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

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 $3{\mathrm{fb}}^{-1}$, collected by the LHCb experiment at center-of-mass energies of $\sqrt{s}=7$ and 8 TeV. 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 $250<m(\chi )<4700\mathrm{MeV}/c^2$ and lifetime $0.1<\tau (\chi )<1000\mathrm{ps}$. 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 $2\times {10}^{-10}$ 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.

Figure 1

Feynman diagram of the decay $B^{+}\to K^{+}\chi ({\mu }^{+}{\mu }^{-})$, where the $\chi$ interacts by mixing with the Higgs boson and then decays to a pair of muons.

Figure 2

Invariant mass distribution for the normalization channel $B^{+}\to K^{+}J/\psi ({\mu }^{+}{\mu }^{-})$ with the results of the fit overlaid. The signal is parametrized with a double-sided crystal ball (dotted red line) [24] and the background with an exponential function (dashed green line).

Figure 3

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 $K_S^0$, $J/\psi$, $\psi (2S)$ and $\psi (3770)$ vetoes (for both distributions) and to the $\varphi$ and $\psi (4160)$ vetoes (only for the prompt decay-time region).

Figure 4

Excluded branching fraction for the $B^{+}\to K^{+}\chi ({\mu }^{+}{\mu }^{-})$ decay as a function of $m(\chi )$ and $\tau (\chi )$ at 95% C.L. Regions corresponding to the fully vetoed $K_S^0$, $J/\psi$, $\psi (2S)$ and $\psi (3770)$ and to the partially vetoed $\varphi$ and $\psi (4160)$ are excluded from the figure. All systematic uncertainties are included in the calculation of the upper limit.

Figure 5

Parameter space of the inflaton model described in Refs. [2, 3, 4]. The region excluded at 95% C.L. by this analysis is shown by the blue hatched area. The region excluded by the search with the $B^0\to K^{*0}\chi ({\mu }^{+}{\mu }^{-})$ decay [8] is indicated by the red hatched area. Direct experimental constraints set by the CHARM and NA48 experiments [7, 29] and regions forbidden by theory or cosmological constraints [4] are also shown.