arXiv subjects
Publications and source records attributed to A. S. Kapoyannis.
The evolution of a hadronic system after its chemical decomposition is described through a model that conserves the hadronic multiplicities to their values at chemical freeze-out. The state of the system is found as function of temperature and the corresponding baryon density is evaluated. The baryon density at thermal decoupling is also computed.
On the basis of universal scaling properties, we claim that in Au+Au collisions at RHIC, the QCD critical point is within reach. The signal turns out to be an extended plateau of net baryons in rapidity with approximate height of the net-baryon rapidity density approximately 15 and a strong intermittency pattern with index s_2=1/6 in rapidity fluctuations. A window also exists, to reach the critical point at the SPS, especially in Si+Si collisions at maximal energy.
The net-baryon density at midrapidity is proposed as an order parameter in the search for the QCD critical point in heavy ion collisions. As a function of the initial energy and the total number of participants, this quantity obeys a scaling law, dictated by the critical exponents of the appropriate universality class. The corresponding scaling variable specifies the proximity of a given experiment to the critical point. Within this framework, measurements at the SPS are discussed and predictions for RHIC and LHC are extracted.
In the vicinity of the quark-hadron critical point, in the phase diagram of QCD, simple power-law relations constrain the mid-rapidity net-baryon density profile, for different heavy-ion processes, in a unifying scheme. The corresponding scaling variable specifies the proximity of a given experiment to the critical point. An important phenomenological aspect of the critical properties in the baryonic sector is the possibility to observe intermittency effects in the phase space distribution of net baryons.
We explore the applicability of the exact renormalization group to the study of tunnelling phenomena. We investigate quantum-mechanical systems whose energy eigenstates are affected significantly by tunnelling through a barrier in the potential. Within the approximation of the derivative expansion, we find that the exact renormalization group predicts the correct qualitative behaviour for the lowest energy eigenvalues. However, quantitative accuracy is achieved only for potentials with small barriers. For large barriers, the use of alternative methods, such as saddle-point expansions, can provide quantitative accuracy.
A generalized Strangeness-incorporating Statistical Bootstrap Model (SSBM) is constructed so as to include indepedent fugacities for up and down quarks. Such an extension is crucial for the confrontation of multiparticle data emerging from heavy ion collisions, wherein isospin symmetry is not satisfied. Two constraints, in addition to the presence of a critical surface which sets the boundaries of the hadronic world, enter the extended model. An analysis pertaining to produced particle multiplicities and ratios is performed for the S+Ag interaction at 200 GeV/nucleon. The resulting evaluation, concerning the location of the source of the produced system, is slightly in favor the source being outside the hadronic domain.
A recently constructed strangeness-including Statistical Bootstrap Model (S-SBM), which defines the limits of the hadronic phase and provides for a phase beyond, is further extended so as to include a factor that describes strangeness suppression. The model is then used to analyse the multiplicity data from collision experiments in which the colliding entities form isospin symmetric systems, the primary focus being on S+S interactions (NA35 collaboration). An optimal set of thermodynamical variables is extracted through a fit to both the inclusive full phase space and midrapidity data. The assumption that the measured particles originate from a thermally and partial-chemically equilibrated source described by the S-SBM is satisfactorily established. The proximity of the thermodynamical variables extracted from the S+S data to the limits of the hadronic phase is systematically investigated. Finally, experimental data from proton-antiproton collisions (UA5 collaboration) are similarly analysed.
The Statistical Bootstrap Model (SBM) is extended to describe hadronic systems which carry the quantum number of strangeness. The study is conducted in the three-dimensional space of temperature, up-down and strange chemical potentials, wherein the existence of a ``critical'' surface is established, which sets the limits of the hadronic phase of matter. A second surface, defined by the null expectation value of strangeness number is also determined. The approach of the latter surface to the critical one becomes the focal point of the present considerations. Two different versions of the extended SBM are examined, corresponding to the values 2 and 4 for the exponent, which determines the asymptotic fall-off of the mass spectrum. It is found that the version with the value 4 has decisive physical advantages. This model is subsequently adopted to discuss (strange) particle ratios pertaining to multiparticle production processes, for which a thermal equilibrium mode of description applies.
We extend the Statistical Bootstrap Model (SBM) in order to describe hadronic systems which carry strangeness. We establish that the hadronic phase can exist only in a region of the three-dimensional space of temperature, up-down and strange chemical potentials, which is bounded by a critical critical surface. We first employ the usual version of the SBM, which corresponds to the value 2 of the exponent which determines the asymptotic fall-off of the mass spectrum and impose strangeness neutrality in order to obtain the relation between the temperature and the chemical potentials of the quarks. We find an unphysical behaviour of the strange chemical potential near the critical surface. Subsequently, we study another version of SBM where the value of the exponent equals 4 and we are led to a realistic as well as acceptable behaviour. In this case we are also able to relate the parameter of the model, the critical temperature at zero strange chemical potential) with the MIT bag constant B.