NuTeV result disagrees with Standard Model

Trevor Johnson writes "New Scientist reports that physicists from the NuTev collaboration at Fermilab have announced a result on their Web page. Bombarding iron nuclei with high-energy neutrinos, they found a 99.75% chance that neutrinos interact slightly differently from the way the Standard Model (the reigning theory which describes the strong, electromagnetic, and weak forces, but not gravity) says they should. Even though the discrepancy is small, it is likely to be significant. There is no theory to explain the difference, but it could indicate a hitherto unknown force. NuTev has submitted a "Physical Review Letter" paper. There is a press release from Fermilab and a plain English version."

Comments

Just the Opposite

[Neil Rubin]

The Sudbury experiment [queensu.ca] this summer is starting to show non-zero neutrino mass, which I understood meant that oscillation between states was going to not happer - or be severely limited.

It's actually precisely the opposite. The modern way to understand this is in terms of quantum fields. The energy density of space depends on the values of these fields. In order for one neutrino to oscillate into another, the expression for this energy density must include a term which contains the anti-neutrino field of one flavor of neutrino and the neutrino field of another flavor (neutrinos come in three types, called flavors). This allows a neutrino of one flavor to be annihilated at the same time as another is created.

Now mass for a particular neutrino flavor comes mainly from a term containing the neutrino and anti-neutrino fields of the same flavor, but also those terms that mix it with other flavors. To be more precise, the measured masses of neutrinos are the eigenvalues of the matrix made up of these terms which multiply anti-neutrino and neutrino fields.

The only matrix with all eigenvalues zero is the zero matrix. Thus, if all neutrinos are massless, there is no mixing. The inverse is not necessarily true.