Search for nucleon decay into charged antilepton plus meson in Super-Kamiokande I and II

H. Nishino et al. (The Super-Kamiokande Collaboration)

Phys. Rev. D 85, 112001 – Published 5 June 2012

Abstract

Searches for a nucleon decay into a charged antilepton ( $e^{+}$ or $μ^{+}$ ) plus a light meson ( $π^{0}$ , $π^{-}$ , $η$ , $ρ^{0}$ , $ρ^{-}$ , $ω$ ) were performed using the Super-Kamiokande I and II data. Twelve nucleon decay modes were searched for. The total exposure is $140.9 kiloton \cdot years$ , which includes a $91.7 kiloton \cdot year$ exposure (1489.2 live days) of Super-Kamiokande-I and a $49.2 kiloton \cdot year$ exposure (798.6 live days) of Super-Kamiokande II. The number of candidate events in the data was consistent with the atmospheric neutrino background expectation. No significant evidence for a nucleon decay was observed in the data. Thus, lower limits on the nucleon partial lifetime at 90% confidence level were obtained. The limits range from $3.6 \times 10^{31}$ to $8.2 \times 10^{33} years$ , depending on the decay modes.

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Received 18 March 2012

DOI:https://doi.org/10.1103/PhysRevD.85.112001

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Vol. 85, Iss. 11 — 1 June 2012

