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Habilitation à diriger des recherches

L'Etrangeté du Plasma de Quarks et de Gluons

Abstract : Like the three other experiments at RHIC (Relativistic Heavy Ion Collider) in the Brookhaven National Laboratory near New York, STAR (Solenoidal Tracker At RHIC) is dedicated to the search of a particular state of nuclear matter, predicted by lattice QCD (Quantum ChromoDynamics) calculations : the Quark Gluon Plasma (QGP). This state, supposed to be that of the Universe a few fraction of second after the Big Bang, should consist according to its first definition (1975) in a matter where quarks and gluons are deconfined and without any interaction. It could be created in laboratory with ultra-relativistic heavy ion collisions allowing to reach extreme temperatures and pressure.
After 20 years of research in Europe and United States, CERN annnnounced (on february 20th, 2000)that a particular state of matter as been created, compatible with a QGP state, but it was not possible to characterize it completely.
RHIC experiments then take over. Today, all the numerous new results that have been collected let us believe that indeed an atypical state of nuclear matter has been reated at RHIC and our understanding of the QGP as a perfect partonic state without any interaction has been reviewed. A new acronym has been defined : sQGP for Strongly Interacting QGP.
This has been obtained by the characterization of the heavy ion collision evolution, from a chemical and dynamical point of view, by comparing effect in heavy ion collisions (for which conditions should be satisfied to form a QGP to collisions at lower energies or involving lighter ions that can not produce the requiered conditions. The QGP is indeed to furtive to be probed directly. My "Habilitation à Diriger des Recherches" presents results of the analyses that I have directed and that contributed to highlight the formation of a new state of matter at RHIC and this new conception of the QGP. Signs of the QGP have been searched with strange particles : resonances of particles containing one strange quark and baryons with two strange quarks.
The production of strange resonances provides indeed information on the hadronization phase (when partons recombined in hadrons) : according to their measurement or not, it is possible to characterize the chemical freeze-out (at which inelastic interactions stop) and cinetic freeze-out (at which elastic interactions stop), if these two freeze-outs coincide or not, if there is some delay between them. The idea is the following : Lambdas(1520) rescatters in a proton and a kaon. Hence, if the time between both freeze-outs is long, products of the desintegration can be rescattered in the dense medium. However, if both freeze-outs coincide or are very close, products of the desintegration are not influenced and the particle mother (resonance) can be identified. Thus, by measuring yields of production of resonances in proton-proton collisions (for which freeze-out coincide)and by comparing to the yields obtained in Au–Au collisions, it appears that indeed at least both freeze-outs are separated by 4 fm/c in Au–Au collisions. This conclusion is an important step in the understanding of ultra_relativistic heavy ion collisions. This analyse was very original within the STAR Collaboration because it was the first study done on strange resonances. Dedicated algorithms have been developed and are used in the collaboration which studies various other resonances or exotic particles.
The production of strange baryons was extensively investigated in the passed experiments since an anormal enhancement of the yields of production is expected if a QGP is formed. CERN experiments have indeed observed an overproduction of strangeness in Pb-Pb collisions but were not able to conlude if a QGP has been created or not since it was possible to explain their results by hadronic gas models. We have realised a similar analysis with the STAR data by comparing the yields of multi-strange baryons (Xi containing two strange quarks), in proton–proton and Au–Au collisiobs at sqrt(s_NN) = 200 GeV. Again, results remain ambigous. These results lead some physicists to the conclusion that strange particle yields can not be viewed as a potential signature of the QGP. However, strangeness comes on stage in a more indirect manner, providing very various information and on the various phases of the collision.
Xi particles indicated firstly that the system created at the full energy at RHIC is thermally and chemically equilibrated (at least in strangeness production. Chemical freeze-out temperatures are close to the temperature predicted by QCD for the phase transition. We have also investigated dynamical collective effects (flow phenomenon) coming from interactions between constituants and lead to an emision of matter in particular directions in phase space. In agreement with their small interaction cross sections, Xi baryons seem to decouple earlier than the lighter particles. However, the fact that baryons suffer a substantial flow, may indicate that they have develop a flow hence that they encountered interactions, before the hadronic phase, differently speaking during the partonic phase. Partons seem to interact ontrarily to the first expectations and predictions of the theoricians 20 years ago.
Moreover, during 2003, the fourth RHIC experiments revealed conjointly the discover of the jet-quenching effect in heavy ion collisions : it corresponds to a suppression of charged particles at high transverse momentum which is nowadays explained by the energy loss of partons in a dense medium. This effect has been studied in the case of Xi particles and showed that jet-quenching affects also baryons and also that they have a different behaviour than the mesons. A particle type dependance has been shown in agreement to coalescence models claiming that hadrons come from quark recombination. This point highlights the fact that quarks may be the relevant degree of freedom.
From these results among others, some theoricians claim the QGP discover at RHIC but experimentalists are more careful and prefer to confirm their results with the study of other signatures.These last five years were particularly exciting with the progress of our knowledges thanks to the wonderful results produced by the fourth RHIC experiments. The QGP has evolved : it is not anymore a perfect plasma without any interactions but a sQGP.
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Habilitation à diriger des recherches
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Submitted on : Tuesday, November 22, 2005 - 4:29:27 PM
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  • HAL Id : tel-00011076, version 1

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Christelle Roy. L'Etrangeté du Plasma de Quarks et de Gluons. Physique Nucléaire Théorique [nucl-th]. Université de Nantes, 2005. ⟨tel-00011076⟩

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