Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Anti-particle to particle ratios in p+p, Cu+Cu and Au+Au collisions at RHIC Gábor I. Veres Eötvös Loránd University, Budapest Massachusetts Institute of Technology, Cambridge Quark Matter 2006 Conference, Shanghai, November 19, 2006 for the collaboration

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Collaboration Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Joshua Hamblen, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Bolek Wysłouch ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER 9 ongoing PhD thesis studies

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Outline Particle ratios: PHOBOS detector and PID methods Particle spectra: Au+Au PID spectra at 62.4 GeV low-p T particle yields baryon/hadron ratios m T scaling centrality dependence of the net proton yield Summary ratios in central Au+Au vs.  s Cu+Cu ratios vs. centrality at 200 and 62 GeV comparison of p+p, Cu+Cu and Au+Au ratios vs. N part

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Motivation Particle ratios: are influenced by baryon pair production, baryon number transport, and final state interactions show how close the system is to ideal baryon-free plasma provide input to quantitative analysis of chemical equilibrium in thermal models z -x y

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, The detector 1m Spectrometer Time of Flight

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, p T (GeV/c) Stopping particles dE/dxTOF Particle ID from low to high p T E loss (MeV) PHOBOS PID Capabilities p (GeV/c) /v (ps/cm) p+p K +K + - ++-++- p (GeV/c)

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Analysis techniques Particles identified via dE/dx in the spectrometer Monte Carlo studies to estimate corrections a cceptance in y and p T : a cceptance  fficiency, corrections PID cuts K  p d e 0 < y < < p T < 0.8 GeV/c p [GeV/c]

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Controlling systematic uncertainties Geometrical a cceptance drops out of the ratios: Ratios measured independently for different: bending directions spectrometer arms Z Z B+B+ B–B– h+h+ h–h– h+h+ h+h+ h+h+ h–h– h–h– h–h– by changing the polarity of the magnetic field

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, p T [GeV/c] Absorption coefficient p p _ GEANT Monte Carlo (FLUKA+Geisha)/2 Corrections Absorption GEANT used 4% for p/p 1.5% for K – /K + _ Secondary particles GEANT used 1% for p/p, 2% for  – /  + Feed-down daughters of weakly decaying particles ( , ,  – ) HIJING particle yields are used track DCA cut (3.5 mm!) keeps correction small 2% for p/p, <1% for K – /K + and  – /  + _ _ DCA {

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Errors on the ratios Statistical errors:  1% (tens of millions of events analyzed) Systematic errors (90% C.L.): beam orbit variations2% PID selection cuts4% 1% track quality cuts3% vertex cuts1% DCA cuts2% 1% trigger selection cuts1% absorption correction3%1% feed-down correction3%0% correction for secondaries1.5%0%2% Total:7.5%6%4.5% p K  

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, New results at 62.4 GeV fit smoothly into the energy evolution of antiparticle/particle ratios Results: energy dependence arXiv:nucl-ex/ submitted to PRC stat.errors only

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Cu+Cu ratios, all 3 species vs. N part Integrated over p T and y p/p ratios: little, if any, centrality dependence (still not understood theoretically) _ Similarly to Au+Au, strong collision energy dependence 200 GeV 62.4 GeV y0y0

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Comparison of systems at 200 GeV p/p ratios do not depend strongly on - system size - centrality _ The study is being currently extended to 62.4 GeV y0y0 p+p: Phys.Rev. C71, (2005)

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Ratios of different species Ratios of different species (like p/  ) are of significant interest: they are used to study the baryon excess observed at higher RHIC energies in the 2 – 4 GeV/c p T range.  For that, identified spectra of all species were analyzed.

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Proton spectra are harder than the meson spectra Time-of-Flight measurement extends the p T reach to 3 GeV/c for protons y0y0 Smooth evolution with centrality arXiv:nucl-ex/ submitted to PRC Identified particle p T -spectra, Au+Au at 62.4 GeV

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Particle production at very low p T Low p T spectra carry information about radial flow, possible anomalous enhancement at very low p T, quantum mechanical effects, scaling laws Unique low-p T coverage of PHOBOS T = 101  6 MeV  surface = 0.72  0.02 y0y0 PHOBOS Au+Au 62.4 GeV arXiv:nucl-ex/ submitted to PRC T = 102  6 MeV  surface = 0.76  0.02 T = 103  6 MeV  surface = 0.78  0.02

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, m T -scaling in d+Au vs. Au+Au at 200 GeV Lack of m T scaling in heavy ion collisions arXiv:nucl-ex/ submitted to PRC J.Phys.G 30 S1143-S1147 (2004 ) PRC 70 (2004) (R)

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, (Anti)proton/hadron ratios for the two charge signs Baryon excess approached at about p T  3 GeV/c y0y0 p>++K+p>++K+ arXiv:nucl-ex/ submitted to PRC PHOBOS ArXiv:nucl-ex/ submitted to PRC y0y0 Smooth energy dependence of p T where p/  +  1 p  + crossing Central A+A

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, y0y0 arXiv:nucl-ex/ submitted to PRC Net protons (p–p) _ The net proton yield is proportional to N part ! p/p centrality independent  p and p  N part as well __

Gábor I. VeresQuark Matter 2006, Shanghai, November 14-20, Summary New results from PHOBOS: antiparticle/particle ratios in Cu+Cu collisions p(p)/h ± ratios, identified spectra in 62.4 GeV Au+Au collisions What have we learned? Centrality-independence: p/p ratios net proton, p and p yield per participant (still not fully understood theoretically) baryon excess in Au+Au collisions at 62.4 GeV m T -scaling holds in d+Au but violated in Au+Au _ _ _