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Figure 1
Calculated cross section of $η$ -nucleon interaction considering $S_{11} (1535)$ resonance in the custom simulation code.
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Figure 2
The differential cross sections of photoproduction of an $η$ meson on a ${}^{12}C$ target with $γ$ energy of 735–765 MeV. The experimental data at MAINZ [29] are shown as circles. The solid line shows the simulated production cross section, and its shaded region corresponds to a factor 2 difference in the cross section of $η$ nuclear effects. The dashed line shows the production cross section without nuclear effects.
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Figure 3
The reconstructed momentum distributions for positrons in the free proton decay of $p \to e^{+} π^{0}$ in SK-I (solid) and SK-II (dashed). The momentum resolution for the positron, as shown by the width of the distribution, is 4.9% in SK-I and 6.0% in SK-II.
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Figure 4
The charged pion momentum distribution for the $n \to e^{+} π^{-}$ MC in SK-I. The solid line and the dashed line show the reconstructed and true pion momentum distributions, respectively. The charged pion momentum can be well reconstructed with a resolution of 9.2% (9.7%) in SK-I (SK-II).
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Figure 5
Number of rings distribution for atmospheric neutrino MC and data. The number of MC events are normalized to the observed data by the number of single-ring $e$ -like events (also in other figures unless otherwise noted).
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Figure 6
Number of events and detection efficiency at each event selection step: (FV) fiducial volume, (1) number of Cherenkov rings, (2) PID, (3) meson invariant mass, (4) number of Michel electrons and (5) total invariant mass and total momentum. The plot shows three nucleon decay mode searches: the mode with highest efficiency ( $p \to e^{+} π^{0}$ ), the mode with the greatest number of rings ( $p \to e^{+} η$ ( $η \to 3 π^{0}$ )) and the mode using PID with both Cherenkov ring pattern and opening angle ( $p \to μ^{+} ρ^{0}$ ). The number of observed events and the estimated background rates agree with each other. Also, the searches in SK-I and SK-II are compared.
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Figure 7
A typical proton decay MC event of $p \to e^{+} π^{0}$ in SK-I. One Cherenkov ring from a positron and two Cherenkov rings from the two $γ$ -rays from $π^{0}$ decay can be observed on the right and the left of the figure, respectively. The size of circles indicate the amount of detected charge. The crosses on the plot show a reconstructed vertex position horizontally and vertically projected on the detector wall. Solid lines show the reconstructed rings. The reconstruction algorithms correctly find all rings and identify their particle for this event.
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Figure 8
The invariant mass of $π^{0}$ distributions for SK-I proton decay MC ( $p \to e^{+} π^{0}$ ), atmospheric neutrino MC, and SK-I data. The shaded histogram is the distribution of free proton decay events. The bars and arrows indicate the $π^{0}$ invariant mass cut threshold.
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Figure 9
The total invariant mass distributions for proton decay MC ( $p \to e^{+} π^{0}$ ), atmospheric neutrino MC, and observed data in SK-I. The shaded histogram is the distribution of free proton decay events. The bars and arrows indicate the total invariant mass cut threshold. Only events which satisfy all other selection criteria except this are plotted. The distribution of atmospheric neutrino MC has the $π^{0}$ invariant mass peak, which is consistent with the observed data.
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Figure 10
Total momentum versus total invariant mass distributions, from left to right: SK-I $p \to e^{+} π^{0}$ ; SK-II $p \to e^{+} π^{0}$ ; SK-I $p \to μ^{+} π^{0}$ ; and SK-II $p \to μ^{+} π^{0}$ , from top to bottom: the proton decay MC; the atmospheric neutrino MC ( $11.25 megaton \cdot years$ for each SK-I and SK-II); and the observed data ( $91.7 kiloton \cdot years$ in SK-I and $49.2 kiloton \cdot years$ in SK-II). These events satisfy the event selection criteria except for the selection by the total momentum and total mass. The boxes in figures indicate the total momentum and mass criteria. The points in the boxes of the atmospheric neutrino MC are shown in a larger size to show the distributions in the signal box. No candidates were found for either $p \to e^{+} π^{0}$ or $p \to μ^{+} π^{0}$ in the data.
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Figure 11
Total momentum versus total invariant mass distributions of proton decay MC ( $SK − I + II$ combined), the atmospheric neutrino MC ( $SK − I + II$ combined; $11.25 megaton \cdot years$ for each SK-I and SK-II) and SK-I ( $91.7 kiloton \cdot years$ ) and SK-II ( $49.2 kiloton \cdot years$ ) data, from top to bottom: $p \to e^{+} η$ ( $2 γ$ ), $p \to μ^{+} η$ ( $2 γ$ ), $p \to e^{+} η$ ( $3 π^{0}$ ) and $p \to μ^{+} η$ ( $3 π^{0}$ ). The boxes in figures indicate the total momentum and mass criteria. These events satisfy the event selection criteria for each mode except for the selection on the total momentum and total mass. Two candidates for $p \to μ^{+} η$ ( $3 π^{0}$ ) were found in the SK-II data.
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Figure 12
The meson invariant mass distributions for the proton decay MC (solid lines), the atmospheric neutrino MC (dashed lines) and the observed data (points) in SK-I (top) and SK-II (bottom), from left to right; $p \to e^{+} η$ mode ( $2 γ$ ), $p \to e^{+} ρ^{0}$ mode and $p \to e^{+} ω$ mode ( $π^{0} γ$ ). Free proton decay events are indicated by shaded histograms. All decay branches of the mesons are filled in the histograms for the proton decay MCs. The other peaks outside of the selection window are events from the other decay branches.
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Figure 13
Total momentum versus total invariant mass distributions of proton decay MC ( $SK − I + II$ combined), the atmospheric neutrino MC ( $SK − I + II$ combined; $11.25 megaton \cdot years$ for each SK-I and SK-II) and SK-I ( $91.7 kiloton \cdot years$ ) and SK-II ( $49.2 kiloton \cdot years$ ) data, from top to bottom: $p \to e^{+} ω$ ( $π^{0} γ$ ), $p \to μ^{+} ω$ ( $π^{0} γ$ ), $p \to e^{+} ω$ ( $3 π$ ) and $p \to μ^{+} ω$ ( $3 π$ ). These events satisfy the event selection criteria for each mode except for the selections on the total momentum and total mass. The boxes in the figures indicate the total momentum and mass criteria. For the $p \to μ^{+} ω$ ( $π^{0} γ$ ) mode, the total invariant mass is equivalent to the meson invariant mass because a muon is invisible. The sharp cutoffs in the total invariant mass distribution correspond to the $ω$ invariant mass cut. For the $p \to μ^{+} ω$ ( $3 π$ ) mode, there are no such sharp cutoffs because the $ω$ invariant mass cut is not applied to this mode. A candidate for $p \to e^{+} ω$ ( $3 π$ ) was found in the SK-I data.
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Figure 14
Total momentum versus total invariant mass distributions of proton decay MC ( $SK − I + II$ combined), the atmospheric neutrino MC ( $SK − I + II$ combined; $11.25 megaton \cdot years$ for each SK-I and SK-II) and SK-I ( $91.7 kiloton \cdot years$ ) and SK-II ( $49.2 kiloton \cdot years$ ) data, from top to bottom: $p \to e^{+} ρ^{0}$ , $p \to μ^{+} ρ^{0}$ , $n \to e^{+} ρ^{-}$ and $n \to μ^{+} ρ^{-}$ . These events satisfy the event selection criteria for each mode except for the selection on total momentum and total mass. The boxes in figures indicate the total momentum and mass criteria. For the $n \to μ^{+} ρ^{-}$ mode, no total invariant mass cut is applied and the sharp cutoffs on the total invariant mass correspond to the $ρ^{-}$ invariant mass cut threshold. A candidate for each $p \to μ^{+} ρ^{0}$ and $n \to e^{+} ρ^{-}$ was found in the SK-I data.
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Figure 15
Total momentum versus total invariant mass distributions of proton decay MC ( $SK − I + II$ combined), the atmospheric neutrino MC ( $SK − I + II$ combined; $11.25 megaton \cdot years$ for each SK-I and SK-II) and SK-I ( $91.7 kiloton \cdot years$ ) and SK-II ( $49.2 kiloton \cdot years$ ) data, for $n \to e^{+} π^{-}$ (top) and $n \to μ^{+} π^{-}$ (bottom). These events satisfy the event selection criteria for each mode except for the selection on the total momentum and total mass. The boxes in the figures indicate the total momentum and mass criteria. A candidate for $n \to μ^{+} π^{-}$ was found in the SK-I data.
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Figure 16
An example of a background event for the $p \to e^{+} π^{0}$ mode in the atmospheric neutrino MC. An electron neutrino interacts with a neutron by CCQE and produces an electron and a proton. One ring from the electron and the other two rings from the decay of a $π^{0}$ , which is produced by a secondary interaction of the proton in water, mimic a proton decay signal from $p \to e^{+} π^{0}$ . Solid line circles show reconstructed rings.
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Figure 17
A candidate event for $p \to e^{+} ω$ ( $3 π$ ) in the SK-I data. Reconstructed total momentum and invariant mass were $137 MeV / c$ and $796 MeV / c^{2}$ , respectively. Solid (dashed) lines correspond to reconstructed rings classified as shower (non shower) type. A Cherenkov ring at the center-bottom of the display was classified as a shower-type ring in the PID algorithm, but its Cherenkov ring edge looks as sharp as a non shower-type ring.
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Figure 18
Explored ranges and lower limits (at 90% confidence level) of nucleon partial lifetime with the results of the previous experiments; IMB-3 [4], KAMIOKANDE- $I + II$ [5] and FREJUS [43].
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