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Search for dark photons produced in 13 TeV $pp$ collisions

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Abstract

Searches are performed for both prompt-like and long-lived dark photons, $A^{\prime}$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using $A^{\prime}\to\mu^+\mu^-$ decays and a data sample corresponding to an integrated luminosity of 1.6 fb$^{-1}$ collected with the LHCb detector. The prompt-like $A^{\prime}$ search covers the mass range from near the dimuon threshold up to 70 GeV, while the long-lived $A^{\prime}$ search is restricted to the low-mass region $214<m(A^{\prime})<350$ MeV. No evidence for a signal is found, and 90% confidence level exclusion limits are placed on the $\gamma$-$A^{\prime}$ kinetic-mixing strength. The constraints placed on prompt-like dark photons are the most stringent to date for the mass range $10.6 < m(A^{\prime}) < 70$ GeV, and are comparable to the best existing limits for $m(A^{\prime}) < 0.5$ GeV. The search for long-lived dark photons is the first to achieve sensitivity using a displaced-vertex signature.

Figures and captions

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.

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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].

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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.

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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{\,MeV} $. The largest signal-like excess occurs at $ m( A^{\prime} ) = 239\mathrm{\,MeV} $ and $\tau( A^{\prime} ) = 0.86\,ps $.

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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.

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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.

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

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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 $\mathrm{\,GeV}$ . 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.

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

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Animated gif made out of all figures.

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Tables and captions

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

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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.