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Neutral Pion Suppression at Forward Rapidities from d+Au Collisions at STAR

Neutral Pion Suppression at Forward Rapidities from d+Au Collisions at STAR. Greg Rakness Penn State University/Brookhaven National Lab for the STAR Collaboration. XIII International Workshop on Deep-Inelastic Scattering (DIS2005) Madison, Wisconsin 28 April 2005. The nucleus:.

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Neutral Pion Suppression at Forward Rapidities from d+Au Collisions at STAR

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  1. Neutral Pion Suppression at Forward Rapidities from d+Au Collisions at STAR Greg Rakness Penn State University/Brookhaven National Lab for the STAR Collaboration XIII International Workshop on Deep-Inelastic Scattering (DIS2005) Madison, Wisconsin 28 April 2005

  2. The nucleus: Naïvely expect density of gluons in nucleus  A1/3... 100 100 Q2(GeV2) Q= 100 GeV 10 x G(x,Q2) 10 1 0.01 0.001 0.1 x Q= 2 GeV [Hirai, Kumano, Nagai, PRC 70 (2004) 044905] 1 10-4 10-3 10-2 10-1 x x = fraction of proton momentum carried by parton Low-x physics = gluons The proton: Accurate determinations of the proton's gluon structure have been extracted from scaling violations in inclusive Deep Inelastic Scattering (DIS)... No collider data to constrain nuclear gluon distributions at low-x... Problem: as x continues to decrease, • the gluon density increases • cross sections must remain finite [J. Pumplin, et al., JHEP 0207 (2002) 012]

  3. The Relativistic Heavy Ion Collider BRAHMS pp2pp PHENIX STAR Au-Au New state of matter QGP De-confinement Deuteron-Au Nuclear modification Gluon saturation Polarized proton-proton Nucleon Spin Structure Spin Fragmentation pQCD RHIC is a QCD lab PHOBOS

  4. Forward Hadron Production at RHIC Nuclear modification factor: Assume factorization to go from DIS to hadron collisions... (xn,xm) nm fn(xn) fm(xm) Suppression of inclusive h production of d+Au relative to p+p at forward rapidities… [I. Arsene, et al. (BRAHMS Coll.) PRL 93 (2004) 242303] “Shadowing” in nuclear DIS emerging in hadronic collisions?

  5. Saturation (Color Glass Condensate) Shadowing • R. Vogt, PRC 70 (2004) 064902. • Armesto, Salgado, and Wiedemann, PRL 94 (2005) 022002. • Jalilian-Marian, NPA 748 (2005) 664. • Kharzeev, Kovchegov, and Tuchin, PLB 599 (2004) 23; PRD 68 (2003) 094013. Parton Recombination Multiple Scattering • Hwa, Yang, and Fries, PRC 71 (2005) 024902. • Qiu and Vitev, PRL 93 (2004) 262301; hep-ph/0410218. Factorization breaking Others? • Kopeliovich, et al., hep-ph/0501260. • Nikolaev and Schaefer, PRD 71 (2005) 014023. Many (recent) descriptions of low-x suppression... A short list (i.e., probably incomplete): • ... Some experimental issues that can be addressed at STAR: • Rapidity dependence? • Isospin? • p+p under control? • What x-values are probed? • Monojet?

  6. STAR <1 radiation length between interaction region and large rapidity region (2.2<<4.5) Integral Matter (Rad. Length) 8 6 4 2 0 3 4 5  TPC: -1.0 <  < 1.0 FPD: ||  3.8 (p+p) ||  4.0 (p+p,d+Au) STAR Detector No WN calorimeter for Run-3 Forward Detector (FPD) • = Pb-glass electromagnetic calorimeter + Preshower

  7. Process breakdown: Kretzer (hep-ph/0410219) STAR gq gg qq qg Dq Dg E [GeV]  ] NLO pQCD compared with forward p + p  0 + X Inclusive forward  production in p+p collisions consistent with NLO pQCD calculations at s = 200 GeV, in contrast to lower s [Bourrely and Soffer, EPJ C 36 (2004) 371]

