Nucleon Decay Research Articles

Nucleon decay with domain-wall fermions

We report on our on-going project to calculate the nucleon decay matrix elements with domain-wall fermions. Operator mixing is discussed employing a non-perturbative renormalization. Bare matrix elements of all the possible decay modes induced by the dimension-six operators are calculated with the direct method, which are compared with the indirect calculation using chiral perturbation theory.

Links between neutrino oscillations, leptogenesis, and proton decay within supersymmetric grand unification

Evidence in favor of supersymmetric grand unification including that based on the observed family multiplet-structure, gauge coupling unification, neutrino oscillations, baryogenesis, and certain intriguing features of quark-lepton masses and mixings is noted. It is argued that attempts to understand (a) the tiny neutrino masses (especially Δm 2(v 2 – v3)), (b) the baryon asymmetry of the Universe (which seems to need leptogenesis), and (c) the observed features of fermion masses such as the ratiom b/mτ, the smallness ofV cb and the maximality of $$\Theta _{\nu _\mu \nu _\tau }^{OSC} $$ seem to select out the route to higher unification based on an effective string-unifiedG(224) =SU(2)L ×SU(2)R ×SU(2)c orSO(10)-symmetry that should be operative in 4D, as opposed to other alternatives. A predictiveSO(10)/G(224)-framework possessing supersymmetry is presented that successfully describes the masses and mixings of all fermions including neutrinos. It also accounts for the observed baryon asymmetry of the Universe by utilizing the process of leptogenesis, which is natural to this framework. It is argued that a conservative upper limit on the proton lifetime within thisSO(10)/G(224)-framework, which is so far most successful, is given by $$\frac{1}{3} - 2$$ x 1034 years. This in turn strongly suggests that an improvement in the current sensitivity by a factor of five to ten (compared to SuperK) ought to reveal proton decay. Implications of this prediction for the next-generation nucleon decay and neutrino-detector are noted.

Nucleon structure from lattice QCD with domain wall fermions

We report the current status of RBCK calculations on nucleon structure with both quenched and unquenched lattice QCD. The combination of domain wall fermions and DBW2 gauge action works well for quantities like isovector vector and axial charges, moments of structure functions 〈χ〉q, di, and 〈1〉σq, and nucleon decay matrix elements. Good chiral symmetry is observed, and translates into continuum-like behavior of non-perturabtive renormalization where explicitly checked. Pion form factor calculation in the same framework is also reported.

Invariant Homology on Standard Model Manifolds

Torus-fibered Calabi-Yau threefolds Z, with base dP_9 and fundamental group pi_1(Z)=Z_2 X Z_2, are reviewed. It is shown that Z=X/(Z_2 X Z_2), where X=B X_{P_1} B' are elliptically fibered Calabi-Yau threefolds that admit a freely acting Z_2 X Z_2 automorphism group. B and B' are rational elliptic surfaces, each with a Z_2 X Z_2 group of automorphisms. It is shown that the Z_2 X Z_2 invariant classes of curves of each surface have four generators which produce, via the fiber product, seven Z_2 X Z_2 invariant generators in H_4(X,Z). All invariant homology classes are computed explicitly. These descend to produce a rank seven homology group H_4(Z,Z) on Z. The existence of these homology classes on Z is essential to the construction of anomaly free, three family standard-like models with suppressed nucleon decay in both weakly and strongly coupled heterotic superstring theory.

SU(4) Instantons on Calabi-Yau Threefolds withZ2 Z2Fundamental Group

Structure group SU(4) gauge vacua of both weakly and strongly coupled heterotic superstring theory compactified on torus-fibered Calabi-Yau threefolds Z with Z_2 x Z_2 fundamental group are presented. This is accomplished by constructing invariant, stable, holomorphic rank four vector bundles on the simply connected cover of Z. Such bundles can descend either to Hermite-Yang-Mills instantons on Z or to twisted gauge fields satisfying the Hermite-Yang-Mills equation corrected by a non-trivial flat B-field. It is shown that large families of such instantons satisfy the constraints imposed by particle physics phenomenology. The discrete parameter spaces of those families are presented, as well as a lower bound on the dimension of the continuous moduli of any such vacuum. In conjunction with Z_2 x Z_2 Wilson lines, these SU(4) gauge vacua can lead to standard-like models at low energy with an additional U(1)_{B-L} symmetry. This U(1)_{B-L} symmetry is very helpful in naturally suppressing nucleon decay.

