First Measurement of theρSpectral Function in High-Energy Nuclear Collisions

Abstract

We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360,000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi+pi- -->rho -->mu+mu- annihilation. The associated space-time averaged spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.

K̄*mesons in dense matter

“…This spectacular increase is much larger than the width of the ρ meson in matter, evaluated theoretically in Refs. [6][7][8][9] or measured recently [12,14,17,18].…”

“…U. In the following, we evaluate the ratio between the nuclear cross sections in heavy nuclei and a light nucleus, for instance, 12 C, as, in this way, many other nuclear effects not related to absorption of the K * − cancel in the ratio [77]. We call this ratio T A ,…”

“…More recent detailed calculations show no shift of the mass of the ρ meson in matter [6][7][8][9][10] and a very small shift for the φ meson [11]. Experimentally the situation has undergone big strides recently, an example being the NA60 Collaboration, which reported a null shift of the ρ mass in the medium [12][13][14] in the dilepton spectra of heavy-ion reactions. Detailed theoretical analyses of the reaction were also done in Refs.…”

Medium modifications of theρmeson in nuclear photoproduction

“…Second, one could concentrate on lepton pairs with low three-momentum, which should enhance the decay fraction inside the nucleus because of the smaller distance they travel from production to decay point. 5 A breakdown of the data in several momentum bins would illuminate the three-momentum dependence of the spectral function, a critical property as indicated above. Third, as the presumably most straightforward option on the experimental side, one could utilize the absolute magnitude of the total cross section in terms of the so-called nuclear transparency ratio.…”

“…On the experimental side, electromagnetic probes (real and virtual photons) are very suitable because no significant distortions occur owing to strong initial-and/or final-state interactions with the nuclear medium. Invariantmass spectra of dileptons in heavy-ion collisions (HICs) at the Super Proton Synchrotron (SPS) [5][6][7] clearly revealed the presence of medium modifications of the ρ-meson spectral function in the hot and dense medium. The most recent NA60 dimuon data [8] enable the average medium effect on the ρ meson to be quantified as an approximately threefold broadening relative to its free width with little (if any) mass shift [9,10].…”