  8. STAR Example model (CGC): y=0 As y grows Kharzeev, Kovchegov, and Tuchin, Phys. Rev. D 68 , 094013 (2003)  Dependence of RdAu Observe significant rapidity dependence, similar to expectations from models which suppress gluon density in heavy nuclei RdAu for p0systematically below linear extrapolation of h data to =4, consistent with expectations that p + p  h is isospin suppressed at large [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)]

  9. For 22 processes PYTHIA Log10(xGluon) TPC Barrel EMC FTPC FTPC FPD FPD Gluon See L.C. Bland, et al., hep-ex/0502040 Measure two hadrons in final state Guzey, Strikman, and Vogelsang, Phys. Lett. B 603 (2004) 173. pQCD • For 2  2 processes: • xBj correlated with h of away-side particle • strong azimuthal correlation expected (back-to-back peak)  Analysis of di-hadron azimuthal and rapidity correlations can give insight on particle production mechanism...

  10. Beam View Top View Coicidence Probability [1/radian] S = Probability of “correlated” event under Gaussian B = Probability of “un-correlated” event under constant s = Width of Gaussian Back-to-back Azimuthal Correlationswith large  Fit LCP normalized distributions and with Gaussian+constant Trigger by forward   ] • E > 25 GeV •  4 ] Midrapidity h tracksin TPC • -0.75 < < +0.75 Leading Charged Particle(LCP) • pT > 0.5 GeV/c LCP

  11. Subprocesses involved: g+g and q+g  q+g+g STAR FPD Soft processes  q g  q g g Forward Inclusive Cross-Section: PYTHIA: a guide to the physics q+g • PYTHIA predictionagrees well with the inclusive 0 cross section at 3-4 • Dominant sources of large xF production from: • q + g  q + g (22) + X • q + g  q + g + g (23)+ X

  12. STAR 25<Ep<35 GeV 25<E<35GeV 45<Ep<55 GeV STAR preliminary 45<E<55GeV Statistical errors only PYTHIA=LO pQCD with parton showers (including detector effects), predicts • S grows with <xF> (<pT,p>) • sS decreases with <xF> (<pT,p>) PYTHIA prediction agrees with p+p data Larger intrinsic kT required to fit data (see Abazov, et al., hep-ex/0409040)  Partonic scattering good language to discuss forward p0 production from p+p collisions at s = 200 GeV...

  13. Expectation from HIJING (PYTHIA+shadowing +nuclear effects) X.N.Wang and M Gyulassy, PR D44(1991) 3501 with detector effects • HIJING predicts clear correlation in d+Au • 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 25<Ep<35GeV 35<Ep<45GeV

  14. STAR  “Mono-jet” PT is balanced by many gluons Dilute parton system (deuteron) Dense gluon field (Au) 25<Ep<35GeV Beam View Top View   • E > 25 GeV •  4 35<Ep<45GeV STAR preliminary Statistical errors only Exploratory d+Au p0 + h + X Correlations Preliminary Data

  15. More d+Au data needed... STAR Fixed as E & pT grows 25<Ep<35GeV 35<Ep<45GeV STAR preliminary Statistical errors only Large 0+h± correlations: • Suppressed at small <xF> , <pT,> Consistent with CGC picture • Consistent in d+Au and p+p at larger <xF> and <pT,> More data needed to measure dependence on pT, h, flavor...

  16. STAR L. C. Bland, et al., hep-ex/0502040 Outlook at STAR: Forward Meson Spectrometer See talk by S. Heppelmann in Spin parallel session, 29 April Physics Motivation: • probing gluon saturation in p(d)+A collisions via… • large rapidity particle production (,,,’,,K0,D0) detected through all  decays • forward  probes gluons with smallest x in Au nucleus • di-jets with large rapidity interval (Mueller-Navelet jets): full EM calorimetry coverage from 4 < h < -1 • disentangling dynamical origins of large xF analyzing power in p+p collisions 2 in  2.2<<4 To be built from existing calorimetry from FNAL E831 (Colorado)

