nEXO and the quest for neutrino-less double beta decay

Neutrinos, the only neutral elementary fermions, have provided many surprises.  Flavor oscillations reveal the non-conservation of the lepton flavor number and demonstrate that neutrino masses are finite; yet they are surprisingly much smaller than those of other fermions (by at least six orders of magnitude!)  It is then natural to ask if the mechanism providing the mass to neutrinos is the same that gives masses to the other (charged) elementary fermions and if neutrinos are described by 4-component Dirac wavefunctions or, as is possible for neutral particles, by 2-component Majorana ones. 

The hypothetical phenomenon of neutrino-less double-beta decay can probe the Majorana nature of neutrinos and the conservation of the total lepton number.   It may also help elucidating the origins of mass in the neutrino sector.  This is the Frontier of neutrino physics.

Following the well-known principle that there is no free lunch in life, interesting half-lives for neutrino-less double-beta decay exceed 10²⁵ years (or ~10¹⁵ times the age of the Universe!) making experiments rather challenging.    I will describe nEXO, a 5-tonne, enriched Xenon experiment with a sensitivity reaching beyond 10²⁸ years, or >100 times the current state of the art.  The nEXO detector derives directly from EXO-200, a very successful, rogue detector built by a collaboration with a heavy SLAC-Stanford participation.

Speaker: Giorgio Gratta, Stanford University

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