Next-generation nucleon decay experimentsT. Nakaya∗a

aFaculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502 Japan

A recent status and future prospect of nucleon decay experiments are summarized. The most stringent limitson proton decays are given by the largest water Cerenkov detector, Super-Kamiokande. A lower limit on thepartial lifetime of a proton is set at 5.4× 1033 years for p → e+π0 at 90% C.L., and 2.2× 1033 years for p → νK+.A next generation experiment with a megaton mass-scale detector can improve the sensitivities up to 1035 yearsfor p → eπ0 and 1034 years for p → νK+. The detector is also suitable as a far detector of a long baseline neutrinooscillation experiment to search for CP violation.

1. INTRODUCTION

Nucleon decay is one of the most dramaticpredictions by a Grand Unified Theory (GUT),in which the strong, weak and electromagneticforces are unified at ultra high energy. The GUTis also attractive to treat quarks and leptons inthe same gauge family. Many models of theGUT predict that proton decay via p → e+π0

is a dominant channel. The p → e+π0 decayis caused by a dimension 6 operator mediatedby a new heavy gauge boson. The operator isexpressed as 1

M2 qqql, where M is the mass ofthe gauge boson and q and l represent a quarkand a lepton, respectively. A partial lifetime (τp)is predicted to be 1034−35( M

1016GeV )4 > 1033

years. Many models of the SUSY GUT pre-dict that a dominant channel of proton decay isp → νK+ by a dimension 5 operator of 1

M qiqjqkll.A prediction of the lifetime is expressed as1015−16( M

1016GeV)2(mSUSY

1TeV) ∼ 1029−35 years. The

sensitivities of current experiments reached to alower range of the predictions, and a future ex-periment can extend the sensitivity to discoverthe signal. Although no evidence of proton decayare found in experiments so far, there are a cou-∗The author thanks the Super-K, K2K, T2K, UNO, andLANNDD collaborations. The author gives special thanksto M. Shiozawa and K. Kobayashi. This work has beensupported by the Ministry of Education, Culture, Sports,Science and Technology, Government of Japan and itsgrants for Scientific Research. The author gratefully ac-knowledge the support by a Grant-in-Aid for the 21stCentury COE “Center for Diversity and Universality inPhysics” in Kyoto University.

ple of hints indicating the GUT. One is the factthat three running coupling constants measuredby LEP could merge at the same energy scale witha SUSY GUT model. Another hint is a tiny andfinite neutrino mass which indicates the existenceof physics at the ultra high energy scale and aconnection between the small neutrino mass andproton decay. A next-generation nucleon decayexperiment is strongly motivated by the indica-tions of GUTs predicting proton decay as directevidence.

2. REVIEW OF CURRENT NUCLEONDECAY EXPERIMENTS

Before introducing a new next-generation nu-cleon decay experiment, we review the status ofcurrent experiments. Two types of detectors havebeen developed for a nucleon decay search: one isa water Cerenkov detector and the other is a fine-grained tracking detector. An example of the wa-ter Cerenkov detector is Super-Kamiokande. Anexample of the fine-grained detector is Soudan 2.In the following sections, the results from theseexperiments are presented after introducing a nu-clear effect.

2.1. Nuclear effectMost experiments use a nucleus target for a nu-

cleon decay search. Therefore, it is important tounderstand an initial condition of nucleons insideof the nucleus and a nuclear effect to a daugh-ter particle from proton decay. With a waterCerenkov detector, the protons are in hydrogen

Nuclear Physics B (Proc. Suppl.) 138 (2005) 376–382

0920-5632/$ – see front matter © 2004 Published by Elsevier B.V.

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doi:10.1016/j.nuclphysbps.2004.11.086

and oxygen. A proton of hydrogen is free butone in oxygen is affected by the Fermi motion,the binding energy, and nucleon-nucleon correla-tion. The 10% of proton decay in oxygen does nothave a peak at the proton mass due to nucleon-nucleon correlation. The daughter particle of thedecay is suffered by a nuclear effect: scattering,absorption and charge-exchange reactions. Theinteraction probability of a π0 in oxygen is shownin Figure 1. It is as high as 60% for the π0 fromthe p → e+π0 decay. Therefore, an experiment

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Figure 1. The interaction probability of a π0 inoxygen as a function of the initial momentum.

with the target of a lighter nucleus, such as hy-drogen and oxygen of water, has an advantagefor the p → e+π0 search. For p → νK+, a heavynucleus, such as iron or argon, can be used asthe target, since the nuclear effect is less sensi-tive to a K meson with the smaller interactioncross-section.

