Search for nucleon decay into charged antilepton plus meson in $0.316\text{megaton}·\text{years}$ exposure of the Super-Kamiokande water Cherenkov detector

We have searched for proton decays into a charged antilepton ($e^{+}$, ${\mu }^{+}$) plus a meson ($\eta$, ${\rho }^0$, $\omega$) and for neutron decays into a charged antilepton ($e^{+}$, ${\mu }^{+}$) plus a meson (${\pi }^{-}$, ${\rho }^{-}$) using Super-Kamiokande I-IV data, corresponding to $0.316\text{megaton}·\text{years}$ of exposure. This measurement updates the previous published result by using 2.26 times more data and improved analysis methods. No significant evidence for nucleon decay is observed and lower limits on the partial lifetime of the nucleon are obtained. The limits range from $3\times {10}^{31}$ to $1\times {10}^{34}\text{years}$ at 90% confidence level, depending on the decay mode.

Figure 1

The effective mass of the decayed neutron in ${}^{16}\mathrm{O}$ for $n\to e^{+}{\pi }^{-}$ as simulated by Monte Carlo.

Figure 2

The ${\pi }^{-}$ interaction cross section on ${}^{12}\mathrm{C}$ as a function of momentum. Open circles indicate existing data [18]; dashed and solid lines show before and after the tune, respectively. The colors correspond to the total cross section and exclusive channels: total (black), quasielastic scattering (green), absorption (red), charge exchange “CX” (blue), and $\pi$ production (light blue). “FSI” stands for final state interaction.

Figure 3

Fraction of meson interactions in the oxygen nucleus in bins of $\eta$ momentum generated by $p\to e^{+}\eta$ MC. $\eta$’s that exit the nucleus without experiencing any interactions are indicated by the portion labeled “No interaction.” If at least one of the pions generated via the baryon resonance of $N(1535)$ is absorbed, it is called “$\pi$ absorption.” If none of the pions are absorbed, it is called “$\pi$ escape.”

Figure 4

Reconstructed invariant mass distribution of the atmospheric-neutrino-induced ${\pi }^0$ events in the observed data (black dot) and the atmospheric neutrino MC samples (red histogram) used in the absolute energy scale error estimation in SK-IV. MC events are normalized by the live time of the observed data (1631 days).

Figure 5

The reconstructed $\eta$ mass distributions for $p\to e^{+}\eta$ MC events for $\eta \to 2\gamma$ search (left) and $\eta \to 3{\pi }^0$ (right) search for each SK period. The histograms show all simulated $p\to e^{+}\eta$ events. For the left plots the eta meson is reconstructed under the assumption of $\eta \to 2\gamma$, and for the right plots the eta meson is reconstructed under the assumption of $\eta \to 3{\pi }^0$. The hatched histograms show, for free proton decays, the true $\eta \to 2\gamma$ events (left) and true $\eta \to 3{\pi }^0$ events (right). The event selections (A1–A2) of Sec. 4a are applied. The inside of the two vertical green lines corresponds to the signal region.

Figure 6

The signal efficiencies (upper) and the number of expected backgrounds (lower, red histogram) and data candidates (lower, black dots) for $p\to e^{+}\eta$ ($\eta \to 2\gamma$), $p\to {\mu }^{+}\eta$ ($\eta \to 2\gamma$), $p\to e^{+}\eta$ ($\eta \to 3{\pi }^0$), and $p\to {\mu }^{+}\eta$ ($\eta \to 3{\pi }^0$) searches from left to right. The results from SK-I to SK-IV are combined. The event selection is defined in the text.

Figure 7

Total invariant mass and total momentum for $p\to l^{+}\eta$ MC (top, cyan for free proton and blue for bound proton), atmospheric neutrino background MC corresponding to about 2000 years live time of SK (middle), and data (bottom) for $p\to e^{+}\eta$ ($\eta \to 2\gamma$), $p\to {\mu }^{+}\eta$ ($\eta \to 2\gamma$), $p\to e^{+}\eta$ ($\eta \to 3{\pi }^0$), and $p\to {\mu }^{+}\eta$ ($\eta \to 3{\pi }^0$) searches from left to right. All the event selections except (A5–A7) and (B5) in Sec. 4a are applied. The results from SK-I to SK-IV are combined. The signal box is shown as a green box. For the $p\to l^{+}\eta$ ($2\gamma$) searches, the total momentum is separated at the horizontal dashed line in the signal box.

