Low-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>p</mml:mi><mml:mi>T</mml:mi></mml:msub></mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>e</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:msup><mml:mi>e</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:math> Pair Production in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>Au</mml:mi><mml:mo stretchy="false">+</mml:mo><mml:mi>Au</mml:mi></mml:mrow></mml:math> Collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><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>200</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>GeV</mml:mi></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">U</mml:mi><mml:mo stretchy="false">+</mml:mo><mml:mi mathvariant="normal">U</mml:mi></mml:mrow></mml:math> Collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msqrt><mml:mrow><mml:msub><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:msqrt><mml:mo>=</mml:mo><mml:mn>193</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>GeV</mml:mi></mml:mrow></mml:math> at STAR
J. AdamCreighton University, Omaha, Nebraska 68178L. AdamczykAGH University of Science and Technology, FPACS, Cracow 30-059, PolandJ. R. AdamsOhio State University, Columbus, Ohio 43210J. K. AdkinsUniversity of Kentucky, Lexington, Kentucky 40506-0055G. AgakishievJoint Institute for Nuclear Research, Dubna 141 980, RussiaM. M. AggarwalPanjab University, Chandigarh 160014, IndiaZ. AhammedVariable Energy Cyclotron Centre, Kolkata 700064, IndiaN. N. AjitanandState University of New York, Stony Brook, New York 11794I. AlekseevAlikhanov Institute for Theoretical and Experimental Physics, Moscow 117218, RussiaD. M. AndersonTexas A&M University, College Station, Texas 77843Y. G.University of Tsukuba, Tsukuba, Ibaraki 305-8571, JapanA. AparinJoint Institute for Nuclear Research, Dubna 141 980, RussiaD. ArkhipkinBrookhaven National Laboratory, Upton, New York 11973E. C. AschenauerBrookhaven National Laboratory, Upton, New York 11973M. U. AshrafTsinghua University, Beijing 100084F. AtetallaKent State University, Kent, Ohio 44242A. AttriPanjab University, Chandigarh 160014, IndiaG. S. AverichevJoint Institute for Nuclear Research, Dubna 141 980, RussiaX. BaiCentral China Normal University, Wuhan, Hubei 430079V. BairathiNational Institute of Science Education and Research, HBNI, Jatni 752050, IndiaK. N. BarishUniversity of California, Riverside, California 92521A. J. BassillUniversity of California, Riverside, California 92521A. BeheraState University of New York, Stony Brook, New York 11794R. BellwiedUniversity of Houston, Houston, Texas 77204A. BhasinUniversity of Jammu, Jammu 180001, IndiaA. K. BhatiPanjab University, Chandigarh 160014, IndiaJ. BielčíkCzech Technical University in Prague, FNSPE, Prague 115 19, Czech RepublicJ. BielčíkováL. C. BlandBrookhaven National Laboratory, Upton, New York 11973I. G. BordyuzhinAlikhanov Institute for Theoretical and Experimental Physics, Moscow 117218, RussiaJ. D. BrandenburgRice University, Houston, Texas 77251A. V. BrandinNational Research Nuclear University MEPhI, Moscow 115409, RussiaD. S. BrownLehigh University, Bethlehem, Pennsylvania 18015J. BryslawskyjUniversity of California, Riverside, California 92521I. BunzarovJoint Institute for Nuclear Research, Dubna 141 980, RussiaJ. M. ButterworthRice University, Houston, Texas 77251H. CainesYale University, New Haven, Connecticut 06520M. Calderón de la Barca SánchezUniversity of California, Davis, California 95616J. M. CampbellOhio State University, Columbus, Ohio 43210D. CebraUniversity of California, Davis, California 95616I. ChakaberiaBrookhaven National Laboratory, Upton, New York 11973P. ChaloupkaCzech Technical University in Prague, FNSPE, Prague 115 19, Czech RepublicF-H. ChangNational Cheng Kung University, Tainan 70101Z. ChangBrookhaven National Laboratory, Upton, New York 11973N. Chankova-BunzarovaJoint Institute for Nuclear Research, Dubna 141 980, RussiaA. ChatterjeeVariable Energy Cyclotron Centre, Kolkata 700064, IndiaS. ChattopadhyayVariable Energy Cyclotron Centre, Kolkata 700064, IndiaJ. H. ChenShanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800X. ChenInstitute of Modern Physics, Chinese Academy of Sciences, Lanzhou, Gansu 730000X. ChenInstitute of Modern Physics, Chinese Academy of Sciences, Lanzhou, Gansu 730000J. ChengTsinghua University, Beijing 100084M. CherneyCreighton