Closer To The Mind of God: Neutrinos Help In Answering One of the Greatest Problems In Physics

One of the greatest scientific questions of all times may soon have an answer and the key might be one of the lightest, most elusive particles known to science. Scientists claim to have got a signature a hint of the answer to the question Where did all the matter of the Universe come from?’ from looking at oscillations of a mysterious particle known as the neutrino.

Here’s the deal about neutrinos

Neutrinos are extremely light, neutral particles. They are generally found in nature moving close to the speed of light. They are extremely weakly interacting particles, and you have a million million million neutrinos going right through your body at this moment per minute. You need not care they are not harmful, interacting with other atoms extremely rarely.


They come in three types or, as scientists call it, flavors: electron, muon and taon. The electron neutrino is the lightest of the lot, with the muon being a bit heavier and the taon topping the charts. Due to its mass, the tao neutrino is hardly seen in nature and we shall be concerned with the electron and muon neutrino only.


In the long neutrino story, there were many crucial junctions, which we shall not be able to get into here. A debate as to whether neutrinos have mass was resolved when a proposed hypothesis neutrino oscillation was observed. Neutrinos did have mass and they did something akin to magic: an electron neutrino could turn into a muon neutrino.

Showing neutrino oscillation

The Kamiokande and the Super-Kamiokande are two of the experiments designed to measure this oscillation. This is where out story begins.

The Super Kamiokande detector. The walls are lined with optical detectors. The inside is filled with ultra-pure water

The T2K Experiment

A new experimental project on this called the T2K was setup in Japan. One of the aims of this was to see whether muon neutrinos became electron neutrinos. The experiment ran from January 2010 till it was rudely interrupted on 11th March 2011 by the Japanese Earthquake. The facility came to no harm and the data is significant enough to analyze. The results came out on the 15th of June, just a couple of days back.

What scientists found was startling. Not only could electron neutrinos turn into muon neutrinos, they could go the other way too something that was only speculated, not confirmed in an experiment. In a muon beam, they found as many as six events of electron neutrinos, while only about 1.5 was expected on average. This is an event rarer than one-in-a-hundred. However, for a discovery, the doubts have to go below the one-in-a-million level and, thus, we can call this nothing more than a signature’. However this is tantalizing.

What’s so tantalizing?

This possibility opens up new avenues of interaction, especially the possibility of asymmetric interactions between neutrinos and the anti-neutrinos (their anti-particles). If the interaction of neutrinos and anti-neutrinos are different on a fundamental scale, then this gives scientists an example of, what is known as, CP violation.

One of the longest outstanding questions of physics is why matter should outnumber anti-matter particles, while physics gives no hint of such an asymmetry at even the most fundamental levels. (That is to say, if we worked with anti-particles rather than particles, we’d use the same physics. Particles’ and anti-particles’ are just labels as far as laws of physics are concerned, just like matter’ and anti-matter’ and there is no unique way to differentiate between them.) CP violation provides the answer, hypothesizing that Nature preferentially creates one more matter particle for every billion or so pairs of matter-antimatter particles. After annihilation, one matter particle out of a billion is left behind and this can account for all the known matter in the universe. (So now, CP violation gives us a unique way to differentiate between matter and antimatter. Just call those particles matter particles’ that are produced in slight excess.)

Journey of a neutrino through to a T2K detector

Here’s the journey a neutrino would have to undertake in order to make it to the last detector in the massive T2K experiment:

  • Muon neutrinos are produced at the Japan Proton Accelerator Research Center in Tokai, Japan.
  • They pass through to a series of near detectors so that their composition can be known before they oscillate.
  • They then fly off and travel for 295 km across Japan to the Super-Kamiokande neutrino detector (which is a huge detector filled with more than 50,000 tons of ultra-pure water and lined with extremely sensitive optical detectors to detect the faintest of flashes). Here we can detect the composition of the beam after the neutrinos have oscillated.
The Super Kamiokande again. Neutrinos interact with the protons in the water and produce electrons or muons, which travel faster than light in water and produce Cerenkov radiation which forms a cone of light on the detector arrays.
Each of the dots represents a detector which has been triggered. Notice the arc of light, which is actually a part of a cone projected on a 2-D surface. This is the Cerenkov Radiation. (Credits: University of Tokyo)

Prof. Dave Wark, head of the UK group for the experiment gave a gem of a statement:

People sometimes think that scientific discoveries are like light switches that click from ‘off’ to ‘on’, but in reality it goes from ‘maybe’ to ‘probably’ to ‘almost certainly’ as you get more data. Right now we are somewhere between ‘probably’ and ‘almost certainly’.

The studies and the data obtained are not confirmatory, but they do provide tantalizing hints to what the answer to the big question of where all the matter in the Universe came from.

Published by

Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.