For the first time ever the ghosts of the particle world, neutrinos, have been explicitly seen to actively change personality. Results presented today by the Tokai to Kamioka (T2K) experiment fills in previously unseen parts of the picture of how our universe works at the smallest scales, but it also raises some interesting questions.

Neutrino particles are ghostly, difficult to see, particles that have real personality issues. They come in three types, known as flavours: electron (ν e ), muon (ν μ ) and tau (ν τ ) neutrinos. The first neutrino experiments used naturally occurring sources of the particles, such as the Sun (electron neutrinos) and cosmic ray particle showers (muon neutrinos), to understand more about how they interacted with the world around them. They seemed to be misbehaving according to either experiment or theory as fewer neutrinos were seen than were predicted. For years neutrinos in nature seemed to be disappearing between being created and then detected in many various experiments that looked for them. After almost 30 years of experimentation all was finally resolved. It was proven that naturally occurring neutrinos were not disappearing, but instead were changing into other types of neutrino which could not be seen, due to having too low an energy.

Beams have now been engineered to further investigate this bizarre characteristic now known as oscillation. These beams are specifically muon-type neutrinos because physicists copied cosmic ray particle showers in nature. Experiments saw the muon-type neutrinos disappearing as expected from natural observation. Because of the disappearance they were assumed to be changing into tau-type neutrinos, which did not have enough energy to produce a tau particle and be directly seen.

For the first time neutrinos have actively been seen to change from one flavour to another rather than just viewing a disappearance. The T2K experiment has seen muon neutrinos change character to become electron neutrinos after a journey of 295km across Japan. The certainty of this measurement is quoted as 7.5 standard deviations from zero or to put in terms of percentage over 99.9999999999936% sure that the appearance is occurring.

History has shown us that the more we understand about neutrinos the more secrets of nature they uncover. The observation made by T2K opens up a whole new way of observing neutrinos. As we continue to piece together the character of the neutrinos we hope to continue uncovering more bizarre secrets; they may even be the key to how the raw material for the Universe was first created.



Ben Still is a particle physicist at Queen Mary, University of London

More information on the EPS HEP meeting is here, and further details of the presentation can be found here.