Low Mass Dimuons Produced in Relativistic Nuclear Collisions

The NA60 experiment has measured low mass muon pair production in In-In collisions at $158A\mathrm{GeV}$ with unprecedented precision. We show that these data are reproduced very well by a dynamical model with parameters scaled from fits to measurements of hadronic transverse mass spectra and Hanbury Brown–Twiss correlations in Pb-Pb and Pb-Au collisions at the same energy. The data are consistent with in-medium properties of $\rho$ and $\omega$ mesons at finite temperature and density as deduced from empirical forward-scattering amplitudes. Inclusion of the vacuum decay of the $\rho$ meson after freeze-out is necessary for an understanding of the mass and transverse momentum spectrum of dimuons with $M\lesssim 0.9\mathrm{GeV}/c^2$.

Figure 1

The imaginary part of the $\rho$-meson propagator ($\mathrm{Im}D$) for temperature $T=150\mathrm{MeV}$ and baryonic density 0.5 and 1 (in units of nuclear matter density) and at momentum $|\overrightarrow{p}|=300\mathrm{MeV}/c$. The imaginary part of the vacuum propagator is also shown.

Figure 2

Mass spectrum in semicentral In-In collisions at SPS [1] compared to theory. The lower panel shows an integration over all $p_T$, the upper panel one over high transverse momenta $1<p_T<2\mathrm{GeV}/c$. Partial contributions arise from $\rho$ decays in vacuum after freeze-out, thermal in-medium $\rho$ and $\omega$ decays, radiation from a thermalized QGP and from thermal four-pion annihilation, and from correlated open charm decay. Because of severe theoretical uncertainties we have not included contributions from Drell-Yan and possible precursor vector mesons which are anyway expected to be small at the energies considered here.

Figure 3

Transverse momentum spectra as obtained from theory for semicentral In-In collision. The acceptance-corrected data are from NA60 [1].