Why Does It Matter If Neutrinos Have Mass?

The Los Angeles Times reports that an experiment in Europe has confirmed that neutrinos have mass . The article suggests that this shows that massive neutrinos may account for a “large proportions” of mass in the universe. As with most science related articles in the mass media, this one involves a bit of hype. First, this experiment is simply a confirmation—albeit an important and very dramatic one—of a conclusion that was already practically certain from other indirect pieces of evidence. Second, the neutrino mass that has been confirmed is too small for neutrinos to be a particularly significant fraction of the mass in the universe. (This was also already known, since the neutrino mass in question was already known by the indirect methods.)

What are neutrinos and why does anyone care about their mass? Three kinds of particles have been observed so far and are the building blocks of the Standard Model of particle physics:

(1) There are particles called “gauge bosons” associated with the basic forces. Two particles exert a force on each other by exchanging one of these gauge boson particles (i.e. by one particle emitting a gauge boson and the other particle absorbing it). The exchange of gauge bosons is said to “mediate” the force. (Perhaps theologians will find that interesting.) Indeed, for all the forces except electromagnetism, two gauge bosons can interact with each other by exchanging a third gauge boson. In that case, gauge bosons are both the things interacting and the thing by which they interact—the things related and the thing relating them. Indeed, in the basic interation, whereby one gauge boson emits another (in which there are three particles involved: one in the initial state and two in the final state) there is a basic symmetry among the three particles involved, so that it is a matter of point of view which is being emitted and which is emitting. (Which may also interest theologians who look for analogies in nature of supernatural realities: ‘subsistent relations’ anyone?) But I digress!

(2) There are particles called “leptons”, which include the familiar electron. There are three “families” of leptons. In the first family there is the electron and the “electron neutrino”. In the second family, there is a particle just like the electron, except about 206 times heavier, called the “muon”, and also the “muon neutrino”. In the third family, there is a particle just like the electron and muon, except about 17 times heavier than the muon, called the “tauon” (aka “tau” or “tau lepton”), and also a “tau neutrino”. I will return to the neutrinos shortly.

(3) There are particles called “quarks”, which also come in three families. In the first family are the u and d quarks; in the second are the much heavier c and s quarks; and in the third are the yet much heavier t and b quarks. The u and d quarks are particularly important, because the protons and neutrons are mostly composed of them.

Why are there three families? No one knows. Ordinary matter needs only the first family particles (electrons and u and d quarks). It would seem that the world would not look much different if there were only one family. (However, the second and third family may play subtle indirect roles that greatly affect how the world looks.) Except for their large masses, the three families are Xerox copies of each other. The first particle discovered that belonged to one of the heavier families was the muon in 1937. The Nobel laureate physicist I.I. Rabi, greeted its discovery with the famous quip, “who ordered that?” a question inspired by dining at Chinese restaurants. To this day, no one knows the answer why there is this seemingly superfluous duplication—or rather triplication—of particles. The numerologically inclined theologian may be tantalized by the fact that the number three crops up repeatedly in fundamental physics: There are three families of quarks and leptons; three non-gravitational forces (strong, weak, and electromagnetic—gravity is very different from the other forces); three gauge bosons of the weak force; three ‘colors’ (analogous to electric charge) for the strong force; the electric charges of quarks are multiples of 1/3 the charges of leptons; there are three dimensions of space (or at least three “large” space dimensions).

Neutrinos are extremely elusive. Because they do not interact via the strong or electromagnetic forces, they ordinarily pass right through matter. This inspired John Updike to write an amusing poem about them many years ago:

Cosmic Gall

Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids through a drafty hall
Or photons through a sheet of glass.
They snub the most exquisite gas,
Ignore the most substantial wall,
Cold-shoulder steel and sounding brass,
Insult the stallion in his stall,
And scorning barriers of class,
Infiltrate you and me! Like tall
And painless guillotines, they fall
Down through our heads into the grass.
At night, they enter at Nepal
And pierce the lover and his lass
From underneath the bed-you call
It wonderful; I call it crass.

The New Yorker Magazine , Inc., 1960

Notice that Updike refers to neutrinos entering the earth at night from Nepal. What he is referring to is the fact that the Sun emits an enormous flux of neutrinos. So, during the day, “solar neutrinos” stream down on us, and at night, they stream up at us, having rained down on the other side of the earth and passed entirely through it. There are two other kinds of neutrinos hitting us all the time. The universe is filled with neutrinos—roughly the same number as there are particles of light. These come at us equally from every direction. There are also lots of neutrinos produced in the atmosphere when highly energetic particles (e.g. protons) from outer space (“cosmic rays”) impinge upon the earth, as they do in copious quantities. These are the so-called “atmospheric” neutrinos”.

If neutrinos pass right through matter, how does one ever detect them? After all, detectors are made of matter, and the neutrinos should pass right through them too. The answer is that neutrinos interact extremely weakly, but do interact. So the vast majority do pass through matter, but on rare occasions one does interact with ordinary matter. (This is governed by quantum mechanics, so that which one interacts is a matter of chance.) By having a huge amount of ordinary matter in one’s detector, one gives neutrinos more opportunities to interact. That is why neutrino detectors typically contain thousands of tons of water or metal or some other material.

In the so-called Standard Model, which is now about 40 years old, neutrinos are massless particles (as “photons” and “gravitons” are also believed to be). Hence John Updike’s statement that “they have no mass”. For a long time, however, theorists speculated that neutrinos may actually have a tiny mass. If neutrinos have mass, then a very curious phenomenon becomes possible: one type of neutrino (say, a “tau neutrino”) can slowly morph into another type (say, a “muon neutrino”). In fact, neutrinos should morph into each other and back again in a periodic way—hence these are called “neutrino oscillations”

Since the 1960s there have been hints in the data that this effect was happening in “solar neutrinos”. Since people understood fairly well how the Sun burns, i.e. the nuclear reactions that power the Sun, they could calculate the rate at which neutrinos are streaming out of the Sun. (This is closely related to how much light is coming out of the Sun.) They started measuring the “flux” of solar neutrinos in the 1960s using the large detectors I was talking about. They saw only about half as many neutrinos as should have been there. An explanation suggested by theorists at the time was that en route from the center of the Sun to the earth about half the solar neutrinos (which start off as electron neutrinos) “oscillated” into muon neutrinos, which the detectors of that time could not see. This turns out to be correct, but because of uncertainties in the precise details of how the Sun burns, this remained unconfirmed until the late 1990s. Meanwhile in 1998 another big discovery was made: In the so-called “atmospheric neutrinos” I mentioned earlier, many muon neutrinos were oscillating into tau neutrinos. These two results showed that neutrinos have non-zero (but tiny) masses. The mass of the heaviest neutrino is roughly 10 million times lighter than an electron.

This fact tells us that there is something going on that lies beyond the original Standard Model. The most likely possibility is that these neutrino masses come from processes that are predicted to exist in so-called “grand unified theories”—theories that unify the three non-gravitational forces. They are therefore windows onto a realm of phenomena that are very hard to get any direct information about. That is one reason why theorists are very interested in them.

What have the experiments the LA Times is talking about now seen? The oscillations of muon and tau neutrinos have (it is reported) now been seen directly in the laboratory. They produced a beam of muon neutrinos (which came from the decay of “muons”) at an accelerator (at CERN) and allowed that beam to propagate for many miles until they were largely morphed into tau neutrinos. Some of those tau neutrinos then interacted and produced “tau leptons” (remember: a heavier version of the electron). These tau leptons were then observed. This was a very direct confirmation of what was shown by more indirect means in 1998.