PHYSICAL REVIEW D 90, 072005 (2014)

Search for proton decay via p → νK þ using 260 kiloton · year dataof Super-Kamiokande
K. Abe,1,26 Y. Hayato,1,26 K. Iyogi,1 J. Kameda,1,26 M. Miura,1,26 S. Moriyama,1,26 M. Nakahata,1,26 S. Nakayama,1,26
R. A. Wendell,1,26 H. Sekiya,1,26 M. Shiozawa,1,26 Y. Suzuki,1,26 A. Takeda,1,26 Y. Takenaga,1 K. Ueno,1 T. Yokozawa,1
H. Kaji,2 T. Kajita,2,26 K. Kaneyuki,2,26,* K. P. Lee,2 K. Okumura,2 T. McLachlan,2 L. Labarga,3 E. Kearns,4,26 J. L. Raaf,4
J. L. Stone,4,26 L. R. Sulak,4 M. Goldhaber,5,* K. Bays,6 G. Carminati,6 W. R. Kropp,6 S. Mine,6 A. Renshaw,6M. B. Smy,6,26 H. W. Sobel,6,26 K. S. Ganezer,7 J. Hill,7 W. E. Keig,7 J. S. Jang,8 J. Y. Kim,8 I. T. Lim,8 J. B. Albert,9K. Scholberg,9,26 C. W. Walter,9,26 T. Wongjirad,9 T. Ishizuka,10 S. Tasaka,11 J. G. Learned,12 S. Matsuno,12 S. N. Smith,12
T. Hasegawa,13 T. Ishida,13 T. Ishii,13 T. Kobayashi,13 T. Nakadaira,13 K. Nakamura,13,26 K. Nishikawa,13 Y. Oyama,13
K. Sakashita,13 T. Sekiguchi,13 T. Tsukamoto,13 A. T. Suzuki,14 Y. Takeuchi,14 K. Ieki,15 M. Ikeda,15 H. Kubo,15
A. Minamino,15 A. Murakami,15 T. Nakaya,15,26 Y. Fukuda,16 K. Choi,17 Y. Itow,17 G. Mitsuka,17 M. Miyake,17P. Mijakowski,29 J. Hignight,18 J. Imber,18 C. K. Jung,18 I. Taylor,18 C. Yanagisawa,18 H. Ishino,19 A. Kibayashi,19
Y. Koshio,19 T. Mori,19 M. Sakuda,19 J. Takeuchi,19 Y. Kuno,20 S. B. Kim,21 H. Okazawa,22 Y. Choi,23 K. Nishijima,24
M. Koshiba,25 Y. Totsuka,25,* M. Yokoyama,25,26 K. Martens,26 Ll. Marti,26 Y. Obayashi,26 M. R. Vagins,26,6 S. Chen,27
H. Sui,27 Z. Yang,27 H. Zhang,27 K. Connolly,27 M. Dziomba,27 and R. J. Wilkes28(Super-Kamiokande Collaboration)
1

Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo,
Kamioka, Gifu 506-1205, Japan
2
Research Center for Cosmic Neutrinos, Institute for Cosmic Ray Research, University of Tokyo,
Kashiwa, Chiba 277-8582, Japan3
Department of Theoretical Physics, University Autonoma Madrid, 28049 Madrid, Spain
4
Department of Physics, Boston University, Boston, Massachusetts 02215, USA
5
Physics Department, Brookhaven National Laboratory, Upton, New York 11973, USA
6
Department of Physics and Astronomy, University of California,
Irvine, Irvine, California 92697-4575, USA
7
Department of Physics, California State University, Dominguez Hills, Carson, California 90747, USA
8
Department of Physics, Chonnam National University, Kwangju 500-757, Korea
9
Department of Physics, Duke University, Durham, North Carolina 27708, USA
10
Junior College, Fukuoka Institute of Technology, Fukuoka, Fukuoka 811-0295, Japan
11
Department of Physics, Gifu University, Gifu, Gifu 501-1193, Japan
12
Department of Physics and Astronomy, University of Hawaii, Honolulu, Hawaii 96822, USA
13High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
14
Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan
15
Department of Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japan
16
Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan
17
Solar Terrestrial Environment Laboratory, Nagoya University, Nagoya, Aichi 464-8602, Japan18
Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook,
New York 11794-3800, USA
19
Department of Physics, Okayama University, Okayama, Okayama 700-8530, Japan20
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
21
Department of Physics, Seoul National University, Seoul 151-742, Korea
22
Department of Informatics in Social Welfare, Shizuoka University of Welfare,
Yaizu, Shizuoka, 425-8611, Japan
23
Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea
24
Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan
25
The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
26
Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced
Study, University of Tokyo, Kashiwa, Chiba 277-8582, Japan
27
Department of Engineering Physics, Tsinghua University, Beijing 100084, China
28
Department of Physics, University of Washington, Seattle, Washington 98195-1560, USA
29
National Centre For Nuclear Research, 00-681 Warsaw, Poland(Received 7 August 2014; published 14 October 2014)*

Deceased.

1550-7998=2014=90(7)=072005(14)072005-1© 2014 American Physical Society

K. ABE et al.

PHYSICAL REVIEW D 90, 072005 (2014)
We have searched for proton decay via p → νK þ using Super-Kamiokande data from April 1996 to

February 2013, 260 kiloton · year exposure in total. No evidence for this proton decay mode is found.
A lower limit of the proton lifetime is set to τ=Bðp → νK þ Þ > 5.9 × 1033 years at 90% confidence level.
DOI: 10.1103/PhysRevD.90.072005