2.2. Super-KamiokandeSuper-Kamiokande (Super-K) is a 50 kiloton

water Cerenkov detector located in a mine 2,700meters of water equivalent underground. A de-tailed description of Super-K is found in [1].Super-K has 11,146 PMTs viewing the inner vol-ume of the detector, which has a fiducial vol-ume of 22.5 kilotons. The pulse height and tim-ing information of PMTs are used to reconstruct

Cerenkov rings and to measure the vertex, direc-tion, energy and particle type of the rings. Atypical vertex and energy resolution for a muonwith 1 GeV is 30 cm and 3%, respectively. Super-K collected data for 1,489 days from 1996 to 2001,corresponding to the exposure of 92 kton · years.

2.2.1. p → eπ0 searchThe p → eπ0(π0 → γγ) events in Super-K are

identified with two or three showering-rings whichare originated by an electron or a γ. The detaileddescription of the analysis is found in [2]. Forthree-ring events, the two rings are required tobe reconstructed as a π0 with the mass between85 and 185 MeV/c2. The events with an electronfrom muon decay are rejected. Figure 2 shows thescatter plot of the total reconstructed momentumversus the invariant mass of the p → eπ0 candi-dates for data and the Monte Carlo (MC) simu-lation. The final events are selected by requiringthe total momentum less than 250 MeV/c andthe mass between 800 and 1,050 MeV/c2 withthe efficiency of 40%. The expected number ofbackground events is 0.3 from atmospheric neu-trino interactions. No events are found in data,and the lower limit on the partial lifetime is setat 5.4× 1033 years at 90 % C.L.

2.2.2. p → νK+ searchThe p → νK+ search is conducted by using two

K+ decay modes; K+ → µ+ν and K+ → π+π0,where the K+ is below the Cerenkov thresholdand not observed. For K+ → µ+ν , a muonwith the momentum of 236 MeV/c is the signalevent. The events are required to have only onemuon-like ring with an electron from muon decay.Figure 3 shows the reconstructed muon momen-tum. No peaks are found at the correspondingmomentum, but flat background events from at-mospheric neutrinos are observed. In order to re-duce the background further, a γ from a 15N∗ de-excitation (called “prompt γ”) is required, wherethe 15N∗ is a daughter product after a proton dis-appears in 16O. The 15N∗ deexcitation emits a6.3 MeV γ with a fraction of ∼ 41 %. The numberof PMT hits by the prompt γ for the p → νK+

candidates is shown in Figure 4. The number ofhits is required to be between 7 and 60 within

T. Nakaya / Nuclear Physics B (Proc. Suppl.) 138 (2005) 376–382 377

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the timing window between 12 and 120 nsec be-fore the µ signal. No events are observed in datawith 0.7 expected background events. The lowerlimit on the partial lifetime is set at 1.1 × 1033

years at 90% C.L.By combining a search with K+ → π+π0 [3],

the combined lower limit is set at 2.2×1033 yearsat 90% C.L.

2.2.3. Summary of the Super-Kamiokandenucleon decay search

In addition to proton decays of p → e+π0 andp → νK+, various decay modes are searched forin Super-K. The list of proton decay searches atSuper-K is shown in Table 1. Most of resultsfrom Super-K are the most stringent limit on the

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proton decay lifetime.

2.3. Soudan 2The Soudan 2 detector is a fine-grained track-

ing calorimeter designed to search for p → νK+

with a good tracking performance [4,5]. The K+

track in p → νK+ is detectable, while Super-Kis not. The detector consists of 224 modules andthe total mass is 0.96 kton, much smaller thanSuper-K. Each module consists of 1.6 mm thicksteel sheets and fine drift tubes, and the size is1 × 1 × 2.7 m3.

The p → νK+ events are selected as follows.For the K+ → µ+ν mode, the events are se-lected to have two charged tracks not identifiedas a proton. The range of the K+ track is re-quired to be less than 50 cm, and the range ofthe µ is between 28 and 58 cm. An electron frommuon decay is required. One event is observedafter all cuts and the expected numbers of back-ground events are 0.21 from atmospheric neutrinointeractions and 0.19 from a neutron produced insurrounding rock by cosmic-rays. The efficiencytimes the branching fraction of K+ → µ+ν is9.0%. For the K+ → π+π0 mode, the event areselected to have two charged tracks and two elec-tromagnetic showers. The range of the K+ isrequired to be less than 50 cm, and the π+ mo-mentum is between 80 and 400 MeV/c, the π0

T. Nakaya / Nuclear Physics B (Proc. Suppl.) 138 (2005) 376–382378

Table 1Summary of Super-K nucleon decay searches withan exposure of 92 kton · years. The ε is the de-tection efficiency, BR is the branching fraction ofthe decay of daughter particles from proton de-cay, Obs is the number of observed events, BG isthe expected number of background events, andτ/BR is the 90% C.L. lower limit on the partiallifetime of a proton.mode ε · BR Obs BG τ/BR

