Search for Proton Decay via p ! e+ 0 in a Large Water Cherenkov Detector(August 26, 1998)

The Super{Kamiokande Collaboration
M.Shiozawa1, B.Viren15 , Y.Fukuda1 , T.Hayakawa1, E.Ichihara1, K.Inoue1 , K.Ishihara1 , H.Ishino1 , Y.Itow1 ,T.Kajita1 , J.Kameda1, S.Kasuga1, K.Kobayashi1, Y.Kobayashi1, Y.Koshio1 , M.Miura1 , M.Nakahata1 , S.Nakayama1,
A.Okada1 , M.Oketa1 , K.Okumura1 , M.Ota1 , N.Sakurai1 , Y.Suzuki1 , Y.Takeuchi1 , Y.Totsuka1 , S.Yamada1 , M.Earl2 ,A.Habig2 , E.Kearns2 , M.D.Messier2 , K.Scholberg2 , J.L.Stone2 , L.R.Sulak2 , C.W.Walter2 , M.Goldhaber3 ,T.Barszczak4, W.Gajewski4 , P.G.Halverson4; , J.Hsu4 , W.R.Kropp4 , L.R. Price4 , F.Reines4 , H.W.Sobel4 ,M.R.Vagins4 , K.S.Ganezer5 , W.E.Keig5 , R.W.Ellsworth6 , S.Tasaka7 , J.W.Flanagan8 ;y , A.Kibayashi8, J.G.Learned8 ,S.Matsuno8 , V.Stenger8 , D.Takemori8 , T.Ishii9 , J.Kanzaki9 , T.Kobayashi9, K.Nakamura9, K.Nishikawa9 ,Y.Oyama9 , A.Sakai9 , M.Sakuda9 , O.Sasaki9 , S.Echigo10 , M.Kohama10 , A.T.Suzuki10 , T.J.Haines11 ; 4 E.Blaufuss12 ,R.Sanford12 , R.Svoboda12 , M.L.Chen13 , Z.Conner13 ;z , J.A.Goodman13 , G.W.Sullivan13 , M.Mori14 ;x , J.Hill15 ,C.K.Jung15 , K.Martens15 , C.Mauger15 , C.McGrew15 , E.Sharkey15, C.Yanagisawa15, W.Doki16 , T.Ishizuka16 ; ,Y.Kitaguchi16 , H.Koga16 , K.Miyano16 , H.Okazawa16, C.Saji16 , M.Takahata16 , A.Kusano17 , Y.Nagashima17 ,M.Takita17 , T.Yamaguchi17 , M.Yoshida17 , S.B.Kim18 , M.Etoh19 , K.Fujita19 , A.Hasegawa19, T.Hasegawa19,
S.Hatakeyama19 , T.Iwamoto19 , T.Kinebuchi19 , M.Koga19, T.Maruyama19 , H.Ogawa19, A.Suzuki19 , F.Tsushima19 ,M.Koshiba20 , M.Nemoto21 , K.Nishijima21 , T.Futagami22 , Y.Hayato22 ;yy , Y.Kanaya22 , K.Kaneyuki22 ,Y.Watanabe22 , D.Kielczewska23 ; 4 , R.Doyle24 , J.George24 , A.Stachyra24 , L.Wai24 , J.Wilkes24 , K.Young24
1 Institute for Cosmic Ray Research, University of Tokyo, Tanashi, Tokyo 188-8502, Japan
2 Department of Physics, Boston University, Boston, MA 02215, USA
3 Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USA
4 Department of Physics and Astronomy, University of California, Irvine Irvine, CA 92697-4575, USA
5 Department of Physics, California State University, Dominguez Hills, Carson, CA 90747, USA
6 Department of Physics, George Mason University, Fairfax, VA 22030, USA
7 Department of Physics, Gifu University, Gifu, Gifu 501-1193, Japan
8 Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822, USA
9 Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
10 Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan
11 Physics Division, P-23, Los Alamos National Laboratory, Los Alamos, NM 87544, USA.
12 Physics Department, Louisiana State University, Baton Rouge, LA 70803, USA
13 Department of Physics, University of Maryland, College Park, MD 20742, USA
14 Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan
15 Department of Physics and Astronomy, State University of New York, Stony Brook, NY 11794-3800, USA
16 Department of Physics, Niigata University, Niigata, Niigata 950-2181, Japan
17 Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
18 Department of Physics, Seoul National University, Seoul 151-742, Korea
19 Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan
20 The University of Tokyo, Tokyo 113-0033, Japan
21 Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan
22 Department of Physics, Tokyo Institute for Technology, Meguro, Tokyo 152-8551, Japan
23 Institute of Experimental Physics, Warsaw University, 00-681 Warsaw, Poland
24 Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
We have searched for proton decay via p ! e+  0 using data from a 25.5 ktonyear exposure of
the Super{Kamiokande detector. We nd no candidate events with an expected background induced
by atmospheric neutrinos of 0.1 events. From these data, we set a lower limit on the partial lifetimeof the proton  =Bp!e+ 0 to be 1:6  1033 years at a 90% con dence level.
13.30.Ce,11.30.Fs,14.20.Dh,29.40.Ka

