Probing the production mechanisms of virtual photon at STAR

Updated on Mon, 2025-05-26 07:16. Originally created by xianwenbao on 2024-06-22 16:13.

Probing the production mechanisms of virtual photon at STAR

PAs : Xianwen Bao, Frank Geurts, Lijuan Ruan, Zebo Tang, Xiaofeng Wang, Zhangbu Xu, Chi Yang, Zaochen Ye, Wangmei Zha

Target Journal: PRL

Abstract:

We present the first measurement of direct virtual photon (γ^∗àe+e-) at low mass region (0.1 < M_ee < 0.28 GeV/c2) in Au+Au collisions with Beam Energy Scan II (BES-II) data from STAR. The production was measured as a function of its invariant mass, transverse momentum (pT) and collision beam energy. A transition of the mass distribution from exponential to power-law shape with increasing pT was observed, indicating an evolution from equilibrium to non-equilibrium radiation. The γ^∗ mass shape in different pT regions and the beam energy dependence of the yields at different mass range reveal its production mechanisms. The observed γ^∗ yield have a strong system-size (dN_ch/dη) dependence. Measured scaling power of 1.44 ± 0.04(stat.) ± 0.04(sys.) of this dependence over different collision energies and centrality classes indicates that QGP and hadron phases both contribute significantly to the photon yields. Such systematic study presents a promising avenue for direct measurement of QGP properties in heavy-ion collisions.

Figures:

Figure1:

Direct virtual photon pT spectrum in Au + Au collision at = 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:

Direct photon yield verse charged particle multiplicity in Au+Au collsions at = 14.6, 19.6, 27, 54.4 GeV

Solid point represent 54.4, 27, 19.6, 14.6 GeV direct photon yield (1.0 < pT < 3.0 GeV/c) 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.

Figure3:

(Top)Direct virtual photon mass distribution in Au+Au collision at = 54.4 GeV in different pT range,
0-80%.

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 virtual photon distribution extracted by the comparison STAR measurement and fitting in different q and β.

The red, blue and pink band represent 1,2 and 3σ band respectively.

Figure4:

Di-electron excess yield divided by yield in Au+Au collision at = 14.6, 19.6, 27, 54.4 GeV in 0-80%.

Black and blue point represent dielectron excess yield within 0.4 < < 0.75 GeV/c2.
Red point represent dielectron excess yield within 0.13 < < 0.4 GeV/c2.
Red band represent 1σ band

Summary:

In conclusion, we present the first measurement of direct virtual photon (0.1 < M_ee < 0.28 GeV/c2) in Au+Au collisions within BES-II data at STAR.

The virtual photon mass shape evolves from exponential at low pT to power law at high pT.

The yield is dominantly from QGP and radiation at higher mass and/or higher beam energies at RHIC BES-II and from Delta Dalitz decay at low mass at SIS energy (HADES).

The yield of (1<pT<3 GeV/c) scales as a power law with the system size and shows no dependence with collision beam energy

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Probing the production mechanisms of virtual photon at STAR