Prompt-like mass spectrum, where the categorization of the data as prompt $\mu^+\mu^-$, $\mu_Q\mu_Q$, and $hh+h\mu_Q$ is determined using the fits described in the text.

Regions of the $ [ m( A^{\prime} ) ,\varepsilon^2]$ parameter space excluded at 90% CL by the prompt-like $ A^{\prime}$ search compared to the best existing limits [27,38].

Ratio of the observed upper limit on $ n_{\rm ob}^{ A^{\prime} }[ m( A^{\prime} ) ,\varepsilon^2]$ at 90% CL to its expected value, where regions less than unity are excluded. There are no constraints from previous experiments in this region.

Three-dimensional distribution of $\chi^2_{\rm DF}$ versus $t$ versus $ m(\mu^+\mu^-)$ , which is fit to determine the long-lived signal yields. The data are consistent with being predominantly due to $b$-hadron decays at small $t$, and due to $ K^0_{\mathrm{ \scriptscriptstyle S}}$ decays for large $t$ and $ m(\mu^+\mu^-) \gtrsim 280\mathrm{ Me V} $. The largest signal-like excess occurs at $ m( A^{\prime} ) = 239\mathrm{ Me V} $ and $\tau( A^{\prime} ) = 0.86 ps $.

Efficiency ratio $\epsilon_{{}^{\gamma^*}}^{{}_{ A^{\prime} }}[ m( A^{\prime} ) ,\tau( A^{\prime} ) ]$ for long-lived dark photons, integrated over decay time. The sharp decrease at larger values of $\varepsilon^2$ is due to the stringent $ {\rm min}[\chi^2_{\mathrm{IP}}(\mu^{\pm})]$ criterion applied in the 2016 trigger.

Long-lived $ A^{\prime} \to \mu^+\mu^-$ candidates (black points) showing $t$ versus $ m(\mu^+\mu^-)$ in bins of $\chi^2_{\rm DF}$, compared to the pulls from binned fits performed without a signal contribution (color axis). Positive pulls denote an excess of data candidates.

Mass spectrum selected by the prompt-like $ A^{\prime} \to \mu^+\mu^-$ trigger.

Example $ {\rm min}[\chi^2_{\mathrm{IP}}(\mu^{\pm})] ^{1/2}$ distributions with fit results overlaid for prompt-like candidates near (left) $ m( A^{\prime} ) =0.5$, (middle) 5, and (right) 50 $\mathrm{ Ge V}$ . The square root of $ {\rm min}[\chi^2_{\mathrm{IP}}(\mu^{\pm})]$ is used in the fits to increase the bin occupancies at large $ {\rm min}[\chi^2_{\mathrm{IP}}(\mu^{\pm})]$ values.

Comparison of the results presented in this Letter to existing constraints from previous experiments (see Ref. [1] for details about previous experiments).

Animated gif made out of all figures.

Summary of mass regions with special treatment in the prompt-like $ A^{\prime}$ search. In all other mass regions from 214 $\mathrm{ Me V}$ to 70 $\mathrm{ Ge V}$ , limits are set and the $p$-values are considered as possible evidence for $ A^{\prime} \to \mu^+\mu^-$ decays.