PACS numbers: 12.10.Dm, 12.60.Jv, 13.30.-a, 29.40.Ka

I. INTRODUCTION
The standard model of particle physics, based on SUð3Þ
for the strong interaction and the unification of SUð2Þ ×Uð1Þ for the electroweak interaction, has been successful inaccounting for many experimental results. However, thestandard model offers no guidance on the unification of thestrong and electroweak forces, and has many other open
questions. Various attempts have been made to resolve theshortcomings by unifying the strong and electroweak
interactions in a single larger gauge group, i.e., a grandunified theory (GUT) [1]. GUTs are motivated by theapparent convergence of the running couplings of thestrong, weak, and electromagnetic forces at a high energy
scale (1015 –1016 GeV). Energy scales this large are out of
the reach of accelerators but may be probed by virtual
processes such as those that govern particle decay. A
general feature of GUTs is the instability of nucleons bybaryon number violating decay. Nucleon decay experiments are direct experimental tests of the general idea of
grand unification.
In GUTs, nucleon decay can proceed via exchange of a
massive gauge boson between two quarks. The favoredgauge-mediated decay mode in many GUTs is p → eþ π 0 .
In the minimal SU(5) GUT, the predicted proton lifetime toeþ π 0 is 10311 years, which has been ruled out by
experimental results from IMB [2], Kamiokande [3], andSuper-Kamiokande [4]. GUT models incorporating supersymmetry [5] (SUSY-GUTs) raise the GUT scale [6],suppressing the decay rate of p → eþ π 0 , thereby allowing
compatibility with the experimental limit. However, SUSYGUTs introduce dimension five operators that enable themode p → ν̄K þ to have a high branching fraction and shortpartial lifetime [7]. In the SUSY SU(5) GUT with minimal
assumptions and TeV scale SUSY particles, the partial
proton lifetime to ν̄K þ is less than 1031 years [8], which has
also been excluded by previously published experimental
constraints [2,3,9]. Nonminimal SUSY SU(5) GUTs [10]or SUSY GUTs based on SO(10) [11] have been constructed that evade this limit, yet still predict partial
lifetimes in the range 1032 to 1035 years, with someparticular models that require the lifetime be less than a
few times 1034 years. The low ends of the lifetime
predictions by these models are probed by this experimental search.
In this paper, our search is for the two-body decay of
proton to a K þ and a neutrino. In most models, (B − L) isconserved and the final state contains an antineutrino;
however we do not detect the neutrino and cannot

distinguish p → νK þ from p → ν̄K þ , nor can we determine the flavor (e, μ, or τ) of the neutrino. In fact, oursearch can be applied to any nearly massless neutral finalstate particle such as a gravitino or axino.
The Super-Kamiokande collaboration published a searchfor p → νK þ with 91.7 kton · years exposure of the firstphase of the experiment, and set a partial lifetime limitτðp → νK þ Þ > 2.3 × 1033 years [12]. In this paper, we
refine the analysis and update the search with 2.8 times
greater detector exposure including later phases of theexperiment.
II. SUPER-KAMIOKANDE DETECTOR
Super-Kamiokande [13] is a large water Cherenkov
detector. It is an upright cylinder in shape, 39 m in diameterand 40 m in height, and it contains 50 kton of pure water. It
lies about 1000 m underneath the top of Mt. Ikenoyama(2700 m water equivalent underground) to reduce cosmic
ray background. The detector is optically separated into
two regions: inner detector (ID) and outer detector (OD).
Cherenkov light in the ID is detected by 20-inch PMTs [14]facing inward, evenly covering the cylindrical inner surface. Cherenkov light from penetrating particles, usuallycosmic ray muons or exiting muons, is detected by 8-inch
PMTs facing outward. The fiducial volume is defined as acylindrical volume with surfaces 2 meters inwards from the
ID PMT plane. The fiducial mass is 22.5 ktons, corresponding to 7.5 × 1033 protons.
Super-Kamiokande started observation in April 1996
with 11,146 PMTs which covered 40% of the ID surface
for photon detection. The observation was continued
until July 2001, with 1489.2 live days, or equivalently,
91.7 kton · years. This period is called Super-KamiokandeI (SK-I). After an accident in 2001, about half of the ID
PMTs were lost and the detector was reconstructed with
5,182 ID PMTs uniformly distributed over the cylindricalsurface decreasing photo coverage to 19%. The PMTs were
thereafter enclosed in acrylic and FRP cases. The period
from December 2002 until October 2005, corresponding to798.6 live days (49.2 kton · years), is called SK-II. After
production and installation of replacement 20-inch PMTs,
the photo coverage was recovered to 40% in 2006. The
period between July 2006 and September 2008, corresponding to 518.1 live days (31.9 kton · years), is definedas SK-III. In the summer of 2008, we upgraded ourelectronics with improved performance including a dataacquisition that records all PMT hit information without
dead time [15]. This has been the configuration of the072005-2

SEARCH FOR PROTON DECAY VIA p → νK þ …
Summary of data sets that are used in this paper.

Live days kton · yr Coverage
SK-I
SK-II
SK-III
SK-IV

1489.2
798.6
518.1
1417.4

91.7
49.2
31.9
87.3

40%
19%
40%
40%

Note

atmospheric neutrino events that survive the proton decay
selection criteria.
A. Proton decay

Half PMT density
New readout electronics

detector since September 2008; it is called SK-IV. In thispaper, we use data until February 2013, corresponding to1417.4 live days (87.3 kton · years). Table I summarizes thedata sets used for the proton decay search in this paper.
The trigger to record an event is based on the coincidenceof the number of hit PMTs exceeding a threshold. For SK IIII, the trigger was implemented in hardware using a signal
proportional to the number of hit PMTs produced by eachfront-end electronics module. For SK-IV, the trigger isimplemented in software. The trigger threshold is less than
10 MeV for all SK periods, and the trigger efficiency forthis proton decay mode is 100%.1
The charge and timing of the PMTs are calibrated using
various calibration sources [16]. The timing resolution of
the 20-inch PMT is about 2.1 nsec at the single photoelectron level. The PMT response, water quality, andreflections from the detector wall are tuned in the SK
detector simulation program using injected light as well as
various control data samples such as cosmic ray muons.

A water molecule contains two free protons and eight
protons bound in the oxygen nucleus. In the decay of a free
proton, the ν and the K þ are emitted opposite each otherwith momenta of 339 MeV=c. In the case of proton decayin oxygen, Fermi momentum, correlation with othernucleons, nuclear binding energy, and kaon-nucleon interactions are taken into account as described below.
We use the Fermi momentum and nuclear binding
energy measured by electron-12 C scattering [18]. Nuclear
binding energy is taken into account by modifying theproton mass. Ten percent of decaying protons are estimatedto be kinematically correlated with other nucleons within
the nucleus [19]. These correlated decays cause the total
invariant mass of the decay products to be smaller than the
proton mass because of the momentum carried by thecorrelated nucleons. Figure 1 shows the invariant mass ofthe products of the decaying proton, K þ and ν and theresulting kaon momenta after the simulation of the proton
decay for both bound and free protons. Correlated decays

Number of events

TABLE I.

PHYSICAL REVIEW D 90, 072005 (2014)

III. SIMULATION

1

For the case of prompt gamma tag (Method 1), with gamma
energy typically 6 MeV, the muon from K þ decay can provide theevent trigger.

