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New Results In Forward Physics at the STAR experiment at RHIC

RHIC pC Polarimeters. Absolute Polarimeter (H  jet). BRAHMS & PP2PP. PHOBOS. Siberian Snakes. Siberian Snakes. PHENIX. STAR. Spin Rotators (longitudinal polarization). Spin Rotators (longitudinal polarization). Pol. H - Source. LINAC. BOOSTER. Helical Partial Siberian Snake. AGS.

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New Results In Forward Physics at the STAR experiment at RHIC

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  1. RHIC pC Polarimeters Absolute Polarimeter (H jet) BRAHMS & PP2PP PHOBOS Siberian Snakes Siberian Snakes PHENIX STAR Spin Rotators (longitudinal polarization) Spin Rotators (longitudinal polarization) Pol. H- Source LINAC BOOSTER Helical Partial Siberian Snake AGS 200 MeV Polarimeter AGS pC Polarimeter Strong AGS Snake New Results In Forward Physicsat the STAR experiment at RHIC L.C. Bland Brookhaven National Laboratory Quantum Field Theory Symposium Saclay, 25 April 2009

  2. What are the goals? • How does the proton gets its spin from quarks and gluons? • Where do conventional descriptions of particle production (e.g., perturbative QCD) apply? • Are there new things to learn about partonic substructures at large x? • Can we learn about low-x structure functions from hadronic interactions? 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].

  3. <z> <xq> <xg> Why do forwardp0production in a hadron collider? Ep p0 p d EN qq qp p Au xgp xqp qg EN (collinear approx.) • Large rapidity p production (hp~4) probes asymmetric partonic collisions • Mostly high-x valence quark + low-x gluon • 0.3 < xq< 0.7 • 0.001< xg < 0.1 • <z> nearly constant and high 0.7 ~ 0.8 • Large-x quark polarization is known to be large from DIS • Directly couple to gluons  probe of low x gluons NLO pQCD Jaeger,Stratmann,Vogelsang,Kretzer

  4. STAR Poletips provide unique windows for forward detectors

  5. STAR Forward Calorimeter Projects F.Bieser2, L.Bland1, E. Braidot7, R.Brown1, H.Crawford2, A.Derevshchikov4, J.Drachenberg6, J.Engelage2, L.Eun3, M.Evans3, D.Fein3, C.Gagliardi6, A. Gordon1, S.Hepplemann3, E.Judd2, V.Kravtsov4, J. Langdon5, Yu.Matulenko4, A.Meschanin4, C.Miller5, N. Mineav4, D.Morozov4, M.Ng2, L.Nogach4, S.Nurushev4, A.Ogawa1, H. Okada1, J. Palmatier3, T.Peitzmann7, S. Perez5, C.Perkins2, M.Planinic8, N.Poljak8, G.Rakness1,3, A.Vasiliev4, N.Zachariou5 1Brookhaven National Laboratory 2University of California- Berkeley 3Pennsylvania State University 4IHEP, Protvino 5Stony Brook University 6Texas A&M University 7Utrecht, the Netherlands 8Zagreb University These people built the Forward Meson Spectrometer (FMS) and/or its components

  6. Early Results from STAR Forward Pion Detector (FPD) View along along axis of single lead-glass detector looking at phototube View along beam from interaction point of left/right symmetric modular arrays of lead-glass detectors

  7. ds - ds AN = ds +ds dσ↑(↓)=differential cross section when proton has spin up (down). Spin Effects for Large-xF Neutral Pion Production Run 6 Data Data: B.I. Abelev et al. (STAR), PRL 101 (2008) 222001 Theory (red): M. Boglione, U. D’Alesio, F. Murgia, PRD 77 (2008) 051502 Theory (blue): C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD 74 (2006) 114013 Even though the kinematics of the SIDIS measurement and the forward p0 data have little overlap, it is possible to account for most of the features of the RHIC data by calculations based on phenomenological fits to the semi-inclusive DIS data

  8. Does pQCD describe particle production at RHIC?Compare cross sections measured for p+pp0 +X at s=200 GeV to next-to-leading order pQCD calculations J. Adams et al. (STAR), PRL 97 (2006) 152302 Particle production cross sections for p+p collisions agree with NLO pQCD down to pT~2 GeV/c over a wide range, 0 < h < 3.8, of pseudorapidity (h = -ln tan /2).

