Why do we exist? Why does anything exist? Last week scientists reported that we may owe our presence to faint, wispy particles – neutrinos.
In the beginning, a massive amount of particles were created in the Universe. In our current understanding of physics, particles should be created along with antiparticles – sort of like an evil twin of a normal particle – being alike in every way except in its electric and magnetic properties.
If this happened, every proton would be created along with an antiproton. An electron would be created along with an antielectron. But then something tragic would happen. Every particle would collide with its antiparticle, annihilating each other in a bright burst of energy. If the same number of particles and antiparticles existed, all the antiparticles would annihilate the particles and nothing would be left.
Obviously, this hasn’t happened. We see stars and planets, you and I.
Instead, the Universe luckily started out with just a bit more matter than antimatter. As the matter and antimatter annihilated, a bit of matter was left over. It’s a little happy fact of physics that we can thank for our existence.
Meet the Ghost: The Neutrino
Smaller than the electron – smaller than the quark – the neutrino is a particle that seems to be ambivalent, not really caring about the matter that it encounters. In fact, near 100 trillion neutrinos pass through you every second. But you could never tell. This near-massless particle is incredibly hard to detect.
Even though you may never know they’re there, they could be the reason you are here. It’s all because neutrinos break the “laws” of the Universe.
Various symmetries can occur in this Universe. Two in particular are charge (C) symmetry and parity (P) symmetry. If a particle has C-symmetry, if it was switched with its antiparticle, thus changing its charge, the laws of physics would remain the same. If it has P-symmetry, it could change to its mirror image.
But neutrinos break both of these symmetries. We only see left-handed neutrinos (imagine a neutrino spinning clockwise) but no right-handed neutrinos, breaking P-symmetry. Furthermore, we only see right-handed antineutrinos, breaking C-symmetry. However, if we combine C and P-symmetry, we get CP-symmetry. Look at a left-handed neutrino in the mirror while changing it to an antiparticle and you get a right-handed antineutrino – which does exist.
This is the first piece of the puzzle. Neutrinos follow CP-symmetry.
Or do they?
There are three types of neutrinos – electron, tau, and muon neutrinos. Often called “flavors”, you can think of them like chocolate, strawberry, or vanilla. But a chocolate neutrino is not always chocolate. Instead, it’s a superposition of all three flavors – sort of like Neapolitan ice cream. As the neutron travels, it oscillates between these three flavors. This ability to switch flavors is the second piece of the puzzle.
Putting the Puzzle Together
If we put these two pieces together, we should expect neutrinos and antineutrinos to oscillate between their three flavors in the same way. This is what the T2K Collaboration, a group of 500 physicists, set out to understand. In their recent article in Nature, they reported on how neutrinos and anti-neutrinos behave. In their experiment, a beam of neutrinos was generated by the Japan Proton Accelerator Research Complex. The resulting neutrino beam traveled almost 300 km underground to a detector under Mount Ikeno, where the resulting neutrinos were detected. By doing this, the researchers could see how often one type of neutrino changed along its voyage.
They found something very interesting. Neutrinos oscillate between their three flavors faster than antineutrinos.
This shows that neutrinos break CP-symmetry after all. And it points to the fact that there is an asymmetry between matter and antimatter.
We already see a CP-imbalance in quarks. This imbalance, however, is not enough to explain why matter “won out” in the early Universe. While the experiment needs to be verified at a higher confidence (the current confidence is 3-sigma), their results could indicate that we all owe our existence to these tiny, ghostly particles.