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Register a new userIn-medium properties of the phi meson with phi N resonant contributions
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Astrid Nathalie Hiller Blin
Publication date
2016
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English
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Nuclear production experiments report missing absorption processes of the in-medium phi meson. Contributions arising from the K-bar K cloud have already been widely studied, and therefore we investigate the phi-meson properties in cold nuclear matter with the additional inclusion of resonant phi N interactions. Two models are considered which dynamically generate N*-like states close to the phi N threshold. We find that these states, together with the non-resonant part of the amplitude, contribute to the phi self-energy with the same order of magnitude as the K-bar K effects. At non-vanishing nuclear density, both models lead to an additional in-medium broadening of the phi, up to around 50 MeV. Furthermore, at least one of the models is compatible with a mass shift to lower energies of up to 35 MeV at threshold and normal matter density. Finally, a double-peak structure appears in the spectral function due to the mixing of resonance-hole modes with the $phi$ quasi-particle peak. These results converge into the direction of the experimental findings.
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We investigate the $phi$ meson nuclear transparency using some recent theoretical developments on the $phi$ in medium self-energy. The inclusion of direct resonant $phi N$-scattering and the kaon decay mechanisms leads to a $phi$ width much larger than in most previous theoretical approaches. The model has been confronted with photoproduction data from CLAS and LEPS and the recent proton induced $phi$ production from COSY finding an overall good agreement. The results support the need of a quite large direct $phi N$-scattering contribution to the self-energy.
The $phi$-meson properties in cold nuclear matter are investigated by implementing resonant $phi N$ interactions as described in effective approaches including the unitarization of scattering amplitudes. Several $N^*$-like states are dynamically generated in these models around $2$ GeV, in the vicinity of the $phi N$ threshold. We find that both these states and the non-resonant part of the amplitude contribute sizably to the $phi$ collisional self-energy at finite nuclear density. These contributions are of a similar strength as the widely studied medium effects from the $bar K K$ cloud. Depending on model details (position of the resonances and strength of the coupling to $phi N$) we report a $phi$ broadening up to about $40$-$50$ MeV, to be added to the $phitobar K K$ in-medium decay width, and an attractive optical potential at threshold up to about $35$ MeV at normal matter density. The $phi$ spectral function develops a double peak structure as a consequence of the mixing of resonance-hole modes with the $phi$ quasi-particle peak. The former results point in the direction of making up for missing absorption as reported in $phi$ nuclear production experiments.
The production of phi mesons in the collisions of 2.83 GeV protons with C, Cu, Ag, and Au at forward angles has been measured via the phi -> K+K- decay using the COSY-ANKE magnetic spectrometer. The phi meson production cross section follows a target mass dependence of A^0.56+/-0.02 in the momentum region of 0.6-1.6 GeV/c. The comparison of the data with model calculations suggests that the in-medium phi width is about an order of magnitude larger than its free value.
A Multi-Phase Transport Model (AMPT) has been extensively used to understand the physics behind the experimental observation. Like other models, the outcome of the AMPT model depends on the initial parameters. Therefore, one needs to choose suitable initial parameters before using the model. Lund string fragmentation function has been used to create quark-antiquarks pairs in the AMPT model. The Lund string fragmentation parameters determine the yields and transverse momentum ($p_{T}$) spectra of particles produced in nucleus-nucleus collisions. The values of Lund string fragmentation parameters were determined by fitting the charged particle yield and $p_{T}$ spectra measured in the experiment. In this paper, we have shown the yield of strange quarks carrying hadrons, e.g. $phi$ mesons, are more sensitive to Lund string fragmentation parameters compared to non-strange pions. Using $phi$ meson spectra measured at RHIC, we have obtained new sets of Lund parameters for Au+Au collisions at $sqrt{s_{mathrm {NN}}}$ = 11.5, 39, and 200 GeV. We have found that using the same set of parameters, we can explain $phi$-meson yield at $sqrt{s_{mathrm {NN}}}$ = 39 and 200 GeV, however, we need a different set of parameters for $sqrt{s_{mathrm {NN}}}$ = 11.5 GeV. This suggests that at low energy, $sqrt{s_{mathrm {NN}}}$ = 11.5 GeV, the underlying mechanism for particle production is different compared to top RHIC energies. We have also predicted invariant yield of $pi$ and $phi$ mesons as a function of $p_{T}$ in U+U collisions at $sqrt{s_{mathrm {NN}}}$ = 196 GeV to be measured by STAR experiment.
Invariant mass spectra of e+e- pairs have been measured in 12 GeV p+A reactions to detect possible in-medium modification of vector mesons. Copper and carbon targets are used to study the nuclear-size dependence of e+e- invariant mass distributions. A significant excess on the low-mass side of the phi meson peak is observed in the low beta gamma (=beta/sqrt(1-beta^2)) region of phi mesons (beta gamma < 1.25) with copper targets. However, in the high beta gamma region (beta gamma > 1.25), spectral shapes of phi mesons are well described by the Breit-Wigner shape when experimental effects are considered. Thus, in addition to our earlier publication on rho / omega modification, this study has experimentally verified vector meson mass modification at normal nuclear density.
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