The first two years of data from the Enriched Xenon Observatory-200 (EXO-200) have been released by an international collaboration of physicists. The experiment looks for evidence of a process known as “neutrinoless double beta decay”, in a sample of isotopically enriched xenon-136. While the EXO-200 collaboration has not yet found any statistically significant evidence for the decay process, they have put an improved lower limit on the half-life of the decay.
They have also shown that they can efficiently suppress background noise from cosmic rays and radioactive decays. Observing any signs of neutrinoless double beta decay would show that neutrinos are “Majorana fermions” (particles that are their own antiparticles). This would constitute discovering a new class of particles that lies beyond the Standard Model of particle physics and would be a major breakthrough in modern physics.
Produced by a neutron undergoing β decay, neutrinos are chargeless particles that interact with matter via the weak force. Although we now have experimental evidence that neutrinos come in three “flavours” – the electron neutrino, the muon neutrino and the tau neutrino – that each have a different mass, researchers have been unable to nail down the individual masses. However, measurements of neutrinoless double β decay – if it were to occur – could be used to determine the absolute mass of a neutrino.
Neutrinoless double β decay is a special case of the common nuclear β decay process wherein the neutron in an unstable nucleus emits an electron and an antineutrino and becomes a proton. A more exotic version of the process, known as “double β decay”, occurs when a nucleus is forbidden to decay through a single β decay. One way for this double decay to happen is for two ordinary β decays to occur, but with no way of measuring the intermediate state between the two decays and with the final nucleus having a larger binding energy than the original nucleus. Two neutrons in the nucleus would be converted to protons and two electrons, with the emission of two electron antineutrinos – this is known as “two-neutrino double β decay” and is predicted by the Standard Model. Two-neutrino double β decay is very rare, thanks to the exceedingly long half-lives of the double β isotopes, above 1020 years. This is more than a billion times longer than the age of the universe itself. Select isotopes do undergo this type of double β decay however (it was first observed in 1986), including xenon-136, which decays, with the emission of two neutrinos, to barium-136. Indeed, the EXO-200 experiment was the first to observe this decay in xenon-136 in 2011.
But the other type of double β decay – the elusive and currently unseen neutrinoless double β decay – is what the EXO-200 collaboration, along with a host of other experiments worldwide, is looking for. This type of decay would only occur if the neutrino was a Majorana particle, first predicted in the 1930s by the equally enigmatic Italian physicist Ettore Majorana, but so far undetected. As neutrinos have no electrical charge, they could conceivably be their own antiparticle. In this case then, the antineutrino emitted from one of the β decays could be absorbed as a neutrino in the other β decay. This process, as observed from outside the nucleus, would result only in the observation of two electrons being emitted, with no neutrinos at all. The electrons would carry all the energy of the decay, unlike normal double β decay, in which the antineutrinos carry away energy. The experimental signature of this decay process is the detection of two electrons, the sum of whose total energy is equal to the mass difference between the parent and daughter nuclei.
The EXO-200 experiment looks for this signature using 200 kg of liquid xenon, enriched to 80% of the 136 isotope, and held in a “time-projection chamber”. The chamber is placed within a cryostat system to help keep the xenon at liquid temperature. The cryostat is then shielded with lead and is located deep in the bowels of a disused salt mine, 641 m underground at the Waste Isolation Pilot Plant in Carlsbad, New Mexico, in the US. This remote underground location is crucial to the experiment’s success because it acts as a shield from background radioactive decay and cosmic rays, while the detector is made up of materials that constitute the lowest possible levels of radioactive contamination.
The EXO-200 experiment has been running for two years, allowing the collaboration to place the most stringent bound on the half-life for neutrinoless β decay. The researchers found that it is greater than 1.1 × 1025 years, at the 90% confidence level, improving on their own previous limit of 1.6 × 1025 years. The collaboration says that the high sensitivity of its measurement “holds promise for further running of the EXO-200 detector and future [neutrinoless double β decay] searches with an improved Xe-based experiment, nEXO”. This long lifetime suggests that neutrinos probably have small masses. Most recent experiments have now set limits to the Majorana neutrino mass at 0.2–0.4 eV. The nEXO experiment is a larger detector, with 5000 kg of xenon that is currently being proposed and simulated, and the current EXO-200 data will help refine its future design.
David Waters, a particle physicist at University College London, says that the EXO-200 “is a beautifully executed experiment and one of the most sensitive currently in operation. Although no signal for neutrinoless double β decay has been seen, experiments such as EXO are getting closer and closer to a promising region of parameter space that is suggested by neutrino oscillation experiments”. Waters, who also works on the another such experiment, the Super Neutrino Ettore Majorana Observatory (SuperNEMO) points out, “The next five years will be a very interesting time with several experiments such as EXO, but also others including SNO+ and SuperNEMO in which the UK plays leading roles, having the potential to make a major discovery that would shed light on some very fundamental questions in particle physics.”
The research is described in Nature.