  9. Inclusive p0 production from dAu J. Adams et al. (STAR), PRL 97 (2006) 152302 Particle production cross sections for d+Au collisions are smaller than expectations from only shadowing at <h> =4.0, and the energy dependence is best described by CGC calculations (A. Dumitru et al. Nucl Phys A765 (2006) 464)

  10. y=0 As y grows Kharzeev, Kovchegov, and Tuchin, Phys. Rev. D 68 , 094013 (2003)  Dependence of RdAu See also J. Jalilian-Marian, Nucl. Phys. A739, 319 (2004) J. Adams et al. (STAR), PRL 97 (2006) 152302 • From isospin considerations, p + p  h is expected to be suppressed relative to d + nucleon  h at large [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.

  11. STAR Fixed h, as E & pT grows Forward + mid-rapidity correlations in d+Au • are suppressed at small <xF> and <pT,p> • consistent with CGC picture • are similar in d+Au and p+p at larger <xF> and <pT,p> • as expected by HIJING J. Adams et al. (STAR), PRL 97 (2006) 152302 25<Ep<35GeV

  12. Run-8 Results from STAR Forward Meson Spectrometer(FMS) Full azimuth spanned with nearly contiguous electromagnetic calorimetry from -1<h<4  approaching full acceptance detector

  13. Guzey, Strikman and Vogelsang Phys. Lett. B603 (2004) 173 • 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 23 (inelastic) parton process contributions thereby reducing uncorrelated background in Df correlation. PYTHIA Simulation

  14. 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 Df 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. • shadowing of gluon structure function is not expected to modify width of away-side peak in Df distribution caused by elastic parton scattering.

  15. STAR Forward Meson Spectrometer • 50 larger acceptance than the run-3 forward pion detector (FPD). • 2p azimuth for 2.5<h<4.0 • Discriminate single g from p0gg up to ~60 GeV North half of FMS before closing Runs 3-6 FPD Run 8 FMS Phys. Rev. Lett. 101:222001 (2008)

  16. “Jet-like” events in the FMS Cone-based clustering of energy  near-side correlations “Jet-shape” distribution of energy within jet-like objects in the FMS as a function of distance from the jet axis. STAR Preliminary Simulation and Data agree well for “jet-like” events Caveat for Run 8: FMS data was acquired with a “high tower” trigger, which creates a bias towards “jets” that derive from a small number of high-z initial fragments N. Poljak, SPIN08 Plot from hep-ex/0901.2828

  17. STAR Detector • Large rapidity coverage for electromagnetic calorimetry (-1<h<+4) spanning full azimuth  azimuthal correlations • Run-8 was the first run for the Forward Meson Spectrometer (FMS)

  18. FMS-FPD comparison • Emulate FPD from run-8 FMS: • FMS photons: x>0cm • |hTPC|<0.725 ; 3.8<hFMS<4.1 • pT(TPC)>0.5 GeV/c • 30<EFMS<55 GeV • leading particle analysis • Normalization requires further systematic studies • pile-up correction • vertex efficiency • run-3/run-8 trigger • Reproduces Gaussian width and many other similarities Run-8 FMS preliminary results/E. Braidot QM09 s =0.949±0.121 s =0.965±0.198 s =1.161±0.132 s =1.073±0.335 J. Adams et al. (STAR), PRL 97 (2006) 152302

  19. Uncorrected Coincidence Probability (radian-1) Azimuthal Correlations (Dh3) p+pp0+h±+X, s=200 GeV • p0 requirements: • pT,p>2.5 GeV/c • 2.8<hp<3.8 • h± requirements: • 1.5<pT,h<pT,p • |hh|<0.9 E. Braidot, QM09 • clear back-to-back peak observed, as expected for partonic 22 processes • fixed and large h trigger, with variable hh  map out Bjorken-x dependence • of greatest interest for forward direct-g trigger

