This year’s Nobel prize for physics was awarded today to two experimentalists Takaaki Kajita of the University of Tokyo and Arthur B. McDonald of Queen’s University in Canada for their independent work that showed that neutrinos have mass. You can read the official announcement form the Royal Swedish Academy of Sciences here.
Of all the known particles, neutrinos are far and away the most elusive and hard to detect. This is because the detection of a particle involves using some property that it has that a detector can grab hold of. But neutrinos have no charge or magnetic moment, the easiest properties to use in detectors, and their masses were so small as to be directly undetectable and for a long time it was assumed that they were exactly zero.
Furthermore, neutrinos only interact with other particles via what is known as the weak interaction, which means that it hardly ever reacts with anything. In fact, at any given time, billions of these little fellas, emitted from the Sun, are passing freely through the Earth (and our bodies) and we have no idea that they are doing so because they don’t affect us in any way. So detecting neutrinos involved building huge detectors deep underground and looking for extremely rare collisions with the liquids in the detectors. These experiments are devilishly difficult because you need huge amounts of detector material, have to eliminate all manner of background noise, and have very long time-scales because the collisions are so rare.
So how do you find out that they have mass? Well, for a long time it was noticed that the number of the most common flavor of neutrino produced by the Sun and detected on the Earth was much lower than the expected values predicted using known nuclear fusion reactions. How could they disappear on their way to the Earth?
According to our theories of particles, neutrinos come in three kinds (called flavors) and this anomaly could be explained if neutrinos had non-zero masses because then one flavor could change to another via the mass matrix, a process known as neutrino oscillations. The two groups headed by these researchers showed that neutrinos did indeed switch flavors in the expected amounts, thus resolving the problem of missing neutrinos. In the process, this implies that neutrinos must have some mass, however small.
I’d only add that neutrinos interact as definite flavors (electron neutrino, muon neutrino, tau neutrino). But particles propagate as definite mass states*. If the flavor and mass ‘axes’ are slightly askew, an electron neutrino, for example, is a particular combination of the mass states. If the masses are different, the three states propagate with different phases, so become different combos as they travel. This varying combo of mass states can be expressed as a varying combo of flavor states, and there is a finite probability that, when the neutrino interacts again, it will no longer be an electron neutrino. Graphs showing probability versus (distance/energy) here.
*Or as coherent superpositions of definite mass states.
Thanks, Mano, I needed that explanation!
So what does this mean -- and I offer no guarantee that I’ll understand the answer 🙂 -- regarding gravity?
This is also helpful though with little re gravity or “dark matter.”
SC @3: I’ve seen estimates of the neutrino contribution to total energy density of much less than 1%, so “not much” would be the short answer, at least in the current cosmological epoch. I can try and find some links tomorrow.
SC@#3,
I agree with Rob that the neutrino mass is not likely to have much effect on gravitational effects.
Another thing: if neutrinos have mass, then they can spin
both ways!.
People said it wasn’t true but I always knew those neutrinos had mass and were the reason I can’t lose weight.
Thanks for the responses, Rob and Mano.
…Now I’m imagining The Neutrino Diet by some charlatan featured on Dr. Oz.
Good one, SC. Here’s a couple more:
One weird particle that scientists use to shed the pounds!
Secrets of Slimming: What Physicists Don’t Want You To Know!