4

10

3

10

2

10
400

600

800

Invariant proton mass in
Number of events

To determine selection criteria for the proton decay
search, and to estimate efficiencies and background rates,
we use proton decay and atmospheric neutrino Monte Carlo(MC) simulations. Because the configuration of the detector is different in SK-I through IV, we generated MC
samples for each period. Proton decay MC samples with
50,000 events are generated in an oversized volume whichis 1 meter outside the fiducial volume boundary, andtherefore 1 meter from the detector wall. This allows us
to include event migration near the fiducial boundary in ourestimates. The selection efficiency is defined as the numberof events fulfilling all requirements divided by number ofgenerated events in the fiducial volume. The MC equivalentof 500 years of atmospheric neutrino exposure are generated for each period. These atmospheric neutrino MC
samples are used for our studies of neutrino oscillations[17]. Because the background rates for the proton decay
studied in this paper are small (less than one event for theentire exposure for two of the analysis techniques), theselarge MC background samples provide fewer than 40

10

10

4

10

3

10

2

1000
16

O

10
0

200

400

600

K+ momentum (MeV/c)

FIG. 1 (color online). The upper figure shows the decaying
proton mass distribution in 16 O and the lower figure shows theK þ momentum distribution from the simulation of p → νK þ . In
the upper figure, the single-bin histogram shows the free protoncase and the broad histogram shows the bound proton case. The
rightmost peak in the bound proton case corresponds to thep-state, located slightly lower than the proton mass by 15.5 MeVof binding energy; the second rightmost peak is the s-state(39 MeV in binding energy). The correlated nucleon decay makes
the longer tail in the lower mass region. In the lower figure, thesingle-bin histogram shows the free proton case (339 MeV=c)
and the broad histogram shows the bound proton case which issmeared by Fermi motion.

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PHYSICAL REVIEW D 90, 072005 (2014)
0.06

2

-38
σ (10 cm)

0.05
0.04
0.03
0.02
0.01
0

0

1

2

0

1

2

3

4

5

6

7

8

9

10

3

4

5

6

7

8

9

10

0.035
0.03
-38
σ (10 cm)produce the broad spectrum below about 850 MeV=c .
In our experiment, the kaon momentum is unobserved
because the kaon is always produced below the Cherenkov
threshold of 560 MeV=c in water. The majority of K þ(89%) are stopped in water and decay at rest. We search forK þ decay at rest into μþ νμ (64% branching fraction) andπ þ π 0 (21% branching fraction).
The position of the decaying proton in 16 O is calculated
according to the Woods-Saxon nuclear density model [20].
The kaon nucleon interactions which are considered
include elastic scattering and inelastic scattering via charge
exchange. The type of interaction is determined using the
calculated mean free path [21]. For kaons, whose momenta
are described by Fig. 1, the probability of charge exchange
for K þ in p → νK þ is 0.14%.
If a nucleon decays in the oxygen nucleus, the remaining
nucleus can be left in an excited state from which it
promptly de-excites by the emission of gamma rays. The
prompt gamma emission processes are simulated based on
Ref. [22]. The dominant gamma ray is 6.3 MeV from the
p3=2 state with 41% branching fraction. The probabilities ofγ emission in this simulation are summarized in Table II.
Other states emitting low energy gamma rays are averaged
and assigned 3.5 MeV γ emission. There are some branches
which are not accompanied by γ emissions, also as shownin Table II.

0.025
0.02
0.015
0.01
0.005
0

Neutrino Energy (GeV)

FIG. 2 (color online). Cross sections of the single K-meson
productions via resonances calculated by NEUT. Upper
plots show neutrino interactions and lower show antineutrino
interactions.

νn → l− ΛK þ
νn → νΛK 0
νp → νΛK þ
ν̄p → lþ ΛK 0

B. Atmospheric neutrinos

ν̄n → ν̄ΛK 0

The SK standard atmospheric neutrino MC used in theprevious neutrino oscillation analyses [17] and proton
decay searches [4,12,23,24] is used in this analysis. It isbased on the Honda atmospheric neutrino flux [25] andNEUT [21] neutrino-nucleus interaction model. Someneutrino interactions which produce K mesons via
resonances could be potential background sources for
p → νK þ search. Cross sections of the single meson
production via resonances are calculated based onRein and Sehgal’s theory [26]. In NEUT, the neutrino
reactions:ν̄p → ν̄ΛK þ

TABLE II. Summary of probabilities of nuclear γ ray emissions
at the de-excitation of the remaining nucleus.
State

Energy of γ

Probability

p3=2
p3=2
s1=2
s1=2
others

6.3 MeV
9.9 MeV7.03 MeV
7.01 MeV
3.5 MeV

41%
3%
2%
2%
16%




11%
25%

Other than γ emission
p=n emission w=o γ
ground state

are taken into account assuming the same cross section
both for νe and νμ . The differential cross sections areshown in Fig. 2.
We simulate propagation of the produced particles andCherenkov light in water by custom code based on
GEANT3 [27]. The propagation of charged pions in water
is simulated by custom code based on Ref. [28] for less than
500 MeV=c and by GCALOR [29] for more than
500 MeV=c.
The equivalent of 500 years of SK atmospheric neutrino
data is simulated for each SK run period. The generated
atmospheric neutrino samples are weighted to include theeffect of νμ disappearance due to νμ -ντ oscillation assuming
Δm2 ¼ 2.5 × 10−3 eV2 and sin2 2θ ¼ 1.0, ignoring theappearance of νe or ντ as a possible background. The final
background event rates for each period are normalized by
the observed total sub-GeV event rate.
IV. DATA SET, REDUCTION AND
RECONSTRUCTION
The vast majority of the triggered events are cosmic raymuons and low energy backgrounds from the radioactivityof materials around the detector wall. Several stages of data

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PHYSICAL REVIEW D 90, 072005 (2014)

reduction were applied to the events before proceeding tofurther detailed event reconstruction processes. Details of
the data reduction and reconstruction can be found in [17].
The fully contained (FC) data sample in the fiducial
volume (FV) is defined by the following cuts:(i) number of hit PMTs in the largest OD hit cluster isless than 10 for SK-I and 16 for other periods;(ii) total visible energy is greater than 30 MeV in ID;(iii) distance of the reconstructed vertex from the ID
PMT surface is greater than 2 meters (corresponding
to 22.5 kton of water volume).
The rate of FCFV events is about 8 events per day.
The contamination of events other than atmospheric neutrinos is estimated to be less than 1% and composed ofcosmic rays that evaded the OD veto and events caused byflashing PMTs.
Reconstruction algorithms are applied to the events
remaining after the reduction process to determine theevent vertex, the number of Cherenkov rings, the particle
type of each ring, the momentum assigned to each ring, andnumber of Michel electrons. As a first step, the event vertex
is defined as the point at which the timing distribution, after
subtraction of the calculated time of flight of the photon
from the vertex (TOF subtraction), has the sharpest peak.
The dominant ring direction is determined from the charge
distribution as a function of angle. Then other ringcandidates are searched for using the Hough transform
method [30], a technique for extracting a particular shapefrom an image, assuming all particles are generated in one
vertex. Each ring candidate is tested against a likelihood
function to remove fake rings before determining the final
number of rings. Each ring is classified as e-like (showering
type as from e; , γ) or μ-like (nonshowering type) based ona likelihood using the ring pattern and Cherenkov opening
angle for the single ring case, and only the ring pattern forthe multiring case.
Michel electrons from the decay of the μ are tagged bysearching for clusters of in-time hits after the primary event.
During the SK-I to SK-III periods, there was an impedancemismatch between cables and electronics which caused asignal reflection at 1000 ns after the main event. The time
period between 800 ns and 1200 ns from the primary eventswas excluded for the decay electron search due to thissignal reflection. For SK-IV, the new electronics have better
impedance matching to avoid signal reflection, and no suchexclusion is required. As a result, the tagging efficiency of
decay electrons from μþ with momentum of 236 MeV=c
has been improved from 85% (SK-I, II, and III) to 99%(SK-IV), which improves the selection efficiency for
p → νK þ .
The momentum for each ring is decided from thecharge spatially inside of 70° from the ring direction andtemporally from −50 nsec to þ250 nsec around the TOFsubtracted main event peak. The number of photoelectrons
from each PMT are corrected by light attenuation in water,