University, Omaha, Nebraska 68178W. ChristieBrookhaven National Laboratory, Upton, New York 11973G. ContinLawrence Berkeley National Laboratory, Berkeley, California 94720H. J. CrawfordUniversity of California, Berkeley, California 94720S. DasCentral China Normal University, Wuhan, Hubei 430079T. G. DedovichJoint Institute for Nuclear Research, Dubna 141 980, RussiaI. M. DeppnerUniversity of Heidelberg, Heidelberg 69120, GermanyA. A. DerevschikovInstitute of High Energy Physics, Protvino 142281, RussiaL. DidenkoBrookhaven National Laboratory, Upton, New York 11973C. DilksPennsylvania State University, University Park, Pennsylvania 16802X. DongLawrence Berkeley National Laboratory, Berkeley, California 94720J. L. DrachenbergLamar University, Beaumont, Texas 77710J. C. DunlopBrookhaven National Laboratory, Upton, New York 11973L. G. EfimovJoint Institute for Nuclear Research, Dubna 141 980, RussiaN. ElseyWayne State University, Detroit, Michigan 48201J. EngelageUniversity of California, Berkeley, California 94720G. EppleyRice University, Houston, Texas 77251R. EshaUniversity of California, Los Angeles, California 90095S. EsumiUniversity of Tsukuba, Tsukuba, Ibaraki 305-8571, JapanO. EvdokimovUniversity of Illinois at Chicago, Chicago, Illinois 60607J. EwiglebenLehigh University, Bethlehem, Pennsylvania 18015O. EyserBrookhaven National Laboratory, Upton, New York 11973R. FatemiUniversity of Kentucky, Lexington, Kentucky 40506-0055S. FazioBrookhaven National Laboratory, Upton, New York 11973P. FedericP. FederičováCzech Technical University in Prague, FNSPE, Prague 115 19, Czech RepublicJ. FedorišinJoint Institute for Nuclear Research, Dubna 141 980, RussiaP. FilipJoint Institute for Nuclear Research, Dubna 141 980, RussiaE. FinchSouthern Connecticut State University, New Haven, Connecticut 06515Y. FisyakBrookhaven National Laboratory, Upton, New York 11973C. E. FloresUniversity of California, Davis, California 95616Ł. FulekAGH University of Science and Technology, FPACS, Cracow 30-059, PolandC. A. GagliardiTexas A&M University, College Station, Texas 77843T. GalatyukTechnische Universitat Darmstadt, Darmstadt 64289, GermanyF. J. M. GeurtsRice University, Houston, Texas 77251S. M. GibsonValparaiso University, Valparaiso, Indiana 46383D. GrosnickValparaiso University, Valparaiso, Indiana 46383D. S. GunarathneTemple University, Philadelphia, Pennsylvania 19122Y. GuoKent State University, Kent, Ohio 44242A. GuptaUniversity of Jammu, Jammu 180001, IndiaW. GurynBrookhaven National Laboratory, Upton, New York 11973A. I. HamadKent State University, Kent, Ohio 44242A. HamedTexas A&M University, College Station, Texas 77843A. HarlenderovaCzech Technical University in Prague, FNSPE, Prague 115 19, Czech RepublicJ. W. HarrisYale University, New Haven, Connecticut 06520L. HePurdue University, West Lafayette, Indiana 47907S. HeppelmannPennsylvania State University, University Park, Pennsylvania 16802S. HeppelmannPennsylvania State University, University Park, Pennsylvania 16802N. HerrmannUniversity of Heidelberg, Heidelberg 69120, Germany
2018lv
ABI
Аннотация
We report first measurements of ${e}^{+}{e}^{\ensuremath{-}}$ pair production in the mass region $0.4<{M}_{ee}<2.6\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ at low transverse momentum (${p}_{T}<0.15\text{ }\text{ }\mathrm{GeV}/c$) in noncentral $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$ and $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{{s}_{NN}}=193\text{ }\text{ }\mathrm{GeV}$. Significant enhancement factors, expressed as ratios of data over known hadronic contributions, are observed in the 40%--80% centrality of these collisions. The excess yields peak distinctly at low ${p}_{T}$ with a width ($\sqrt{⟨{p}_{T}^{2}⟩}$) between 40 and $60\text{ }\text{ }\mathrm{MeV}/c$. The absolute cross section of the excess depends weakly on centrality, while those from a theoretical model calculation incorporating an in-medium broadened $\ensuremath{\rho}$ spectral function and radiation from a quark gluon plasma or hadronic cocktail contributions increase dramatically with an increasing number of participant nucleons. Model calculations of photon-photon interactions generated by the initial projectile and target nuclei describe the observed excess yields but fail to reproduce the ${p}_{T}^{2}$ distributions.
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