Search for Majorana neutrinos with the first two years of EXO-200 data

Abstract

Many extensions of the Standard Model of particle physics suggest that neutrinos should be Majorana-type fermions, but this assumption is difficult to confirm. Observation of neutrinoless double-beta decay (0νββ), a spontaneous transition that may occur in several candidate nuclei, would verify the Majorana nature of the neutrino and constrain the absolute scale of the neutrino mass spectrum. Recent searches carried out with 76 Ge (GERDA experiment) and 136 Xe (KamLAND-Zen and EXO-200 experiments) have established the lifetime of this decay to be longer than 10 25 yr, corresponding to a limit on the neutrino mass of 0.2-0.4 eV. Here we report new results from EXO-200 based on 100 kg·yr of 136 Xe exposure, representing an almost fourfold increase from our earlier published datasets. We have improved the detector resolution at the 136 Xe double-betadecay Q-value to σ/E = 1.53% and revised the data analysis. The obtained half-life sensitivity is 1.9 · 10 25 yr, an improvement by a factor of 2.7 compared to previous EXO-200 results. We find no statistically significant evidence for 0νββ decay and set a half-life limit of 1.1 · 10 25 yr at 90% CL. The high sensitivity holds promise for further running of the EXO-200 detector and future 0νββ decay searches with nEXO. Majorana fermions, a class of neutral spin-1/2 particles described by 2-component spinors, have been an element of quantum field theory since its inception [1,2]. Electrons and other spin-1/2 elementary particles with distinct antiparticles, however, are described by 4-component Dirac spinors. Majorana quasiparticles may have been observed in condensed matter systems [3] where neutrality is achieved through the collective action of electrons and holes. Among the known elementary particles, only neutrinos are Majorana fermion candidates, owing to their intrinsic neutrality. Confirmation of this property would imply the non-conservation of lepton number, an additive quantum number that, unlike charge or color, is not related to any known gauge symmetry. As yet, lepton number has been empirically found to be conserved. Neutrinos are also remarkable for their small, yet finite, masses [4] that are generally difficult to explain, but arise naturally in many extensions [5,6] of the Standard Model of particle physics (SM). A generic consequence of many such extensions is that neutrinos should be of the Majorana variety.The most sensitive probe for Majorana neutrinos is a nuclear process known as neutrinoless double-beta decay (0νββ), whereby a nucleus decays by emitting two electrons and nothing else, while changing its charge by two units [7]. A related double-beta decay process, known as two-neutrino double-beta decay (2νββ), is allowed by the SM and has been observed in many nuclei, 136 Xe among them [8,9]. It provides, however, no direct information on the Majorana/Dirac question. The exotic 0νββ can be distinguished from the 2νββ by measuring the sum energy of the two electrons that is peaked at the Q-value for the former and is a continuum for the latter....

Neutrinoless double beta decay in chiral effective field theory: lepton number violation at dimension seven

“…The dimension-six and -seven operators enter through the external sources, l µ , r µ , s, and p. Contributions from the dimension-six tensor operator require two additional derivatives which increase the chiral index by two. As such, the dominant contribution from C (6) T comes from the pion-nucleon sector which is discussed below.…”

“…The observation of 0νββ would have far reaching implications: it would demonstrate that neutrinos are Majorana fermions [2], shed light on the mechanism of neutrino mass generation, and give insight on leptogenesis scenarios for the generation of the matterantimatter asymmetry in the universe [3]. The current experimental limits on the halflives are already impressive [4][5][6][7][8][9][10][11][12][13], at the level of T 0ν 1/2 > 5.3 × 10 25 y for 76 Ge [12] and T 0ν 1/2 > 1.07 × 10 26 y for 136 Xe [13], with next generation ton-scale experiments aiming at a sensitivity of T 0ν 1/2 ∼ 10 27−28 y. By itself, the observation of 0νββ would not immediately point to the underlying physical origin of LNV.…”

“…(3.5), (3.6), and (3.7) need to be evolved from the matching scale µ ∼ m W to scales µ ∼ 2 GeV, where the matching to chiral perturbation theory and LQCD calculations is performed. The vector operators, C (6) VL, VR and C (7) VL, VR , consisting of quark non-singlet axial and vector currents, do not run in QCD. 3 The renormalization group equations (RGEs) of the scalar and tensor operators below µ = m W are given by…”

“…The coefficients C (6,7,9) ij are all defined to be dimensionless. Keeping the lepton flavor structure, the matching coefficients for the dimension-six operators at the electroweak scale are given by 2 …”

Search for Neutrinoless Double-Beta Decay ofTe130with CUORE-0

“…The authors thank J. Feintzeig, L. Gladstone, P. Mosteiro, and V. Singh for carefully reviewing the manuscript, M. Nastasi for preparing the 60 Co calibration sources, and F. Iachello for helpful discussions concerning the NME literature. [39], KamLAND-Zen [40] independently), and (c) 76 Ge (combined limit from Gerda, IGEX, HDM [41]). The vertical arrows aim to emphasize the range currently probed with each isotope.…”