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Register a new userThe low energy frontier: What is exciting about physics below the top RHIC energy
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Marcus Bleicher
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These proceedings summarize my plenary talk at Quark Matter 2011 with a focus on the future perspectives of the low energy programs at RHIC, FAIR, NICA and CERN.
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A state-of-the-art 3+1 dimensional cascade + viscous hydro + cascade model vHLLE+UrQMD has been applied to heavy ion collisions in RHIC Beam Energy Scan range $sqrt{s_{rm NN}}=7.7dots 200$ GeV. Based on comparison to available experimental data it was estimated that an effective value of shear viscosity over entropy density ratio $eta/s$ in hydrodynamic stage has to decrease from $eta/s=0.2$ to $0.08$ as collision energy increases from $sqrt{s_{rm NN}} = 7.7$ to $39$ GeV, and to stay at $eta/s=0.08$ for $39lesqrt{s_{rm NN}}le200$ GeV. In this work we show how an equation of state with first order phase transition affects the hydrodynamic evolution at those collision energies and changes the results of the model as compared to default scenario with a crossover type EoS from chiral model.
We study $eta$ photoproduction off the deuteron ($gamma dtoeta pn$) at a special kinematics: $sim 0.94$ GeV of the photon beam energy and $sim 0^circ$ of the scattering angle of the proton. This kinematics is ideal to extract the low-energy $eta$-nucleon scattering parameters such as $a_{eta N}$ (scattering length) and $r_{eta N}$ (effective range) because the $eta$-nucleon elastic scattering is significantly enhanced. We show that if a ratio $R$, the $gamma dtoeta pn$ cross section divided by the $gamma ptoeta p$ cross section convoluted with the proton momentum distribution in the deuteron, is measured with 5% error, ${rm Re}[a_{eta N}]$ (${rm Re}[r_{eta N}]$) can be determined at the precision of $simpm$0.1 fm ($simpm$0.5 fm), significantly narrowing down the currently estimated range of the parameters. The measurement is ongoing at the Research Center for Electron Photon Science (ELPH), Tohoku University.
We investigate possibility of emission of the bremsstrahlung photons in nuclear reactions with hypernuclei for the first time. A new model of the bremsstrahlung emission which accompanies interactions between $alpha$ particles and hypernuclei is constructed, where a new formalism for the magnetic momenta of nucleons and hyperon inside hypernucleus is added. For first calculations, we choose $alpha$ decay of the normal nucleus $^{210}{rm Po}$ and the hypernucleus $^{211}_{Lambda}{rm Po}$. We find that (1) emission for the hypernucleus $^{211}_{Lambda}{rm Po}$ is larger than for normal nucleus $^{210}{rm Po}$, (2) difference between these spectra is small. We propose a way how to find hypernuclei, where role of hyperon is the most essential in emission of bremsstrahlung photons during $alpha$ decay. As demonstration of such a property, we show that the spectra for the hypernuclei $^{107}_{Lambda}{rm Te}$ and $^{109}_{Lambda}{rm Te}$ are essentially larger than the spectra for the normal nuclei $^{106}{rm Te}$ and $^{108}{rm Te}$. Such a difference is explained by additional contribution of emission to the full bremsstrahlung, which is formed by magnetic moment of hyperon inside hypernucleus. The bremsstrahlung emission formed by such a mechanism, is of the magnetic type. A new formula for fast estimations of bremsstrahlung spectra for even-even hypernuclei is proposed, where role of magnetic moment of hyperon of hypernucleus in formation of the bremsstrahlung emission is shown explicitly. Such an analysis opens possibility of new experimental study of properties of hypernuclei via bremsstrahlung study.
We study dielectron production in proton-proton collisions at top RHIC beam energy within an extended statistical hadronization model. The invariant mass spectrum of correlated dielectron pairs is evaluated in the low invariant mass region and calculated results are compared with the PHENIX experiment. The model is found to be able to describe the data very well up to invariant masses of 1 GeV with few adjustable parameters.
Ultra-peripheral collisions (UPCs) involving heavy ions and protons are the energy frontier for photon-mediated interactions. UPC photons can be used for many purposes, including probing low-$x$ gluons via photoproduction of dijets and vector mesons, probes of beyond-standard-model processes, such as those enabled by light-by-light scattering, and studies of two-photon production of the Higgs.
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