When you are your own opposite

I was reading in a physics magazine earlier in the week about the nature of neutrinos. These are extremely numerous elementary particles, but only interact very weakly with anything.  Around a million billion pass through you each second, almost all originating from our sun, but few of them are likely to interact with you enroute. A typical neutrino will travel through space for many many lightyears before it interacts with anything.  Consequently studying neutrinos is pretty tough science. There are a few experiments in the world to detect neutrinos, such as the IceCube detector at the South Pole (Don’t you love the name?) Here’s Jenni Adams from the University of Canterbury talking about what it takes to detect them.

I first came across neutrinos in my undergraduate studies in a Particle Physics course. Here they were discussed in terms of beta-decay, one of the more common forms of radioactive decay. We see beta-decay for example with the element yttrium (I love that name too). The particular isotope of yttrium with 39 protons and 51 neutrons in its nucleus is unstable – a neutron is able to change to a proton and emit an electron (the ‘beta particle’) and an anti-neutrino.

While I knew about protons, neutrons and electrons, the neutrino (or in this case, anti-neutrino) was something new.

Now, we don’t know much about neutrinos, really, because they are so hard to observe. We know there are three types of them, and that they have a very small mass (though what the possible values of mass are is only partly known), that they have no electric charge, and they have a ‘spin’ of 1/2.  And we know that a neutrino must have an ‘opposite’ version – an anti-neutrino, because that’s how stuff works in this universe. Anti matter is much the same as matter, it’s just an ‘opposite’. If we bring together a particle and its antiparticle, they can annihilate each other in a puff of energy.  It’s a staple of science fiction stories, but it’s also highly useful science fact – for example, anti-electrons (positrons) are produced during positron emission tomography (the ‘PET’ scan now commonly found in hospitals).

But, what I found out while reading my physics magazine earlier in the week, was that no-one is yet sure whether or not a neutrino is its own antiparticle. That is, there’s the possibility that a neutrino and an anti-neutrino are actually the same thing. This isn’t crazy, for example a photon (a ‘particle’ of light) is its own antiparticle. If this is the case, a neutrino would be the first and probably only example of an elementary particle being a ‘Majorana particle’ – a particle of non-integer spin being its own antiparticle.

There is a potential way of finding out. There are some elements that undergo,very occasionally, ‘double-beta’ radioactive decay, where two neutrons change to protons and emit electrons and anti-neutrinos simultaneously. Now, if the neutrino is its own antiparticle, two anti-neutrinos could annihilate each other. Thus, if one observes double-beta decay without any neutrinos one can conclude that they are their own opposite. It’s an extremely hard experiment to do, but there are research groups trying just that.

In practical terms, it’s unlikely to mean very much to our daily lives, but for our understanding of the universe it would be very significant indeed.







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