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Measurement of direct virtual photon production in Au + Au collisions from RHIC Beam Energy Scan-II

Updated on Wed, 2025-05-07 04:09. Originally created by xianwenbao on 2024-06-22 16:13.

 

  Measurement of direct virtual photon production in Au+Au collisions from RHIC Beam Energy Scan-II

PAs :Xianwen Bao, Zhangbu Xu, Chi Yang

Target Journal: PRL

Abstract:

       We present the first measurement of direct virtual photon in Au+Au collisions with Beam Energy Scan II (BES-II) data from STAR. The di-electron were constructed at eta mass region (0.1 < < 0.28 GeV/c2) and low transverse momentum(1.0 < < 5.0 GeV/c), originating by direct virtual photon from initial hard scattering to hadron gas stage. The observed direct virtual photon yield have a strong dependence, with a measured alpha value of 1.44 0.04(stat.) 0.04(sys.) within different collision energies and centrality classes. Furthermore, the production mechanism of direct virtual photon was first studied by direct virtual photon mass distribution in different regions, observed a transition of production mechanism from equilibrated system to non-equilibrated system with pT increasing, presenting a promising avenue for deepening our understanding of QGP in heavy-ion collisions.

Figures:


Figure1:

Direct virtual photon pT spectrum in Au + Au collision at 𝑠NN√(s_NN ) = 14.6, 19.6, 27, 54.4 GeV in different centralities. The vertical bars and boxes around data points represent the statistical and systematic uncertainty

Figure2:

Solid point represent 54.4, 27, 19.6, 14.6 GeV direct photon yield respectively, open point represent 2.76 and 5.02 GeV results from ALICE and 200, 62.4, 39 GeV results from PHENIX, pink line corresponds to the fit data point from STAR and acquire 1σ band.

(Left): The yield represent 1.0 < pT < 3.0 GeV/c

(Right): The yield represent 1.5 < pT < 3.0 GeV/c




Figure3:

(Top) Red point is the dielectron mass spectrum in different pT region, red line is the direct virtual photon spectrum from 4 component fit and red band represent 1σ band.

(Bottom) The constraints on direct virtual photon mass spectrum. The red, blue and pink band represent 1,2 and 3σ band respectively.

Figure4:

Black and blue point represent dielectron excess yield within 0.4 < 𝑀𝑒𝑒M_ee < 0.75 GeV/c2.
Red point represent dielectron excess yield within 0.13 < 𝑀𝑒𝑒 M_ee  < 0.4 GeV/c2.
Red band represent 𝑝/𝜋0〖p/π〗_0 1σ band

Summary:

       In conclusion, we present the first measurement of direct virtual photon in Au+Au collisions within BES-II data at STAR. Our findings reveal the direct photon yield have a strong dependence with QGP system size which not only advance our understanding of QGP but also the virtual photon distribution in different transverse momentum, reveal virtual photon production mechanism from early stage to the later stage.
       The excess yield of di-electron at low mass region can provide a good base line for the QCD phase diagram and understand the properties of matter under extreme conditions in the future. Furthermore, the discrepancy between STAR and PHENIX still not be fixed up to now.

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