The Importance of MiniBooNE

After five years of intense construction involving over 50 people, MiniBooNE began collecting data in the summer of 2002.

MiniBooNE’s primary goal is to definitively detect neutrino oscillations, a few occurrences of which were seen in 1995 in the Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos , New Mexico. Confirming the LSND result would have an important consequence: it would require physicists to rewrite a large part of their current understanding of how our universe is constructed, a picture known as the “ Standard Model .” Because Fermilab's accelerator will feed protons to MiniBooNE in large quantity--about a billion, trillion protons a year--MiniBooNE will be able to detect neutrino oscillations in large numbers. In doing so, it will be able to measure the exact details of neutrino oscillations with large precision.

The Standard Model is meant to explain the interactions between elementary particles in our universe. It includes a listing of the most basic particles known--which now numbers a few dozen--their properties, such as mass--and the four forces by which they interact with one another. Solidified over the period from about 1960 to 1990, the Standard Model is capable of explaining almost all experiments that physicists have carried out, and to an astonishing accuracy.

Rewriting the Standard Model?
But if MiniBooNE confirms neutrino oscillations, part of the Standard Model will need to be rewritten. That’s because oscillations of muon neutrinos into electron neutrinos will imply that a fourth, additional kind of neutrino must exist, one beyond the three known “flavors” (electron neutrino, muon neutrino, and tau neutrino). In turn, that will imply a fourth type of electron-like particle, one beyond the electron, muon, and tau particles (a particle family collectively called “leptons”).  

How many more flavors might exist? Probably only one, at most. Scientists who study the large-scale structure and evolution of the universe--its cosmology--have calculated that, based on the details of how our universe is evolving, there is at most one additional lepton besides the three already known. Still, finding a fourth lepton and its associated neutrino would be quite surprising.

For decades physicists believed that neutrinos of all flavors were massless. But the presence of neutrino oscillations also implies that at least some of the different types of neutrinos must, in fact, have mass. This mass, if it exists, is expected to be quite small, perhaps a million times smaller than the electron. (The electron itself is already quite light: it accounts for only about 0.05 percent of a hydrogen atom’s mass, and it would take about a trillion trillion electrons to weigh as much as an amoeba.)

So if neutrinos have a mass, it will be very small indeed. But even that small mass would be different from the Standard Model. But because there are so many neutrinos in the universe--about a trillion just passed through your body in the last second--they may still be a significant portion of the mass of the universe. And, in fact, scientists have been puzzled because they have been unable to account for almost all of the mass that the universe is expected to have, based on the way it expands and the way its galaxies rotate. Neutrinos with mass might be a sizeable fraction of such “missing mass,” sometimes also called “dark matter.”

Hunting more than oscillations
MiniBooNE will also investigate other areas of science. One is as a supernova detector. Supernovae are exploding stars that occur at the end of a typical star’s lifetime. After a collapse inward, the star suddenly radiates out immense amounts of energy in the form of photons (particles of light) and in neutrinos. Supernovae have been seen throughout history, as in 1054 A.D., and astronomers expect about three such exploding stars occur in our galaxy every 100 years.

One of the more recent notable supernovae occurred in 1987 and was named “SN1987A .” It occurred about 160,000 light-years from earth (a billion billion miles), and in fact several neutrino detectors then in operation, such as Kamiokande II in Japan, detected about two dozen neutrinos from SN1987A.

MiniBooNE can efficiently operate as a supernova neutrino detector without interfering with its main task of testing the LSND signal. About 230 electron-antineutrinos would be expected to be seen by MiniBooNE if a future supernova exploded in our galaxy, about ten times more than were observed by Kamiokande II in 1987. It could thus serve as an important cross check with other supernova neutrino detectors that exist around the world (especially if, as is sometimes the case, other detectors have been shut down for repairs). And because supernova neutrinos arrive first, such combined information might tell astronomers where to look to see the light from a supernova and all them to build upon their theories of how stars evolve and die.

Rare particles
MiniBooNE could also help explain a puzzling result found by particle physics in 1995. Physicists in England found a small anomaly in the distribution of neutrinos from the decays of muons and pions. Over the years physicists have learned not to ignore such relatively small discrepancies, because they often provide hints and clues of new phenomenon. This particular discrepancy, called the “KARMEN timing anomaly” after the experimental group that detected it, has never been explained, although more recent experiments have not seen evidence of the anomaly.

One explanation of the anomaly involves a new, apparently rare and as yet unknown particle physicists call a Q0 (read “Q-zero”). The Q 0 would be have no electric charge, would have to be very massive, and would have to interact with other particles only weakly. MiniBooNE will be capable of broadly exploring for the KARMEN timing anomaly, and perhaps either prove the existence of the Q0 or ruling it out.

MiniBooNE will also be able to more precisely measure the extent to which muon neutrinos act as a tiny magnetic, its so-called "magnetic moment." As well, it may be able to look for high-energy protons that come from high-energy solar flares, explosions on the sun's surface that cast energy into space and towards the sun.

Finally, as with any experiment done on the frontiers of science, MiniBooNE has the possibility of uncovering new and exotic phenomena never before seen, or even suspected. Curiosity is a motivation for all scientists, and it has time and time again paid off in exciting new discoveries that have opened windows into unexpected corners of nature. As exciting as the boom in neutrino physics has been over the last several years, no one yet believes that all the secrets of these ghostly particles have yet been revealed. MiniBooNE aims to keep the secrets coming.

-- David Appell