Observing nucleon decay in lead perchlorate

Lead perchlorate, part of the OMNIS supernova neutrino detector, contains two nuclei, 208Pb and 35Cl, that might be used to study nucleon decay. Both would produce signatures that will make them especially useful for studying less-well-studied neutron decay modes, e.g., those in which only neutrinos are emitted.

Science from detection of neutrinos from supernovae

The neutrinos emitted from supernovae contain information about the physics of stellar collapse and of the nature of the neutrinos themselves. Several large detectors exist that will be capable of observing some subset of those neutrinos. In addition, we have designed OMNIS, the Observatory for Multiflavour NeutrInos from Supernovae. OMNIS will detect the neutrinos from (a) neutral-current interactions from νe, νμ, , ντ and , and (b) charged-current interactions from high-momentum νe, with lead nuclei. It will utilize two types of detectors: (1) lead slabs alternating with vertical planes of neutron detectors, in which neutrons produced by neutrino–lead interactions will be detected, and (2) lead perchlorate, in which both the resulting neutrons and Cerenkov light will be detected. OMNIS will measure neutrino masses below 100 eV, provide new information on MSW or vacuum oscillations from νμ/ντ to νe, especially to Θ13, and possibly diagnose the process of collapse to a black hole. It will observe the late-time evolution of the neutrino distributions, and possibly see predicted late-time effects, e.g. a phase transition from neutron-star matter to kaon-condensed matter or quark matter. OMNIS is also sensitive to some modes of nucleon decay that should make it possible to improve significantly on present limits for those modes. Of crucial importance to OMNIS is an experiment, using neutrinos from a stopped pion beam, to determine the flavour and energy-dependent response of lead to neutrinos. This will provide important input into cross section calculations for which few data currently exist. We plan to perform this experiment using one of the lead perchlorate detector modules from OMNIS.

PROBING GRAND UNIFICATION THROUGH NEUTRINO OSCILLATIONS, LEPTOGENESIS, AND PROTON DECAY

Evidence in favor of supersymmetric grand unification including that based on the observed family multiplet-structure, gauge coupling unification, neutrino oscillations, baryogenesis, and certain intriguing features of quark-lepton masses and mixings is noted. It is argued that attempts to understand (a) the tiny neutrino masses (especially Delta m^2 (nu_2 -nu_3)), (b) the baryon asymmetry of the universe (which seems to need leptogenesis), and (c) the observed features of fermion masses such as the ratio m_b/m_tau, the smallness of V_cb and the maximality of theta_{nu_mu-nu_tau}, seem to select out the route to higher unification based on an effective string-unified G(224) = SU(2)_L x SU(2)_R x SU(4)^c or SO(10)-symmetry, operative in 4D, as opposed to other alternatives. A predictive framework based on an effective SO(10) or G(224) symmetry possessing supersymmetry is presented that successfully describes the masses and mixings of all fermions including neutrinos. It also accounts for the observed baryon asymmetry of the universe by utilizing the process of leptogenesis, which is natural to this framework. It is argued that a conservative upper limit on the proton lifetime within this SO(10)/G(224)-framework, which is so far most successful, is given by (1/3-2) x 10^34 years. This in turn strongly suggests that an improvement in the current sensitivity by a factor of five to ten (compared to SuperK) ought to reveal proton decay. Implications of this prediction for the next-generation nucleon decay and neutrino-detector are noted.

HYPER-KAMIOKANDE — A NEXT GENERATION WATER CHERENKOV DETECTOR

A next generation water Cherenkov detector at Kamioka with a total mass of ~1 Mton is called Hyper-Kamiokande. If the beam intensity of a 50-GeV proton synchrotron now under construction at Tokai is upgraded to 4 MW in future, a long baseline neutrino oscillation experiment with Hyper-Kamiokande as a far detector will open the possibility of measuring CP violation in the neutrino sector. Also, Hyper-Kamiokande will allow to extend nucleon decay search to τp/B(p → e+π0) > 1035 yr and [Formula: see text]. Aiming at the realization of Hyper-Kamiokande, various R&D efforts are in progress.

