Search for dark photons at future e$^+$e$^-$ colliders

Abstract

In a class of theories, dark matter is explained by postulating the existence of a `dark sector', which interacts gravitationally with ordinary matter. If this dark sector contains a U(1) symmetry, and a corresponding `dark' photon ($A_{D}$) , it is natural to expect that this particle kineticly mix with the ordinary photon, and hence become a `portal' through which the dark sector can be studied. The strength of the mixing is given by a mixing parameter $(ε)$. This same parameter governs both the production and the decay of the $A_{D}$ back to SM particles, and for values of $ε$ not already excluded, the signal would be a quite small, and quite narrow resonance: If $ε$ is large enough to yield a detectable signal, its decay width will be smaller than the detector resolution, but so large that the decay back to SM particles is prompt. For masses of the dark photon above the reach of Belle II, future high energy e$^+$e$^-$ colliders are ideal for searches for such a signal, due to the low and well-known backgrounds, and the excellent momentum resolution and equally excellent track-finding efficiency of the detectors at such colliders. This contribution will discuss a study investigating the dependency of the limit on the mixing parameter and the mass of the $A_{D}$ using the $A_{D}\rightarrowμ^{+}μ^{-}$ decay mode in the presence of standard model background, using fully simulated signal and background events in the ILD detector at the ILC Higgs factory. In addition, a more general discussion about the capabilities expected for generic detectors at e$^+$e$^-$ colliders operating at other energies will be given.

Figures (8)

• Figure 1: Expected Dark photon limits from 2019 EPPSU briefing-book EuropeanStrategyforParticlePhysicsPreparatoryGroup:2019qin. (a) Original figure, with all experiments considered and on a logarithmic mass-scale; (b) the same on a linear mass scale (up to the reach of the higgs factories), only showing the relevant experiments on this scale (Belle II, ILC 250 and HL-LHC).
• Figure 2: (a): Cross-section of $\hbox{${\, \mathrm e}^+ {\mathrm e}^- \to$} \gamma_{ISR} A_D \rightarrow \mu^+ \mu^- \gamma_{ISR}$, for the beam polarisations given in the legend; (b) Branching ratio of $A_D \rightarrow \mu^+ \mu^-$ (c): Cross-section times branching ratio of $\hbox{${\, \mathrm e}^+ {\mathrm e}^- \to$} \gamma_{ISR} A_D \rightarrow \mu^+ \mu^- \gamma_{ISR}$, for the beam polarisations given in the legend; (d) Total width of $A_D$.
• Figure 3: Di-muon mass distributions for $m_{A_D}$ = 150 GeV from full simulation of ILD: (a): Generated di-muon mass distribution (red), and reconstructed (blue), for a dark photn with mass 150 GeV. (b): as (a), but also including all backgrounds; (c) is the same as (b) zoomed into the signal region.
• Figure 4: (a) The efficiency to find both muons from the decay of a $A_D$ vs. $m_{A_D}$; (b) The polar angle of the $\mu^-$ versus that of the $\mu^+$ of the generated $\hbox{${\, \mathrm e}^+ {\mathrm e}^- \to$} \gamma_{ISR} A_D \rightarrow \mu^+ \mu^- \gamma_{ISR}$ events, for $m_{A_D}$ = 10, 100, 150 and 200 GeV (clock-wise, from upper-left). The green square indicates the acceptance of the tracking system of ILD, and the red one indicates the coverage of the barrel tracking system;
• Figure 5: (a): Momentum resolution for charged particles in ILD from full detector simulation; (b): Momentum of muons from $A_D$ decays at several values of $m_{A_D}$.