for the Collaboration Adam Trzupek The Henryk Niewodniczański Institute of Nuclear Physics Polish Academy of Sciences Kraków, Poland The 2007 Europhysics Conference on High Energy Physics Manchester, England, July 2007 Results from the PHOBOS experiment at RHIC

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 2 PHOBOS 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 PANMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITYUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 3 Outline PHOBOS detector – Data: p+p, d+Au, Cu+Cu, Au+Au at 20 – 200 GeV Charged particle multiplicities – Factorization of energy and centrality dependence in Au+Au and Cu+Cu collisions Azimuthal anisotropy of produced particles in Au+Au and Cu+Cu collisions – Participant eccentricity scaling p T - Spectra of identified particles – Very low p T data – a handle on radial flow Summary

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 4 PHOBOS Detector Multiplicity Detector (Octagon, Rings) -5.4 <  < 5.4, 0 <  < 2  1m Octagon Ring Counters

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 5 1m ZDC Spectrometer TOF ZDC Triggering PHOBOS Detector Multilayer Spectrometer, TOF midrapidity, 0.03 GeV/c< p T < 5 GeV/c

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 6 Charged hadron dN ch /d  distribution (PHOBOS) PRL 91 (2003) , PRC 74 (2006) , PRC 72 (2005) d+Au centrality 19.6 GeV62.4 GeV130 GeV 200 GeV preliminary Au+Au Cu+Cu Energy dependence: Height increases Width increases (in  space) Centrality dependence: Height increases Species dependence: Same systematic trends

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 7 Au+Au and Cu+Cu at the same N part ( = 200 GeV) For the same N part (system size) dN ch /d  shape is very similar for Au+Au and Cu+Cu collisions Cu+Cu central Preliminary 3-6%, N part = 100 Au+Au midcentral 35-40%, N part = 99 Cu+Cu midcentral Preliminary 15-25%, N part = 61 Au+Au peripheral 45-55%, N part = 56 N part - number of participating nucleons PHOBOS: NPA 774 (2006) 113

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 8 Charged particle yields in Au+Au and Cu+Cu at midrapidity PRC 74 (2006) , NPA 774 (2006) 113 PHOBOS preliminary No centrality dependence for N part > 40 Energy and centrality dependences of charged hadron yields factorize Particle density per participant pair Ratio of charged hadron yield at 200 GeV to yields at lower energies (200/X) Increase in particle production per participant with N part s

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 9 PRL 94, (2005), PRL 96 (2006) Ratio of charged hadron yields at 200 and 62.4 GeV Charged Particle p T Spectra = 0.25 GeV/c = 1.25 GeV/c = 2.5 GeV/c = 3.38 GeV/c = 3.88 GeV/c Au+Au Cu+Cu p+p midrapidity No centrality dependence for p T = 0.2 – 4 GeV/c Factorization of energy and centrality dependence is valid at different transverse momenta.

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 10 Energy independence of charged particle yields from moderate to high rapidities NPA 774 (2006) 113 Extended longitudinal scaling

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 11 Azimutal anisotropy of produced particles Pressure gradients lead to azimuthal anisotropy Elliptic flow is the second harmonic in the Fourier expansion of azimuthal particle distribution Reaction plane x z y M. Kaneta dN/d(  0 ) = N 0 (1 + 2v 1 cos (  0 ) + 2v 2 cos (2(  0 )) +... )

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 12 v 2 in Au+Au and Cu+Cu (  dependence) Centrality: 0-40% PRL 98 (2007) , PRC 72 (2005) , PRL 94 (2005) broad  range several energies for Cu+Cu v 2 is large and grows with energy shape (in  ) for Au+Au and Cu+Cu similar 0-40%, charged particles Au+Au Cu+Cu

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 13 v 2 in Au+Au and Cu+Cu (centrality dependence) Charged particles, |  | < 1 decreases with centrality for central collisions v 2 is non-zero (larger in Cu+Cu) PRL 98 (2007) , PRC 72 (2005)

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 14 Standard and Participant eccentricity b Au+Au 00 Initial overlap geometry Visible in final measured particle azimuthal angular distributions Standard eccentricity: minor axis along b, (b  x) Participant eccentricity: for the same b, interaction points vary from event-to-event minor axis not along b, (b  x)

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester GeV Does using make a difference? YES increases for smaller systems For central Cu+Cu: > PRL 98 (2007)

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 16 Eccentricity scaled v 2 in Au+Au and Cu+Cu v 2 scaled by participant eccentricity Cu+Cu Au+Au unifies average v 2 in Au+Au and Cu+Cu v 2 scaled by standard eccentricity Cu+Cu Au+Au 200 GeV PRL 98 (2007)

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 17 Eccentricity scaled v 2 in Au+Au and Cu+Cu v 2 scaled by participant eccentricity Cu+Cu Au+Au unifies average v 2 in Au+Au and Cu+Cu v 2 scaled by standard eccentricity Cu+Cu Au+Au 200 GeV PRL 98 (2007)

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 18 p T dependence of v 2 / Au+Au and Cu+Cu at matched N part unifies v 2 (p T ) in Au+Au and Cu+Cu nucl-ex/ midrapidity

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 19 The collision geometry controls the dynamical evolution of heavy ion collisions Au+Au and Cu+Cu at matched N part Pseudorapidity dependence of v 2 / unifies v 2 (  ) in Au+Au and Cu+Cu More information on the dynamical evolution can be obtained from identified particle p T spectra

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 20 p T (GeV/c) Stopping particles dE/dxTOF Particle ID from low to high p T PHOBOS Particle Identification E loss (MeV) p (GeV/c) /v (ps/cm) PRC 70 (2004) , PRC 75 (2007) p+p K +K + - ++-++- p (GeV/c)   KK p p

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 21 Identified particle p T -spectra, Au+Au at 62.4 GeV Smooth evolution with centrality Proton spectra are harder than the meson spectra Time-of-Flight measurement extends p T reach to 3 GeV/c for protons y0y0 PRC 75 (2007)

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 22 Particle production at very low p T Unique low-p T coverage of PHOBOS y0y0 PHOBOS Au+Au 62.4 GeV T = 101  6 MeV  surface = 0.72  0.02 T = 102  6 MeV  surface = 0.76  0.02 T = 103  6 MeV  surface = 0.78  0.02 No enhanced production at very low p T p T - spectra consistent with transverse expansion of the system PRC 75 (2007)

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 23 m T -scaling in d+Au vs. central Au+Au Lack of m T scaling in central heavy ion collisions J.Phys.G 30 S1143-S1147 (2004 ) PRC 70 (2004) (R) m T - Scaling  the same slope of m T –spectra PRC 70 (2004) , PRC 75 (2007)

Adam Trzupek – INP PAN, Kraków HEP’07 Manchester 24 Summary dN ch /d  for Au+Au and Cu+Cu – Similar at the same N part – Factorization of centrality and energy dependence – Extended longitudinal scaling Elliptic Flow – v 2 for A+A is large and continues to grow with energy – Participant eccentricity is relevant for the azimuthal anisotropy – Scaling of v 2 /  part for Cu+Cu and Au+Au p T -Spectra of Identified Particles – No enhanced production at very low p T in central Au+Au collisions – Lack of m T scaling in central Au+Au collision consistent with transverse expansion of the system The collision geometry controls the dynamical evolution of heavy ion collisions