Sensitivity of the FCC-ee to the decay of a dark photon into a $μ^+μ^-$ pair

TL;DR

This work assesses FCC-ee sensitivity to a dark photon $A^{\prime}$ produced via $e^+e^-\to\gamma A^{\prime}$ with $A^{\prime}\to\mu^+\mu^-$ over $m_{A^{\prime}}$ from $0.4$ to $360$ GeV, using a parametrised IDEA detector and full event kinematics. It combines prompt and long-lived search strategies across all FCC-ee runs, employing MG5_aMC@NLO, PYTHIA8, DELPHES, and an XGBoost-based multivariate analysis to optimize signal discrimination; the prompt analysis yields a 95% CL $\epsilon$ reach around $(0.5-0.85)\times10^{-3}$, while the long-lived channel probes $m_{A^{\prime}}\lesssim1.5$ GeV with $\epsilon$ down to $\mathcal{O}(10^{-5})$. The combined FCC-ee reach surpasses HL-LHC extrapolations for $m_{A^{\prime}}$ above ~20 GeV and provides a complementary, novel sensitivity in the low-mass, long-lived region not covered by collider or beam-dump experiments. The results underscore the critical role of precision tracking, vertexing, and kinematic reconstruction in unlocking dark-sector probes at future lepton colliders.

Abstract

The production of a dark photon $A^{\prime}$ at the proposed CERN FCC-ee collider is investigated. The study addresses the associated production $e^+e^-\rightarrowγA^{\prime}$ followed by the decay $A^{\prime}\rightarrowμ^+μ^-$. The 95% CL sensitivity on the mixing between the photon and the dark photon is evaluated in the $m_{A^{\prime}}$ mass range 0.4-360 GeV, based on a parametrised simulation of the IDEA detector. The study is performed both for prompt and long-lived decays of the $A^{\prime}$.

Figures (20)

• Figure 1: Value of $BR\rightarrow\mu^+\mu^-$ as calculated in Curtin:2014cca (blue) and leading order calculation as implemented in MG5aMC@NLO (orange) as a function of $m_{A^{\prime}}$.
• Figure 2: Left: Cross-section for $A^{\prime}$ production in pb divided by $\epsilon^2$ as a function of $m_{A^{\prime}}$. Right: Width in GeV of the $A^{\prime}$ divided by $\epsilon^2$ as a function of $m_{A^{\prime}}$.
• Figure 3: $L_{A^{\prime}}$ in the $m_{A^{\prime}}-\epsilon$ plane calculated for the process $e^+e^-\rightarrow\gamma A^{\prime}$ at $\sqrt{s}=91.2$ GeV. The red and the green line bound the area in the parameter space where a long-lived $A^{\prime}$ search can be performed, and the blue line shows the parameters corresponding to the production of 3 events for the expected FCC-ee statistics of $6\times10^{12}$$Z$ bosons.
• Figure 4: Relative mass resolution for muon pairs from the of $A^{\prime}$ multiplied by 1000 as a function of $m_{\mu^+\mu^-}$.
• Figure 5: Distribution of the variables $M_{cut}$ truncated at 10 (left) and $\log_{10}(M_{cut})$ for signal (blue) and background (orange) for $m_{A^{\prime}}=40$ GeV.