The first results from the Enriched Xenon Observatory 200 (EXO-200) on the search for neutrinoless double beta decay show no evidence for this hypothesised process, which would shed new light on the nature of the neutrino. Located in the US Department of Energy’s Waste Isolation Pilot Plant in New Mexico, EXO-200 is a large beta-decay detector. In 2011 it was the first to measure two-neutrino double beta decay in 136Xe; now it has set a lower limit for neutrinoless double beta decay for the same isotope.
Double beta decay, first observed in 1986, occurs when a nucleus is energetically unable to decay via single beta decay, but can instead lose energy through the conversion of two neutrons to protons, with the emission of two electrons and two antineutrinos. The related process without the emission of antineutrinos is theoretically possible but only if the neutrino is a “Majorana” particle, i.e. it is its own antiparticle.
EXO-200 uses 200 kg of 136Xe to search for double beta decay. Xenon can be easily purified and reused, and it can be enriched in the 136Xe isotope using Russian centrifuges, which makes processing large quantities feasible. It also has a decay energy – Q-value – of 2.48 MeV, high enough to be above many of the uranium emission lines. Using 136Xe as a scintillator gives excellent energy resolution through the collection both of ionization electrons and of scintillation light. Finally, using xenon allows for complete background elimination through tagging of the daughter barium ion. This tagging, combined with the detector’s location more than 650 m underground and the use of materials selected and screened for radiopurity, ensures that other traces of radioactivity and cosmic radiation are eliminated or kept to a minimum. The latest results reflect this low background activity and high sensitivity – as only one event was recorded in the region where neutrinoless double beta decay was expected.
In the latest result, no signal for neutrinoless double beta decay was observed for an exposure of 32.5 kg/y, with a background of about 1.5 × 10–3 kg–1y–1keV–1. This sets a lower limit on the half-life of neutrinoless double beta decay in 136Xe to greater than 1.6 × 1025 y, corresponding to effective Majorana masses of less than 140–380 meV, depending on details of the calculation (Auger et al. 2012).
The EXO collaboration announced the results at Neutrino 2012, the 25th International Conference on Neutrino Physics and Astrophysics, held in Kyoto, on 3–9 June. This dedicated conference for the neutrino community provided the occasion for many neutrino experiments to publicize their latest results. In the case of the MINOS collaboration, these included the final results from the first phase of the experiment, which studies oscillations between neutrino types.
In 2010 the MINOS collaboration caused a stir when it announced the observation of a surprising difference between neutrinos and antineutrinos. Measurements of a key parameter used in the study of oscillations – Δm2, the difference in the squares of the masses of two oscillating types – gave different values for neutrinos and antineutrinos. In 2011, additional statistics brought the values closer together and, with twice as much antineutrino data collected since then, the gap has now closed. From a total exposure of 2.95 × 1020 protons on target, a value was found for muon antineutrinos of Δm2 = 2.62+0.31–0.28(stat.)±0.09(syst.) and the antineutrino “atmospheric” mixing angle was constrained with sin22θ greater than 0.75 at 90% confidence level (Adamson et al. 2012). These values are in agreement with those measured for muon neutrinos.
Since its debut in 2006, the OPERA experiment in the Gran Sasso National Laboratory has been searching for neutrino oscillations in which muon-neutrinos transform into τ-neutrinos as they travel the 730 km of rock between CERN, where they originate, and the laboratory in Italy. At the conference, the OPERA collaboration announced the observation of their second τ-neutrino, after the first observation two years ago. This new event is an important step towards the accomplishment of the final goal of the experiment.
Results on the time of flight of neutrinos from CERN to the Gran Sasso were also presented by CERN’s director for research and scientific computing, Sergio Bertolucci, on behalf of four experiments. All four – Borexino, ICARUS, LVD and OPERA – measure a neutrino time of flight that is consistent with the speed of light. The indications are that a measurement by OPERA announced last September can be attributed to a faulty element of the experiment’s fibre-optic timing system.
Further reading
M Auger et al. 2012 EXO collaboration arXiv:1205.5608v1 [hep-ex].
P Adamson et al. 2012 MINOS collaboration arXiv:1202.2772v1 [hep-ex].
