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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 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 ${250 < m(\chi) < 4700\,\rm MeV/c^2}$ 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.

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diagram.pdf

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).

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fit_doubleSided_dCB-LL_triggSig_Jpsipm50MeV_BconstrTree_newTrigg_PAPER_log.pdf

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).

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diMuon_mass_PAPER.pdf

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.

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excluded_limit2D.pdf

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 [7,30] and regions forbidden by theory or cosmological constraints [4] are also shown.

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Inflaton_parameter_space_log_PAPER.pdf

Animated gif made out of all figures.

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Supplementary Material [file]

Supplementary material full pdf

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supplementary.pdf

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

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.