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Register a new userImproved quark coalescence model for spin alignment and polarization of hadrons
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We propose an improved quark coalescence model for spin alignment of vector mesons and polarization of baryons by spin density matrix with phase space dependence. The spin density matrix is defined through Wigner functions. Within the model we propose an understanding of spin alignments of vector mesons $phi$ and $K^{*0}$ (including $bar{K}^{*0}$) in the static limit: a large positive deviation of $rho_{00}$ for $phi$ mesons from 1/3 may come from the electric part of the vector $phi$ field, while a negative deviation of $rho_{00}$ for $K^{*0}$ may come from the electric part of vorticity tensor fields. Such a negative contribution to $rho_{00}$ for $K^{*0}$ mesons, in comparison with the same contribution to $rho_{00}$ for $phi$ mesons which is less important, is amplified by a factor of the mass ratio of strange to light quark times the ratio of $leftlangle mathbf{p}_{b}^{2}rightrangle $ on the wave function of $K^{*0}$ to $phi$ ($mathbf{p}_{b}$ is the relative momentum of two constituent quarks of $K^{*0}$ and $phi$). These results should be tested by a detailed and comprehensive simulation of vorticity tensor fields and vector meson fields in heavy ion collisions.
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We study the production of charmed hadrons $D^{0}$ and $Lambda_c^+$ in relativistic heavy-ion collisions using an improved quark coalescence model. In particular, we extend the usual coalescence model by letting a produced hadron to have the same velocity as the center-of-mass velocity of coalesced constituent quarks during hadronization to take into account the effect of collective flow in produced quark-gluon plasma. This results in a shift of charmed resonances of higher masses to larger transverse momenta ($p_T^{}$). Requiring all charm quarks of very low $p_T^{}$ to be converted to hadrons via coalescence and letting charm quarks not undergoing coalescence to hadronize by independent fragmentation, we obtain a good description of the measured yield ratio $Lambda_c^+/D^0$ as a function of $p_T^{}$ in $text{Au} + text{Au}$ collisions at $sqrt{s_{NN}}^{}=200$~GeV by the STAR Collaboration at the Relativistic Heavy Ion Collider.
The string melting version of a multi-phase transport model is often applied to high-energy heavy-ion collisions since the dense matter thus formed is expected to be in parton degrees of freedom. In this work we improve its quark coalescence component, which describes the hadronization of the partonic matter to a hadronic matter. We removed the previous constraint that forced the numbers of mesons, baryons, and antibaryons in an event to be separately conserved through the quark coalescence process. A quark now could form either a meson or a baryon depending on the distance to its coalescence partner(s). We then compare results from the improved model with the experimental data on hadron $dN/dy$, $p_{_{rm T}}$ spectra, and $v_2$ in heavy-ion collisions from $sqrt{s_{_{rm NN}}}=62.4$ GeV to $5.02$ TeV. We show that, besides being able to describe these observables for low-$p_{_{rm T}}$ pions and kaons, the improved model also better describes the low-$p_{_{rm T}}$ baryon observables in general, especially the baryon $p_{_{rm T}}$ spectra and antibaryon-to-baryon ratios for multistrange baryons.
We propose an improved quark coalescence model for spin alignment of vector mesons by spin density matrix with phase space dependence. Within this model we propose an understanding of spin alignments of vector mesons $phi$ and $K^{*0}$ in the static limit: a large positive deviation of $rho_{00}$ for $phi$ mesons from $1/3$ may come from the electric part of the vector $phi$ field, while a negative deviation of $rho_{00}$ for $K^{*0}$ mesons may come from the electric part of vorticity fields. In the low-$p_T$ region, $rho_{00}$ for $K^{*0}$ mesons is proportional to $p_T^2$, which is qualitatively agree with experimental results.
Identifying hadronic molecular states and/or hadrons with multi-quark components either with or without exotic quantum numbers is a long standing challenge in hadronic physics. We suggest that studying the production of these hadrons in relativistic heavy ion collisions offer a promising resolution to this problem as yields of exotic hadrons are expected to be strongly affected by their structures. Using the coalescence model for hadron production, we find that compared to the case of a non-exotic hadron with normal quark numbers, the yield of an exotic hadron is typically an order of magnitude smaller when it is a compact multi-quark state and a factor of two or more larger when it is a loosely bound hadronic molecule. We further find that due to the appreciable numbers of charm and bottom quarks produced in heavy ion collisions at RHIC and even larger numbers expected at LHC, some of the newly proposed heavy exotic states could be produced and realistically measured in these experiments.
We use a thermal model with single freeze-out to determine longitudinal polarization of $Lambda$ hyperons emitted from a hot and rotating hadronic medium. We consider the top RHIC energies and use the model parameters determined in the previous analyses of particle spectra and elliptic flow. Using a direct connection between the spin polarization tensor and thermal vorticity, we reproduce earlier results which indicate a quadrupole structure of the longitudinal component of the polarization three-vector with an opposite sign compared to that found in the experiment. We further use only the spatial components of the thermal vorticity in the laboratory system to define polarization and show that this leads to the correct sign and magnitude of the quadrupole structure. This procedure resembles a non-relativistic connection between the polarization three-vector and vorticity employed in other works. In general, our results bring further evidence that the spin polarization dynamics in heavy-ion collisions may be not directly related to the thermal vorticity. The additional material explains the construction of the hydrodynamicaly consistent gradients of fluid velocity and temperature in thermal models with the help of the perfect-fluid equations of motion.
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