Thermal dileptons to probe the baryon-rich QCD matter in the forward region of LHC energy heavy-ion collisions

Abstract

We investigate thermal dilepton production from a quark-gluon plasma (QGP) with finite baryon chemical potential ($μ_{\text{B}}$) in central Pb-Pb collisions at $\sqrt{s_{\text{NN}}}=5.02~\text{TeV}$. Recent studies suggest that sizable baryon densities can be achieved at forward rapidity even at LHC energies. We incorporate finite $μ_{\text{B}}$ into a (3+1)-dimensional hydrodynamic framework and find that $μ_{\text{B}}$ exceeds 500 MeV around $η_\text{s} = 6$ during the medium evolution. Using this framework, we calculate thermal dilepton spectra over a wide rapidity range and evaluate the impact of finite $μ_{\text{B}}$ on dilepton production. A suppression of 3-4% is observed in the forward-rapidity region $5.2 < y < 7.2$ due to the reduced quark-antiquark abundance at finite baryon density. We further examine the effective temperature extracted from dilepton mass spectra in the intermediate-mass region $1.2 < M_{\ell \ell} < 2.6~\text{GeV}$ . The effective temperature remains strongly correlated with the underlying hydrodynamic temperature and retains sensitivity to the early high-temperature stage of the QGP evolution. These results demonstrate that forward-rapidity dileptons remain effective thermometers while providing sensitivity to finite baryon density at the LHC.