PMT acceptance, dark noise, time variation of gain, andtrack length in the case of μ-like rings. If a Michel electron
is within 250 nsec of the parent particle, the time window
for momentum determination is shortened and the charge
sum is corrected from the nominal þ250 nsec case. The
momentum scale is checked by cosmic ray muons, Michelelectrons from the cosmic ray muons which stop in the
inner detector, and also the invariant mass distributions of
π 0 events produced in atmospheric neutrino interactions.
The uncertainty on the momentum scale is less than 3% for
the entire period.
An additional precise vertex fitter is applied only forsingle-ring events. The expected charge for each PMT iscalculated using the result of particle identification (e-likeor μ-like) and using the momentum estimate. The expectedcharge is compared with the observed charge by varying
the vertex along the particle direction. The estimatedvertex resolution for FC single-ring sub-GeV events isabout 30 cm.
V. ANALYSIS
If a proton decays by p → νK þ, the K þ itself is not
visible in a water Cherenkov detector since its velocity is
below Cherenkov threshold. However, the K þ can be
identified by its decay products in the decay modes
K þ → μþ ν and K þ → π þ π 0 . Being two-body decay processes, the daughter particles have monochromatic momentum in the K þ rest frame: 236 MeV=c for μþ νμ and205 MeV=c for π þ π 0. There are three established methods
for the p → νK þ mode search [9]: (Method 1) search forsingle-muon events consistent with K þ decay with agamma ray promptly emitted at the time of K þ production;(Method 2) search for an excess of muon events with amomentum of 236 MeV=c in the momentum distribution;and (Method 3) search for π þ π 0 events with a momentumof 205 MeV=c.
A. Method 1: K þ → μþ ν, tag by prompt gamma ray
In this scenario, the Super-K detector should observe asingle μ-like ring preceded by PMT hits due to a nuclear
de-excitation gamma ray. Figure 3 shows a graphical event
display of the PMT hit pattern for an example event, as aMonte Carlo simulation of the proton decay. A prompt
gamma ray, a muon, and a Michel electron peak should beobserved in order, as seen in another example in Fig. 4,
where the time of each particle is labeled as used in theanalysis. To search for events tagged by the prompt gamma
ray, single-ring μ-like events are selected by requiring thefollowing criteria:(A-1) a fully contained event with one nonshowering(μ-like) ring,(A-2) there is one Michel decay electron,(A-3) the reconstructed muon momentum is between 215
and 260 MeV=c,

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PHYSICAL REVIEW D 90, 072005 (2014)

10 1

10 0
-50
0
50 100 150
Residual PMT Hit Time (ns)FIG. 3 (color online). An example graphical event display of asimulated proton decay passing all of the criteria for Method 1.
The single Cherenkov ring was produced by the muon from kaon
decay and fit with momentum 231 MeV=c. The color of the hit
PMTs represents the residual hit time after subtracting thetime-of-flight of Cherenkov light in water from the vertex to
the PMT. The hit PMTs associated with a 6 MeV prompt gamma
are colored cyan. The decay electron was detected after the
displayed event.

Number of hits(A-4) the distance between the vertices of the muon and theMichel electron is less than 200 cm,(A-5) the TOF-subtracted timing distribution for the muonvertex is required to have a minimum goodness-offit (>0.6),
50
45
40
35
30
25
20
15
10
5
0

μ

Proton decay MC
e

γ
0

200

400

600

40

Number of hits

35
30
25
20
15
10
5
0

-50

-25

0

25

50

T-TOF (nsec)

FIG. 4. Number of hits versus hit time for a typical proton
decay MC event selected by Method 1. In the upper figure, three
hit clusters due to prompt γ, the μ from kaon decay (τ ¼ 12 ns),
and the muon-decay electron (τ ¼ 2.2 μ sec) can be seen in order.
The lower figure is expanded around the μ hit cluster and tγ , t0 ,and tμ are shown as a demonstration.

(A-6) the pattern of the single nonshowering ring is more
likely to be a muon than a proton: Lpr − Lμ < 0, Lpr ,Lμ are likelihood functions assuming a proton and amuon, which are described later,(A-7) gamma hits are found: 8 < N γ < 60 for SK-I, III, andVI, 4 < N γ < 30 for SK-II(A-8) the time difference from the gamma tag to thekaon decay is consistent with the kaon lifetime:tμ − tγ < 75 nsec,
The cut criteria (A-4) and (A-5) are applied to reject
atmospheric neutrino events with a high momentum recoil
proton (above Cherenkov threshold) accompanied by aninvisible muon or charged pion (below the Cherenkov
threshold) which produces a Michel electron in its decay
chain. Since the particle type of the single nonshowering
Cherenkov ring is assumed to be that of a muon, the vertex
accuracy is worse when the Cherenkov ring is from a recoil
proton. The inaccurate vertex determination causes incorrect TOF subtraction of the Cherenkov light. As a result,recoil protons may create a false peak in the time distribution of hit PMTs, which can fake a prompt gamma ray.
The event may include Michel electrons from the decayof the invisible muon, but the distance between themisreconstructed vertex and the Michel electron is typicallylarge.
The proton identification criterion (A-6) is a refinement
to the methods in our previous paper for rejecting single
proton ring events. It is used for reduction of single proton
ring events. The algorithm [31] makes a likelihood function
assuming a muon (Lμ ) and a proton (Lpr ) by using theCherenkov angle and the width of the Cherenkov ring.
Figure 5 shows the likelihood function. The upper figure isfor the sample requiring cuts (A-1) through (A-5). Data
and MC agree well. The lower figure is the same distribution after applying cuts (A-7) and (A-8) additionally.
Background events are efficiently reduced by (A-6).
After cuts (A-1) through (A-6), a distribution of hits (N)vs time after TOF subtraction (t) is made. To search for the
prompt gamma ray, three quantities of time must be
defined. The first is tμ , which represents the time associatedwith the detection of the muon, or equivalently, the decay of
the kaon. The second is t0 , which is the start time to searchbackwards to find the prompt gamma ray hits. The third istγ , which is the associated time of the gamma ray detection.
PMTs outside of a 50° cone with respect to the muon
direction are masked and tμ is defined as the time wheredN=dt is maximum. The signal of the gamma ray is so tiny,
compared to the muon, that it can easily be hidden by muonhits. To avoid this, the gamma finding is started earlier thanthe muon hits. To determine t0 , dN=dt is calculated fromthe muon peak time into the past. Muon hits are dominant
while dN=dt is increasing; t0 is defined as the latest time
before the muon peak at which the number of hits decreases
or stays the same. Then, in the N-versus-t distribution, atime window with 12 nsec width is slid backward from t0 .