Figure 8

Total momentum for $p\to l^{+}\eta$ MC (upper, open histogram for bounded protons and shaded histogram for free protons), atmospheric neutrino MC (lower, red histogram), and data (lower, black circles) for $p\to e^{+}\eta$ ($\eta \to 2\gamma$), $p\to {\mu }^{+}\eta$ ($\eta \to 2\gamma$), $p\to e^{+}\eta$ ($\eta \to 3{\pi }^0$), and $p\to {\mu }^{+}\eta$ ($\eta \to 3{\pi }^0$) searches from left to right. All the event selections except (A5–A7) and (B5) in Sec. 4a are applied. The atmospheric neutrino MC is normalized to data by area. Results from SK-I to SK-IV are combined. The signal region corresponds to the left side from the vertical green solid line. The vertical dashed line for $p\to l^{+}\eta$ ($2\gamma$) corresponds to the total momentum separation cut value. Total momenta for the $p\to {\mu }^{+}\eta$ ($\eta \to 3{\pi }^0$) data candidates are $188\mathrm{MeV}/\mathrm{c}$ and $239\mathrm{MeV}/\mathrm{c}$.

Figure 9

The signal efficiencies (upper) and the number of expected backgrounds (lower, red histogram) and data candidates (lower, black dots) for $p\to e^{+}{\rho }^0$ (left) and $p\to {\mu }^{+}{\rho }^0$ (right) searches. The results from SK-I to SK-IV are combined. The event selection is defined in the text.

Figure 10

Total invariant mass and total momentum for $p\to l^{+}{\rho }^0$ MC (top, cyan for free proton and blue for bound proton), atmospheric neutrino background MC corresponding to about 2000 years live time of SK (middle), and data (bottom) for $p\to e^{+}{\rho }^0$ (left) and $p\to {\mu }^{+}{\rho }^0$ (right) searches. All the event selections except (C5) in Sec. 4b are applied. The results from SK-I to SK-IV are combined. The signal box is shown as a green box.

Figure 11

Total momentum for $p\to l^{+}{\rho }^0$ MC (upper, open histogram for bounded protons and shaded histogram for free protons), atmospheric neutrino MC (lower, red histogram), and data (lower, black circles) for $p\to e^{+}{\rho }^0$ (left) and $p\to {\mu }^{+}{\rho }^0$ (right) searches. All the event selections except (C5) in Sec. 4b are applied. The atmospheric neutrino MC is normalized to data by area. Results from SK-I to SK-IV are combined. The signal region corresponds to left side from the vertical green line. Total momenta are $61\mathrm{MeV}/\mathrm{c}$ and $90\mathrm{MeV}/\mathrm{c}$ for the $p\to e^{+}{\rho }^0$ data candidates and $179\mathrm{MeV}/\mathrm{c}$ for the $p\to {\mu }^{+}{\rho }^0$ data candidate.

Figure 12

The signal efficiencies (upper) and the number of expected backgrounds (lower, red histogram) and data candidates (lower, black dots) for $p\to e^{+}\omega$ ($\omega \to {\pi }^0\gamma$), $p\to {\mu }^{+}\omega$ ($\omega \to {\pi }^0\gamma$), $p\to e^{+}\omega$ ($\omega \to 3\pi$), and $p\to {\mu }^{+}\omega$ ($\omega \to 3\pi$) searches from left to right. The results from SK-I to SK-IV are combined. The event selection is defined in the text.

Figure 13

Total invariant mass and total momentum for $p\to l^{+}\omega$ MC (top, cyan for free proton and blue for bound proton), atmospheric neutrino background MC corresponding to about 2000 years live time of SK (middle), and data (bottom) for $p\to e^{+}\omega$ ($\omega \to {\pi }^0\gamma$), $p\to {\mu }^{+}\omega$ ($\omega \to {\pi }^0\gamma$), $p\to e^{+}\omega$ ($\omega \to 3{\pi }^0$), and $p\to {\mu }^{+}\omega$ ($\omega \to 3{\pi }^0$) searches from left to right. All the event selections except (D5) and (E5) in Sec. 4c are applied. The results from SK-I to SK-IV are combined. The signal box is shown as a green box.

Figure 14

Total momentum for $p\to l^{+}\omega$ MC (upper, open histogram for bounded protons and shaded histogram for free protons), atmospheric neutrino MC (lower, red histogram), and data (lower, black circles) for $p\to e^{+}\omega$ ($\omega \to {\pi }^0\gamma$), $p\to {\mu }^{+}\omega$ ($\omega \to {\pi }^0\gamma$), $p\to e^{+}\omega$ ($\omega \to 3\pi$), and $p\to {\mu }^{+}\omega$ ($\omega \to 3\pi$) searches from left to right. All the event selections except (D5) and (E5) in Sec. 4c are applied. The atmospheric neutrino MC is normalized to data by area. Results from SK-I to SK-IV are combined. The signal region corresponds to the left side from the vertical green line. Total momentum for $p\to e^{+}\omega$ ($\omega \to 3\pi$) data candidate is $124\mathrm{MeV}/\mathrm{c}$.