(%) (1032 yr)

p → e+π0 40 0 0.2 54p → µ+π0 32 0 0.2 43p → e+η 17 0 0.2 23p → µ+η 9 0 0.2 13p → e+ρ 4.2 0 0.4 5.6p → e+ω 2.9 0 0.5 3.8p → e+γ 73 0 0.1 98p → µ+γ 61 0 0.2 82p → ν + K+ 22•K+ → νµ+

(µ spectrum) 34 3.8(prompt γ) 8.6 0 0.7 11

•K+ → π+π0 6.0 0 0.6 7.9n → ν + K0 2.0•K0 → π0π0 6.9 14 19.2 3.0•K0 → π+π− 5.5 20 11.2 0.8

p → e+ + K0 10.7•K0 → π0π0 9.2 1 1.1 8.7•K0 → π+π−

(2-ring) 7.9 5 3.6 4.0(3-ring) 1.3 0 0.1 1.7

p → µ+ + K0 13.9•K0 → π0π0 5.4 0 0.4 7.1•K0 → π+π−

(2-ring) 7.0 3 3.2 4.9(3-ring) 2.8 0 0.3 3.7

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Figure 4. The number of PMT hits within thetiming region of the prompt γ for the p →νK+, K+ → µ+ν candidates.

momentum is between 40 and 390 MeV/c. Thereconstructed invariant mass of the π0 is requiredto be between 10 and 290 MeV/c2, and the massof the K+ is between 100 and 660 MeV/c2. Noevents are observed, with 1.14 expected back-ground events, which are 1.05 events from theatmospheric neutrinos and 0.09 events from theneutrons. By combining two K+ decay modes,the lower limit on the partial lifetime of a protonis set at 4.3 × 1031 years at 90% C.L.

In Table 2, the sensitivities of p → νK+ arecompared between Soudan 2 and Super-K. Super-K is fifty times more sensitive than Soudan 2,thanks to the larger mass.

2.4. Other nucleon decay searchBesides Super-K and Soudan 2, there are sev-

eral results from other experiments. A model-independent limit on nucleon decay is set toτp(n) > 1.6 × 1025 years by studying Xe isotopesof 2.5 × 109 years old by looking for reactions of130Te → 129Te (β − decay) → 129Xe for neutrondecay and 130Te → 129Sb (β−decay) → 129Xe forproton decay [6]. The decay of n → ννν searchis conducted by the Kamiokande collaboration tosearch for a γ ray in the energy range between19 and 50 MeV from the deexcitation of 15O [7].The lower limit on the partial lifetime is set at

T. Nakaya / Nuclear Physics B (Proc. Suppl.) 138 (2005) 376–382 379

Table 2A comparison of the sensitivities for p → νK+

between Soudan 2 and Super-K. The ε is the de-tection efficiency, BR is the branching fraction ofthe decay of daughter particles from proton de-cay, # Events is the number of observed eventsand # BG is the expected number of backgroundevents. The limit is calculated at 90% C.L.

Soudan 2 Super-KExposure (kton · years) 3.56 91.6ε · BR (K+ → µ+ν) 9.0 % 8.6 %ε · BR (K+ → π+π0) 5.5 % 6.0 %# Events (K+ → µ+ν) 1 0# Events (K+ → π+π0) 0 0# BG (K+ → µ+ν) 0.4 0.7# BG (K+ → π+π0) 1.1 0.6Limit (1032 years) 0.43 22

τn/BR(n → ννν) > 4.9 × 1026 years at 90%C.L. An invisible decay of a proton is searchedfor with SNO data by measuring neutrons fromthe proton disappearance in a deuteron [8]. Thepartial lifetime of τp/BR(p → invisible) is set at3.5× 1028 years at 90 % C.L. In the near future,KamLAND [9], a one-kiloton liquid scintillatordetector, has the potential to improve the sensi-tivity of these nucleon decays.

3. FUTURE EXPERIMENTS

There are several proposals for a future nu-cleon decay experiment. Most of them arebased on the water Cerenkov technology due toa great success of Super-K, and the other is aliquid Ar TPC. The target mass of the next-generation water Cerenkov detector is aroundone megaton (Mton). The proposed detectors areHyper-Kamiokande (Hyper-K) [10] at Kamiokain Japan, TITAND [11] in an ocean in Japan,UNO [12] in the USA, and MEMPHYS [13] atFrejus in France. A liquid Ar TPC of a few hun-dred kilotons is proposed as LANNDD [14] atWIPP in the USA.

3.1. Hyper-KamiokandeThe sensitivities of Hyper-K are presented

as representative of the next-generation water

Cerenkov detector, since most of them have thesimilar performance. The schematic picture ofHyper-K is shown in Figure 5. Hyper-K has100, 000 ∼ 200, 000 photo sensors, and the total(fiducial) mass is ∼ 1, 000 (∼ 500) kilotons.

Figure 5. The schematic picture of the Hyper-Kamiokande detector.

