# Search for long-lived scalar particles in $B^+ \to K^+ \chi (\mu^+\mu^-)$ decays

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## Abstract

A search for a long-lived scalar particle $\chi$ is performed, looking for the decay ${B^+ \to K^+ \chi}$ with ${\chi \to \mu^+\mu^-}$ in $pp$ collision data corresponding to an integrated luminosity of $3 {\rm fb}^{-1}$, collected by the LHCb experiment at centre-of-mass energies of $\sqrt{s}=7$ and 8TeV. This new scalar particle, predicted by Hidden Sector models, is assumed to have a narrow width. The signal would manifest itself as an excess in the dimuon invariant mass distribution over the Standard Model background. No significant excess is observed in the accessible ranges of mass ${250 < m(\chi) < 4700 \rm MeV/c^2}$ and lifetime ${0.1 < \tau(\chi) < 1000 \rm ps}$. Upper limits on the branching fraction $\mathcal{B}(B^+ \to K^+ \chi (\mu^+\mu^-))$ at 95 confidence level are set as a function of $m(\chi)$ and $\tau(\chi)$, varying between $2\times10^{-10}$ and $10^{-7}$. These are the most stringent limits to date. The limits are interpreted in the context of a model with a light inflaton particle.

## Figures and captions

 Feynman diagram of the decay $B ^+ \rightarrow K ^+ \chi (\mu ^+\mu ^- )$, where the $\chi$ interacts by mixing with the Higgs and then decays to a pair of muons. diagram.pdf [37 KiB] HiDef png [38 KiB] Thumbnail [29 KiB] *.C file Invariant mass distribution for the normalization channel $B ^+ \rightarrow K ^+ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} (\mu ^+\mu ^- )$ with the results of the fit overlaid. The signal is parametrised with a double-sided Crystal Ball (dotted red line) [26] and the background with an exponential function (dashed green line). fit_do[..].pdf [25 KiB] HiDef png [213 KiB] Thumbnail [159 KiB] *.C file Distribution of the $m(\mu ^+\mu ^- )$ in the first (black) and second (red) decay-time region of the search. The binning scheme reflects the dimuon mass scanning procedure and the bin width corresponds to the mass-dependent dimuon resolution. Empty regions correspond to the $K ^0_{\mathrm{ \scriptscriptstyle S}}$ , ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ , $\psi {(2S)}$ and $\psi (3770)$ vetoes (for both distributions) and to the $\phi$ and $\psi(4160)$ vetoes (only for the prompt decay-time region). diMuon[..].pdf [22 KiB] HiDef png [226 KiB] Thumbnail [205 KiB] *.C file Excluded branching fraction for the $B ^+ \rightarrow K ^+ \chi (\mu ^+\mu ^- )$ decay as a function of $m(\chi)$ and $\tau(\chi)$ at 95% CL. Regions corresponding to the fully-vetoed $K ^0_{\mathrm{ \scriptscriptstyle S}}$ , ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ , $\psi {(2S)}$ and $\psi (3770)$ and to the partially-vetoed $\phi$ and $\psi(4160)$ are excluded from the figure. All systematic uncertainties are included in the calculation of the upper limit. exclud[..].pdf [514 KiB] HiDef png [1 MiB] Thumbnail [437 KiB] *.C file Parameter space of the inflaton model described in Refs. [2,3,4]. The region excluded at 95% CL by this analysis is shown by the blue hatched area. The region excluded by the search with the $B ^0 \rightarrow K ^{*0} \chi (\mu ^+\mu ^- )$ decay [8] is indicated by the red hatched area. Direct experimental constraints set by the CHARM experiment [7] and NA48 experiments [30,7] and regions forbidden by theory or cosmological constraints [4] are also shown. Inflat[..].pdf [47 KiB] HiDef png [1 MiB] Thumbnail [496 KiB] *.C file Animated gif made out of all figures. PAPER-2016-052.gif Thumbnail

## Supplementary Material [file]

 Supplementary material full pdf supple[..].pdf [2 MiB] This ZIP file contains supplemetary material for the publication LHCb-PAPER-2016-052. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in variuous formats Fig1-S.pdf [220 KiB] HiDef png [131 KiB] Thumbnail [111 KiB] *C file Fig2a-S.pdf [1 MiB] HiDef png [2 MiB] Thumbnail [630 KiB] *C file Fig2b-S.pdf [15 KiB] HiDef png [146 KiB] Thumbnail [144 KiB] *C file Fig2c-S.pdf [15 KiB] HiDef png [139 KiB] Thumbnail [129 KiB] *C file Fig2d-S.pdf [15 KiB] HiDef png [139 KiB] Thumbnail [132 KiB] *C file Fig3a-S.pdf [16 KiB] HiDef png [258 KiB] Thumbnail [217 KiB] *C file Fig3b-S.pdf [61 KiB] HiDef png [585 KiB] Thumbnail [335 KiB] *C file Fig4-S.pdf [514 KiB] HiDef png [1 MiB] Thumbnail [438 KiB] *C file Fig5a-S.pdf [16 KiB] HiDef png [258 KiB] Thumbnail [216 KiB] *C file Fig6-S.pdf [27 KiB] HiDef png [777 KiB] Thumbnail [439 KiB] *C file

Created on 22 February 2020.