The last decade has seen the beginning of a revolution in our understanding of the most elusive of the fundamental particles: the neutrinos (ν). Currently, particle physics understands the universe in terms of the "Standard Model," which tries to describe everything in terms of twelve fundamental particles and four forces. The theory is known to be incomplete, but has been frustratingly good at predicting the results of experiments. We now believe that neutrinos may be the rebels we need to break the iron grip of the Standard Model and lead us to new physics.
The Standard Model’s twelve fundamental particles are divided into six quarks and six leptons. Quarks are never alone and bind together to form such particles as protons and neutrons. In contrast, “lepton” is Greek for loose change, and, accordingly, you will find them off by themselves. Three of these leptons have electric charge: the electron, the muon, and the tau. The other three are the neutral neutrinos. They come in three flavors, corresponding to their charged partner: the electron neutrino, muon neutrino, and tau neutrino.
Of all the fundamental particles, neutrinos have the elusive nature that makes them good candidates for the rebels we need. Through simple energy conservation, we have never observed their mass, so in the Standard Model neutrinos are mass-less. Because they have no electric charge, they can only be detected when they interact through the weak force and produce their charged partners. However, as its name implies, the weak force behind these interactions means that very few interactions occur. Therefore, you need multiple tons of material to detect a handful of neutrinos per day.
Little wonder, then, that in the 1930’s Pauli decried, “I have done a terrible thing. I have invented a particle that cannot be detected.” However, if we could observe a quantum effect called “neutrino oscillation,” we would then confirm that neutrinos have mass, challenging the Standard Model. The observation of this effect would be the equivalent of dumping tea into Boston Harbor: a great start to a revolution.
"Our rebel neutrinos have thrown the tea into the harbor; proof of neutrino oscillation is proof that they have mass, which goes beyond the Standard Model as we know it."
Neutrino oscillation is a manifestation of the dual wave and particle nature of matter, the hallmark of quantum mechanics. A neutrino particle of a particular flavor is created, but what travels through space is a neutrino matter wave composed of three waves with very close frequencies. Think about a symphony tuning up and two violinists playing the same note slightly out of tune; you hear that note get louder and softer as the sound waves interfere. In neutrino oscillation, we define the electron neutrino as when the note is loudest. A person farther away will, however, hear another combination of waves, with a slightly different flavor composition.
So, we have three neutrinos and three neutrino mass states, or, in other words, three matter waves, which interfere. The three free parameters of the theory are the “mixing angles” that define the amount of each flavor a particular matter wave has. Experiments finally became sensitive enough in the last decade to detect enough neutrinos to observe neutrino oscillation, and two of the mixing angles have been measured. Our rebel neutrinos have thrown the tea into the harbor; proof of neutrino oscillation is proof that they have mass, which goes beyond the Standard Model as we know it. So, now that we can push this boundary, what next? The answer is tied up in the third mixing angle and a possible difference in the oscillation of neutrinos and antineutrinos. The greatest failing of the Standard Model is that it cannot explain why there is more matter than antimatter in the universe. Physicists are particularly interested in this fundamental problem, and many believe that differences in neutrino and antineutrino interactions are responsible for the matter/antimatter asymmetry. Observation of these differences would be the equivalent of the shots fired at Lexington and Concord. Unfortunately, the last mixing angle must be large enough to make the measurement feasible, and many theories say that it will be too small.
The Conrad group is part of the team that built Double Chooz, which is the first in a series of next generation experiments specially designed to measure this last mixing angle. As the source of electron-flavored antineutrinos, it uses two of the world’s most powerful nuclear reactors, found at France’s Chooz Nuclear Power Plant.
The Double Chooz detector is centered around eight tons of mineral oil doped with 0.1% gadolinium. The antineutrinos from the power plant are detected when an antineutrino hits a proton making a positron (an antielectron) and a neutron. The positron annihilates with an electron, creating one flash of light. The neutron is then captured by a gadolinium nucleus within about 30 microseconds, creating a second flash of light. This process is called “inverse beta decay,” and not much can mimic its signature two flashes of light in quick succession. This light is observed with 390 photomultiplier tubes, devices that use the photoelectric effect to turn light into electronic pulses. We expected to find about 50 antineutrinos a day in our detector.
Since inverse beta decay is only sensitive to electron-flavored antineutrinos, if neutrinos oscillate we will measure a deficit. Those which became muon- or tau-flavored will not be detected. The size of the deficit is a function of the distance between the source and the detector, and the energy of the neutrinos. We started taking data in April 2011, analyzed it through September, and obtained a result in November by comparing to a prediction from a detailed model of the reactor (this model is the thesis of our graduate student, Christopher Jones).
We measured a 5.6% deficit. This is at least five times larger than most theories predicted and is now confirmed by the Daya Bay and RENO experiments. Like Paul Revere on his famous ride, we are continuing to take data to improve the result, but it is time to raise the alarm. The last mixing angle is large, and a measurement of the matter/antimatter asymmetry in neutrinos is feasible in the next decade. The revolution is here and things are getting exciting!