Minos observes neutrino mass

More evidence for neutrino mass has been seen by the MINOS Experiment in the Soudan mine in Minnesota.

More precisely, the experiment extends and corroborates evidence for neutrino mass that has been gathering over the past 40 years, speeding up mightily in the last decade. The first evidence was the deficit of neutrinos from the sun, first observed by Ray Davis in his experiment in the Homestake mine (inspired by and in collaboration with the late John Bahcall, seen here with Davis in the mine), followed by several other experiments worldwide. One possible explanation for this deficit is that, contra the standard model of particle physics (as was), neutrinos have a small mass. This changes their interactions with normal matter, and means that, when they pass through a medium such as the gas of the sun, they can be converted from one type of neutrino (in this case, the “electron neutrinos” that are created in the center of the sun) to another (either mu or tau neutrinos), via a process known as neutrino osciallation and the MSW Effect.

Because of the vast difficulty of detecting neutrinos, this solar neutrino deficit was not really confirmed until much more recent experiments, Kamiokande in Japan (whose developer, Masatoshi Koshiba, received the 2002 Nobel along with Davis) and SNO in Canada, which finally convinced quashed all remaining doubt.

As I mentioned above, there are three kinds (“flavors”) of neutrinos, corresponding to each of the elementary particles electron, muon and tau — together, these six particles (and their antiparticles) are known as leptons. So really, we need to measure three different kinds of neutrino oscillations. Because of the oscillations, these neutrinos do not actually have a definite mass! Instead, quantum mechanics allows us to make combinations of these flavored neutrinos to make three generations of neutrinos of definite mass. In fact, MINOS is the first experiment to measure the oscillation between the second and third generations.

Essentially, all of these experiment work the same way: they count the number of neutrinos that interact with a target some long distance away from a source. For the solar experiments, the source is the sun; for MINOS and other experiments like it, the source is a beam at Fermilab. By counting the number of detected neutrinos (which is a tiny fraction of the number that pass through the detector), they can determine the fraction that oscillate into another flavor, undetectable by this experiment, and measure the parameters that describe the mixing. In fact, there are quite a few such parameters — for the aficionados, the masses are described by a 3x3 matrix — but MINOS itself measures two numbers, an angle describing how much the different generations are mixed, and, curiously, the difference in the squares of the masses of the generations. Minos’ main result is that δm223=0.0031±0.0007 eV2, and that there seems to be quite strong (“large angle”) mixing between the states. [Thanks to Ned Wright for pointing out a three order-of-magnitude typo here!]

Such a result has clear implications for particle physics. The current standard model has massless neutrinos, and any extensions, such as those that generate the supersymmetric particles that CERN’s LHC will hunt for, must also give the neutrinos a mass. Cosmologically, the situation is less clear. These neutrinos are sufficiently light to have very little impact on the evolution of the Universe, although we expect it to be detectable in the long run due to the massive neutrinos’ ability to wash out a small amount of the growth of structure in the universe as they rush around at high speeds, dragging other particles with them through their gravitational interactions. And, of course, Outer Space and Inner Space are deeply connected — the early instants of the Universe are governed by that same model of particle physics, so ultimately the changes will reverberate here.

You can see more detailed results in the slides from a talk given yesterday at Fermilab.

Update: In his link to this post, Chad Orzel questions “whether they can use the mass difference they measure to determine an actual mass”. Since they measure only difference between masses (between squared masses, actually), they can’t measure masses individually. So all three neutrinos could be light, but still orders of magnitude apart in their masses, or they could all be relatively heavy, with only small differences (compared to the smallest mass) between them. The best bet for a direct neutrino mass measurement (aside from cosmological probes!), is probably the KATRIN experiment, which uses beta decay (which releases an electron and an antineutrino) of tritium (an isotope of hydrogen with one proton and two neutrons) to measure the mass of the electron neutrino.