  20. leading p0 subleading p0 Azimuthal Correlations (Dh=0) Preliminary • Near-side and away-side correlations observed for forward-p0 pair • Clear increase of width of away-side peak as Dh increases

  21. Azimuthal Correlations (Dh3) “GSV” Selection 2.5 GeV/c<pT(<h>=3) 1.5 GeV/c<pT(|h|<0.9)<pT(<h>=3) “GSV” Selection p+p d+Au p0+p0+X sdAu – spp = 0.09±0.04 Uncorrected Coincidence Probability (radian-1) p0+h±+X Df=fp-flead E. Braidot, QM09 “GSV” selection leads to clear back-to-back peak with similar pp/dAu widths as expected by pQCD

  22. p+p d+Au p0+p0+X Uncorrected Coincidence Probability (radian-1) p0+h±+X Df=fp-flead Azimuthal Correlations (Dh3) “GSV” Selection 2.5 GeV/c<pT(<h>=3) 1.5 GeV/c<pT(|h|<0.9)<pT(<h>=3) “lower-pT” Selection sdAu – spp = 0.09±0.04 “lower-pT” Selection 2.0 GeV/c<pT(<h>=3) 1.0 GeV/c<pT(|h|<0.9)<pT(<h>=3) sdAu – spp = 0.19±0.03 E. Braidot, QM09 Evidence of pT dependent azimuthal broadening of signal

  23. Au-Side Multiplicity Dependence p0bEMC Au d east BBC multiplicity p0FMS • Select events on east BBC phototube charge sum • -5.0 < hBBC < -3.4 (Au side) • 18 counts on average from minimum ionizing particle

  24. low SQ mid SQ high SQ mid high low fFMS-fbEMC SQ Au-Side Multiplicity Dependencep0FMS+p0EMC events “lower-pT” Selection: 2.0 GeV/c<pT(<h>=3), 1.0 GeV/c<pT(|h|<0.9)<pT(<h>=3) p+p Uncorrected Coincidence Probability (radian-1) d+Au • No forward multiplicity dependence of p+pp0+p0 (FMS-EMC) correlations • Multiplicity dependence of d+Au p0+p0 (FMS-EMC) correlations is observed

  25. Near-Side g+p0 Correlations in FMS Observation of large-xFw production p+pg+p0+X, s = 200 GeV STAR Preliminary For this plot simulation is background only Run 8 data Pythia+GEANT (background only) (simulation generation ongoing) Fit is gaussian + cubic polynomial  μ=0.784±0.008 GeV σ=0.087±0.009 GeV Scale=1339±135 Events Etrip>30 GeV pT,trip>2.5 GeV/c pT(g cluster)>1.5 GeV/c pT(p0)>1 GeV/c Comparison to Run 8 dAu data and transverse SSA may be interesting. Significant (10s) wp0g signal seen in the data. A. Gordon, Moriond 2009

  26. p+pJ/+Xe+e¯ + X, s=200 GeV 2.5<hclus<4.0 pT,clus>1 GeV/c Epair > 60 GeV 4.5s significance Large-xF J/ ObservationC. Perkins (for STAR), Quark Matter 2009 p+pJ/+g+Xe+e¯+g+X, s=200 GeV 2.9s significance • high-xF J/ may have implications for intrinsic charm at large Bjorken-x in proton • use to benchmark simulations for future transverse-spin Drell-Yan experiment

  27. Conclusions • RHIC run 8 (dAu/pp) was a big success with 20x increase of forward acceptance at STAR enabling the study of pT depenence of azimuthal correlations. • Run-8 FMS results reproduce run-3 FPD Gaussian widths and other features. • Comparison of Dfp0(FMS)+p0(EMC) for pp and dAu indicates azimuthal broadening in dAu. • Data are qualitatively consistent with a pT dependent picture of gluon saturation of the gold nucleus.

  28. Outlook • Extract Dfp0+p0 for two forward p0 for d+Au collisions • Complete scan of pT range (from GSV to run-3) • Scan Dh for elastic parton scattering  x dependence of nuclear gluon density • Clustering: towards p0+jet or jet+jet • Complete absolute normalizations and systematics studies

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