******** CTCD/NTHU HEP Seminar ********

Speaker ：Dr. Van Que Tran (NCTS)

Title ：Search for Long-Lived Dark Photons from Dark Radiation at the LHC

Time ：12:30-13:30, Thursday, 2026/3/26

Place ：R521, 5F, General Building II, Nat'l Tsing Hua Univ.

Host ： Prof. Kingman Cheung (NTHU) & Prof. Norihiro Iizuka (NTHU)

Abstract:
In this talk, I will present a novel production mechanism for long-lived dark photons at the LHC, arising from dark radiation emitted in SM Z-boson decays into dark matter. The effective coupling between the Z-boson and dark matter is generated radiatively via a one-loop triangle diagram involving the SM top-quark and a new colored scalar mediator. We show that dark photons produced through this dark radiation channel can dominate over conventional sources—such as meson decays and proton bremsstrahlung—across wide regions of parameter space, particularly for small kinetic mixing and for dark photon masses well above the GeV scale. Exploiting this enhanced production mechanism, we analyze the sensitivity of dedicated long-lived particle detectors, including FASER2, FACET, and MATHUSLA. We find that these experiments can significantly surpass existing bounds and probe regions of dark photon parameter space that are inaccessible in the conventional framework.
We also discuss complementary constraints from invisible Z decays, direct detection experiments, monojet searches, and the requirement of the observed thermal relic abundance.