  17. New FMS Calorimeter Loaded On a Rental Truck for Trip To BNL Lead Glass From FNAL E831

  18. Conclusions Forward p0 production at s = 200 GeV: • ... is consistent with partonic scattering calculations in p+p collisions  Inclusive cross section agrees with NLO pQCD and PYTHIA  Large Dh correlations agree with PYTHIA  Selects collisions of high-x quarks with low-x gluons • ... is different in exploratory d+Au collisions (d-side):  Inclusive yield normalized to p+p is suppressed  Trend expected in models that suppress gluon density in nuclei Shows evidence of isospin effects Large Dh correlations are suppressed relative to p+p  Direction of suppression qualitatively consistent with CGC ... More data and quantitative theoretical understanding are needed to make definitive physics conclusions… The tools are coming into place to study low-x physics at RHIC

  19. Backup transparencies

  20. Similar analyses performed prior: CERN ISR [J. Singh, et al., (CHLM Collab.) NP B140, 189 (1978)] • 0 production at s=45 GeV, 0.55<pT<1.05 GeV/c, xF>0.3 BNL RHIC [S. S. Adler, et al., (PHENIX Collab.) PRL 91, 241803 (2003)] • 0 production at s=200 GeV, 1<pT<14 GeV/c, xF=0 Inclusive 0 cross section vs. pT at fixed xF CONCLUSIONS: • For 0.3<xF<0.5,  production: • ~ (1/pT)6 • independent of xF • At pT=2 GeV/c, 0 production: • ~ (1-xF)5 • ISR at pT=1.05 GeV/c, xF>0.3: • +: 0.12 (1-xF)4.35 + 0.41 (1-xF)2.93 • : 0.55 (1-xF)5.24 + 0.09 (1-xF)3.02 At pT=2GeV/c:

  21. Near-Term Future Plans reconstruction of HIJING + GEANT simulations Simulations suggest that forward detection is feasible in centrality-averaged Cu+Cu collisions at s=200 GeV. In addition to establishing RCuCu at large rapidity, the FPD can trigger full STAR readout to examine particle correlations with large-rapidity . This can be useful to study flavor dependence of recoil jets at midrapidity. L.C. Bland, HardProbes

  22. For a few cases…: xBj at RHIC and LHC Collinear partons with momentum fractions x+, x elastically scatter to   For  New kinematic regimes will soon be explored in nuclei both at RHIC and at the LHC…

  23. qg+gq qq gg pT(GeV/c) P? [GeV] Analysis of h: KKP, NP B597, 337 (2001) NLO pQCD compared with midrapidity p+p : S. S. Adler, et al., PRL 91, 241803(2003) Process breakdown: Kretzer (hep-ph/0410219) Partonic scattering good model to describe p+p collisions at s = 200 GeV pT(GeV/c)

  24. charged hadrons p+p midrapidity production cross sections in comparison to NLO pQCD Direct photons PHENIX, hep-ex/0502006 Fixed order pQCD calculations agree with data for several different reactions...

  25. √s=52.8GeV Ed3dp3[b/GeV3] Ed3dp3[b/GeV3]      Data-pQCD difference at pT=1.5GeV/c Forward 0 Production in p + p collisions at s << 200 GeV √s=23.3GeV 2 NLO calculation with different scale: pT and pT/2 xF xF Bourelly 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 , √s in addition to pT

  26. azimuthal asymmetry in particle yields from a transversely polarized beam on an unpolarized target = STAR Large Analyzing Powers at RHIC at  = 3.8 STAR collab., PRL 92, 171801 (2004) Similar to FNAL E704 result at s = 20 GeV In agreement with several models including different dynamics: First measurement of AN for forward 0 production at s=200GeV • Sivers: spin and k correlation in initial state (related to orbital angular momentum?) • Collins: Transversity distribution function & spin-dependent fragmentation function • Qiu and Sterman (initial-state) / Koike (final-state) twist-3 pQCD calculations

  27.  q g +  q g g Partonic Correlations from PYTHIA Large energy deposited at =3.8 • one parton in hard scattering with peak in forward direction +broad  range • other parton spread over broad  range

  28. p + p 0 + X 35<E<45 25<E<35 [GeV] 45<E<55 E 0 0.5 0 0.5 MGeV/c2] <z> <xq> <xg> Some advantages of  mesons/experimental details... Identified  allows absolute energy calibration to ~1%... Efficiency driven by geometrical acceptance of calorimeter...  carries most of the energy of the fragmenting parton… …and pions are well described by NLO pQCD calculations over a broad rapidity window at large s…

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