Invisible wisps of matter, around us and through us
In my June article, I briefly mentioned the Standard Model of particle physics that describes the fundamental particles and how they interact. Today, I want to focus on the neutrino – a ghostly particle that plays an important role in understanding our universe.
The Standard Model says that ordinary matter is primarily composed of two quarks, up (u) and down (d), electrons and their partner, electron-neutrinos. A proton is two u-quarks and one d-quark, and a neutron is two d-quarks and one u-quark. Protons and neutrons combine to form nuclei, and with the electrons, atoms. But what is the role of the electron-neutrino?
I told you about the Weak force, which is the force responsible for radioactive decay where one particle can change into another. For example, in a nucleus, a neutron can transform into a proton and emit an electron. This is referred to as beta-decay.
Note that electric charge is a conserved quantity, so the total charge must be the same before and after an interaction. Energy must also be conserved.
Solving the mystery
About 100 years ago, it was observed that the electron energy in these decays varied. This violated energy conservation and caused great concern.
To “solve” the problem, Wolfgang Pauli hypothesized the existence of another particle that would share the energy. It had to be neutral, very light (or no mass at all) and hardly interact with matter, since it was not observable. Sometime later, Enrico Fermi named it the “neutrino,” which means “little neutral one.”
It turns out that there are lots of neutrinos. But due to the very low interaction rate with matter, it took physicists 26 years to confirm their existence. The chances of a neutrino interacting as it travels through the Earth is about 1 in 100 billion.
We now also know that anti-neutrinos exist, and, in fact, it is an anti-neutrino that is emitted in neutron decay.
A charming story
The Standard Model contains more than the readily observable particles mentioned above. The up and down quarks, the electron and electron-neutrino constitute a “generation.” There is a second generation composed of so-called strange and charm quarks, a particle called a muon that is just like an electron but about 200 times more massive and a muon-neutrino. And finally, a third generation that includes the “top” and “bottom” quarks, the tau (heavier still than the muon) and the tau-neutrino. So, we have three types or “flavors” of neutrinos.
There is no mechanism in the Standard Model that suggests that neutrinos have mass, but it was confirmed quite recently that indeed they do. This result was largely driven by what was referred to as “the Solar Neutrino Problem.” The nuclear interactions in the sun generate a mind-boggling flood of electron neutrinos. Every second, tens of billions of neutrinos are streaming through your body.
In the 1960s, scientists calculated and then experimentally tried to verify the number of neutrinos coming from the sun. The experiment only observed about one-third of what was theoretically predicted. Of course, initially they suspected the experiment and/or the calculation. This went on for the last half of the 20th century.
It was suggested that one way to “solve” the problem was that neutrinos “oscillated,” changed flavors, on their trip from the sun to the Earth. It took 30 years to confirm that this was the case.
Still more questions
The study of neutrinos has led to four Nobel prizes and questions still remain. Neutrinos have mass, but what is the hierarchy? Does mass increase with each generation like the quarks? The only difference between matter and antimatter is electric charge. Neutrinos are neutral, so is the neutrino its own anti-particle? If so, how do we distinguish between the two? Perhaps there is a new symmetry that would be outside the Standard Model?
Such is the world of physics. Answering one question only leads to many more.
For more information on neutrinos, check out https://icecube.wisc.edu/outreach/neutrinos.
Steve Gourlay is a career scientist with a PhD in experimental particle physics. He recently retired after working at the Fermi National Accelerator Laboratory, CERN (the European Center for Nuclear Research) and the Lawrence Berkeley National Laboratory. Send questions and comments to him at sgpntz@outlook.com.
Steve Gourlay
Steve Gourlay is a career scientist with a PhD in experimental particle physics. He recently retired after working at the Fermi National Accelerator Laboratory, CERN (the European Center for Nuclear Research) and the Lawrence Berkeley National Laboratory. Send questions and comments to him at sgpntz@outlook.com.