RESULTS FROM SUPER-KAMIOKANDE ON ATMOSPHERIC NEUTRINO AND LIMITS ON MATTER INSTABILITY

Latest atmospheric neutrino results from 1489 days exposure of the Super-Kamiokande detector are presented. Our data are well explained by two flavor νμ↔ντ oscillations. In the three flavor oscillation case, the result shows no evidence for non zero θ13. Other models, ντ-νs admixture and neutrino decay, are strongly constrained. Moreover, the appearance searches for tau neutrino interactions are performed. Our data are consistent with ντ appearance. For the nucleon decay, p → e+π0 and [Formula: see text] are searched for, but no evidence for nucleon decays has been found. Experimental lower limits for nucleon partial lifetime are obtained to be 5.7×1033 years for p → e+π0 and 2.0×1033 years for [Formula: see text].

Nobel Lecture: Birth of neutrino astrophysics

In giving this talk I am very much helped by the preceding talk because I can skip some of the topics. If you want further information, please refer to my review article, ‘‘Observational Neutrino Astrophysics’’ (Koshiba, 1992). I am to talk about the birth of neutrino astrophysics, but before the birth, there was a very important event, which was just described by Professor Davis (Nobel Lecture, Davis, 2003). It was the radiochemical work using the reaction ne1 37Cl going to e137Ar. He found that the observed neutrino flux was only one-third of what was theoretically expected. This could be considered as the conception of neutrino astrophysics and was in fact the impetus for us to begin seriously working on solar neutrinos. I will talk about two experiments. The first was carried out at the original KamiokaNDE, which might be called an Imaging Water Cerenkov detector with 20% of its surface covered by photomultipliers. The total mass of the water inside this detector was 3000 tons. Its cost was about 3 million U.S. dollars. This, mind you, was meant to be the feasibility experiment on the astrophysical detection of solar neutrinos. The second experiment was carried out at Super-KamiokaNDE, the same type of detector but with a better light sensitivity, that is, 40% of the entire surface was covered by photocathodes and the total mass of the water was 50 000 tons. Its cost was about 100 million U.S. dollars. This was considered to be the full-scale solar neutrino observatory. Both the facilities are situated about 1000 meters underground in the Kamioka Mine. The capital letters NDE at the end of the two names originally stood for Nucleon Decay Experiment. However, because of the detection of various neutrinos by these detectors, people started calling them Neutrino Detection Experiments. Figure 1 shows the interior of KamiokaNDE. You can see arrays of photomultipliers on the sidewalls as well as on the top and at the bottom. When we were preparing for this KamiokaNDE experiment, we heard that a much bigger experiment, but of similar type, was being planned in the United States (Bionta et al., 1983). We had to think very seriously about the competition with this bigger detector. Both experiments aimed at the de-

Gauged baryon parity and nucleon stability

We show that the Standard Model Lagrangian, including small neutrino masses, has an anomaly-free discrete Z6 symmetry. This symmetry can emerge naturally from (I3R+Li+Lj−2Lk) gauge symmetry (Li is the ith lepton number) and ensure the stability of the nucleon even when the threshold of new physics Λ is low. All ΔB=1 and ΔB=2 (B is the baryon number) effective operators are forbidden by the Z6 symmetry. ΔB=3 operators are allowed, but they arise only at dimension 15. We estimate the lifetime for “triple nucleon decay” resulting from these operators and find that Λ can be as low as 102 GeV. We suggest a simple mechanism for realizing reasonable neutrino masses and mixings even with such a low scale for Λ.

Neutrino democracy and other phenomenology from 5D SO(10)

We present a five-dimensional supersymmetric SO(10) model compactified on an orbifold S (1)/ Z 2× Z 2′. The gauge symmetry G 422≡ SU(4) c × SU(2) L × SU(2) R , realized on one of the fixed points (branes), is spontaneously broken to the MSSM via the Higgs mechanism. Employing a flavor U(1) symmetry and suitably extending the ‘matter’ sector enables us to understand large mixings in the neutrino sector via a democratic approach, vs. the small CKM mixings. A residual R -symmetry on G 422 brane helps eliminate the troublesome dimension five nucleon decay, while the U(1) symmetry plays an essential role in suppressing dimension six decay. For rare leptonic decays we expect BR( μ→ eγ)∼BR( τ→ μγ)∼BR( τ→ eγ).