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In the Standard Model, which is the modern paradigmof elementary particle physics, protons are assumed to
be stable [1]. In Grand Uni ed Theories (GUTs), however, the decay of the proton is one of the most dramatic predictions of various models [2{5]. In the past
two decades, several large mass underground detector
experiments have looked for proton decay but no clear
evidence has been reported [6{10]. In general, GUTs
predict many modes of proton decay. In many models,the p ! e+  0 mode is dominant and there are several
GUTs which predict a decay rate within the observable
range of Super{Kamiokande (see, for example [11{13]).
This decay mode has a characteristic event signature, inwhich the electromagnetic shower caused by the positronis balanced against the two showers caused by the gamma
rays from the decay of the  0 . This signature enables
us to discriminate the signal events clearly from atmospheric neutrino background. In this letter, we report
the results of our search for proton decay via p ! e+  0
in 414 live-days of data corresponding to an exposureof 25.5 ktonyear (8:52  1033 protonyears) in Super{
Kamiokande.
Super{Kamiokande is a large water Cherenkov detector located in a mine 2700 meters-water-equivalent below
the peak of Mt. Ikenoyama in Kamioka, Japan. The
detector holds 50 ktons of ultra-pure water containedin a cylindrical stainless steel tank and separated into
two regions: a primary inner volume viewed by 11146
50 cm photomultiplier tubes (PMTs) and a veto region,
surrounding the inner detector, viewed by 1885 20 cm
PMTs. More details can be found in Ref. [14].
The data sample we use for this analysis consists ofevents which are fully contained within the inner detector
and is identical to that used for the atmospheric neutrino
analysis [14]. The essential criteria of this selection are as
follows: (1) no signi cant outer detector activity, (2) the
total number of photoelectrons (p.e.'s) in the inner detector is greater than 200, (3) the ratio of the maximum
number of p.e.'s in a single PMT to the total number of
p.e.'s in the event is less than 0.5, (4) the time intervalfrom the preceding event is greater than 100 sec.
Essentially 100% of the cosmic ray muons are eliminated by criterion (1). Criterion (2) corresponds to alower momentum cut of 22 MeV/c for electrons and 190
MeV/c for muons. Criterion (3) removes spurious electrical noise events. Criterion (4) removes electrons fromthe decay of stopping cosmic ray muons as well as thenoise events caused by unwanted PMT signals following
highly energetic events (\after pulsing"). By applying
these criteria, the number of events is reduced from about
400 million to about 12,000. In addition all events which
follow a previous event within 30 sec are tagged as anelectron from the decay of a muon.
After criteria (1){(4) are applied, further reduction isdone by scanners using an interactive graphic event display to eliminate most of the few remaining cosmic raymuons or electronic noise events. About 6,000 events areclassi ed as fully contained events. In this analysis, only
events with a tted vertex inside of the ducial volume
are used. This ducial volume is de ned as all points
which are more than 2 m from the inner detector wall,
giving a 22.5 kiloton ducial mass. This volume cut removes any remaining entering cosmic ray muon eventsand assures that the performance of the reconstruction
algorithms is uniform throughout the ducial volume.
We observe 3468 fully contained events in the ducial
volume. The ineÆciency to recover p ! e+  0 candidates
due to the criteria (1){(4) and scanning is estimated tobe less than 0.1%.
All measurement of physical quantities of an event suchas vertex position, the number of Cherenkov rings, momentum, particle type and the number of decay electrons, is automatically performed [14] by reconstruction
algorithms. The vertex position is estimated by nding
the position at which the timing residual ((photon arrivaltime)-(time of ight)) distribution is most peaked. Usinga Monte Carlo (MC) simulated data sample, the vertex
resolution for p ! e+  0 events is estimated to be 18 cm.
To nd the rings in an event, the charge, viewed from thevertex as a function of f; g, is Hough transformed [15].
The resulting space is searched for peaks, giving the ring
centers. With p ! e+  0 MC, 44% of the simulated events
passing proton decay selection criteria (described below)
are classi ed as 3{ring events and 56% are classi ed as2{ring events. The 2{ring classi cation is primarily forevents with one of the two rings taking only a small
fraction of the  0 's energy or overlapping too much withother rings.
The particle identi cation (PID) classi es a particle asa showering particle (e ; ) or a nonshowering particle( ;   ), using the photon distribution of its Cherenkov
ring. For single ring events, the particle misidenti cation
probability is estimated to be less than 1.0% using the atmospheric neutrino MC. This is con rmed with stopping
cosmic ray muons and their associated decay electrons.
The PID performance was also checked using a 1 kton
water Cherenkov detector with e and  beams from the
12 GeV proton synchrotron at KEK [16]. However, themisidenti cation probability di ers between single ringand multi ring events due to overlapping rings. Using a
p ! e+  0 MC sample this misidenti cation is estimatedto be 2%.
An appropriate momentum cut will reject atmospheric
neutrino background but accept proton decay events.
The momentum is estimated from the total sum of p.e.'s
detected within a 70Æ half opening angle from the reconstructed ring direction. The number of p.e.'s collectedin each PMT is corrected for light attenuation in water,
PMT angular acceptance, and PMT coverage. In the momentum reconstruction, we assume that showering particles are electrons and nonshowering particles are muons.
For single ring events, the reconstructed momentum res2