072005-6

SEARCH FOR PROTON DECAY VIA p → νK þ …

PHYSICAL REVIEW D 90, 072005 (2014)60

10

muon-like

Number of Events

Number of events

70

proton-like

50
40
30
20
10
0
-500

10
1
10
10

-250

0

250

2

-1

SK-I/III/IV

-2

500

10

10

10

10

2

Number of Events

Number of events

10

1

10

10

-1

-2

-500

10
1
-1

10

SK-II

-2

10
-250

0

250

500

Lpr-L

Number of γ hists

2

FIG. 5 (color online). The likelihood distribution to separate μ
and proton. The negative region is μ-like and positive is protonlike. In the upper figure, the atmospheric ν MC (red boxes) is
compared with data merging SK-I/III/IV (black bars) requiring
cuts (A-1) through (A-5), and they are in good agreement. The
lower figure shows the same distributions after γ-tagging cuts; isreduced by the likelihood cut. The blue histogram overlays the
proton decay MC with arbitrary normalization.

FIG. 6 (color online). Number of γ ray hit distributions. The
upper figure shows sum of SK-I, III, and IV which have 40%
photo coverage, the lower figure corresponds to SK-II with 19%
photo coverage. Bars, squares, and histogram correspond to data,atmospheric ν MC normalized to livetime of data, and proton
decay MC with arbitrary normalization, respectively. The signal
regions are indicated by arrows. The peaks at small numbers ofhits are due to dark hits of the PMTs.

The associated time of the gamma ray candidate, tγ , isdefined as the middle of the time window where the numberof hits in the window is maximum, N γ .
Figure 6 shows the N γ distribution for SK-I, III, and IV in
the upper figure and SK-II in the lower figure after all cuts
except (A-7). An arrow in the figure shows the signal
region, i.e., with cut (A-7) applied; there are no data in thisregion.
Table III show background rate per Megaton · year andexpected events estimated by atmospheric ν MC, observed
events in data, and efficiencies evaluated by the proton
decay MC for each reduction step. SK-I, III, and IV whichhave 40% photo-coverage are merged and results of SK-II
with 19% photo-coverage are separately shown.
The selection efficiencies, expected number of background, and observed number of events are summarizedin Table V. The efficiency in SK-IV is higher than the otherperiods because the tagging efficiency for Michel electrons
has been improved thanks to the new electronics described in
the detector section. The total expected background for
260 kton · year exposure is 0.4 events, and no events areobserved. The dominant neutrino interaction in the background expectation comes from ΔS ¼ 0 kaon production(48%): νp → νK þ Λ, where the Λ decays to unobserved
proton and π − . If the neutrino interaction is accompanied byde-excitation gamma rays, the event has the same configuration as the proton decay signal. The second most prevalent
background is νμ charged current quasielastic scattering
accompanied by de-excitation gamma rays (25%).

The dominant systematic error for the selection of signal
is uncertainty in the de-excitation gamma ray emission
probabilities. They are estimated to be 15% for the 6.3 MeV
gamma ray and 30% for the others [22], and they contribute
19% to the overall systematic uncertainty on the selection
efficiency. The other systematic uncertainties come fromevent reconstruction: energy scale, particle ID, ringcounting, fiducial volume, water scattering and attenuation
parameters, and they range in size from 1% to 3%. In total,
22% is the systematic error of the selection. The uncertainty
in the background rate comes from atmospheric neutrino
flux and the cross section of neutrino interactions. The
uncertainty in the neutrino flux is conservatively estimatedto be 20% [25]. By changing the cross section of charged
current quasielastic scattering, neutral current elastic scattering, and single π production interaction by 30%, andthe deep inelastic scattering by 50%, a 10% uncertainty inbackground rate is found. The total systematic error for the
background in Method 1 is estimated to be 25%.
There were several improvements in our analysis since
our last paper about p → νK þ was published in 2005 [12].
As described in the reconstruction section, the time window
for hits used to calculate momentum is changed if a Michelelectron is closer than 250 nsec from the parent particle.
This is a new algorithm which prevents the overestimationof the momentum due to including PMT hits from theMichel electron. Previously, the vertices of those events
tended to be misfit in the forward direction since the precise
fitter used the expected charge for each PMT based on an

072005-7

K. ABE et al.

PHYSICAL REVIEW D 90, 072005 (2014)

TABLE III. Event rates per Megaton · year and expected numbers of event from atmospheric ν MC, observed numbers of event in
data, and signal efficiencies estimated from proton decay MC, after each step. SK-I/III/IV with 40% photo-coverage and SK-II with 19%
are shown separately.
SK-I/III/IV

SK-II

Criterion

Bkg. Rate

Exp. Bkg.

Data

Signal Eff.

Bkg. Rate

Exp. Bkg

Data

Signal Eff.

A-1
A-2
A-3
A-4
A-5
A-6
A-7
A-8

35240.8
24865.7
2496.62443.72400.32302.71.34
1.11

7432.35244.2
526.5
515.4
506.2
485.6
0.28
0.24

7497
5240
531
520
514
488
0
0

0.575
0.520
0.494
0.485
0.479
0.436
0.084
0.084

34910.6
22239.7
2161.02067.82030.01931.55.84
2.75

1717.61094.2
106.3
101.7
99.9
95.0
0.29
0.14

1712
1051
91
87
82
78
0
0

0.566
0.473
0.440
0.420
0.414
0.368
0.063
0.062

overestimated momentum with larger Cherenkov angle.
This resulted in more TOF to be subtracted for hits
backward of the particle direction and, as a result, it
sometimes made fake prompt γ signals in atmospheric νinteractions. The improvement of the momentum calculation algorithm reduced the atmospheric ν background by
a factor of three and eliminated a candidate event in the
SK-IV data that would have survived based on theuncorrected algorithm. As a result, the expected background for the gamma-tag method in SK-I is reduced fromthe value in the previous paper 0.7 events, to 0.2 events
while maintaining signal efficiency. The new selection
criterion (A-6) further rejects 60% of the atmosphericbackground (after all other cuts) while losing only 8%
efficiency. As a result, the expected background for
Method 1 is greatly reduced, finally to 0.08 events for
the SK-I period.

