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Exploring the saturation in cold nuclear matter at RHIC

This article provides an introduction to the RHIC (Relativistic Heavy Ion Collider) and discusses the legacy of its first three years. It explores the nontrivial initial state effects and presents outlooks for the future. The article summarizes the findings presented at the Moriond 2004 conference.

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Exploring the saturation in cold nuclear matter at RHIC

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  1. E.Kistenev, BNL Exploring the saturation in cold nuclear matter at RHIC • Introduction to RHIC; • Legacy of the first three years; • Hints of the nontrivial initial state effects; • Outlooks for the not so distant future • Summary Moriond 2004

  2. RHIC’s Experiments STAR • 3.83 km circumference • Two independent rings • 120 bunches/ring • 106 ns crossing time • Any nuclear species on ~any other species • Energy: • 500 GeV for p-p • 200 GeV for Au-Au(per N-N collision) • Luminosity • Au-Au: 2 x 1026 cm-2 s-1 • p-p : 2 x 1032 cm-2 s-1(polarized) Moriond 2004

  3. NLO DGLAP fits can fit HERA data accurately • but: • have too many free parameters (>10) • breaks down at very low x (negative exponents in parton distribution functions) RHIC may produce a solution RHIC is special • Atomic number A introduces new scale Q2 ~ A1/3 Q02 • Gluon density is increased relative to the proton by A1/3 ; • Compared to fixed target heavy ion facilities • ECM increased by order-of-magnitude • Accessible x (parton momentum fraction)decreases by ~ same factor • Access to perturbative phenomena • Direct photons • Jets • Non-linear dE/dx Ideal enviroment to study partonic structure functions and evolution Moriond 2004

  4. PDS in x->0 limit: discovery or just measurement target rest frame probe rest frame lc ~1/x lc ~1/x Transverse size of the quark-antiquark cloud is determined by r ~ 1/Q~ 2 10-14cm/ Q (GeV) Transverse size of the quark-antiquark cloud is determined by r ~ 1/Q~ 2 10-14cm/ Q (GeV) r/ ggg 1/Q gluons overlap and fuse -> saturating gluon density in the initial state (scale Qs) Color dipole interacts with saturated nuclear glue • Implications for Qs2<Q2< Qs4/2QCD • - Npart scaling of minijet production, “monojets”-- 21 gluon fusion • Kharzeev, Levin, McLerrean • Nuclear dijet decorrelation • Nikolaev, Zakharov Moriond 2004

  5. 130 GeV 200 GeV dNchg / d per part. pair PHENIX Npart Npart D.Kharzeev and M. Nardi, Phys.Lett. B503, 121 (2001) D.Kharzeev and E.Levin, Phys.Lett. B523, 79 (2001) In the saturation limit (McLerran et al.) linear factorization breaks down and one can describe the proton or nucleus in terms of classical gluon fields Color Glass Condensate CGC is not a state of matter like QGP, it is a Fock state of the wavefunction. The factorization breakdown does not affect suppression in the central rapidity region but may contribute to the total multiplicity. Moriond 2004

  6. The signs of DGLAP breakdown due to saturation effects should be relatively easy to identify in hard processes: large scale in calculations makes perturbative QCD applicable to establish a base line: high momentum transfer (Q2), high invariant mass (M), high transverse momentum (pT) It is calculable DIS at HERA In the limit x2<<x1 Q2=sx1x2 nA collisions to reach very low x values in nuclei x Moriond 2004

  7. PRL91, 072301(2003) Legacy of the first three years Moriond 2004

  8. Further clues from dAu Au + Au Experiment d + Au Control Experiment Final Data Preliminary Data Dramatically different and opposite centrality evolution of Au+Au experiment from d+Au control allows to claim that suppression observed in central region is clearly a final state effect. Moriond 2004

  9. First hint of gluon saturation in the low-x part of nucleus (Au) partonic wave function… RICH:  / K separation20 GeV/c Proton ID up to35 GeV/c R.Debbe, QM 2004 BRAMS Moriond 2004

  10. As of today Physics at low x is being explored by all four RHIC experiments • BRAHMS • charged hadrons; • STAR • charged hadrons, 0 ‘s; • PHENIX • hadrons, muons, J/Y; • PHOBOS • charged particles; All four experiments have plans for major upgrades related to low x physics (via Forward Studies) Moriond 2004

