159,338 research outputs found

    Difficulties in probing density dependent symmetry potential with the HBT interferometry

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    Based on the updated UrQMD transport model, the effect of the symmetry potential energy on the two-nucleon HBT correlation is investigated with the help of the coalescence program for constructing clusters, and the CRAB analyzing program of the two-particle HBT correlation. An obvious non-linear dependence of the neutron-proton (or neutron-neutron) HBT correlation function (Cnp,nnC_{np,nn}) at small relative momenta on the stiffness factor γ\gamma of the symmetry potential energy is found: when γ0.8\gamma \lesssim 0.8, the Cnp,nnC_{np,nn} increases rapidly with increasing γ\gamma, while it starts to saturate if γ0.8\gamma \gtrsim 0.8. It is also found that both the symmetry potential energy at low densities and the conditions of constructing clusters at the late stage of the whole process influence the two-nucleon HBT correlation with the same power.Comment: 11 pages, 4 figure

    Numerical study of mach number effects in compressible wall-bounded turbulence

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    The aim of this work is to improve the present understanding of compressibility effects in wall-bounded turbulence and to provide data for improving models. A family of wall-bounded compressible flows has been investigated using direct numerical simulation (DNS). The research is divided into two aspects: a study of the intrinsic compressibility effects in isothermal-wall channel flow and a study of the impinging shock/turbulent boundary-layer interaction. For the channel flow, an energy sink is introduced in the energy equation to effectively eliminate the compressibility effects caused by mean-property variation, isolating the intrinsic compressibility effects induced by fluctuations of the density and temperature (and pressure, dilatation, etc) fields. Centreline Mach numbers, Mcl, up to 6.2 have been considered, for which we find that both explicit compressibility terms in the TKE equation such as the pressure-dilatation and dilatational dissipation, and the implicit compressibility such as Reynolds-stress-anisotropy tensor begin to become important. An oblique shock/turbulent boundary-layer interaction at free stream Mach number M ? = 2 is also investigated. Central to this work is the need to quickly obtain a fully developed turbulent boundary-layer over the shortest possible downstream distance, for which a quasi-deterministic inflow strategy is used. Then an oblique shock is impinged on to the fully developed turbulence boundary-layer and the flow separates. Explicit and implicit compressibility effects become important and the TKE budget is altered completely within the interaction zone. The interactions of shock with the separation bubble and the velocity are also addressed

    A model comparison of resonance lifetime modifications, a soft equation of state and non-Gaussian effects on ππ\pi-\pi correlations at FAIR/AGS energies

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    HBT correlations of ππ\pi^--\pi^- pairs at FAIR/AGS energies are investigated by using the UrQMD transport model and the CRAB analyzing program. Three different possible sources (treatment of resonance lifetimes, a soft equation of state and non-Gaussian effects) to understand the HBT RO/RSR_O/R_S puzzle are investigated. Firstly, we find that different treatments of the resonance decay time can not resolve the HBT time-related puzzle, however it can modify the HBT radii at low transverse momenta to some extent to explain the data slightly. Secondly, with a soft equation of state with momentum dependence, the measured transverse momentum dependent HBT radii and RO/RSR_O/R_S ratio can be described fairly well. Thirdly, non-Gaussian effects are visible in the calculated correlation function. Using the Edgeworth expansion, one finds that the non-Gaussian effect is strongest in the longitudinal direction and weakest in the sideward direction.Comment: 18 pages, 6 figures. To be published in J.Phys.

    The role of qqˉq\bar q components in the N(1440) resonance

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    The role of 5-quark components in the pion and electromagnetic decays and transition form factors of the N(1440) is explored. The qqqqqˉqqqq\bar q components, where the 4-quark subsystem has the flavor-spin symmetries [4]FS[22]F[22]S[4]_{FS}[22]_F[22]_S and [4]FS[31]F[31]S[4]_{FS}[31]_F[31]_S, which are expected to have the lowest energy of all qqqqqˉqqqq\bar q configurations, are considered in detail with a nonrelativistic quark model. The matrix elements between the 5-quark components of the N(1440) and the nucleon, qqqqqˉqqqqqˉqqqq\bar q\to qqqq\bar q, play a minor role in these decays, while the transition matrix elements qqqqqˉqqqqqqq\bar q\to qqq and qqqqqqqqˉqqq\to qqqq\bar q that involve quark antiquark annihilation are very significant. Both for the electromagnetic and strong decay the change from the valence quark model value is dominated by the confinement triggered qqˉq\bar q annihilation transitions. In the case of pion decay the calculated decay width is enhanced substantially both by the direct qqˉπq\bar q \to \pi and also by the confinement triggered qqˉπq\bar q\to \pi transitions. Agreement with the empirical value for the pion decay width may be reached with a \sim 30% qqqqqˉqqqq\bar q component in the N(1440).Comment: 23 pages revte