polution is estimated to be (2:5= E (GeV) + 0:5)% forelectrons and 3% for muons. For multi-ring events, thefraction of p.e.'s in each PMT due to each ring is determined using the expected p.e. distribution. Then,the momentum for each ring is determined by the same
method used for single ring events. The reconstructed
momentum resolution is 10% for each ring in the p !
e+  0 events.
The energy scale stability is checked by the reconstructed mean energy of decay electrons from stopping
cosmic ray muons. It varies within 0:5% over the exposure period. The absolute energy scale was checked withmany calibration sources such as electrons from a linear
accelerator [17], decay electrons from stopping cosmic raymuons, stopping cosmic ray muons themselves, and thereconstructed mass of  0 events observed in atmospheric
neutrino interactions. From comparisons of these sources
and MC simulation, the absolute calibration error is estimated to be smaller than 2:5%.
Finally, the eÆciency for detection of decay electrons isestimated to be 80% for + and 63% for  by a Monte
Carlo study. The di erence in these eÆciencies is due to capture on 16 O. This eÆciency was con rmed to an
accuracy of 1.5% using stopping cosmic ray muons.
The main sources of background for this analysis areatmospheric neutrino interactions which could mimic a
p ! e+  0 event. To estimate the number of background
events, we have developed a detailed MC simulation of
atmospheric neutrino interactions, meson propagation in
the 16 O nucleus, and propagation of secondary particles,
as well as Cherenkov photons, in the detector water [14].
We use the atmospheric neutrino ux of Honda et al. [18].
For neutrino interactions in the detector, the following
types of interaction are simulated: quasi elastic scattering, single{ production, multi{ production, and coherent single{ production for both charged current (CC)and neutral current (NC). For the p ! e+  0 mode, CC production is the most important background because
it could produce an e accompanied by a  0 . We use
Rein-Sehgal's model to simulate single{ production [19].
The pion cross-sections in 16 O are calculated with themodel by Oset et al. [20]. Propagation of produced particles and Cherenkov light in water is simulated with a
GEANT [21] based custom detector simulator. Propagation of charged pions in the detector water is simulated
by a custom simulator [22] for less than 500 MeV/c pions and by the CALOR [23] simulator for more than 500
MeV/c pions. For the p ! e+  0 MC, the same simulator
is used. In this, as well as the atmospheric neutrino MC,
the Fermi motion of protons, the nucleon binding energy,
and pion interactions in 16 O are considered.
The observed atmospheric neutrino avor ratio ( =e )in Super{Kamiokande is signi cantly smaller than the
expected value [14]. For comparison of data and atmospheric neutrino MC, the neutrino MC sample is normalized to the number of observed atmospheric neutrino

