NA60 results on thermal dimuons

Abstract

The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158 A GeV In-In collisions. A strong excess of pairs above the known sources is observed in the whole mass region 0.2 < M < 2.6 GeV. The mass spectrum for M < 1 GeV is consistent with a dominant contribution from π + π − → ρ → μ + μ − annihilation. The associated ρ spectral function shows a strong broadening, but essentially no shift in mass. For M > 1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differences to Drell-Yan pairs. The slope parameter T eff associated with the transverse momentum spectra rises with mass up to the ρ, followed by a sudden decline above. The rise for M < 1 GeV is consistent with radial flow of a hadronic emission source. The seeming absence of significant flow for M > 1 GeV and its relation to parton-hadron duality is discussed in detail, suggesting a dominantly partonic emission source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Plancklike shape of the mass spectra at low p T and the lack of a e-mail: sanja.damjanovic@cern.ch polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on ω in-medium effects.PACS 25.75.-q · 12.38.Mh · 13.85.Qk

Massive Yang–Mills for vector and axial-vector spectral functions at finite temperature

“…The only in-medium hadronic spectral function which is experimentally accessible is that of the light vector mesons, dominated by the ρ, which can be probed via low-mass dilepton spectra in heavy-ion collisions [3][4][5][6][7]. Theory calculations reveal that the experimental data are compatible with a ρ melting scenario whereby the spectral peak broadens without a significant shift in its mass, see, e.g., Refs.…”

Dilepton production from RHIC to the LHC

“…With its short lifetime the ρ spectral function should be very sensitive to the predicted restoration of chiral symmetry inside the fireball. Enhancements of the invariant-mass spectrum of dileptons above the "cocktail" of ordinary hadron decays had been observed in heavy-ion collisions by the CERES and NA60 experiments at the SPS [1,2]. The latter was able to precisely determine that the enhancement was consistent with a broadening of the ρ spectral function, but could not be described by a "dropping" mass scenario as originally predicted by Brown and Rho [3].…”

The problem of anomalous dilepton yield in relativistic heavy-ion collisions

“…Because they are not known with precision in the present setting, particularly their off-shell values, the relative weights are exploited as free parameters in the hadronic 'cocktail' [5,30,31]. This is the procedure used both by NA60 and PHENIX and we shall follow it here too.…”

“…In order to include a proper description of the p T spectrum a more realistic statistical description would be needed. Nevertheless the general features of the LPB-induced modifications should be already visible in a simplified description to be compared to the experimental data averaged over space-time and momenta for NA60 [31] or acceptance-corrected for PHENIX [7]. This simple model is appropriate only for central collisions and moderate values of p T .…”

“…The corresponding data for central collisions is shown in Fig.7 along with the contribution from the 'cocktail' scaled by the 2.3 factor and using m ρ = 750 MeV. The 'cocktail' ρ can be obtained from (3.1) by setting ζ = 0 and adjusting the effective temperature to T ≃ 300 MeV [31]. However, even after optimization of the constant to reproduce central ρ production, the agreement of the data with the 'cocktail' ρ is poor reflecting the long standing problem of the insufficient dilepton yield.…”