The Neutrino Riddle: Identical Particle and Anti-Particle?

Neutrinos are elusive particles: It took many years to detect them. Decades passed until scientists could prove that they actually have mass. However, the mysterious particles hold further secrets, e.g. the question why and how neutrinos swap families (see press release 2011/09/06). One of the unsolved neutrino problems is the nature of the relationship between the neutrino and its anti-particle, the anti-neutrino. There are indications that unlike all other elementary particles neutrinos and their counterparts might be identical. Recently, an international collaboration including Prof. Peter Fierlinger and Dr. Mike Marino from the Excellence Cluster Universe at the Technische Universität München has taken an important experimental step in “profiling” the hunted particle. The research work was published at http://arxiv.org/abs/1108.4193 and submitted to Physical Review Letters.

One of the keys to the anti-neutrino riddle is radioactive beta decay, when a neutron decays emitting a proton, an electron and an anti-neutrino. Actually, scientists are looking for a related decay scheme, neutrinoless double-beta decay, which if found would prove that the neutrino and anti-neutrino are the same particle. The experimental setup used by the EXO collaboration is an underground detector situated in New Mexico, USA.

Currently it is taking data by studying decay processes in the noble gas Xenon, or to be more precise, in the isotope Xenon-136. Recently, the EXO collaboration reached an important milestone by observing for the first time the two-neutrino double-beta decay of Xenon-136, a process whereby Xenon-136 becomes Barium-136 and emits two electrons and two anti-neutrinos.  This particular process is a close relative of the neutrinoless decay and its observation is important both as a test of the functionality of the detector and also as an indication that theoretical models used to understand the 136-Xenon nucleus are accurate. The accuracy of these models will be important when trying to understand any possible observation of the neutrinoless decay mode. 

The two-neutrino double-beta decay half-life was measured by the EXO collaboration to be 2.11 x 10²¹  years, making it the slowest decay ever directly observed in a detector.  For comparison, this half-life is more than one hundred billion times the age of the Universe – 13.7 billion years – and means that in a drinking glass full of liquid Xenon-136, only a few atoms per day would actually decay by this process. To find neutrinoless double-beta decay, the EXO collaboration must continue to look quite hard as current experimental limits indicate that the decay is even rarer, at least 1,000 times slower than its two-neutrino relative.