Figure 15

Positron momentum for $p\to e^{+}\omega$ MC (upper, open histogram for bounded protons and shaded histogram for free protons), atmospheric neutrino MC (lower, red histogram), and data (lower, black circles) for $p\to e^{+}\omega$ ($\omega \to 3\pi$) search. The event selections (E1–E4) of Sec. 4c are applied. The atmospheric neutrino MC is normalized to data by area. Results from SK-I to SK-IV are combined. The inside of the two vertical green lines corresponds to the signal region.

Figure 16

The signal efficiencies (upper) and the number of expected background events (lower, red histogram) and data candidates (lower, black dots) for $n\to e^{+}{\pi }^{-}$ (left) and $n\to {\mu }^{+}{\pi }^{-}$ (right) searches. The results from SK-I to SK-IV are combined. The event selection is defined in the text.

Figure 17

Total invariant mass and total momentum for $n\to l^{+}{\pi }^{-}$ MC (top), atmospheric neutrino background MC corresponding to about 2000 years live time of SK (middle), and data (bottom) for $n\to e^{+}{\pi }^{-}$ (left) and $n\to {\mu }^{+}{\pi }^{-}$ (right) searches. All the event selections except (F4) in Sec. 4d are applied. The results from SK-I to SK-IV are combined. The signal box is shown as a green box.

Figure 18

Total momentum for $n\to l^{+}{\pi }^{-}$ MC (upper), atmospheric neutrino MC (lower, red histogram), and data (lower, black circles) for $n\to e^{+}{\pi }^{-}$ (left) and $n\to {\mu }^{+}{\pi }^{-}$ (right) searches. All the event selections except (F4) in Sec. 4d are applied. The atmospheric neutrino MC is normalized to data by area. Results from SK-I to SK-IV are combined. The signal region corresponds to the left side from the vertical green line. Total momentum for $n\to {\mu }^{+}{\pi }^{-}$ data candidate is $205\mathrm{MeV}/\mathrm{c}$.

Figure 19

The signal efficiencies (upper) and the number of expected backgrounds (lower, red histogram) and data candidates (lower, black dots) for $n\to e^{+}{\rho }^{-}$ (left) and $n\to {\mu }^{+}{\rho }^{-}$ (right). The results from SK-I to SK-IV are combined. The event selection is defined in the text.

Figure 20

Total invariant mass and total momentum for $n\to l^{+}{\rho }^{-}$ MC (top), atmospheric neutrino background MC corresponding to about 2000 years live time of SK (middle), and data (bottom) for $n\to e^{+}{\rho }^{-}$ (left) and $n\to {\mu }^{+}{\rho }^{-}$ (right) searches. All the event selections except (G6) in Sec. 4e are applied. The results from SK-I to SK-IV are combined. For the $n\to e^{+}{\rho }^{-}$ search, the signal box is shown as a green box. For the $n\to {\mu }^{+}{\rho }^{-}$ search, no total mass cut is applied in the analysis and the sharp cutoffs on the total mass correspond to the ${\rho }^{-}$ mass cut threshold.

Figure 21

Total momentum for $n\to l^{+}{\rho }^{-}$ MC (upper), atmospheric neutrino MC (lower, red histogram), and data (lower, black circles) for $n\to e^{+}{\rho }^{-}$ (left) and $n\to {\mu }^{+}{\rho }^{-}$ (right) searches. All the event selections except (G6) in Sec. 4e are applied. The atmospheric neutrino MC is normalized to data by area. Results from SK-I to SK-IV are combined. The signal region corresponds to the left side from the vertical green line. Total momenta of the data candidates are $173\mathrm{MeV}/\mathrm{c}$, $235\mathrm{MeV}/\mathrm{c}$, $239\mathrm{MeV}/\mathrm{c}$, $246\mathrm{MeV}/\mathrm{c}$ for $n\to e^{+}{\rho }^{-}$ search and $129\mathrm{MeV}/\mathrm{c}$ for $n\to {\mu }^{+}{\rho }^{-}$ search.

Figure 22

Explored ranges and lower limits of nucleon partial lifetime with the results of the other experiments [27, 28, 29] and the previous SK analysis [11]. The $p\to l^{+}{\pi }^0$ results [7] are also included.