Measurements of dielectron production in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Au</mml:mi><mml:mo>+</mml:mo><mml:mi>Au</mml:mi></mml:mrow></mml:math> collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:msub><mml:mi>s</mml:mi><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:msub></mml:msqrt><mml:mo>=</mml:mo><mml:mn>27</mml:mn></mml:mrow></mml:math>, 39, and 62.4 GeV from the STAR experiment
M. I. Abdulhamid, B. E. Aboona, J. Adam, L. Adamczyk, J. R. Adams, Y. Hu, M. M. Aggarwal, Z. Ahammed, D. M. Anderson, E. C. Aschenauer, S. Aslam, J. Atchison, V. Bairathi, W. Baker, J. Ball, K. N. Barish, R. Bellwied, P. Bhagat, A. Bhasin, S. Bhatta, J. Bielčík, J. Bielčíková, J. D. Brandenburg, J. M. Butterworth, X. Z. Cai, H. Caines, M. Calderón de la Barca Sánchez, D. Cebra, J. Ceska, I. Chakaberia, P. Chaloupka, B. K. Chan, Z. Chang, A. Chatterjee, D. Chen, J. Chen, C. H. Cheng, Z. Chen, J. Cheng, Y. Cheng, S. Choudhury, W. Christie, X. Chu, H. J. Crawford, M. Csanád, G. Dale-Gau, Arpita Das, M. Daugherity, I. M. Deppner, A. Dhamija, L. Di Carlo, L. Didenko, P. Dixit, X. Dong, J. L. Drachenberg, E. Duckworth, J. C. Dunlop, J. Engelage, G. Eppley, S. Esumi, O. Evdokimov, A. Ewigleben, O. Eyser, R. Fatemi, S. Fazio, C. Feng, Y. Feng, E. Finch, Y. Fisyak, F. A. Flor, C. Fu, C. A. Gagliardi, T. Galatyuk, F. J. M. Geurts, N. Ghimire, S. M. Gibson, K. Gopal, X. Gou, D. Grosnick, Yuhan Guo, A. Gupta, W. Guryn, A. Hamed, Y. Han, S. Harabasz, M. D. Harasty, J. W. Harris, H. Harrison-Smith, W. He, X. H. He, Y. He, N. Herrmann, L. Holub, C. Hu, Q. Hu, Y. Hu, B. Huang, H. Z. Huang, H. Z. Huang, S. Huang
Abstract
We report systematic measurements of dielectron (${e}^{+}{e}^{\ensuremath{-}}$) invariant-mass ${M}_{ee}$ spectra at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}$ = 27, 39, and 62.4 GeV taken with the STAR detector at the Relativistic Heavy Ion Collider. For all energies studied, a significant excess yield of dielectrons is observed in the low-mass region ($0.40<{M}_{ee}<0.75 \mathrm{MeV}/{c}^{2}$) compared to hadronic cocktail simulations at freeze-out. Models that include an in-medium broadening of the $\ensuremath{\rho}$-meson spectral function consistently describe the observed excess. In addition, we report acceptance-corrected dielectron-excess spectra for $\mathrm{Au}+\mathrm{Au}$ collisions at midrapidity ($|{y}_{ee}|\phantom{\rule{4pt}{0ex}}<$ 1) in the 0--80% centrality bin for each collision energy. The integrated excess yields for $0.4<{M}_{ee}<0.75\phantom{\rule{4pt}{0ex}}\text{GeV}/{c}^{2}$, normalized by the charged particle multiplicity at midrapidity, are compared with previously published measurements for $\mathrm{Au}+\mathrm{Au}$ at $\sqrt{{s}_{NN}\phantom{{}^{1}}}$ = 19.6 and 200 GeV. Models that include an in-medium broadening of the $\ensuremath{\rho}$-meson spectral function consistently describe the observed excess. The normalized excess yields in the low-mass region show no significant collision energy dependence. The data, however, are consistent with model calculations that demonstrate a modest energy dependence.