Puzzling Neutrino Results at Fermilab

Particle physics is slow, expensive, and labor-intensive. That's because studying the most fundamental forces and bits of matter in the universe often means boosting things to very close to the speed of light (the expensive part), accumulating collision events rarer than sightings of snow leopards (the slow part) and building accelerators, detectors, and software to handle the staggering amount of data needed to sift phenomena that occur at the level of parts per billion or trillion (the laborious part).

Neutrino physics exhibits all of these features to the greatest degree. Neutrinos are the least reactive of particles, interact only via the weak nuclear force, and must be made artificially at reactors or accelerators. The neutrino's reticence, however, makes it a good probe for studying the weak force which, unlike electromagnetism or gravity, does not operate over large distances and cannot be measured with a handy machine like a gravimeter or voltmeter.

Instead a sense of the weak force must be pieced together by observing how it mediates (via the charged W bosons and the neutral Z boson) a variety of interactions among quarks, leptons (such as electrons and muons), and neutrinos.

At Fermilab the NuTeV experiment does this by shooting neutrinos and antineutrinos at a target wherein the neutrinos, if they interact at all, do so by scattering from a quark in one of two ways. It can exchange a W boson (in which case the neutrino must turn into a muon); the shuttling W constitutes a tiny charged current. Or the neutrino can retain its identity (not change into a muon) by exchanging a Z boson, which constitutes a tiny neutral current.

By observing how often the neutral current events occur relative to the charged currents events, one can calculate a parameter called the weak mixing angle, which is an indication of how much of the combined electroweak force is electromagnetic in nature (the part of the force which respects "parity," that is, cannot tell left from right) and how much of it is really the weak force (the part of the force which does differentiate between left and right).

The NuTeV measured value for theta (actually the square of the sine of theta) is 0.2277 while the theoretical value is 0.2227. The discrepancy in the rate of neutral current interactions is tiny but interesting because it amounts to a 3-standard-deviation departure.

(Herewith a short statistical discussion about how to deal with the measurement of a value over N trials: the mean value is the sum of all the measurements, divided by N. The variance is the sum of the square of the difference between each measurement and the mean, all divided by N. The standard deviation, is the square root of the variance. Thus the standard deviation, often signified by the Greek letter sigma, is an indication of much individual measurements depart from the mean.)

A three-sigma result (in this case the center-point of the gaussian-shaped measurement distribution lies 3 standard deviations from the theoretical value) is taken by scientists as a significant but not conclusive sign that something interesting is happening.

The NuTeV determination is not the most precise measurement of theta ever made, but it is the most precise measurement made with neutrino interactions, and as NuTeV scientist Kevin McFarland (585-242-9585, ksmcf@pas.rochester.edu) says, there is always the chance that neutrino behavior is different from that of other particles.

Even if this departure holds up, the standard model is by no means in trouble. More likely the experimental results might suggest the existence of particles not seen before, such as the "leptoquark," a hypothetical particle that turns quarks into leptons and vice versa, or the Z-prime boson, a heavier cousin of the Z boson. (Results announced at Fermilab seminar, 26 Oct; text, submitted to Physical Review Letters, available at Rochester website.)