events at Super{Kamiokande in the following manner.
The number of e ( ) CC events is normalized by theratio of the number of single ring events with a showering(nonshowering) PID in the data to the number of single
ring events with a showering (nonshowering) PID in theatmospheric neutrino MC. For NC events, the same normalization factor as that of the e CC events is used.
To extract the p ! e+  0 signal from the event sample,
these selection criteria are de ned: (A) 6800 p.e. < total
p.e. < 9500 p.e., (B) the number of rings is 2 or 3, (C) all
rings have a showering PID, (D) 85 MeV/c2 <  0 invariant mass < 185 MeV/c2 , (E) no decay electron, (F) 800
MeV/c2 < total invariant mass < 1050 MeV/c2 and total
momentum < 250 MeV/c. Criterion (A) roughly corresponds to a total energy of 800 MeV to 1100 MeV. Criterion (C) selects e and . Criterion (D) only applies to
3-ring events. Here, at least one pair of rings must give a
reconstructed invariant mass which is consistent with theestimated  0 mass resolution of 135  35MeV/c2 . Criterion (E) is required since the desired e+ and  0 particles
produce no decay electrons. In criterion (F), the totalrings
momentum is de ned as Ptot = j all
p~i j where ~piiis reconstructed momentum vector of i-th ring. The total
2 Ptot2 whereinvariant mass is de ned as Mtot = Etot
all rings
total energy Etot = i
jp~i j. Criterion (F) checks
that the total invariant mass and total momentum correspond to the mass and momentum of the source proton,
respectively.

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Finally we apply the same criteria to the data to search
for the p ! e+  0 signal. No events survive all criteria asshown in Fig. 1-(c). Furthermore, the mass distributionof data in Fig. 2 is well reproduced by that of the neutrino MC and the event rates at di erent stages in the
reduction for data and neutrino MC match, as shown in
Fig. 3.
The primary result is based on cuts using kinematicreconstruction. Because a systematic error in this reconstruction could miss p ! e+  0 candidates a looser
analysis is performed as a consistency check. The selection criteria are as follows: (1) Visible energy is within
200 MeV of the proton rest mass energy. (2) The lightanisotropy is less than 30%. (3) No electrons from muon
decays follow the primary products. The visible energy ofan event is the total energy assuming all Cherenkov lightis from electromagnetic showers. The light anisotropyof an event is an estimate of the total momentum imbalance and is de ned as the magnitude of the charge
weighted vector sum of the directions from the vertex
to each PMT. For a typical single ring event the lightanisotropy is 75%. This simple analysis nds 4 eventswith a background of 3.5, which is consistent with the 0candidates and 0.1 background of the primary analysis.
From these results, we conclude we do not nd any evidence for proton decay via the mode p ! e+  0 . Therefore we set a lower limit on the partial decay lifetime.
The method used to calculate this limit takes both theestimated background and systematic uncertainties into
account [24]. This calculation gives a limit on the partial
lifetime for p ! e+  0 of,  =Bp!e+ 0 > 1:6  1033 years
at a 90% CL. If uncertaintes are ignored the limit would
increase by about 5%.
In calculating the limit, the parameter with the dominant uncertainty is the detection eÆciency. This uncertainty is primarily due to imperfectly known pion{
nucleon cross sections in 16 O nuclei and is estimated by

