hit in • Number of photons found >= 2 20MeV Beam pipe Mass resolution ~ FPD • Fiducial volume > 1/2 cell width from edge • Energy sharing zggE1E2/(E1 ~2% We understand gainE2) < 0.7level h • Absolute gain determined from p0 peak position for each tower almost Efficiencies is Limit with zgg<0.5 cut correction) purely Gain stability (before Try both geometrically determined • Energy dependent gain correction • Run/luminosity from reconstructiondependent gain correction Energy • Checking1gCluster f of MC(PYTHIA+GEANT) (PYTHIA+GEANT) h with MC FPD position known Gain stability (after correction) Geometrical limit f relative to STAR 2gCluster High tower sorted mass distributions 9 May 2005 L.C.Bland, ECT* Workshop 21 2nd moment of cluster (long axis) ppp0X cross sections at 200 GeV The error bars are point-to-point systematic and statistical errors added in quadrature The inclusive differential cross section for p0 production is consistent with NLO pQCD calculations at 3.3 < η < 4.0 The data at low pT are more consistent with the Kretzer set of fragmentation functions, similar to what was observed by PHENIX for p0 production at midrapidity. D. Morozov (IHEP), XXXXth Rencontres de Moriond - QCD, NLO pQCD calculations by Vogelsang, et al. March 12 - 19, 2005 9 May 2005 L.C.Bland, ECT* Workshop 22 STAR -FPD Preliminary Cross Sections Similar to ISR analysis J. Singh, et al Nucl. Phys. B140 (1978) 189. d 3 E 3 (1 xF ) pT N B dp N 5 B6 9 May 2005 L.C.Bland, ECT* Workshop 23 PYTHIA: a guide to the physics Forward Inclusive p0 Cross-Section: Subprocesses involved: q+g g+g and q+g q+g+g STAR FPD Soft processes • PYTHIA prediction agrees well with the inclusive p0 cross section at h3-4 • Dominant sources of large xF p0 production from: p0 ● q + g q + g (22) p0 + X q g p0 ● q + g q + g + g (23) p0 + X q g 9 May 2005 L.C.Bland, ECT* Workshop g 24 Probing low-x gluon densities Forward inclusive particle production in p+p and d+Au Particle correlations in p+p and d+Au 9 May 2005 L.C.Bland, ECT* Workshop 25 Parton Densities in the Proton Deep inelastic scattering (DIS) of electrons and muons is the primary source of information about the quark and gluon structure of the proton. Deep inelastic scattering Kinematics defined for electron(muon) scattering from a fixed proton target. Global analyses use world data from DIS, neutrino scattering, Drell-Yan,… to determine parton distribution functions (PDF). 9 May 2005 L.C.Bland, ECT* Workshop 26 Determining the gluon density The gluon density is determined by applying QCD evolution equations to account for the Q2 dependence (scaling violations) of structure functions measured in DIS. At low-x, the full QCD evolution equations can be simplified to approximate the gluon distribution by F2 ( x, Q ) 2 g (2 x) (ln Q 2 ) i.e., determine g(2x) by measuring the lnQ2 slope of F2(x,Q2) at fixed x. K. Prytz, Phys. Lett. B311 (1993) 286 9 May 2005 L.C.Bland, ECT* Workshop 27 Gluons in the Proton • DIS results from HERA ep collider provide accurate determination of xg(x) for the proton in the range 0.001<x<0.2 • the low-x gluon density is large and continues to increase as x0 over the measured range J. Pumplin, D.R. Stump, J. Huston, H.L. Lai, P. Nadolsky, W.K. Tung JHEP 0207 (2002) 012. 9 May 2005 L.C.Bland, ECT* Workshop 28 Nuclear Gluon Density e.g., see M. Hirai, S. Kumano, T.-H. Nagai, Phys. Rev. C70 (2004) 044905 and data references therein World data on nuclear DIS constrains nuclear modifications to gluon density only for xgluon > 0.02 9 May 2005 L.C.Bland, ECT* Workshop 29 New Physics at high gluon density 1. Shadowing. Gluons hiding behind other gluons. Modification of g(x) in nuclei. Modified distributions needed by codes that hope to calculate energy density after heavy ion collision. 2. Saturation Physics. New phenomena associated with large gluon density. • Coherent gluon contributions. • Macroscopic gluon fields. • Higher twist effects. • “Color Glass Condensate” Figure 3 Diagram showing the boundary between possible “phase” regions in the t=ln(1/x) vs plane Edmond Iancu and Raju Venugopalan, review for Quark Gluon Plasma 3, . R.C. Hwa and X.