for the Collaboration Charged Hadron Spectra and Ratios in d+Au and Au+Au Collisions from PHOBOS Experiment at RHIC Adam Trzupek The Henryk Niewodniczański Institute of Nuclear Physics Polish Academy of Sciences Kraków, Poland 4th Budapest Winter School on Heavy Ion Collisions (December 1st-3rd 2004) in Budapest, Hungary nucl-ex/ , 2004

Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Bruce Backer, Russell Betts, Abigail Bickley, Richard Bindel, Andrzej Budzanowski,Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski, Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Steve Gashue, Clive Halliwell, Joshua Hamblen, Adam Harington, Michael Hauer, George Heintzelman, Conor Henderson, David Hofman, Richard Hollis, Roman Holynski, Burt Holzman, Aneta Iordanova, Erik Johnson, Jay Kane, Judith Katzy, Nazim Khan, Wojtek Kucewicz, Piotr Kulinich, Chia Ming Kuo, Jang Wo Lee, Willis Lin, Steven Manly, Don McLeod, Alice Mignerey, Rachid Nouicer, Gerrit van Nieuwenhuizen, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Louis Remsberg, Mike Reuter, Christof Roland, Gunther Roland, Leslie Rosenberg, Joe Sagerer, Pradeep Sarin, Paweł Sawicki, Helen Seals, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Jaw-Luen Tang, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Robin Verdier, Gábor Veres, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Wozniak, Alan Wuosmaa, Bolek Wyslouch, Jinlong Zhang ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKÓWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER PHOBOS Collaboration

PHOBOS Detector T0 counter Spectrometer SpecTOF TOF multiplicity, vertex and calorimeter detectors are not labeled (see Russell Betts talk) Magnet Paddle Trigger counter Paddle Trigger counter T0 counter

p T = ~5 GeV/c track curvature in B field => p,charge dE/dx in Si, ToF => mass p T = GeV/c low-p particles stop in silicon wafers => p, mass B field negligible => no charge identification p T and PID Measurement in PHOBOS Spectrometer 10 cm z -x PHOBOS Spectrometer dipole magnetic field of 2T at maximum 16 layers of silicon wafers fine/optimal pixelization, precise dE measurement collision vertex close to spectrometer near mid-rapidity coverage 70 cm Z [cm] X[cm] A B C D E F Be pipe...

PID Measurement in PHOBOS Spectrometer p T > 0.2 GeV/cp T = GeV/c E tot =  dE i, i=A,...,E M p i = E i dE i /dx M p =  /K separation: p T < ~0.6 GeV/c p(p) separation: p T < ~1.2 GeV/c  K p

Energy Dependence of Antiparticle to Particle Ratios p/p K – /K + A+A central, near mid-rapidity  PHOBOS 130 GeV PRL 87 (2001)  PHOBOS 200 GeV PRC 67 (2003) particle ratios increase with energy net baryon density is rapidly decreasing GOOD CONDITIONS FOR QGP FORMATION at  s NN = 200 GeV in Au+Au central collisions: baryochemical potential:  B = 27  2 MeV energy density:  = ~ 5 GeV /fm 3,  0 = 1 fm, nucl-ex/

in AA collisions “BULK” of hadrons is produced at low transverse momentum “TAIL” of transverse momentum distribution at high-p T originates from hard partonic scatterings 0.2<y   <1.4 Charged Hadron Transverse Momentum Distributions in Au+Au collisions at  s NN = 200 GeV centrality: 0-15% mid-rapidity  PRC RC in press nuc-ex/  PLB 578 (2004) 297 PLB 578 (2004) 297 TAIL BULK invariant yields particle density

parton nucleus t = - a few fm/c parton t = 0 fm/c hard partonic scattering t = + a few fm/c hadronization jet of hadrons leading hadron of high p T t = + a few fm/c scattered partons pass through hot and dense medium Hard partonic scatterings occur early in AA collision. Scattered partons can probe the dense and hot medium created in AA collision detector nucleus if scattered partons loose energy then the number of leading hadrons will be suppressed (”jet quenching”) High-p T Probes

R AA „hard collisions” „soft collisions” p T (GeV/c) Nuclear Modification Factor R AA R AA =1 (N coll scaling), lack of nuclear effects, small cross section for hard partonic scattering N coll - number of binary inelastic NN interactions in AA,m, NN data: p+ p (UA1) at 200 GeV p+ p (ISR) at 62.4 GeV

R AuAu for Charged Hadrons in Au+Au Collisions at  s NN = 200 GeV N coll scaling suppression of high-p T hadron production is observed strongest effect is seen in most central collisions PLB 578 (2004) 297 p T (GeV/c) 45-50%35-45% 25-35% 15-25% 6-15% 0-6% R AuAu mid- peripheral central 1 d 2 N AuAu / dp T d  d 2 N NN / dp T d  R AuAu =