The p → e+π0 search at Hyper-K is expectedto be background-limited by atmospheric neutri-nos. The expected number of background eventsis three events per Mton · years. The numberof background events depends on the property ofneutrino interactions, and it is important to studyneutrino interactions by a neutrino beam to esti-mate the more accurate background level [15]. InFigure 6, the sensitivity on p → e+π0 at Hyper-K is shown with an exposure assuming the samedetector performance and the same analysis asSuper-K. With the exposure of 10 Mton·years, thesensitivity on the p → e+π0 search reaches above1035 years at 90% C.L. Above 10 Mton · years,the sensitivity is no more proportional to the ex-posure because of background.

In order to suppress the background more, thetotal momentum cut is further tightened. Thetotal momentum of the p → e+π0 MC eventsin Figure 2 shows a broad distribution due tothe Fermi momentum, a nuclear effect to the π0,and nucleon-nucleon correlation in oxygen. Byselecting events with the momentum less than100 MeV/c, we can reduce the background to be

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Figure 6. The sensitivity on p → e+π0 by Hyper-K.

0.15 events per Mton ·years, and effectively selecta free proton in water with the total efficiencyof 17 %. With the exposure of 20 Mton · years,we can observe a clear mass peak of p → e+π0,as shown in Figure 7, with the proton lifetime of1035 years. Though the background is more sup-pressed, the sensitivity with this cut is the sameas that in the standard analysis in Figure 6 at theexposure of 10 Mton · years because of the lowerefficiency.

The sensitivity on p → νK+ is also studied byusing the Super-K MC simulation. The sensitiv-ity versus exposure is shown in Figure 8. Themost sensitive mode to discover p → νK+ isK+ → µ+ν with a prompt γ tag. With the ex-posure of 10 Mton · years, the sensitivity on thep → νK+ search reaches to 3 × 1034 years at90% C.L. The expected number of background is6 events per Mton · years, while the backgroundis 2,100 events without the γ tag and 22 eventsfor K+ → π+π0. A serious background of thekaon production by an atmospheric neutrino isexpected to be one event per Mton · years with aprompt γ. This background could limit the sen-sitivity on this proton decay.

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Figure 7. The reconstructed invariant mass dis-tribution of p → e+π0 MC events with the tight-ened momentum cut of < 100 MeV/c for the ex-posure of 20 Mton ·years. The dots are the signalMC events with the proton life time of 1035 years,and the histogram is the atmospheric neutrinobackground.

3.2. LANNDDLANNDD is another proposal of proton decay

search by using a liquid Ar TPC, whose technol-ogy is recently established by the ICARUS collab-oration [16]. The sensitivity of LANNDD is notgood for p → e+π0 because of the nuclear effect,but LANNDD has an excellent reconstruction ca-pability of the K+ track in p → νK+ events,which is used to suppress background. In orderto reach the same sensitivity of 3 × 1034 yearsas the water Cerenkov detector, LANNDD musthave a fiducial volume of 300 ktons with 4.5 yearsrunning time. It is a challenge to construct sucha gigantic liquid Ar TPC which is almost two or-ders of magnitudes larger than ICARUS. A seriesof R&D for LANNDD is planed to start from a50 litter µ−LANNDD. Though it may take a longtime to construct a big liquid Ar TPC, it wouldbe worthwhile to continue the research.

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Figure 8. The sensitivity on p → νK+ by a waterCerenkov detector.

4. ADDITIONAL PHYSICS POTEN-TIAL

A natural extension of the proton decay detec-tor is that it be used as a neutrino detector withthe large mass. Hyper-K, MEMPHYS, TITANDand UNO are designed to observe atmosphericneutrinos, the neutrinos from an accelerator, andsupernova neutrinos. For atmospheric neutrinos,the oscillation characteristics of L/E dependencecan be clearly observed. For supernova neutri-nos, the supernova explosion within 1 Mpc canbe observed, and the expected rate of the super-nova is every 10 ∼ 15 years. For an acceleratorneutrino beam, the experiment also works as afar detector of the long baseline neutrino oscilla-tion experiment. The most fascinating feature isto search for CP violation in neutrino oscillationby using a neutrino beam. Such an experimentis proposed as a J-PARC 4 MW neutrino beamwith Hyper-K [17], a BNL super-beam with UNO,a CERN-SPL super-beam or β beam with MEM-PHYS. The sensitivity on a CP-violating phase δin J-PARC is expected to reach down to 20 de-grees at the 3 σ sensitivity for sin2 2θ13 > 0.01,with 2% systematic errors.

5. CONCLUSION

A summary of proton decay experiments is pre-sented. Discovery of proton decay is one of themost important subjects in physics as a direct ev-idence of a GUT. Although no candidate eventsare found so far, continuous research for protondecay is a key to opening a new window to nature.

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