Title: Gluon imaging through spin interference with exclusive J/ψ photoproduction in Au+Au and isobar ultraperipheral collisions

PA's: Ashik Ikbal Sheikh, Kaiyang Wang, Declan Keane, Zhangbu Xu, Prithwish Tribedy, Daniel Brandenburg and Wangmei Zha

Target Journal: Physical Review Letters 

Paper draft: 

Abstract: The linear polarization of photons in Ultraperipheral collisions (UPC) aids in imaging the nucleus by utilizing spin interference in vector meson photoproductions. The photoproduced vector meson,  J/ψ, exhibits a longer lifetime (2160 fm/c) and a non-localized wave function, offering a distinctive chance to investigate the spin interference effect. This interference phenomenon probes gluon distribution of colliding nuclei. We present the measurements of the interference pattern for photoproduced  J/ψ in Au+Au and isobar UPC at a collision energy of $\sqrt{s_{_\mathrm{NN}}}=200$ GeV with the STAR experiment at RHIC.

Introduction: 

    FIG. 1: UPC quasi-real photons are linearly polarized. The photons fluctuate into quark anti-quark pair and interacts with the Pomeron of the other nucleus which produces a Vector Meson (e.g., ρ, J/ψ, etc ). Vector mesons are short lived particles, so they decay and the decay daughters are preferentially emitted along the Vector Meson's spin direction due to angular momentum conservation. In UPC vector meson production process, there is an ambiguity that which nucleus provides photon and which one serves as a target. So there are two possible paths as shown in the cartoons here. In path1, nucleus 1 provides photon and nucleus 2 serves as target. In path2 it is just the other way around. These two paths are indistinguishable in experiment. Both the paths have amplitudes or wave functions since it’s a quantum particle. The wave functions from the two indistinguishable paths may interfare and make the cos(2φ) modulation observable. Since the vector mesons have spin, it is spin-dependent interference.

J/ψ reconstruction:  

   FIG. 2. J/ψ is reconstructed in e⁺e^- channel. The  e⁺e^- invariant mass distribution in Au+Au UPC at 200 GeV. The two-photon background is also shown underneath the J/ψ peak. The relative signal and background contributions have been estimated and listed on the plot.

 FIG. 3. Experimental observable for vector meson spin interference effect is the angle between J/ψ  and one of the decay electrons, we call it φ. 

Summary and Conclusion: