Speaker
Description
In-medium modifications of vector spectral functions are anticipated to carry imprints of restoration of chiral symmetry at high temperature/baryon density. Strong modifications have been indeed measured via dimuon production at the CERN SPS, whereas it remains inconclusive how to quantify those changes as the direct consequence of chiral symmetry restoration. Given the difficulty that it is elusive to construct the axial-vector spectrum in heavy-ion experiments, the key phenomenon is that the vector meson mixes with the axial-vector meson in a medium via pion loops, known as chiral mixing. The chiral mixing effect can be quantified as a model-independent theorem at low temperature [1], whereas at the critical temperature the chiral symmetry restoration compels the vanishing chiral-mixing [2]. On-going experiments and recent development of hydrodynamic model bring us a unique opportunity to get insight into chiral symmetry restoration from investigation of the dilepton invariant mass spectra.
In this work, a case study is carried out by utilizing the following 3 scenarios: (A) chiral restoration absent, (B) chiral restoration present and (C) false chiral restoration. The scenario-B describes the in-medium spectral function of the rho meson from zero to the chiral crossover temperature [2]. The scenario-C is often argued to generate an increase of the dilepton yield, which is incorrect since the procedure does not capture the change of QCD ground state.
We combine the in-medium spectral function in the chiral effective field theory with state-of-the-art fluid dynamical simulations with temperature-dependent shear and bulk viscosities [3]. We find an increase of the yield at 1.2-1.4 GeV due to the degenerate $\rho$ and $a_1$ mesons, whereas the scenario of false restoration leads to a substantial overestimate at 1.2 GeV and this also causes an underestimate below and above the $\rho$ peak.
[1] M. Dey, V. L. Eletsky, and B. L. Ioffe, Phys. Lett. B 252, 620 (1990).
[2] M. Harada, C. Sasaki, and W. Weise, Phys. Rev. D 78, 114003 (2008).
[3] K. Okamoto and C. Nonaka, Phys. Rev. C 98, no.5, 054906 (2018); H. Fujii, K. Itakura, K. Miyachi and C. Nonaka, Phys. Rev. C 106, no.3, 034906 (2022).
| Category | Theory |
|---|
