Limit on ν¯emass from observation of the β decay of molecular tritium

Abstract

We report the most sensitive direct upper limit set on the mass my of the electron antineutrino. Our measurements of the shape of the fi decay spectrum of free molecular tritium yield, under the assumption of no new physics other than that of mass, a central value for mv of -147 ±68 ±41 eV^, which corresponds to an upper limit of 9.3 eV (95% confidence level) on mv. The result is in clear disagreement with a reported value of 26(5) eV.PACS numbers: 23.40.Bw, 14.60.GhThat the mass of the electron neutrino (or antineutrino) could be determined from the shape of p spectra has been known since Fermi's formulation of the theory of p decay. A group at the Institute for Theoretical and Experimental Physics (ITEP) in Moscow have reported [1] from their studies of the tritium p spectrum that the Ve mass lies in the range 17-40 eV, with revolutionary implications for particle physics and cosmology. Not only are massive neutrinos incompatible with the otherwise successful minimal standard model of particle physics, a neutrino mass in that range would be sufficient both to close the Universe gravitationally and to account qualitatively for observational evidence for dark matter. Other experiments [2][3][4], however, do not support the ITEP claim.When tritium decays to ^He, the orbital electrons redistribute themselves over the set of eigenstates of the residual molecule. The resulting energy spread impressed on the outgoing p must be very precisely calculated if serious errors in interpreting the data are to be avoided. Such calculations can be carried out with some confidence for atomic and molecular tritium, in contrast to the multielectron solid sources used heretofore [1][2][3]. Our experiment at the Los Alamos National Laboratory is the first to make use of a gaseous source of T2 to capitalize on the simplicity of the two-electron system. The gaseous source also confers minimal, well understood energy-loss corrections, and freedom from backscatter; together with detailed measurements of the instrumental resolution function, energy-loss, energy efficiency, and other effects, it has made possible a reliable mass measurement at the 10-eV level.In an earlier paper [4] we described our apparatus briefly and reported our initial result, rriy < 27 eV. The source is a tube fed at its midpoint with a steady flow of T2 gas and placed on the axis of a solenoidal magnetic field whose strength decreases monotonically toward the extraction end. Electrons from p decay near the axis are guided to the object point of a toroidal-field magnetic spectrometer of 5-m focal length. The most significant of many improvements [5] made recently are the elimina-tion of electron trapping in the source and replacement of the single-element proportional counter in the spectrometer by an octagonal array of Si microstrip detectors, each with twelve pads. Signals from pads at the same location along the dispersion axis are combined to form twelve simultaneously acquired spectra, each independently calibrated by a ^^Kr'" spectrum similarly formed.Tritium...

Neutrino mass problem: the state of the art

“…A remarkable feature of the detector manufactured from 76 Ge (and of the TPC detector manufactured from 136 Xe) is that it combines two functions, that of the emitter and of a registering device, while in contrast to 136 Xe the detection process that uses 76 Ge has an exceptionally high resolution power DEaE 9 3 Â 10 À3 and practically 100% efficiency. The 2b0n-decay of 76 Ge produces a monochromatic line at 2038X58 AE 0X31 keV in the energy spectrum for an electron pair.…”

“…In particular, Robertson et al [76] were the first to use molecular tritium T 2 as a gaseous source, where a molecule is a two-electron system, which is much simpler for calculations than the manyelectron molecules of solid sources.…”

“…The 2b0n-decay of 76 Ge produces a monochromatic line at 2038X58 AE 0X31 keV in the energy spectrum for an electron pair.…”

“…Today the most sensitive experiment in the study of double beta decay is that of the Heidelberg ± Moscow Collaboration, an experiment that uses the 2b-decay source with 76 Ge enriched to 86%. Klapdor-Kleingrothaus [109] and Balysh et al [110] obtained the most precise value of T 1a2 2b2n and an upper bound on T 1a2 2b0n , which leads to a lower bound on the Majorana neutrino mass (m n`0 X6 eV).…”

“…The Heidelberg ± Moscow Collaboration had 19.2 kg of germanium enriched in 76 Ge, which was used to manufacture five high-purity p-type semiconductor detectors with a total mass of 11.5 kg. At present all five detectors are operational in the underground laboratory near Gran Sasso, Italy at a depth of 3500-m water equivalent.…”