-N. Wang (eds.), World Scientific, 2003 [hep-ph/0303204]. 9 May 2005 L.C.Bland, ECT* Workshop 30 FPD Detector and pº reconstruction • robust di-photon reconstructions with FPD in d+Au collisions on deuteron beam side. • average number of photons reconstructed increases by 0.5 compared to p+p data. 9 May 2005 L.C.Bland, ECT* Workshop 31 h Dependence of RdAu Ed 3 inelastic dp3 dAu 1 dAu RdAu pp N binary dAu Ed 3 inelastic 2 197 pp dp3 pp y=0 As y grows 1 dAu G. Rakness (Penn State/BNL), Kharzeev, Kovchegov, and Tuchin, RdAu 2 197 Moriond - QCD, XXXXth Rencontres de pp Phys. Rev. D 68 , 094013 (2003) March 12 - 19, 2005 See also J. Jalilian-Marian, Nucl. Phys. A739, 319 (2004) • From isospin considerations, p + p h is expected to be suppressed relative to d + nucleon h at large h [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)] • Observe significant rapidity dependence similar to expectations from a “toy model” of RpA within the Color Glass Condensate framework. 9 May 2005 L.C.Bland, ECT* Workshop 32 Constraining the x-values probed in hadronic scattering Guzey, Strikman, and Vogelsang, Phys. Lett. B 603, 173 (2004). For 22 processes Log10(xGluon) FTPC TPC FTPC FPD Barrel EMC FPD Log10(xGluon) Collinear partons: hGluon + ● x = p /s (e +h1 + e+h2) T ● x = p /s (e h1 + eh2) • FPD: |h| 4.0 T • TPC and Barrel EMC: |h| < 1.0 CONCLUSION: Measure two particles in the final state to constrain • Endcap EMC: 1.0 < h < 2.0 the x-values probed • FTPC: 2.8 < h < 3.8 9 May 2005 L.C.Bland, ECT* Workshop 33 Back-to-back Azimuthal Correlations with large h Beam View Top View Fit ffpfLCP normalized Trigger by distributions and with f ] forward p0 Gaussian+constant • Ep > 25 GeV Coicidence Probability • hp 4 [1/radian] ] Midrapidity h tracks in TPC • -0.75 < h< +0.75 Leading Charged Particle(LCP) • pT > 0.5 GeV/c ffpfLCP S = Probability of “correlated” event under Gaussian B = Probability of “un-correlated” event under constant s = Width of Gaussian 9 May 2005 L.C.Bland, ECT* Workshop 34 STAR PYTHIA (with detector effects) predicts • “S” grows with <xF> and <pT,p> • “s” decrease with <xF> and <pT,p> 25<Ep<35GeV PYTHIA prediction agrees with p+p data Larger intrinsic kT required to fit data 45<Ep<55GeV Statistical errors only 9 May 2005 L.C.Bland, ECT* Workshop 35 Expectation from HIJING (PYTHIA+nuclear effects) X.N.Wang and M Gyulassy, PR D44(1991) 3501 with detector effects • HIJING predicts clear correlation in d+Au 25<Ep<35GeV • Small difference in “S” and “s” between p+p and d+Au • “B” is bigger in d+Au due to increased particle multiplicity at midrapidity 35<Ep<45GeV 9 May 2005 L.C.Bland, ECT* Workshop 36 dAu Correlations: probing low x “Mono-jet” p0 PT is balanced by Dilute parton system many gluons (deuteron) Dense gluon field 25<Ep<35GeV (Au) Beam View Top View f p0 STAR • Ep > 25 GeV Preliminary 35<Ep<45GeV • hp 4 Statistical errors only 9 May 2005 L.C.Bland, ECT* Workshop 37 dAu Correlations: probing low x Large h p0+h± correlations • Suppressed at small <xF> , <pT,p> Consistent with CGC picture 25<Ep<35GeV Fixed has E & pT grows •Consistent in d+Au and p+p at larger <xF> STAR and <pT,p> Preliminary 35<Ep<45GeV More data are needed… Statistical errors only 9 May 2005 L.C.Bland, ECT* Workshop 38 Plans for the Future 9 May 2005 L.C.Bland, ECT* Workshop 39 STAR Forward Meson Spectrometer NSF Major Research Initiative (MRI) Proposal -submitted January 2005 [hep-ex/0502040] 9 May 2005 L.C.Bland, ECT* Workshop 40 Three Highlighted Objectives In FMS Proposal (not exclusive) 1. A d(p)+Aup0p0+X measurement of the parton model gluon density distributions xg(x) in gold nuclei for 0.001< x <0.1. For 0.01<x<0.1, this measurement tests the universality of the gluon distribution. 