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Top Comments: My Very Brief Career as a Particle Physicist [1]

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Date: 2023-02-19  

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Nearly 40 years ago, when I was in graduate school, I shared an apartment with another graduate student who had just switched fields from particle physics to planetary sciences. Despite the fact that I was pursuing a graduate degree in chemistry, I read popular accounts of advances in particle physics, and discussed them with my apartment-mate.

At the time, there was a problem regarding solar neutrinos, that is neutrinos emitted by the Sun as products of nuclear processes taking place at its core. Neutrinos were originally conceived as massless and electrically neutral particles that are produced in the process of nuclear decay brought about by the weak nuclear force. As massless particles, they would travel at the speed of light. They were originally invoked to fix an apparent violation of the conservation of angular momentum in these nuclear decay processes. For example, a neutron, left to its own devices, decays into a proton and an electron. All three of these particles carry a spin angular momentum of ½. The process as described starts with a single particle with spin ½, and then ends with two particles with spin ½ + ½ = 1, which violates angular momentum conservation. Physicist Wolfgang Pauli proposed the existence of the neutrino specifically as a particle that would make up the difference. So if the the neutrino created by the process had a spin of ½, but was oriented in the opposite direction of the other two particles, you get ½ + ½ — ½ = ½, and angular momentum is again conserved.

Because neutrinos respond only to the weak nuclear force, they pass through most matter as though it wasn’t there. Approximately 100 trillion neutrinos pass through you every second, but you’d never know it because nothing happens as they pass. As such, they are a convenient and immediate probe into what’s going on at the Sun’s core, because they emerge promptly, unlike any other sort of signal. Even light generated in the core takes many years to get out due to its scattering within the Sun. The problem was that the number of neutrinos observed to be emitted by the Sun was much smaller than the number predicted by theory, too small by a factor of three, and theorists were scrambling to try to figure out why this was. This was the solar neutrino problem.

Now there are three different “flavors” of neutrino, each associated with one of the three electron-like particles that had been discovered by that time: In order of increasing mass, these are the electron, the muon, and the tau particle. Each of these has an electrical charge of -1 and responds to the weak nuclear force, but not the strong nuclear force. Each specific neutrino only interacts with its particular type of electron-like particle: the electron, and the electron neutrino, the muon and the muon neutrino, and the tau particle and the tau neutrino. The neutrinos generated by the Sun were all expected to be electron neutrinos, because those are the dominant negatively-charged particles within the Sun, and so detectors for solar neutrinos were designed to detect electron neutrinos. However, an idea floating among theorists at the time was called “neutrino oscillation,” which conceived that as neutrinos travel through space, they can change their flavors among the three possible ones. Somebody then realized that such behavior could account for the missing solar neutrinos: an electron neutrino produced in the Sun’s core would have equal chance of being any one of the three different flavors by the time it got to Earth, thus explaining why detectors for just electron neutrinos were only seeing one-third as many as they were expecting.

However, there is a consequence to neutrino oscillation: If neutrinos oscillate between their different flavors, theory requires that they have mass, contrary to how they were originally conceived (as massless). That mass is a very small one (indeed, so small that 40 years later, it still hasn’t been measured), but it doesn’t change the implication that these particles must be traveling at something slightly less than the speed of light.

In the original conception of the neutrino, the only difference between a neutrino and an antineutrino was its spin. A neutrino has a left-handed spin, that is the spin vector is pointing in the direction opposite to that of the particle’s motion, while for an antineutrino, the spin vector is pointing in the same direction as the particle is moving. So back almost 40 years ago, as I was thinking about all this, I had an epiphany. If the neutrino has mass, and its speed is less than the speed of light, then, in principle, it is possible to move faster than the neutrino in its direction of motion. If you manage to do that, the neutrino will appear to be moving in the opposite direction from what is observed while (relatively) motionless. This means that the direction of the neutrino’s spin relative to its motion is reversed, making it an antineutrino! In other words, the neutrino is its own antiparticle!

So when I next saw my apartment-mate, I very excitedly declared “If the neutrino has mass, that means it’s its own antiparticle!” His response: “Yeah, but so what?” I was crestfallen. I thought I had independently discovered an important detail regarding neutrinos, but apparently not in his judgement. Perhaps it will come as no surprise that he ultimately became a professor at Princeton while I became a professor at Nowhere State University. But I found it gratifying when, many years later, I discovered that I wasn’t the only person concerned about this point, and that the question has not yet been fully settled. Still, it’s best I leave particle physics to the physicists.

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