Speaker
Description
Dark photons ($Z_D$) with direct vector ($g_V$) and axial ($g_A$) couplings to Standard Model (SM) fermions provide a well-motivated portal to physics beyond the SM. In contrast to kinetic mixing scenarios, which are strongly constrained, models with direct fermion couplings can remain consistent with current experimental limits. The top quark, as the heaviest SM fermion, offers a sensitive probe of such interactions. This study investigates dark photon production in association with a top-quark pair in the Large Hadron Collider (LHC), $pp \to t\bar{t}Z_D$ with $Z_D \to \ell^+ \ell^-$ ($\ell = e,\mu$). The analysis considers three top decay channels: hadronic, semileptonic, and dileptonic. The dilepton invariant mass $m_{\ell\ell}$ from the dark photon decay is used as the primary discriminating observable. The visible production rate is evaluated across the mass range $20~\mathrm{GeV} \leq m_{Z_D} \leq 1000~\mathrm{GeV}$ for benchmark values of coupling, demonstrating that the cross section scales with $g_V^2 + g_A^2$ and decreases rapidly with increasing mediator mass. The expected decay width remains consistently narrow across the scan, with $\Gamma_{Z_D}/m_{Z_D} \sim 0.08$. Beyond $m_{\ell\ell}$, additional kinematic observables including $p_T^{Z_D}$, $\Delta\phi_{Z_D,t}$, and $\Delta R_{Z_D,t}$ are being investigated to further enhance signal-background discrimination. Projected exclusion limits in the $(m_{Z_D}, g_V, g_A)$ parameter space are under study to establish the sensitivity reach of the $t\bar{t}Z_D$ channel at the High-Luminosity LHC.
| Apply for student award at which level: | PhD |
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| Consent on use of personal information: Abstract Submission | Yes, I ACCEPT |