Research in Relativistic Nuclear Physics at the Faculty of Physics of the University of Bucharest

1. Research in Relativistic Nuclear Physics at the Faculty of Physics of the University of Bucharest Alexandru JIPA Atomic and Nuclear Physics Department, Faculty of Physics, University of Bucharest, ROMANIA

2. Introduction • * 1970 – JINR Dubna – first relativistic nuclear collisions using accelerator systems – members of the Atomic and Nuclear Physics Chair were involved (Prof. C.Beşliu, Prof. N.Ghiordănescu) – studies related to the cumulative effect • *’70’s – ’80’s – many Diploma Thesis in Relativistic Nuclear Physics related to the results obtained in symmetric and asymmetric nucleus-nucleus collisions at 4.5 A GeV/c – SKM 200 Collaboration, mainly • *’80’s – Ph.D. Thesis • ’90’s – Relativistic Nuclear Physics course – students specialized in Nuclear and Particle Physics • ~1996-present - Master studies in Nuclear and Particle Physics – 3 semesters – 3 courses: Relativistic Nuclear Physics, Anomal States and Phase Transitions in Nuclear Matter, Processing of the Information at Complex experimental set-ups • ~ 3 Diploma Theses, 3 Master Theses, 1 Ph.D. Theses per academic year with subjects from Relativistic Nuclear Physics

3. International Collaborations • Direct involving • SKM 200 Collaboration (JINR Dubna) • MARUSYA Collaboration (JINR Dubna) • BRAHMS Collaboration (BNL Upton, New York) • NA50 Collaboration (CERN Geneva) – only at the beginning of ’90’s • Indirect involving • ALICE (CERN Geneva) – Ph.D. students, members of different Romanian research institutes • ATLAS (CERN Geneva) – Ph.D. students, members of different Romanian research institutes • CMS (CERN Geneva) – Ph.D. students, members of different Romanian research institutes • BECQUEREL Collaboration (JINR Dubna) - Ph.D. students, members of different Romanian research institutes

4. National Collaborations • Institute of Space Sciences – many students have a job and excellent conditions to continue their works in the filed; also, they are to possibility to work for Ph.D. Thesis (Advisors: Prof. Călin Beşliu (14), Prof. Alexandru Jipa (7)) • Institute of Nuclear Physics and Engineering – Applied Nuclear Physics Department, mainly • University of Civil Marine Constanţa • Different High Schools

5. Main research studies and results • Global characterization of He-A_T, C-A_T, O-A_T, Ne-A_T collisions at 4.5 A GeV/c (multiplicities, participants, momentum spectra, cross sections) • Inverse slopes for pions (temperatures), baryonic chemical potential, baryonic and energy densities • Space-time characterization of the particle source, correlations in the particle source (fireball) • Investigations for thermal equilibration in fireball, hydrodynamic flow (global analysis: jets of nuclear matter),

6. Main research studies and results (2) • Transverse momentum - longitudinal momentum correlations (connections with anomal states – cumulative effect) • Behaviour of the antiparticle to particle ratios for stopped charged particles in streamer chamber • Some evidences for the influence of the nuclear medium on the particle properties (modification of the rest mass with the increase of the density) • Studies on intermittent behaviour

7. Results obtained in BRAHMS Collaboration

8. Kpnd,Kpnd,

9. Two independent rings ~3.8 km in circumference RHIC experiments • Run 1: June - September 2000 First Physics Run Au+Au @ two energies SNN = 56 and 130 GeV • Run 2: July 2001- January 2002 Au+Au @ SNN = 200 GeV (maximal design energy) p+p (reference data) • Run 3: December 2002- May 2003 d+Au @ SNN = 200 GeV p+p @ SNN = 200 GeV

10. Run 4: December 2003- May 2004 Au+Au @ SNN = 200 GeV; Au+Au @ SNN = 63 GeV; p+p@ SNN = 200 GeV Run 5: December 2004- May 2005 Cu+Cu@ SNN = 200 GeV; p+p@ SNN = 200 GeV

14. Nuclear Modification Factor Yield(AA) RAA = NCOLL(AA)  Yield(NN) Scaled N+N reference RAA<1  Suppression relative to scaled NN reference • Data do not show suppression • Enhancement (RAA>1) due to initial state multiple scattering (“Cronin effect”) Known in p+A collisions

15. At Mid-Rapidity (=0) High pt Suppression in Au+Au

16. d-Au, sNN =200 GeV Charged particle multiplicities for the centrality ranges of 0-30% and 30-60%. The square points and circular points from SiMA and TMA detectors, respectively, while the triangles are from the BBC detectors.

19. d-Au Nuclear Modification factor at  ~3.2

20. Evolution of the nuclear modification factor measured by BRAHMS for the 10% most central d-Au collisions at sqrt(s_NN) = 200 GeV, as o function of pseudorapidity

21. Central to peripheral ratio Rcp as a function of pseudorapidity measured by BRAHMS for d-Au collisions at the RHIC top energy (central to peripheral, semi-central to peripheral) • – In central collisions – increased Cronin effect • additional suppression at forward rapidities – suppression mechanism that scales • with the collision centrality • Colour Glass Condensate • pQCD and string breaking – for soft coherent particle production

22. The difference between positive and negative hadrons in dAu at 4 degrees This measured difference (≥2 at 3GeV/c) is not easily explained if pion production is dominant. (NLO pQCD) It has been early dubbed as “beam fragmentation”

23. The experimental results from BRAHMS RHIC are consistent with formation of a hot dense system that: • Exhibits a high degree of reaction transparency leading to the formation of a near baryon free central region • There is an appreciable energy loss of the colliding nuclei, so the conditions for the formation of a very high energy density zone with approximate balance between matter and antimatter in a rapidity interval of -1.5;+1.5 around mid-rapidity are present • There are indications that the initial energy density is considerable large, so it is difficult to consider that the hadrons are isolated and well defined entities • Relative abundances of different particles suggest chemical equilibrium at a temperature around 175 MeV • Small values of the chemical potential are observed • General conditions for formation of a deconfined system of quarks and gluons appear, but ….

24. otherfeatures defining the quark-gluon plasma are absent or are not been identified up to now (vanishing interactions between quarks, characteristics of the chiral symmetry restoration, clear phase transition behaviour of the system … Main questions: * The properties of the matter as it is created in high energy nucleus-nucleus collisions clearly bears the imprint of a system characterized by quark and gluon degree of freedom over a range larger than the characteristic dimensions of the nucleon? * The color change is effective at distances larger than those of conventional confined objects? (high p_T suppression could be a such candidate, but there are some problems – the magnitude of the observed effect can not be reproduce) There is no doubt that the experiments at RHIC have revealed a plethora of new phenomena for the most part have come as a surprise. In this sense it is clear that the matter that is created at RHIC differs from anything that has been seen before. What name to give it must await our deeper understanding of this matter White paper of the BRAHMS Collaboration

26. Other Romanian physicists participating in BRAHMS: Dr. Dan Argintaru, Dr. Florin Constantin, Dr. Daniel Felea, Asist.Prof.Dr.Marius Calin, Ciprian Mitu, Mihai Potlog, Silvia Ochesanu, Costin Caramarcu Dr. Rory Clarke – postdoc Other Ph.D. students and Master Students involved in the researches in the Relativistic Nuclear Physics field: Claudian Grigorie, Ileana Stefan, Bogdan Iliescu, Amelia Horbuniev, Cristian Bordeianu, Valentin Grossu, Madalin Cherciu, Tiberiu Esanu, Rodica Dinu