The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity ∣yee∣+ Au collisions at √sNN = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened ρ spectral function for Mee < 1.1 GeV/c2. The integrated dielectron excess yield at √sNN = 19.6 GeV for 0.4 < Mee < 0.75 GeV/c2, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in ln+ln collisions at √sNN = 17.3 GeV. For √sNN = 200 GeV, the normalized excess yield in central collisions is higher than that at √sNN = 17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au + Au collisions at √sNN = 200 GeV is longer than those in peripheral collisions and at lower energies.

Document Type


Publication Date


Notes/Citation Information

Published in Physics Letters B, v. 750, p. 64-71.

© 2015 The Authors. Published by Elsevier B.V.

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.

Due to the large number of authors involved, only the first 10 and the ones affiliated with the University of Kentucky are listed in the author section above. The authors of this article are collectively known as STAR Collaboration. To see a full list of authors, please download this article or visit: http://dx.doi.org/10.1016/j.physletb.2015.08.044.

Digital Object Identifier (DOI)


Funding Information

This work was supported in part by the Offices of NP and HEP within the U.S. DOEOffice of Science, the U.S. NSF, CNRS/IN2P3, FAPESPCNPq of Brazil, the Ministry of Education and Science of the Russian Federation, NNSFC, CAS, MoST and MoE of China, the Korean Research Foundation, GA and MSMT of the Czech Republic, FIAS of Germany, DAE, DST, and CSIR of India, the National Science Centre of Poland, National Research Foundation (NRF-2012004024), the Ministry of Science, Education and Sports of the Republic of Croatia, and RosAtom of Russia.