  11. PHOBOS 2003 – near complete h-coverage Poster by C. Henderson TOF Walls TOF StartCounter “SpecTrig” SpectrometerTrigger StartCounter h 0 -5 -4 -3 -2 -1 1 2 3 4 5 dN/dh Octagon Spectrometer N Rings P Rings Moriond 2004

  12. Charged and neutral multiplicity Transverse Energy in EMCal Related observables in PHENIX Hit multiplicity in BBC Muons and hadrons in muon spectrometers Collision Region (not to scale) Moriond 2004

  13. J/Y dA from PHENIX R. Granier ”J/Psi Production and Nuclear Effects for dAu and pp Collisions at RHIC” Au d • Suppression in deuteron direction consistent with some shadowing but can’t distinguish among various models • Anti-shadowing in Au direction • Overall absorption • *Centrality dependence unique measurement from RHIC Moriond 2004

  14. Rapidity dependence of the high Pt hadron yield suppression (compilation) Moriond 2004

  15. Heavy quarks and shadowing(PHENIX) • probing gluon field in the initial state measuring total yield; • - probing media effects measuring inv. s Scaled down by ncoll Yield of is consistent with binary scaling in d+Au and Au+Au collisions. Moriond 2004

  16. pQCD vs CGC(*). • We got hints of what to come. The goal now is to answer a fundamental question if those hints point to the breakdown of DGLAP evolution and advent of “dynamic shadowing” : CGC . • The NLO DGLAP must be further tuned (NNLO) to describe • transverse energy flow; • forward (Mueller-Navelet) jets; • jet azimuthal correlations; • forward particles and energy flow together (measure of the range of compensation for forward partons) • in pp interactions. Establish base-line for dynamical effects in the high Pt yields in dAu collisions. • Use hard-scattering processes, the cleanest is • g q->g J • to find if DGLAP really breaks down. (*) see A.Accardi talk at QM2004 Moriond 2004

  17. Can it be done at RHIC Typical cross section at pT~5-6 GeV/c is d2N/dptdh~ 10-6 Kinematics x yield restrict such measurements to h ~ 3.5 and pT ~ 6 GeV/c • Dream detector…… • Large solid angle • Momenta in the range 20-60 GeV/c (photons, hadrons, jets) • PID in the range 20-60 GeV/c Moriond 2004

  18. BRAMS – aiming to increase acceptance Magnet Calorimeter Rich 8 4 1.5 Tracking Such a design can probably give a factor 10 in solid angle and improved rates but ……. philosophy is unchanged measuring forward jets would require radically different design. Moriond 2004

  19. Muon from hadron decay Hole in pole tips gives access to |h|> 2 Muon from W RPC Forward Silicon Tracker 4 layers ofSilicon pixels Upgraded Muon Trigger Momentum Cut: Tracking*muID Timing from RPC for background rejection W/Si Calorimeter Issues: impact on muon phyiscs occupancy pileup from underlying event Muons from hadrons Muons from Ws pmuon STAR and PHENIX – similar goals, different constrains Moriond 2004

  20. Summary • first hints of saturated gluon partonic density in fast moving nuclei are observed in dAu collisions at 200 GeV at RHIC. All four experiments reported new data on related observables to Quark-Matter 2004 Conference in January 2004; • the suppression of the forward yields is found consistent in value for J/Y, charged hadrons, inclusive muons and p0’s measured by different experiments; • presently running RHIC experiments able to see only tip of the iceberg, further progress requires dedicated upgrades; • by chance central particle production at RHIC is largely unaffected by saturation so the matter effects potentially related to QGP are really pronounced. At LHC CGC will (may) become a dominant player even in y=0 region. Just be aware. Moriond 2004

  21. Particle production at RHIC • is the realization of “pre-existing” (dense) gluon fields F(x); • In the low x limit Parton Saturation (~Extreme shadowing ~ Extreme kT ) results in F(x) described as coherent Yang-Mills field (Color Glass Condensate); • Saturation scale QS2 increases with nuclearsize and/or overlap • F = F (x, Q/QS) • Saturation condition r s ~ 1 • here  =  s(Q2) / Q2 •   xG(x, Q2) nucleon •  QS2 ~ aS A xG(x,Q2) /R2 ~ A1/3 Moriond 2004

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