New limits on nucleon decays into invisible channels with the BOREXINO counting test facility

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Abstract

The results of background measurements with the second version of the BOREXINO Counting Test Facility (CTF-II), installed in the Gran Sasso Underground Laboratory, were used to obtain limits on the instability of nucleons, bounded in nuclei, for decays into invisible channels (inv): disappearance, decays to neutrinos, etc. The approach consisted of a search for decays of unstable nuclides resulting from N and NN decays of parent 12C, 13C and 16O nuclei in the liquid scintillator and the water shield of the CTF. Due to the extremely low background and the large mass (4.2 t) of the CTF detector, the most stringent (or competitive) up-to-date experimental bounds have been established: τ(ninv)>1.8×1025 yr, τ(pinv)>1.1×1026 yr, τ(nninv)>4.9×1025 yr and τ(ppinv)>5.0×1025 yr, all at 90% C.L.

Introduction

The baryon (B) and lepton (L) numbers are considered to be conserved in the Standard Model (SM).14 However, no symmetry principle underlies these laws, such as, e.g., gauge invariance, which guarantees conservation of the electric charge. Many extensions of the SM include B and L violating interactions, predicting the decay of protons and neutrons bounded in nuclei. Various decay mechanisms with ΔB=1, 2 and Δ(BL)=0, 2 have been discussed in the literature intensively [2], [3]. A novel baryon number violating process, in which two neutrons in a nucleus disappear, emitting a bulk majoron nnχ, was proposed recently [4]; the expected mean lifetime was estimated to be ∼1032–39 yr. Additional possibilities for the nucleon (N) decays are related to theories which describe our world as a brane inside higher-dimensional space [5], [6]. Particles, initially confined to the brane, may escape to extra dimensions, thus disappearing for the normal observer; the characteristic proton mean lifetime was calculated to be τ(p)=9.2×1034 yr [7]. Observation of the disappearance of e, N, NN would be a manifestation of the existence of such extra dimensions [6].

No evidence for nucleon instability has been found to date. Experimental searches [8] with the IMB, Fréjus, (Super)Kamiokande and other detectors have been devoted mainly to nucleon decays into strongly or electromagnetically interacting particles, where lower limits on the nucleon mean lifetime of 1030–33 yr were obtained [9]. At the same time, for modes where N or NN pairs disappear or they decay to some weakly interacting particles (neutrinos, majorons, etc.), the experimental bounds are a few orders of magnitude lower. Different methods were applied to set limits for such decays15 (see Table 1 for summary):

(1) Using the limit on the branching ratio of spontaneous fission of 232Th under the assumption that p or n decay in 232Th will destroy the nucleus [10]. The bound on the mean lifetime obtained in this way can be considered independent of the p or n decay mode, since the 232Th nucleus can be destroyed either by strong or electromagnetic interactions of daughter particles with the nucleus or, in the case of N disappearance, by subsequent nuclear deexcitation process.

(2) Search for a free n created after p decay or disappearance in the deuterium nucleus (d=pn) in a liquid scintillator enriched in deuterium [11] or in a volume of D2O [12], [16], [17].

(3) Geochemical [13] or radiochemical [14] search for daughter nuclides which have appeared after N decays in the mother nuclei (valid for decays into invisible channels).

(4) Search for prompt γ quanta emitted by a nucleus in a de-excitation process after N decays within the inner nuclear shell [18] (valid for invisible channels).

(5) Considering the Earth as a target with nucleons which decay by emitting electron or muon neutrinos; the νe, νμ can be detected by a large underground detector [19], [20] (valid for decay into neutrinos with specific flavors).

(6) Search for bremsstrahlung γ quanta emitted due to a sudden disappearance of the neutron magnetic moment [21] (limits depend on the number of emitted neutrinos).

(7) Study of radioactive decay of daughters (time-resolved from prompt products), created as a result of N or NN decays of the mother nuclei, incorporated into a low-background detector (valid for decay into invisible channels). This method was first exploited by the DAMA group with a liquid Xe detector [15].

In the present Letter we use the same approach to search for N and NN instability with the Counting Test Facility, a 4.2 t prototype of the multiton BOREXINO detector for low energy solar neutrino spectroscopy [23]. The preliminary results were presented in [24].

Section snippets

Technical information about CTF and BOREXINO

BOREXINO, a real-time 300 t detector for low-energy neutrino spectroscopy, is nearing completion in the Gran Sasso Underground Laboratory (see [23] and references therein). The main goal of the detector is the measurement of the 7Be solar neutrino flux via νe scattering in an ultra-pure liquid scintillator, while several other basic questions in astro- and particle physics will also be addressed.

The Counting Test Facility (CTF), installed in the Gran Sasso Underground Laboratory, is a

Theoretical considerations

The decay characteristics of the daughter nuclides, resulting from N and NN decays in parent nuclei—12C, 13C and 16O—contained in the sensitive volume of the CTF liquid scintillator or in the water shield, are listed in Table 2.

After the disappearance of one or two nucleons in the parent nuclide, one or two holes appear in the nuclear shells; these holes will be filled in a subsequent nuclear de-excitation process, unless the nucleons reside on the outermost shells. If the initial excitation

Conclusions

Using the unique features of the BOREXINO Counting Test Facility—the extremely low background, the large scintillator mass of 4.2 t and the low energy threshold—new limits on N and NN decays into invisible channels (disappearance, decays to neutrinos, majorons, etc.) have been set: τ(n→inv)>1.8×1025 yr,τ(p→inv)>1.1×1026 yr,τ(nn→inv)>4.9×1025 yrandτ(pp→inv)>5.0×1025 yrwith 90% C.L. Comparing these values with the data of Table 1, one can see that the established limits for nn and pp decays are

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