Abstract
We measured direct photons for pT<5GeV/c in minimum bias and 0%–40% most-central events at midrapidity for Cu+Cu collisions at sNN=200GeV. The e+e− contribution from quasireal direct virtual photons has been determined as an excess over the known hadronic contributions in the e+e− mass distribution. A clear enhancement of photons over the binary scaled p+p fit is observed for pT<4GeV/c in Cu+Cu data. The pT spectra are consistent with the Au+Au data covering a similar number of participants. The inverse slopes of the exponential fits to the excess after subtraction of the p+p baseline are 285±53(stat)±57(syst)MeV/c and 333±72(stat)±45(syst)MeV/c for minimum bias and 0%–40% most-central events, respectively. The rapidity density, dN/dy, of photons demonstrates the same power law as a function of dNch/dη observed in Au+Au at the same collision energy.1 MoreReceived 13 May 2018DOI:https://doi.org/10.1103/PhysRevC.98.054902Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasRelativistic heavy-ion collisionsPhysical SystemsLeptonsPhotonsTechniquesHadron collidersParticles & FieldsNuclear Physics
Highlights
Direct photons are excellent probes for understanding the time evolution of the hot and dense matter created in ultrarelativistic heavy-ion collisions [1,2]
The inverse slopes of the exponential fits to the excess after subtraction of the p+p baseline are 285 ± 53(stat) ± 57(syst) MeV/c and 333 ± 72(stat) ± 45(syst) MeV/c for minimum bias and 0%–40% most-central events, respectively
Low-pT direct photons have been measured by using the vcoirltluisailonpshointon√msNeNth=od20f0orGMeVB
Summary
Direct photons are excellent probes for understanding the time evolution of the hot and dense matter created in ultrarelativistic heavy-ion collisions [1,2]. The virtual photon method [6] and the external conversion method [7], have been established to measure direct photons at low pT (pT < 5 GeV/c). Further understanding of the thermal properties of the created hot medium can be realized through the systematic study of low-pT direct photon production within a wide range of system size and collision energy. We photons in Cu+Cu present the collisions amt e√assuNrNem=e2n0t 0of low-pT direct GeV with the virtual photon method. This measurement may provide additional information on the system size dependence of low-pT direct-photon production.
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