The Nature of the Neutrino
Janet Conrad is in the business of chasing ghosts, but theyre not ordinary specters. With the mysterious squarely in her sights, the Columbia physicist is working to unravel the secrets of a world that is all around us yet completely transparent, an intricate part of the universe but one just beyond the frontier of human knowledge.
Conrads target is the neutrino, a wispy, subatomic particle whose true nature has eluded physicists for decades. By way of elucidation, Conrad and her colleagues have built an instrument that hopes to capture some of the intricacies of the neutrino, to catch it changing partners in its elementary dance through the universe. By observing oscillations of the neutrino, spying it as it exchanges costumes from one character into another, Conrad and her experimental collaboration hope to help settle basic questions about the neutrino and its place in the cosmic fandango.
But that discovery was only the beginning. Soon it was found that neutrinos come in two different flavors, the electron neutrino and the muon neutrino, distinguished by the nuclear reactions that give rise to their creation. Two years ago, a third type of neutrino was found, long expectedthe tau neutrino. Together, physicists refer to the three flavors of neutrinos as a family.
A lot of people have questioned it, but theyre never been able to find a reason why it was wrong. . . . For a long time people said, Well, we dont know what to do with this result, so well have to throw it out and not include it in our study. But thats obviously not the right way to handle physics.
Conrad and her Columbia colleague Mike Shaevitz decided to pursue the controversial signal. Together they developed an idea for a new neutrino experiment and joined forces with some scientists from the Los Alamos experiment.
Mini-BooNE is a large detector set up to detect neutrino oscillationsspecifically, a change of the muon neutrino into an electron neutrino. Set at the Fermilab National Accelerator Laboratory in Illinois, it consists of a forty-foot sphere filled with 800 tons (250,000 gallons, or thirty tanker trucks) of mineral oil.Basically, says Conrad, its baby oil.
Conrad explains neutrino oscillations with an analogy. If two flutes play the same note but one is slightly mistuned, youll hear a reverberating harmonic sound that Conrad mimics as a wah-wah. Or strike a tuning fork, and strike another that is identical except for a tiny piece of gum on its tines. The added mass of the gum will alter the forks frequency, and together the two forks will produce their own wah-wah. In the quantum world, where particles act like waves, the oscillating noise represents the neutrinos oscillations, and the added gum their mass difference. Its a property called wave interference, and its seen in waves as diverse as those of water, sound, and light. What were really seeing here, says Conrad, is an interference effect.
To search for neutrino oscillations, Mini-BooNE will look for a characteristic pattern of light inside the tank. A beam of muon neutrinos, produced by interactions of protons from Fermilabs Booster ring, will enter the tank. A precious few of the muon neutrinos will collide with the carbon atoms in the mineral oil and produce a highly energetic particle called a muon, which is essentially a heavy electron. The muon will produce light, called Cerenkov light, as it shoots through the mineral oil and out of the Mini-BooNE tank.
Over the course of a year, about a million of the vast number of muon neutrinos that enter Mini-BooNEs tank should collide with carbon atoms. However, a few of these muon neutrinos are expected to have oscillated before this collision. If the prior Los Alamos experiment is correct, about a thousand electron neutrinos will collide with the carbon atoms, producing a different Cerenkov light pattern that will serve as Mini-BooNEs important distinguishing signal.
The Cerenkov light is the key. Cerenkov light is a shock wave phenomenon produced by any particle that travels through a medium (mineral oil, in this case) at a speed faster than light would travel through the mediumthe electromagnetic equivalent of a sonic boom. (This does not violate Einsteins dictum that nothing can travel faster than the speed of light, which is true only in a vacuum.)
In Mini-BooNEs tank, any entering muon neutrino from Fermilabs muon neutrino beam will collide with a carbon atom and produce a highly energetic muon. This muon will hurry through the mineral oil, producing a cone of Cerenkov light that, in turn, will travel to the edges of the tank. There, an array of photomultiplier tubesessentially, inverse light bulbswill receive the light and produce an electrical signal.
Should any muon neutrinos in the muon neutrino beam have oscillated to electron neutrinos before entering the tank, they will, once inside the tank, collide with carbon atoms and produce, instead of a muons, electrons. These electrons will scatter (bounce around) because they are so much lighter than the escaping muons and will quickly come to a halt as they collide with mineral oil atoms. This produces a Cerenkov cone with fuzzy inner and outer edges, smeared by the bouncing around. After two years of taking data, the group hopes to have results by summer 2004.
If neutrinos do indeed oscillate, basic principles of quantum physics imply that they also have mass. Because there are so many neutrinos in the universe, even a small neutrino mass could constitute a significant portion of the mass of the universe. That excites cosmologists who study the universe at large, who have found that part of the mass of the universe is missing; their determination of the rate of expansion of the universe is too slow to jibe with the amount of matter they observe in the universe. Neutrinos could account for at least part of that missing mass.And if Mini-BooNE confirms neutrino oscillations, its results, together with previous findings from other experiments, would have yet another implication: a new, fourth flavor of neutrino. Such an enlargement of the flavor family would rock the field of high-energy physics.
We would find ourselves forced into a situation where we have to introduce a fourth neutrino, says Conrad. We know that the fourth neutrino cant be one that interacts the way all the other neutrinos do, or we would have seen it. Since we dont see it, that means that, if it exists, it must not interact. Physicists call such a neutrino sterile. In fact, Mini-BooNE might see such sterile neutrinos if their initial muon neutrinos simply disappear, having oscillated to the fourth flavor.
A fourth neutrino might confound experimental physicists, but it could be good news for theoretical physicists. A fourth flavor of neutrino, Conrad notes, routinely falls out of esoteric theories that try to unify the three microscopic forces (the nuclear force, the electromagnetic force, and the weak force responsible for radioactive decay) with the macroscopic force of gravity.
Conrad says the netherworld of particle physics, where particles oscillate and dance and, like Alice in Wonderland, nothing is quite what it seems, captures her imagination. I find the idea of these tiny universes that you create just so exciting. That there could be something so completely different if you go to very high energies and to very, very short time scales from the world that we actually live in. . . . I end up anthropomorphizing these particles, and they become a part of my life. A true physicist wouldnt have it any other way.
Janet Conrad and the Joy of Physics: