An Inconvenient New Neutrino?

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 down 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.

—John Updike, “Cosmic Gall”

The Standard Model, which is a theory of nearly everything (gravity being an exception) likes things in threes. There are, for example, three colors of quarks, just as there were three neutrinos types—until very recently: research conducted by the Mini Booster Neutrino Experiment (MiniBooNE) at the Fermi National Accelerator Laboratory in Batavia, Illinois, suggests the possible existence of a fourth type of neutrino that does not fit into any theoretical model.

Neutrinos have always been my favorite particles, ever since I learned about them in graduate school in the 1950s. They reminded me of that eccentric uncle you’ve never seen, but who you know exists because your aunt speaks of him affectionately as a slightly rogue member of the family. Even the invention of neutrinos is a bizarre tale.

It had been known before 1930 that some nuclei can decay into a daughter that carries one more positive charge along with an electron that carries away the related negative charge (that is, a neutral neutron turns into a positively charged proton, and an electron, with a negative charge, then restores the charge balance). These were the only two visible decay products. But instead of emitting a single predictable energy when the nuclei decayed, the electrons emerged with a spectrum of energies. This was utterly baffling and some very celebrated physicists, such as Niels Bohr, proposed that energy might not be conserved in this process—a very radical idea.

Wolfgang Pauli was an eccentric and often acerbic genius, who once damned another scientist’s paper by saying it was “not even wrong,” and dismissed an aspiring physicist with the words: “So young and already so unknown.” There was a meeting of scientists in Tübingen to which Pauli addressed the following letter, dated December 4, 1930. Some of it is rather technical, but relish the flavor!

Dear Radioactive Ladies and Gentlemen,
As the bearer of these lines, to whom I graciously ask you to listen, will explain to you in more detail, how because of the “wrong” statistics of the N and Li⁶ nuclei and the continuous beta spectrum, I have hit upon a desperate remedy to save the “exchange theorem” of statistics and the law of conservation of energy. Namely, the possibility that there could exist in the nuclei electrically neutral particles, that I wish to call neutrons, which have spin 1/2 and obey the exclusion principle and which further differ from light quanta in that they do not travel with the velocity of light. The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton masses. The continuous beta spectrum would then become understandable by the assumption that in beta decay a neutron is emitted in addition to the electron such that the sum of the energies of the neutron and the electron is constant…
I agree that my remedy could seem incredible because one should have seen these neutrons much earlier if they really exist. But only the one who dare can win and the difficult situation, due to the continuous structure of the beta spectrum, is lighted by a remark of my honoured predecessor, Mr. Debye, who told me recently in Bruxelles: “Oh, it’s well better not to think about this at all, like new taxes.” From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge.
Unfortunately, I cannot appear in Tübingen personally since I am indispensable here in Zurich because of a ball on the night of 6/7 December. With my best regards to you, and also to Mr. Back.
Your humble servant,
W. Pauli

I will not try to explain the matters of spin and statistics, but will focus on mass, charge, and energy. I will also explain Pauli’s name for his new particle, which did not stick. First, the hypothetical particle could not have a charge because all the charge was carried off by the electron. It had to have a very small mass because, otherwise, energy conservation would not allow the decay at all. But since there were now three particles in the final state, one could have a spectrum of electron energies. Pauli did not think that this particle, which he called the “neutron,” could ever be detected, so he never published the contents of the letter.

The scene shifts to Enrico Fermi in Rome. Fermi was the first one to take this idea seriously enough to actually devise a theory. In Fermi’s theory, the electron and neutrino do not pre-exist in the nucleus, but are created at the time of the decay. This is the theory we have used ever since. In Italian, the neutron was known as the neutrone—the big neutral one—so Fermi called it the neutrino, the little neutral one.

For the next several decades—and this was true when I took that course in graduate school—the neutrino was treated as a speculation. Indeed, in their classic text of 1952, John Blatt and Victor Weisskopf wrote: “The fact that the neutrino escapes observation is unfortunate but not unexpected.” But this all changed in 1956 when Clyde Cowan and Frederick Reines announced the observation of neutrinos. They made use of the huge flux of neutrinos—strictly, they are anti-neutrinos, but that need not concern us here—produced by a nuclear reactor, in which the fission fragments are radioactive and can undergo electron decay. This gigantic flux of neutrinos escaping from the reactor can be used to create very rare but observable neutrino interactions. In normal conditions on the Earth’s surface, there is but a tenth of this flux (still a huge amount)—which is the fact that inspired Updike’s poem.

The next great advance came from the Sun and involved the astrophysicist John Bahcall and the experimenter Raymond Davis. The nuclear reactions that make the Sun shine produce a neutrino flux, as I have indicated, about a tenth of the reactor’s. But how to detect it? This was the genius of the late experimental physicist Ray Davis, whom I knew when we were colleagues at the Brookhaven National Laboratory. Starting in the mid-1960s, he put a huge tank of what amounted to cleaning fluid 5,000 feet down the Homestake Mine in Lead, South Dakota. The basic component of the cleaning fluid is chlorine, and he hoped that, from time to time, a neutrino would interact with a chlorine molecule and turn it into an argon molecule. The reason to place the tank so far underground was to shield it from influences like cosmic rays. But what to expect?

Here is where Bahcall came in. He was an expert on models of nuclear reactions in the Sun and could predict how many events Davis should expect. Sure enough, there were events, but only about a third of the number that had been predicted. How could this be explained? By this time, it was known that there were two “flavors” of neutrinos—one emitted with electrons, and one emitted with muons, which are, for all practical purposes, heavy electrons. It had also been noted by theorists that if these neutrinos had mass, they could oscillate into each other. But a muon neutrino cannot make Davis’s transformation, which produces an electron along with the argon—as the effect was explained, though it makes no sense to me. But if there were two kinds of neutrinos, could there be more? Indeed, a third type was found in the 1970s and is associated with another particle called the tauon.

The recent Fermi Lab experiment has things in common with the one Ray Davis conducted. Instead of using cleaning fluid, MiniBooNE’s scientists use some 800 metric tonnes of mineral oil. The neutrinos—muon neutrinos, in this case—are supplied by the decay of an elementary particle called the K meson (or kaon). What is measured are the reactions caused when these particles oscillate into electron neutrinos. The claim is that there are more neutrinos than the Standard Model predicts—and this is the possible evidence of a new neutrino.

When the muon was discovered, in 1936, the then-future Nobel laureate I.I. Rabi famously asked, “Who ordered that?” The Standard Model is a tight mathematical structure, with a specific symmetry, that simply does not allow for another neutrino. If these results from MiniBooNE prove correct, it will once again show just how odd neutrinos are. But as the Italians say, Se non è vero, è ben trovato. Even if it’s not true, it’s a good story.