is much smaller than the other, sometimes those eventsare misidentified as a single-ring event. A special π 0
reconstruction algorithm is used to search for proton decay
candidates within the single-ring event sample. The π þdoes not make a clear Cherenkov ring due to its low
momentum. However, hit activity in the opposite directionof the π 0 , caused by the π þ , is used to identify the K þ →π þ π 0 signal. The following selection criteria are used:(C-1) FC events with one or two rings and all rings are
e-like,(C-2) one Michel decay electron from the muon producedby π þ decay,(C-3) the reconstructed invariant mass of the π 0 candidate is
between 85 and 185 MeV=c2 ,100

B. Method 2: K þ → μþ νμ , monoenergetic muon search80

Since most of the K s stop in water, the monoenergeticμþ s from kaon decays would lead to an excess peak in themuon momentum distribution of atmospheric neutrino
background. To avoid using the same events as in
Method 1, Method 2 requires all the criteria in Method
1 except: the requirements in momentum (A-3) are relaxed
to allow a spectrum fit, and the gamma hits (A-7) must not
be present. We search for an excess of muon events in the
momentum distribution by fitting the data with the proton
decay signal expectation over the atmospheric neutrino
background events. The signal and background normalizations are free parameters in the fit. Figure 7 shows themuon momentum distribution of data from SK-I to SK-IV
compared with MC. No significant excess is observed in
the signal region, defined by vertical lines.
C. Method 3: K þ → πþ þ π0
In Method 3, π 0 events with a momentum of 205 MeV=c
are selected. An example event display is shown in Fig. 8.
The π 0 decays into two photons; if the energy of one photon

Number of Events

þ

60

40

20

0
200

225

250

275

300

P (MeV/c)

FIG. 7 (color online). Muon momentum distribution for
260 kton · year. Dots, boxes, and histogram correspond to data,atmospheric ν MC, and proton decay MC, respectively. The data
are fit by the background plus signal by free normalization. No
excess above the expected background is observed. The normalization of the proton decay MC histogram shown is at theupper limit allowed by the fit.

072005-8

SEARCH FOR PROTON DECAY VIA p → νK þ …

PHYSICAL REVIEW D 90, 072005 (2014)16
14

pe/ev/deg

12
10
8
6
4
2
0

0

50

100

150

2

pe/ev/deg

1.75

10 3
10 2

1.5
1.25
1
0.75
0.5

10 1

0.25
0

10 0
-1000

-500

0

500

1000

FIG. 8 (color online). An example graphical event display of asimulated proton decay passing all of the criteria for Method 3.
The two Cherenkov rings were produced by gamma rays from π 0
decay and reconstructed to an invariant mass of 155 MeV=c2 andmomentum of 209 MeV=c. The color of the hit PMTs represents
the residual hit time after subtracting the time-of-flight of
Cherenkov light in water from the vertex to the PMT. The π þdoes not make a Cherenkov ring that is reconstructed, however
PMT hits due to this particle are present opposite to the π 0
direction, visible in the upper left of the ID barrel region.
0(C-4) the reconstructed momentum of the π candidate is
between 175 and 250 MeV=c,(C-5) the residual visible energy, defined as energy not
associated with the π 0 nor the π þ is low: Eres <12 MeV for two-ring events and Eres < 20 MeV forsingle-ring events (Eres is described later),(C-6) the likelihood for the photon distribution is consistent
with that expected for signal events: Lshape > 2.0 for
two-ring events and Lshape > 3.0 for single-ring
events in SK-I/III/IV; Lshape > 1.0 for SK-II. Lshapeis explained later.
(C-7) there is visible energy backwards from the π 0
direction consistent with a low momentum
π þ ∶ 10 MeV < Ebk < 50 MeV. A detailed description of Ebk is given later.
A special π 0 reconstruction algorithm is applied to thesingle-ring sample, which was developed for rejecting
single-ring π 0 background from CC νe appearance inneutrino oscillations. The π 0 algorithm forces a fit to thebest second ring by comparing the observed and the
expected light patterns under the assumption of twoelectromagnetic showers and reconstructs the invariant
mass and momentum of the π 0 candidate. Then (C-3)
and (C-4) can be applied even for the single-ring samples.
After selecting single π 0 candidates in the signal
momentum region, and requiring a Michel electron, further
cuts are applied to find the tiny Cherenkov light from the

0

20

40

60

Angle to π+ (deg)Residual PMT Hit Times (ns)FIG. 9 (color online). Charge distribution as a function of angle
to the π þ direction which is defined as opposite the reconstructed
π 0 direction. The upper figure shows the distribution for thesignal MC in which K þ decays into π þ and π 0 . The bump around
23° in the signal is made by Cherenkov light of π þ . The lower
figure shows only the region from 0° to 60° for signal MC (blue)
and atmospheric ν MC (hatched red) after the (C-1)-(C-5) criteria
are required. These histograms show the average response fromnumerous Monte Carlo events.

π þ . Figure 9 shows the photoelectron distribution versus
angle for proton decay Monte Carlo events. The angle iscalculated from the opposite direction of the reconstructed
π 0 , which can be assumed as the π þ direction. The small
bump around 23° comes from π þ .
The Cherenkov light in an event is then divided into three
regions: (i) a π 0 dominant region, inside of 90° from each γ
direction, (ii) a π þ dominant region, inside of 35° from the
backwards direction from the reconstructed π 0 momentum
vector, and (iii) a residual region. These regions areillustrated in Fig. 10.
Visible energy sums are calculated for regions (2) and (3)by masking region (1) to define Ebk and Eres to be used in
criteria (C-5) and (C-7), respectively. Nonzero Ebk is used
to identify the presence of the π þ . By requiring low Eres, we
reject background events with additional final state particles that produce Cherenkov light. In the case of a singlering π 0 candidate, only the ring direction found by thedefault reconstruction is masked as region (1), because theother ring candidate found by the π 0 special algorithm isoften found at a large angle from the existing ring direction
due to asymmetric decay, and if the 90° cone of theadditional ring is masked, the π þ direction may be also
masked. In this case, the missing γ may exist in region (2)or (3), so the cut value for Eres is looser than in the two-ringcase. The shape of the angular distribution seen in Fig. 9 isalso useful to separate signal and background. Based on
Fig. 9, the expected photoelectrons are generated assuming

072005-9

K. ABE et al.

PHYSICAL REVIEW D 90, 072005 (2014)
Re
gio
n

Number of events

(3
)

10

Region (1)Region (2)90°

2R sample
1

10

10

-1

-2

35°
0

Number of events

Region (3)90°Region (1)SK PMT array surface

FIG. 10 (color online). An illustration showing each region forK þ → π þ π 0 . Region (1) is an area which is inside of 90° fromeach γ direction, region (2) is an area which is inside of 35° frombackward of reconstructed π 0 direction, and region (3) is defined
as residual part.

