As with many features of particle physics, the science behind the neutrino is relatively contemporary. It was only in the 50’s that we first experimentally detected the particle and now 60 years on the neutrino holds enough weight (joke) to bag a Nobel Prize (2015 – Metamorphosis in the particle world). Today I offer a very brief introduction to our little friend, explain the ideas behind the 2015 Nobel Prize and discuss why neutrinos will continue to be an area of scientific focus. I find it terribly exciting how fashionable particle physics has become in recent decades; at a surface level it is far easier to be seduced by the big stuff since it looks so beautiful and is much more observable. The little stuff is shy, mysterious but fundamentally the material from which reality is constructed. Shall we?

What is a neutrino

The reason the neutrino skulked by undetected for so long, is because it almost does not exist at all. There is hardly anything to it (that’s where the -ino comes from, meaning small). Yet despite the low key presence of the particle, it’s abundance is truly staggering – it is estimated that if you hold your hand to sunlight for one second a billion neutrinos will pass through your hand, more if you have fat hands of course. The neutrino is by far the most abundant particle in the universe, gliding through you right now and you don’t feel a thing. This is the raw beauty of particle physics – it’s like a whole new universe you have never seen, where the rules are totally different to your quotidian experiences taking place on a stage not parsecs away but at the end of your limb.

First we must define the features of the neutrino before we can get to the discussion. If the periodic table is the cookbook of the elements, then the standard model is something like the seeds to grow the ingredients. We have discussed the standard model in various different posts – today we focus on the bottom row of the leptons, where you can see three different types of neutrinos. Because someone somewhere in history decided the study of physics should be more whimsical, these different types are known as flavors; we are now wedded to this labeling. So the flavors of neutrino we have to play with are electron, muon and tau neutrinos (which are the only flavors we currently know of). As you can see above each of the neutrinos is a similar sounding particle – we have an electron and an electron neutrino, a muon and a muon neutrino and a tau and a tau neutrino. Don’t worry about the names – names are nothing more than a convenient labeling system to transport information from A to B. What we are interested in is the properties.

The Standard Model

All flavors of neutrinos listed above have the following properties:

• No electric charge (a key difference between an electron, and an electron neutrino);
• Exceptionally low (but not zero) mass;
• Half integer spin (see here); and
• Interacts via the weak interaction (and gravity).

It is precisely the properties – and moreover the differences in properties that allow us to detect when we have one particle or another. Much like when we look into a bag of marbles and see different colors we are viewing  essentially the same underlying thing with light reflecting differently, particles may well be the same. It is totally possible that we can unify all particles to be fundamentally the same thing, with the only real difference being the properties – different manifestations of the same thing. So I think it is healthy not to consider the properties of a particle as some set of features of another separable entity – but rather the properties are the particle.

Why do we care about neutrinos

The neutrino plays a fundamental role in many of the interactions so key to modern physics – including as discussed the nuclear processes which take place in the sun. There is so much we are yet to understand about the neutrino, the most abundant particle in the universe that it seems implausible that there are not some major scientific breakthroughs available from a deeper understanding. For one – it is theorized that there may well be another type of neutrino; the sterile neutrino. In short this neutrino would not be charged under the weak interaction, it would only interact via gravity. This would be very hard to detect – which is why we have our best minds trying to do it right now! Confirming the existence of this particle, and obtaining a deep understanding would heavily enrich our studies into dark matter and the overall formation of the universe. We need to find extra things hiding in the shadows tying the universe together. Not bad going.

Because of their ability to travel huge distances interacting with very little neutrinos are still knocking about from the big bang – quite amazingly! The fact that neutrinos can linger from reactions which eclipse any human history make them vital in the physical archaeology of the universe. Neutrinos are a gift from the past, a clue for those in the future to unravel. Currently detecting and studying neutrinos is innovative and cool, as shown in the feature photo to this post which is a neutrino telescope, a huge vat of water and photo-multipliers which detect the light given off when neutrinos pass through the water.

Flavor oscillations and the noble prize

We know exactly what particles can be created from certain reactions – it’s a discussion for another post, but there is a sort of accounting you can do where providing certain things always add up it is allowable. We also know, thanks to many different techniques what reactions are taking place in the sun. So it is very easy to use our theoretical models to calculate exactly what neutrinos will be created in the suns reaction and then detect them on earth. This is what we did and the flavors were all wrong – they were not in a ratio that could allow us to say the theoretical models were sound, unless something was missing. This is a huge problem – it is central in particle physics.

It was discovered that the missing information was flavor oscillation, or rather neutrinos can change flavor when they travel! There is a very small difference in velocity between neutrinos and this is what gives rise to this wonderful effect. As the neutrino travels it may change from one type to another, and this will increase as it travels further and further but then, after a time it will recede again back the other way. Not only is the flavor of the neutrino changing, it is oscillating sinusoidally from one to another. When you account for this strange effect, you end up with the exact right quantity of different types of neutrinos on Earth from the sun. Phew. It’s a bit like throwing an apple through the air and seeing it become an orange, then a peach and then back to an orange before becoming an apple again. I’m quite glad I am not quantum sized it sounds terrifying.

Closing remarks

Finally – apologies for fairly low levels of activity on the blog in recent weeks – RTU have been taking a well deserved break, which you can see below!