2. Characterization of correlated pion cross sections as a function of Q2 (pT2) to search for the onset of gluon saturation effects associated with macroscopic gluon fields. (again d-Au) 3. Measurements with transversely polarized protons that are expected to resolve the origin of the large transverse spin asymmetries in reactions for forward p0 production. (polarized pp) 9 May 2005 L.C.Bland, ECT* Workshop 41 FMS Design • FMS increases areal coverage of forward EMC from 0.2 m2 to 4 m2 • FMS to be mounted at roughly the same distance from the center of the STAR interaction region as the FPD, FPD Calorimeters and would face the Blue beam • Addition of FMS to STAR provides nearly continuous EMC from -1<h<+4 9 May 2005 L.C.Bland, ECT* Workshop 42 STAR detector layout with FMS TPC: -1.0 < h < 1.0 FTPC: 2.8 < h < 3.8 BBC : 2.2 < h < 5.0 EEMC:1 < h < 2 BEMC:-1 < h < 1 FPD: |h| ~ 4.04.0~3.7 FMS: 2.5<h< & 9 May 2005 L.C.Bland, ECT* Workshop 43 FMS MRI Proposal Details • Full azimuthal EM coverage 2.5<h<4.0 – Extending STAR coverage to -1<h<4.0 • 684 3.8 cm 3.8 cm 45 cm lead glass inner cells (IHEP, Protvino). • 756 5.8 cm 5.8 cm 60 cm Schott F2 lead glass outer cells (FNAL-E831). • New Photinis XP2202 (outer cells) • Cockroft Walton Bases. • Readout Electronics 9 May 2005 L.C.Bland, ECT* Workshop 44 Frankfurt, Guzey and Strikman, J. Phys. G27 (2001) R23 [hep-ph/0010248]. • constrain x value of gluon probed by high-x quark by detection of second hadron serving as jet surrogate. • span broad pseudorapidity range (-1<h<+4) for second hadron span broad range of xgluon • provide sensitivity to higher pT for forward p0 reduce 23 (inelastic) parton process contributions thereby reducing uncorrelated background in f correlation. 9 May 2005 L.C.Bland, ECT* Workshop 45 Pythia Simulation d+Au p0+p0+X, pseudorapidity correlations with forward p0 HIJIING 1.381 Simulations • increased pT for forward p0 over run-3 results is expected to reduce the background in f correlation • detection of p0 in interval -1<h<+1 correlated with forward p0 (3<h<4) is expected to probe 0.01<xgluon<0.1 provides a universality test of nuclear gluon distribution determined from DIS • detection of p0 in interval 1<h<4 correlated with forward p0 (3<h<4) is expected to probe 0.001<xgluon<0.01 smallest x range until eRHIC • at d+Au interaction rates achieved at the end of run-3 (Rint~30 kHz), expect 9,700200 (5,600140) p0p0 coincident events that probe 0.001<xgluon<0.01 for “no shadowing” (“shadowing”) scenarios. 9 May 2005 L.C.Bland, ECT* Workshop 46 Disentangling Dynamics of Single Spin Asymmetries Spin-dependent particle correlations Collins/Hepplemann mechanism Sivers mechanism asymmetry is requires transversity and spin- present for forward jet or g dependent fragmentation Large acceptance of FMS will enable disentangling dynamics of spin asymmetries 9 May 2005 L.C.Bland, ECT* Workshop 47 Timeline Completion By Fall 2006 9 May 2005 L.C.Bland, ECT* Workshop 48 Other Possible Applications of FMS • forward p0/g reconstruction in heavy-ion collisions Reconstruction of HIJING/GSTAR simulations • direct photon detection at large rapidity • reconstruction of other mesons decaying to g or e produced in p+p or d(p)+Au (and heavy-ion?) collisions hgg wp0g Kshort p0p0 4g hgg? Limited sample of events obtained J/ e+e? in Cu+Cu run with good view of interaction region 9 May 2005 L.C.Bland, ECT* Workshop 49 Summary / Outlook • Large transverse single spin asymmetries are observed for large rapidity p0 production for polarized p+p collisions at s = 200 GeV AN grows with increasing xF for xF>0.35 AN is zero for negative xF • Large rapidity p0 cross sections for p+p collisions at s = 200 GeV is in agreement with NLO pQCD, unlike at lower s. Particle correlations are consistent with expectations of LO pQCD (+ parton showers). • Large rapidity p0 cross sections and particle correlations are suppressed in d+Au collisions at sNN=200 GeV, qualitatively consistent with parton saturation models. • Plan partial mapping of AN in xFpT plane in RHIC run-5 • Propose increase in forward calorimetry in STAR to probe low-x gluon densities and establish dynamical origin of AN (complete upgrade by 10/06). 9 May 2005 L.C.Bland, ECT* Workshop 50 New FMS Calorimeter Lead Glass From FNAL E831 Loaded On a Rental Truck for Trip To BNL 9 May 2005 L.C.Bland, ECT* Workshop 51

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