High-p T Suppression Final state effects? energy loss in medium initial state effects possible in d+Au no final state effects no suppression in d+Au collisions indicates that final state effects are responsible for suppression in Au+Au central Au+Au: d +Au: Initial state effects? gluon saturation: suppression of high parton density (g+g-> g) Color Glass Condensate d+Au at 200 GeV is a control experiment

R dAu for Charged Hadrons,  s NN = 200 GeV d+Au control experiment indicates that suppression of particle production in central Au+Au collisions at  s NN = 200 GeV is a consequence of final state effects PRL 91 (2003) R dAu Au+Au mid-rapidity, 0.2<y  <1.4 medium created in Au+Au collisions is strongly interacting no suppression in d+Au collisions

Low-p T Spectra of Identified Charged Particles in Central Au+Au at  s NN = 200 GeV m T =  p T 2 +m h 2 no enhancement in low-p T yields for pions is observed flattening of (p+p) spectra down to very low p T, consistent with transverse expansion of the system |T= 229 MeV for (  + +  - ) 293 MeV for (K + + K - ) 392 MeV for (p + p) PRC RC in press nucl-ex/  medium created in Au+Au collisions is strongly interacting

R AuAu for Charged Hadrons at  s NN = 62.4 GeV nucl-ex/ (Au+Au, 62.4 GeV) RHIC Physics Run 2004 R AuAu at 62.4 GeV is significantly higher than at 200 GeV for all centralities within the studied p T range R AuAu

Energy Dependence of R AA nucl-ex/ central Pb+Pb and Au+Au collisions, near mid-rapidity at high-p T : R AA > 1 at  s NN = 17.2 GeV R AA < 0.2 at  s NN = 200 GeV smooth evolution of R AA with energy

p T (GeV/c) yields normalized by N part weakly depend on centrality Nuclear Modification Factor R AA Npart N coll scaling 45-50%25-35%15-25%0-6% N part - number of participating (wounded) nucleons in AA b(fm) Au+Au Glauber Model N coll ~ N part 4/3 nucl-ex/ , Au+Au:  62.4 GeV,  200 GeV 1 d 2 N AA / dp T d  d 2 N NN / dp T d  R AA Npart =

yield per participant (or R AA Npart ) changes by less than 25% for both energies in centrality range from 60 to 340 participants. centrality evolution is the same at both energies: R AA Npart = R PC Npart (N part ) * f(  s NN ) Factorization of Energy and Centrality Dependence of R AA Npart at  s NN = 62.4 and 200 GeV nucl-ex/

Summary Au+Au: almost net-baryon free environment, energy density ~ 5 GeV/fm 3 strong suppression of high-p T charged hadron yields in central collisions at 200 GeV (~ 5 times at p T ~ 5 GeV) no evidence for enhanced production of very low-p T pions flattening of p+p spectra at low-p T, strong radial flow in the system, R AuAu at 62.4 GeV is significantly higher than R AuAu at 200 GeV factorization of energy and centrality dependence of R AuAu Npart approximate N part scaling of hadron yields d+Au: no suppression of charged hadron yields at high-p T (at mid-rapidity) suppression in central Au+Au is final state effect

Conclusions STRONGLY INTERACTING, HIGH DENSITY AND ALMOST NET-BARYON FREE MEDIUM IS CREATED AT THE HIGHEST RHIC ENERGY IN CENTRAL Au+Au COLLISIONS particle ratios high-p T suppression low-p T spectra

Triggering on Collisions & Centrality Coincidence between Paddle counters at  t = 0 defines a valid collision Paddle + ZDC timing reject background PP Negative Paddles Positive Paddles Au x z PN Positive ZDC Negative ZDC Negative Cerenkov Positive Cerenkov Au CentralPeripheral HIJING +GEANT Glauber calculation Model of paddle trigger Data Data+MC

R dAu as a Function of Pseudo-rapidity (  = - ln tan(  /2)) nucl-ex/ , PRC in press positive  is in deuteron direction with increasing , R AA decreases model constraints: Color Glass Condensate

PRC in press nucl-ex/ d+Au Scale uncertainty: 15% Not feed-down corrected Au+Au Spectra normalized at 2 GeV/c m T Scaling in d+Au vs Au+Au

R AA at low energy ( fixed target experiments) Initial state effects R SAu R PbPb R pA Cronin effect E lab = 200 AGeV,  s NN = 19.4GeV Pb+Pb: E lab =158 AGeV,  s NN = 17.3 GeV R SS multiple scatterings p T broadening => R AA >1

Factorization of R AA (  s NN,centrality) For b<10.5 fm: Centrality  N coll 62.4 GeV 200 GeV p T (GeV/c) nucl-ex/

EPS Aachen 38 Theory Calculations Cronin Effect: X.N. Wang, Phys. Rev C61, (2000). Attributed to initial state multiple scattering. Implemented by Q 2 (p t ) dependent Gaussian k t broadening Energy loss applied: M. Gyulassy, I. Vitev, X.N Wang and B.W. Zhang; nucl-th/ dE/dx o is the only free parameter. It is determined by fitting to STAR central R AA (p t )