30

1

10

-1

10

-2

10

20

40

Eres (MeV)1

SK-I/III/IV
2R sample

-1

-2

10

-20

5
-20

0

40

10

10

0

30

between data and simulation. Figures 12–15 show Eres,
Lshape , and Ebk distributions after all cuts except for the cut
itself.
Table IV shows background rates per megaton · year
exposure, expected events estimated by atmospheric ν MC,
observed number of events in data, and efficiencies for eachstep. Results for SK-II, which has 19% photo-coverage are

Number of events

20

40

FIG. 12 (color online). Eres distributions for the two-ring(upper) and single-ring (lower) samples after all cuts except(C-5) in all periods. Red histograms are atmospheric ν MC, andblue histograms are proton decay MC, respectively. No dataremain after cuts. Arrows in the figures show signal regions.

5

10

30

Eres (MeV)

10

0

20

1R sample0

Number of events

Number of events Number of events Number of events

signal and background, and a likelihood function (Lshape ) is
calculated for use in (C-6). Then π þ can be tagged by using
deposited energy (Ebk ) and shape of charge distribution(Lshape ). The shape of the angular distribution is slightly
different in the single-ring and two-ring cases, and also
depends on photo-coverage, so the cut value is tuned
separately for SK-II. Figure 11 shows Eres , Lshape , andEbk distributions of the two-ring sample of SK − I þ III þ
IV data and the atmospheric ν MC normalized by livetime,after cuts from (C-1) through (C-4), with good agreement

0

10

10

20

Lshape
15
10

1

0

20

0

20

SK-I/III/IV
1R sample

-1

10

-2

10

5
-20
0

0

20

Lshape40

Ebk (MeV)

FIG. 11 (color online). Eres (upper), Lshape (middle), and Ebk(lower) distributions for the two-ring sample in SK-I,III, and IV
after cuts from (C-1) through (C-4). Black crosses correspond todata and red histograms show the atmospheric ν MC normalizedby livetime.

FIG. 13 (color online). Lshape distributions for the two-ring(upper) and single-ring (lower) samples for SK-I,III, and IV, after
all cuts except (C-6). Black dots correspond to data, red histograms are atmospheric ν MC normalized to the livetime of data,
and blue histograms are proton decay MC, respectively. Arrows
in the figures show signal regions.

072005-10

Numbwe of events

SEARCH FOR PROTON DECAY VIA p → νK þ …
SK-II
2R sample

1

10

10

-1

-2

Numbwe of events

-10

10

0

10

0

10

SK-II
1R sample

1

10

PHYSICAL REVIEW D 90, 072005 (2014)

-1

-2

-10

LshapeFIG. 14 (color online). Lshape distributions for the two-ring(upper) and single-ring (lower) samples for SK-II after all cuts
except (C-6), respectively. Red histograms are atmospheric ν MC,
and blue histograms are proton decay MC. No data remain after
cuts. Arrows in the figures show signal regions.

Number of events

10

2R sample

1
-1

10

momentum μ, kaon production (37%) described in
Method 1, and neutral current multi-π production (11%).
The uncertainty for the π 0 fitter is rather large, 18% [32],but is only applied to the fraction of the single-ring π 0sample (19%) with a final systematic error of 4.5%. The
leading systematic uncertainty in the signal efficiency of
Method 3 is in the π þ interaction in water, estimated to be
5%. Including other reconstruction errors as listed for
Method 1, the total systematic uncertainty in the signal
efficiency of Method 3 is estimated to be 9.5%. The
systematic uncertainty for the background in Method 3
is estimated to be 29% based on the uncertainties inatmospheric neutrino flux and cross section following
the same procedure as described for Method 1.
Compared to our previous publication, Method 3 isimproved in efficiency by the addition of the single-ring π 0sample which occupies 19% of the selected events in thesignal MC. The angle cut at 40° applied to calculate thecharge sum in region (2) in the previous paper was rather
loose (refer to Fig.
pﬃﬃﬃﬃ 9). The cut value was tuned to 35° by
maximizing S= N to reduce more background. The newvariable Lshape also reduces background. As a result, theefficiency of Method 3 is increased by 30% and thebackground is reduced by 30% from the previously
published result for SK-I. The efficiencies, backgrounds,
and observed events are summarized in Table V. The
efficiency in SK-IV is larger than the other periods becauseof the improvement in efficiency for Michel electron
tagging.

-2

10

D. Lifetime limit

-3

10

0

20

40

In the absence of any excess signal above the background expectation, we calculate the lower limit on the
proton partial lifetime using a Bayesian method [33] toincorporate systematic uncertainty. The calculation method
used in our previous publication [12] is applied in thisanalysis.
For Methods i ¼ 1 and 3, where ni is the number ofcandidate events in the ith proton decay search, theconditional probability distribution for the decay rate isexpressed as:

Number of events

10

1R sample

1
-1

10

-2

10

0

20

40

Ebk (MeV)

ZZZ

FIG. 15 (color online). Ebk distributions for the two-ring(upper) and single-ring (lower) samples after all cuts except(C-7) in all periods. Black dots correspond to data, red histograms
are atmospheric ν MC, and blue histograms are proton decay MC,
respectively. Arrows in the figures show signal regions.

separately shown in the table. The selection efficiencies,
expected numbers of background, and observed numbers ofevents for each period are summarized in Table V. The total
expected background for 260 kton · year exposure is 0.6
events. No events are observed in the SK data. The
dominant neutrino interaction modes in the backgroundare charged current single π production (38%) with lowPðΓjni Þ ¼e−ðΓλi εi þbi Þ ðΓλi εi þ bi Þni
ni !

× PðΓÞPðλi ÞPðεi ÞPðbi Þdλi dεi dbi

ð1Þ

where λi is the true detector exposure, εi is the true
detection efficiency including the meson branching ratio,
and bi is the true number of background events. The decayrate prior probability distribution PðΓÞ is 1 for Γ ≥ 0 andotherwise 0.
The prior probability distributions incorporating uncertainties in detector exposure Pðλi Þ, efficiency Pðεi Þ, andbackground Pðbi Þ, are expressed as:

072005-11

K. ABE et al.

PHYSICAL REVIEW D 90, 072005 (2014)

TABLE IV. Event rates per Megaton · year and expected numbers of event from atmospheric ν MC, observed numbers of event in data,and signal efficiencies estimated from proton decay MC, for method 3. SK-I/III/IV with 40% photo-coverage and SK-II with 19% areshown separately.
SK-I/III/IV
Criterion

Bkg. Rate

Exp. Bkg.
10940.3
1112.6
211.7
47.6
36.2
2.13
0.44

SK-II
Data

Signal Eff.

