To date, the best limits on the half-life come from GERDA, which today reports a lower limit of 8 × 1 0 2 5 years and from a fifth experiment, KamLAND-Zen, at the Kamioka Observatory in Japan, which uses xenon-136 and achieved a limit of 1. Finally, CUORE, also located at Gran Sasso, studies TeO 2 crystals made of natural tellurium, 34% of which is the double-beta decay isotope tellurium-130. EXO-200, at the Waste Isolation Pilot Plant in the US, analyzes liquid xenon enriched with xenon-136. GERDA, located at Italy’s Gran Sasso underground laboratory, and the MAJORANA Demonstrator, at the US’s Sanford Underground Research Facility, both look for the decays in materials enriched with germanium-76. (In 0 □ □ □ decay, the two electrons carry away all of the decay energy, which is the mass difference between the initial and final nucleus.) Where the experiments differ is in which isotope they use and how they measure the electron energies. The four experiments all determine the decay half-life (the inverse of the decay rate) in roughly the same way: by monitoring a large number of atoms of a given double-beta decay isotope and looking for a peak in the two-electron energy. The discovery that lepton number isn’t conserved might also point physicists toward an explanation for the observed asymmetry between matter and antimatter. But detecting the decay, no matter which mechanism causes it, would tell us that neutrinos are Majorana particles and that there are new particles allowing the nonconservation of lepton number. If nature prefers the second option (heavy particles), the relation between the decay rate and neutrino masses is more complicated. If nature chooses the first scenario (virtual Majorana neutrinos), the decay rate is proportional to the square of a mass called m □ □, which is a weighted average of the masses of the three neutrino mass states. 1, center) or of some new heavy particle (Fig. They involve the creation and destruction of either a virtual Majorana neutrino (Fig. Various mechanisms for this neutrinoless process are possible. But if neutrinos are Majorana particles, double-beta decay can occur without the emission of antineutrinos, meaning the lepton number changes by 2. Lepton number is therefore conserved because the electrons and antineutrinos have opposite lepton number. 1, left), which is allowed in certain isotopes, two neutrons transform into two protons plus two electrons and two antineutrinos. In the process of two-neutrino beta decay (Fig. Now if neutrinos are Majorana particles, then they violate the conservation of lepton number-the quantum number that is assigned to all leptons and is 1 for electrons and neutrinos and − 1 for their respective antiparticles. These theories explain the light neutrino masses as being inversely proportional to a large mass scale set by other particles that have yet to be seen. Most such extensions of the model say that the neutrinos are Majorana particles-meaning they are their own antiparticles. This vast discrepancy suggests that the origin of neutrino mass is different from that of all other fermions, involving physics that goes beyond the standard model. The particles, which exist in three possible mass states, are about 1 0 6 times lighter than the next lightest fermion, the electron. These new results invite a discussion of why detecting 0 □ □ □ decay is of interest and what physicists might learn as the experiments become more sensitive.Ī striking feature of neutrinos is their extremely small mass. Several of these experiments should reach the 1 0 2 6 level soon, thus catching up with a fifth experiment. Four experimental collaborations are reporting new lower limits on the decay’s half-life, all of which exceed 1 0 2 5 years. It would also explain why neutrinos are so light. This hypothetical decay would show that neutrinos are their own antiparticles and that a fundamental law-the conservation of lepton number-is violated in nature. But researchers are also searching for new physics in the “low-energy” environment of the nucleus through a process known as neutrinoless double-beta ( 0 □ □ □) decay. The search for physics beyond the standard model-our current best description of fundamental particles and the interactions between them-is a top priority at high-energy particle accelerators. Different models for the decay describe it in terms of the creation and destruction of a Majorana neutrino (center) or of an unknown heavier particle (right). If neutrinos are Majorana particles then a neutrinoless form of this double-beta decay should be allowed. APS/ Alan Stonebraker Figure 1: “Two-neutrino” double-beta decay (left) is allowed in certain isotopes and involves the transformation of two neutrons into two protons, two electrons, and two antineutrinos.
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