# 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. Fig1.pdf [359 KiB] HiDef png [224 KiB] Thumbnail [144 KiB] *.C file 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]. Fig2.pdf [159 KiB] HiDef png [301 KiB] Thumbnail [153 KiB] *.C file 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. Fig3.pdf [259 KiB] HiDef png [1 MiB] Thumbnail [471 KiB] *.C file 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 $\tau( A^{\prime} ) = 0.86 ps$. Fig4.pdf [66 KiB] HiDef png [397 KiB] Thumbnail [215 KiB] *.C file 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. Fig5.pdf [149 KiB] HiDef png [1 MiB] Thumbnail [671 KiB] *.C file 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. Fig6a.pdf [28 KiB] HiDef png [270 KiB] Thumbnail [205 KiB] *.C file Fig6b.pdf [30 KiB] HiDef png [293 KiB] Thumbnail [218 KiB] *.C file Fig6c.pdf [32 KiB] HiDef png [309 KiB] Thumbnail [227 KiB] *.C file Fig6d.pdf [32 KiB] HiDef png [299 KiB] Thumbnail [219 KiB] *.C file Mass spectrum selected by the prompt-like $A^{\prime} \to \mu^+\mu^-$ trigger. Fig7.pdf [96 KiB] HiDef png [205 KiB] Thumbnail [136 KiB] *.C file 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. Fig8a.pdf [14 KiB] HiDef png [118 KiB] Thumbnail [116 KiB] *.C file Fig8b.pdf [15 KiB] HiDef png [121 KiB] Thumbnail [123 KiB] *.C file Fig8c.pdf [15 KiB] HiDef png [144 KiB] Thumbnail [145 KiB] *.C file Comparison of the results presented in this Letter to existing constraints from previous experiments (see Ref. [1] for details about previous experiments). Fig9.pdf [155 KiB] HiDef png [211 KiB] Thumbnail [137 KiB] *.C file Animated gif made out of all figures. PAPER-2017-038.gif Thumbnail

## Tables and captions

 Summary of mass regions with special treatment in the prompt-like $A^{\prime}$ search. In all other mass regions from 214 $\mathrm{ Me V}$ to 70 $\mathrm{ Ge V}$ , limits are set and the $p$-values are considered as possible evidence for $A^{\prime} \to \mu^+\mu^-$ decays. Table_1.pdf [35 KiB] HiDef png [70 KiB] Thumbnail [30 KiB] tex code

Created on 11 July 2020.