This deserves a Nobel prize
This is a picture of Super-Kamiokande
, one of the neutrino detectors that won this year's physics Nobel prize. It's a tank buried 1 kilometer deep in a mine in Japan. The tank holds 50,000 tons of ultra-pure water, surrounded by 11,146 machines that can detect tiny flashes of light. When a neutrino zipping through space happens to hit a water molecule, it makes a flash of light --- and Super-Kamiokande records it.
Here you see some people on a raft working on the detectors. The winners of the Nobel prize were Takaaki Kajita
and Arthur B. McDonald
, who worked at another neutrino detector in Canada. But these big experiments involve huge teams of people!
These teams, and their machines, deserve a Nobel prize because they proved something we'd begun to suspect much earlier.
There are 3 different kinds of neutrinos: electron, muon and tau neutrinos. Nuclear fusion in the Sun makes electron neutrinos... but we saw only about 1/3 as many as expected. This made physicists suspect that electron neutrinos were turning into the other 2 kinds of neutrinos as they went from the Sun to Earth.
But proving this was very hard. And it's only possible if neutrinos have mass!
You see, time doesn't pass for a massless particle, since special relativity says time slows down for you when you're moving fast, and it comes to a halt if you're moving at the speed of light. So, a massless particle can't turn into something else until it hits another particle.
As early as the 1950s we knew that neutrinos were almost
massless. So, we thought they were
massless. But now, thanks to these experiments, we know neutrinos really do change from one kind into another. So, we know they have a tiny but nonzero mass.
Here's what the Nobel prize committee says about it:The discovery that neutrinos can convert from one flavour to another and therefore have nonzero masses is a major milestone for elementary particle physics. It represents compelling experimental evidence for the incompleteness of the Standard Model as a description of nature. Although the possibility of neutrino flavour change, i.e. neutrino oscillations, had been discussed ever since neutrinos were first discovered experimentally in 1956, it was only around the turn of the millennium that two convincing discoveries validated the actual existence of neutrino oscillations: in 1998, at Neutrino ’98, the largest international neutrino conference series, Takaaki Kajita of the Super-Kamiokande Collaboration presented data showing the disappearance of atmospheric muon-neutrinos, i.e. neutrinos produced when cosmic rays interact with the atmosphere, as they travel from their point of origin to the detector. And in 2001/2002, the Sudbury Neutrino Observatory (SNO) Collaboration, led by Arthur B. McDonald, published clear evidence for conversion of electron-type neutrinos from the Sun into muon- or tau-neutrinos. These discoveries are of fundamental importance and constitute a major breakthrough.
I would put it this way: in the old
Standard Model, neutrinos were massless. In the new improved Standard model, they have a nonzero mass.
In fact, there's a whole 3 × 3 matrix of numbers, the neutrino mass matrix
, which says what neutrinos do as they're flying through empty space. These numbers actually say how the neutrinos interact with the Higgs boson. This determines their masses, but also how the 3 kinds turn into each other.
We don't know why the numbers in this matrix are what they are. We may never know. But maybe someday someone will figure it out. Physics is full of slow-burning mysteries like this.
For the full story, go here:http://www.nobelprize.org/nobel_prizes/physics/laureates/2015/press.html
The neutrino mass matrix is also called the Pontecorvo–Maki–Nakagawa–Sakata matrix
. In 1962, right after the muon neutrino was discovered, Ziro Maki, Masami Nakagawa and Shoichi Sakata speculated that electron and muon neutrinos could turn into each other, and invented a 2 × 2 matrix to describe this. And even earlier, in 1956, Bruno Pontecorvo had considered the possibility that neutrinos and antineutrinos could turn into each other.
If you want to see the numbers in this matrix, go here:https://en.wikipedia.org/wiki/PMNS_matrix #physics