From Fig. 1-(a), the detection eÆciency of p ! e+  0
events is estimated to be 44%. The absorption, charge
exchange and scattering of  0 's in the 16 O nucleus are
the dominant contribution to the detection ineÆciency.
To estimate the background from atmospheric neutrino
interactions, we generate a MC sample of 900 ktonyear.
By applying the proton decay selection criteria to thissample, we estimate the number of background events inthe signal region to be 0:1 event in 25.5 ktonyear as seenin Fig. 1-(b).
10BFIG. 3. The event rate after each proton decay selection
criterion (see text) for data ( lled circles) and atmospheric
neutrino Monte Carlo (empty circles). There is no event in
the data after criterion F and only the 90% C.L. upper limit
is shown in the last bin.

FIG. 1. The total invariant mass and total momentum distributions after criteria (A){(E) (see text) for 3 samples: (a)p ! e+  0 Monte Carlo, (b) atmospheric neutrino Monte
Carlo corresponding to 900 ktonyear, (c) data corresponding to 25.5 ktonyear. The boxed region in each gure shows
the criterion (F) for the p ! e+  0 signal.

1200

FIG. 2. The total invariant mass distributions of data (circles), normalized atmospheric neutrino Monte Carlo corresponding to 10 years (unshaded histogram), and p ! e+  0Monte Carlo normalized to one event (shaded histogram)which satisfy the criteria (B){(E) (see text) and have a total reconstructed momentum < 250 MeV/c.

4comparing with another detailed model (based on [25]).
This uncertainty is estimated to be 15%. In addition, systematic di erences in the energy scale for data and MC
contributes 1%, lack of uniformity in the detector gain
contributes 2% and tting resolution contributes 5% tothe uncertainty in the detection eÆciency. The total uncertainty in the detection eÆciency is then 16%. The
statistical uncertainty in the background is 60% due tothe small number of MC background events passing thecuts. Finally, the uncertainty in the exposure is negligible.
In this letter, we have reported the results of a proton
decay search in Super{Kamiokande. We have no evidenceof the proton decaying via the mode p ! e+  0 in the 25.5
ktonyear data. We set the most stringent limit on thepartial lifetime of the proton to be 1:6  1033 years at
a 90% CL, which should be compared with the previous
experimental results, 2:6  1032 years [7] and 5:5  1032
years [8].
We gratefully acknowledge the cooperation of the
Kamioka Mining and Smelting Company. The Super{
Kamiokande experiment was built from, and has been
operated with, funding by the Japanese Ministry of Education, Science, Sports and Culture, and the UnitedStates Department of Energy.

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 Present address: NASA, JPL, Pasadena, CA 91109, USA
y Present address: Accelerator Laboratory, High Energy

Accelerator Research Organization (KEK), Tsukuba,
Ibaraki 305-0801, Japanz Present address: Enrico Fermi Institute, University of
Chicago, Chicago, IL 60637 USA
x
Present address: Institute for Cosmic Ray Research, University of Tokyo, Tanashi, Tokyo 188-8502, Japan Present address: Dept. of System Engineering, Shizuoka
University Hamakita, Shizuoka 432-8561, Japanyy
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