Search for Dinucleon Decay into Kaons in Super-Kamiokande

A search for the dinucleon decay $pp\to K^{+}{K}^{+}$ has been performed using $91.6\mathrm{kton}·\text{yr}$ data from Super-Kamiokande-I. This decay provides a sensitive probe of the $R$-parity-violating parameter ${{\lambda }_{112}}^{\prime \prime }$. A boosted decision tree analysis found no signal candidates in the data. The expected background was $0.28\pm 0.19$ atmospheric neutrino induced events and the estimated signal detection efficiency was $12.6\%\pm 3.2\%$. A lower limit of $1.7\times {10}^{32}\text{years}$ has been placed on the partial lifetime of the decay ${}^{16}\mathrm{O}\to {}^{14}\mathrm{C}{K}^{+}{K}^{+}$ at 90% C.L. A corresponding upper limit of $7.8\times {10}^{-9}$ has been placed on the parameter ${{\lambda }_{112}}^{\prime \prime }$.

Figure 1

Feynman diagram for $pp\to K^{+}{K}^{+}$ in a supersymmetric framework. ${{\lambda }_{112}}^{\prime \prime }$ appears in two interaction vertices.

Figure 2

Cartoon illustration of an expected event geometry for the final state $pp\to K^{+}{K}^{+}\to {\mu }^{+}{\nu }_{\mu }{\pi }^{+}{\pi }^0$. The Fermi momentum of the parent nucleons will actually distort the back-to-back symmetry of the kaon vectors. Dashed lines indicate particles undetectable in a water Cherenkov detector.

Figure 3

Two highly discriminating variables used as inputs for the BDT. Top: Reconstructed vertex separation of kaon-decay product candidates (e.g., two ${\mu }^{+}$ candidates). Plots are generated after precuts. Bottom: Reconstructed momentum of $K^{+}$ candidates, which assumes the candidate particles have a mass of $493.7\mathrm{MeV}/c^2$. Plots are generated after precuts and selection on event category. The number of entries per event in a histogram depend on the BDT event category. The integral of the $pp\to K^{+}{K}^{+}$ Monte Carlo is normalized to the atmospheric neutrino Monte Carlo.

Figure 4

Final output of the decision tree. Events to the right of the cut at 0.12 are considered signal candidates. All data points are consistent with background; no signal candidates are found in the data. Final signal efficiency: $12.6\%\pm 3.2\%$. Final expected background contamination: $0.28\pm 0.19$ events.