Evidence for the production of thermal muon pairs with masses above 1 GeV/c 2 in 158 A GeV indium-indium collisions

Abstract

The yield of muon pairs in the invariant mass region 1 < M < 2.5 GeV/c 2 produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of prompt muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are indea

Thermal Dileptons from Hot and Dense Strongly Interacting Matter

“…The results are perfectly consistent with no enhancement of open charm, but rather match the level expected from upscaling the NA50 results on charm in pA collisions. The excess observed in the IMR, confirming the result previously found by NA50, is really prompt, with an enhancement over Drell-Yan production of dimuons by a factor of 2.4±0.08 [15]. The excess can now be isolated in the same way as done in the LMR, by subtracting the expected known sources, here Drell-Yan and open charm as plotted in Fig.1, from the total data.…”

“…The NA60 apparatus [15] combines the muon spectrometer previously used by NA10/NA38/NA50 with a novel radiation-hard, silicon-pixel vertex telescope, placed inside a 2.5 T dipole magnet between the target region and the hadron absorber. The matching of the muon tracks in the two spectrometers, both in coordinate and momentum space, greatly improves the dimuon mass resolution, reduces the combinatorial background from π and K decays, and allows to measure the dimuon offset with respect to the primary interaction vertex to tag open charm decays.…”

“…The combinatorial background has been determined with an accuracy of about 1% by using a mixed-event technique [15], the much weaker background of incorrect ("fake") matches between the two spectrometers by an overlay Monte Carlo method. After subtraction of the total background, the resulting net spectrum contains about 440 000 events, exceeding previous results by up to 3 orders of magnitude in effective statistics, depending on mass.…”

“…The subtracted data for the η, ω and φ themselves are subject to the same further steps as the excess data. The (extended) IMR part is shown in the right panel of Fig.1 [15]. The use of the silicon vertex tracker has allowed to disentangle prompt and offset dimuons resulting from simultaneous semi-muonic decays of (correlated) D andD mesons.…”

“…A large step forward in technology, leading to completely new standards of the data quality in this field, has now been achieved by the "thirdgeneration" experiment NA60 at the CERN SPS. The central results have already been published [11][12][13][14][15][16][17]. The present paper shortly reviews them, but places also particular emphasis on the quantitative understanding of the thermal-radiation part in connection with the medium modification of hadrons (one of the main topics of the workshop), just to create confidence towards a community which is usually exposed to "elementary" reactions rather than heavy-ion collisions.…”