Bkg. Rate

Exp. Bkg

Data

Signal Eff.

10945
1126
200
42
37
2
0

257
0.191
0.153
0.131
0.120
0.098
0.087

53574.1
5421.11266.7
244.1
178.1
15.9
3.42

2635.8
266.7
62.3
12.0
8.76
0.780
0.17

2615
300
56
10
7
0
0

0.277
0.191
0.137
0.114
0.101
0.775
0.067

C-1
C-2
C-3
C-4
C-5
C-6
C-7

51874.1
5275.41003.9
225.6
171.8
10.1
2.09

TABLE V.

Summary of the proton decay search with selection efficiencies and expected backgrounds for each detector period.
SK-I

SK-II

SK-III

SK-IV

Eff.(%)
BKG=Mt · yr
BKG
OBS

91.7
7.9  0.1
0.8  0.2
0.08
0

49.2
6.3  0.1
2.8  0.5
0.14
0

31.9
7.7  0.1
0.8  0.3
0.03
0

87.3
9.1  0.1
1.5  0.3
0.13
0Pμ spec.

Eff.(%)
BKG=Mt · yr
BKG
OBS

33.9  0.3
2107  39
193
177

30.6  0.3
1916  35
94.3
78

32.6  0.3
2163  40
69.0
85

37.6  0.3
2556  47
223.1
226

πþπ0

Eff.(%)
BKG=Mt · yr
BKGOBS

7.8  0.1
2.0  0.4
0.18
06.7  0.1
3.4  0.6
0.17
07.9  0.1
2.3  0.4
0.09
010.0  0.1
2.0  0.3
0.18
0

Exp:ðkton · yrsÞPrompt γ

Pðλi Þ ¼ δðλi − λ0;i Þ;ð2ÞPðεi Þ ¼ exp ½−ðεi − ε0;i Þ2 =2σ 2ε;i ð0 ≤ εi ≤ 1; otherwise 0Þ;ð3Þ

× PðΓÞPðλj ÞPðεj ÞPðbÞPðbshape;j Þdλj dεj dbdbshape;j ;ð5ÞZ ∞ −b0 0 b0;ie ðb Þ
−ðbi Ci − b0 Þ2
db0Pðbi Þ ¼
expb0;i !
2σ 2b;i0ð0 ≤ bi ; otherwise 0Þ;PðΓjm1 ; m2 ; m3 ÞZZZ Y3e−ðΓλj εj þbshape;j bÞ ðΓλj εj þ bshape;j bÞmj
¼
mj !
j¼1

ð4Þ

where λ0;i is the estimated exposure, ε0;i is the estimatedefficiency, b0;i is the estimated number of background
events in 500 years MC, Ci is the MC-to-exposurenormalization factor, and σ ε;i and σ b;i are the uncertainties
in detection efficiency and background, respectively.
To combine Method 2, the remaining events are
divided into three momentum bins: 200–215 MeV=c,
215–260 MeV=c, and 260–305 MeV=c and the numberof observed events are denoted as m1 , m2 , m3 instead of ni .
Then, the nucleon decay rate probability, PðΓjm1 ; m2 ; m3 Þ,
is calculated as:

where j ¼ 1; 2; 3 corresponds to each momentum bin, PðbÞis defined as one for b > 0 and otherwise 0, and εj denotes
the efficiency for each bin. The number of background
events, bshape;j , is bj normalized by b2. The uncertainty
function of the background shape Pðbshape;j Þ is defined by aGaussian function for the 1st and 3rd bin, and a delta
function for the 2nd bin. The uncertainties of the background are estimated to be 7% and 8% respectively fromthe difference of MC models.
We combine all three searches to calculate the lower limitof the nucleon decay rate, Γlimit as:

072005-12R Γlimit QN¼3i¼1 PðΓjni ÞdΓ
QN¼3;
CL ¼ RΓ¼0∞
i¼1 PðΓjni ÞdΓ
Γ¼0

ð6Þ

SEARCH FOR PROTON DECAY VIA p → νK þ …

PHYSICAL REVIEW D 90, 072005 (2014)

where N ¼ 3 is the number of independent searchmethods,2 and CL is the confidence level, taken to be
90%. The lower lifetime limit is given by:
τ=Bp→νKþ ¼

N
1 X½ε0;i · λ0;i :Γlimit i¼1

ð7Þ

The result of the limit calculation combining the three
search methods is

IV. There are several improvements in the analysis and we
succeeded to reduce backgrounds drastically and toincrease efficiencies. However, we do not find any evidence
for proton decay in this exposure, therefore we have set alimit on the partial lifetime as 5.9 × 1033 years, which ismore than 2.5 times more stringent than our previous
publication. The nonobservation of proton decay into thismode constrains, but does not exclude, recent SUSY GUT
models.
ACKNOWLEDGMENTS

τ=Bp→νKþ > 5.9 × 1033 years;
at the 90% confidence level. If only the results of the low
background searches are used, Methods 1 and 3, the lower
limits of proton lifetime are estimated to be 2.5 × 1033 and2.6 × 1033 years, respectively. The lifetime limit fromthe muon momentum spectrum fit, Method 2, is 0.8 ×1033 years.
VI. CONCLUSION
The proton decay search for p → νK þ was carried out
with 260 kton · year exposure, including SK-I, II, III, andfor i ¼ 2, thePðΓjm1 ; m2 ; m3 Þ
2secondsearchmethod,PðΓjn2 Þ ≡[1] H. Georgi and S. L. Glashow, Phys. Rev. Lett. 32, 438(1974).
[2] C. McGrew et al., Phys. Rev. D 59, 052004 (1999).
[3] K. S. Hirata et al., Phys. Lett. B 220, 308 (1989).
[4] M. Shiozawa et al., Phys. Rev. Lett. 81, 3319 (1998);
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We gratefully acknowledge the cooperation of the
Kamioka Mining and Smelting Company. The SuperKamiokande experiment has been built and operated fromfunding by the Japanese Ministry of Education, Culture,
Sports, Science and Technology, the U.S. Department ofEnergy, and the U.S. National Science Foundation. Someof us have been supported by funds from the Korean
Research Foundation (BK21), the National Research
Foundation of Korea (NRF-20110024009), the State
Committee for Scientific Research in Poland (Grant No.
1757/B/H03/2008/35), the European Union FP7 (DSlaguna-lbno PN-284518 and ITN invisibles GA-2011289442), the Japan Society for the Promotion of
Science, and the National Natural Science Foundation ofChina under Grants No. 10575056.

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