Phys. Rev. D 90, 072005 (2014)

Credit: © KAMIOKA OBSERVATORY, ICRR, THE UNIVERSITY OF TOKYO

Attempts to unify our understanding of strong and electroweak forces are hampered by the fact that the energies involved are too high to be probed using accelerators. But unified theories also predict nucleon decay where the standard model does not, turning the spotlight — or search light — to proton decay experiments. After nearly two decades on the case, we still have no evidence of nucleon decay, but the Super-Kamiokande Collaboration has now set a new limit on the lifetime of a proton.

The search involved four separate phases, spanning April 1996 to February 2013, carried out in a large water Cherenkov detector (pictured), which is buried beneath Mount Ikenoyama to reduce cosmic background radiation. The past decade of the experiment witnessed a number of improvements, including a reduction in background radiation and a massive increase in efficiency. The results of the first phase of detection, published in 2005, set a preliminary lower limit on proton lifetime. The new figure is more than twofold the original estimate — placing strong constraints on unified theories, and in particular, those incorporating supersymmetry.