cern.ch

Improved limit on the branching fraction of the rare decay $K^0_{\scriptscriptstyle S}\to\mu^+\mu^-$

[to restricted-access page]

Abstract

A search for the decay $K^0_{\scriptscriptstyle S}\to\mu^+\mu^-$ is performed, based on a data sample of proton-proton collisions corresponding to an integrated luminosity of 3 fb$^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The observed yield is consistent with the background-only hypothesis, yielding a limit on the branching fraction of ${\cal B}(K^0_{\scriptscriptstyle S}\to\mu^+\mu^-) < 0.8 (1.0) \times 10^{-9}$ at $90\% (95\%)$ confidence level. This result improves the previous upper limit on the branching fraction by an order of magnitude.

Figures and captions

Reconstructed mass for $ K ^0_{\mathrm{ \scriptscriptstyle S}} \rightarrow \pi^+\pi^-$ decays in trigger-unbiased events, computed assuming the muon (dashed red line) or pion (solid blue line) mass for the final-state tracks. Candidates satisfy the selection criteria described in the text.

Fig1.pdf [15 KiB]
HiDef png [186 KiB]
Thumbnail [72 KiB]
*.C file
Fig1.pdf

Fits to the reconstructed kaon mass distributions, for the two most sensitive $\text{BDT}_{\rm cb}$ bins in the two trigger categories, TOS $ _{\mu}$ and TOS $ _{\mu\mu}$ . The fitted model is shown as the solid blue line, while the combinatorial background and $ K ^0_{\mathrm{ \scriptscriptstyle S}} \rightarrow \pi^+\pi^-$ double misidentification are overlaid with dotted red and dashed green lines, respectively. For each fit, the pulls are shown on the lower smaller plots.

Fig2a.pdf [18 KiB]
HiDef png [235 KiB]
Thumbnail [77 KiB]
*.C file
Fig2a.pdf
Fig2b.pdf [18 KiB]
HiDef png [223 KiB]
Thumbnail [73 KiB] *.C file
Fig2b.pdf
Fig2c.pdf [17 KiB]
HiDef png [230 KiB]
Thumbnail [74 KiB] *.C file
Fig2c.pdf
Fig2d.pdf [17 KiB]
HiDef png [226 KiB]
Thumbnail [74 KiB] *.C file
Fig2d.pdf

Confidence level of exclusion for each value of the $ K ^0_{\mathrm{ \scriptscriptstyle S}} \rightarrow \mu^+\mu^-$ branching fraction. The regions corresponding to $90\%$ and $95\%$ CL are emphasised in green (dark shading) and yellow (light shading), respectively.

Fig3.pdf [13 KiB]
HiDef png [116 KiB]
Thumbnail [38 KiB]
*.C file
Fig3.pdf

Animated gif made out of all figures.

PAPER-2017-009.gif
Thumbnail
thumbnail_PAPER-2017-009.gif

Tables and captions

Values of the single candidate sensitivity $\alpha_{ij}$ and the number of candidates $N^{K}_{ij}$ compatible with the $ K ^0_{\mathrm{ \scriptscriptstyle S}}$ mass (reconstructed mass in the range $[492,504]\, {\mathrm{\,MeV\!/}c^2} $), for each $\text{BDT}_{\rm cb}$ bin $i$ and trigger category $j$. Only statistical uncertainties are given. The first uncertainty is uncorrelated, while the second is fully correlated among the $\text{BDT}_{\rm cb}$ bins of the same trigger category.

Table_1.pdf [58 KiB]
HiDef png [97 KiB]
Thumbnail [16 KiB]
tex code
Table_1.pdf

Relevant systematic uncertainties on the branching fraction. They are separated, using horizontal lines, into relative uncertainties on (i) $\alpha_{ij}$, (ii) on the signal yield from the signal model used in the mass fit, and (iii) on the branching fraction, obtained combining the two categories, from the background model.

Table_2.pdf [46 KiB]
HiDef png [107 KiB]
Thumbnail [15 KiB]
tex code
Table_2.pdf

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