Supernovae neutrinos and the r-process and ν-processes of nucleosynthesis

The neutrinos emitted from supernovae contain information both about the physics of stellar collapse and of the nature of the neutrinos. Several detectors exist that will be capable of observing some subset of those neutrinos. To complement these detectors, we have designed OMNIS, the Observatory for Multiflavor NeutrInos from Supernovae. OMNIS will provide new information on stellar collapse and neutrino transformations, possibly diagnose the process of collapse to a black hole, and observe the late-time evolution of the neutrino distributions. OMNIS' ability to measure the spectra of the neutrinos will have an important effect on our understanding of the r- and ν-processes of nucleosynthesis. OMNIS is also sensitive to some modes of nucleon decay that could significantly improve present limits.

Torus-Fibered Calabi-Yau threefolds with non-trivial fundamental group

We construct smooth Calabi-Yau threefolds Z, torus-fibered over a dP_9 base, with fundamental group Z_2 X Z_2. To do this, the structure of rational elliptic surfaces is studied and it is shown that a restricted subset of such surfaces admit at least a Z_2 X Z_2 group of automorphisms. One then constructs Calabi-Yau threefolds X as the fiber product of two such dP_9 surfaces, demonstrating that the involutions on the surfaces lift to a freely acting Z_2 X Z_2 group of automorphisms on X. The threefolds Z are then obtained as the quotient Z=X/(Z_2 X Z_2). These Calabi-Yau spaces Z admit stable, holomorphic SU(4) vector bundles which, in conjunction with Z_2 X Z_2 Wilson lines, lead to standard-like models of particle physics with naturally suppressed nucleon decay.

OMNIS, the Observatory of Multiflavor neutrInos from Supernovae

OMNIS, the Observatory for Multiflavor NeutrInos from Supernovae, is being designed to detect supernova neutrinos via their interactions with lead nuclei. The information obtained will impact our understanding of the stellar collapse process, neutrino mass and oscillations, nucleosynthesis, black hole astrophysics, and nucleon decay.

Nucleon decay matrix elements for domain-wall fermions

We report on the nucleon decay matrix elements with domain-wall fermions in quenched approximation. Results from direct and indirect method are compared with a focus on the process of a proton decaying to a pion and a lepton. We discuss the renormalization necessary for the matching to the continuum theory. Preliminary results for the renormalized chiral lagrangian parameters are presented.

Signatures of nucleon disappearance in large underground detectors

For neutrons bound inside nuclei, baryon instability can manifest itself as a decay into undetectable particles (e.g., $\it n \to \nu \nu \bar{\nu} $), i.e., as a disappearance of a neutron from its nuclear state. If electric charge is conserved, a similar disappearance is impossible for a proton. The existing experimental lifetime limit for neutron disappearance is 4-7 orders of magnitude lower than the lifetime limits with detectable nucleon decay products in the final state [PDG2000]. In this paper we calculated the spectrum of nuclear de-excitations that would result from the disappearance of a neutron or two neutrons from $^{12}$C. We found that some de-excitation modes have signatures that are advantageous for detection in the modern high-mass, low-background, and low-threshold underground detectors, where neutron disappearance would result in a characteristic sequence of time- and space-correlated events. Thus, in the KamLAND detector [Kamland], a time-correlated triple coincidence of a prompt signal, a captured neutron, and a $\beta^{+}$ decay of the residual nucleus, all originating from the same point in the detector, will be a unique signal of neutron disappearance allowing searches for baryon instability with sensitivity 3-4 orders of magnitude beyond the present experimental limits.

Testing orbifold models of supersymmetric grand unification

In a model of supersymmetric SU(5) grand unification with a spatial dimension described by the orbifold S1/(Z2 × Z′2), proton decay is naturally suppressed at all orders. This is achieved by a suitable implementation of the discrete symmetries on the brane. But baryon number violating interactions are present in this model. We propose a few possible experimental tests of this model which exploit the effect of the baryon number violating couplings on low-energy observables such as neutron–antineutron oscillations, double nucleon decay into two kaons, hadronic decay widths of the Z boson and tt̄ production cross section in Run II of the Tevatron.

The Super-Kamiokande detector

Super-Kamiokande is the world's largest water Cherenkov detector, with net mass 50,000 tons. During the period April, 1996 to July, 2001, Super-Kamiokande I collected 1678 live-days of data, observing neutrinos from the Sun, Earth's atmosphere, and the K2K long-baseline neutrino beam with high efficiency. These data provided crucial information for our current understanding of neutrino oscillations, as well as setting stringent limits on nucleon decay. In this paper, we describe the detector in detail, including its site, configuration, data acquisition equipment, online and offline software, and calibration systems which were used during Super-Kamiokande I.