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In the context of future electron-ion collision experiments, particularly the Electron-Ion Collider (EIC) and the Electron-Ion Collider in China (EicC), investigating exclusive photoproduction processes is of paramount importance. These processes offer a unique opportunity to probe the gluon structure of nuclei across a broad range of Bjorken-$x$, facilitating measurements of nuclear shadowing and searches for gluon saturation and/or the color glass condensate. This paper explores the potential of utilizing neutron tagging from Coulomb excitation of nuclei to effectively determine centrality for exclusive photoproduction in electron-ion collisions. By developing the equivalent photon approximation for fast-moving electrons, this work incorporates a coordinate-space-dependent photon flux distribution, elucidating the relationship between the photon transverse momentum distribution and collision impact parameter. Leveraging spatial information from the photon flux, the differential cross section for Coulomb excitation of nuclei is derived. Our calculations demonstrate that neutron tagging can significantly alter impact parameter distributions, thereby providing a robust method for centrality manipulation in electron-ion collisions. This study contributes essential baseline and strategies for exploring the impact parameter dependence of exclusive photoproduction, offering novel insights for experimental design and data analysis. Ultimately, it provides additional information to better visualize the gluon distribution within the nucleus.

Significant excesses of $e^+e^-$ pair production at very low transverse momentum ($p_T <$ 0.15 GeV/c) were observed by the STAR collaboration in hadronic heavy-ion collisions. Such enhancement is assumed to be a sign of photon-photon production in heavy-ion collisions with hadronic overlap, based on comparisons with model calculations for spherical Au + Au collisions. However, there is a lack of calculations for $e^+e^-$ pair production from coherent photon-photon interactions in hadronic U + U collisions, due to the deformity of Uranium nuclei. In this article, we present calculations for $e^+e^-$ pair photoproduction at $\sqrt{s_{NN}}$ = 193 GeV in both spherical and deformed U + U collisions within STAR detector acceptance using the equivalent photon approximation (EPA). We conduct event-by-event analysis to investigate the effects of initial nuclear deformation on pair production. Our numerical results show good agreement with experimental data for the 40%--60% and 60%--80% centrality classes in U + U collisions, and the differences between spherical and deformed configurations are approximately 3\%. We also calculate the yields of the photoproduced $e^+e^-$ pair in hadronic deformed Ru + Ru and Zr + Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV. The results reveal that the ratios of the yields of Ru to Zr exhibit very small differences ($<$ 1%) between spherical and deformed cases., Comment: 8 pages, 5 figures, 4 tables

Extensive studies have been conducted in the past few decades to investigate potential signatures of higher-order QED effects in high-energy electromagnetic scattering processes. In our previous work, we have identified evidence of higher-order corrections in the total cross-section for the Breit-Wheeler process in relativistic heavy-ion collisions. However, the presence of higher-order QED corrections cannot be unambiguously proven solely based on total cross-section measurements due to substantial experimental and theoretical uncertainties. The objective of this paper is to explore the sensitivity of specific differential observables in the Breit-Wheeler process to higher-order QED effects in high-energy heavy-ion collisions. These investigations will provide guidance in determining the presence or absence of higher-order QED processes by conducting precise measurements in future experiments., Comment: 8 pages, 4 figures, 1 table

The collective observables play critical roles in probing the properties of quark-gluon-plasma created in relativistic heavy-ion collisions, in which the information on initial collision geometry is crucial. However, the initial collision geometry, e.g., the reaction plane, cannot be directly extracted in the experiment. In this paper, we demonstrate the idea of determining the reaction plane via the feature of linear polarization of the coherent photoproduction process and discuss the advantages of the proposed approach in comparison with traditional methods.

Frequency combs have applications that extend from the ultra-violet into the mid-infrared bands. Microcombs, a miniature and often semiconductor-chip-based device, can potentially access most of these applications, but are currently more limited in spectral reach. Here, we demonstrate mode-locked silica microcombs with emission near the edge of the visible spectrum. By using both geometrical and mode-hybridization dispersion control, devices are engineered for soliton generation while also maintaining optical $Q$ factors as high as 80 million. Electronics-bandwidth-compatible (20 GHz) soliton mode locking is achieved with low pumping powers (parametric oscillation threshold powers as low as 5.4 mW). These are the shortest wavelength soliton microcombs demonstrated to date and could be used in miniature optical clocks. The results should also extend to visible and potentially ultra-violet bands., Comment: Seung Hoon Lee, Dong Yoon Oh, Qi-Fan Yang, Boqiang Shen and Heming Wang contributed equally to this work