Next Target: The Fourth Neutrino and Maybe More

by Mike Perricone

Alexandre Dumas's classic adventure, The Three Musketeers, actually centers on the story of D'Artagnan -- a fourth musketeer-in-waiting.

Does the classic Standard Model tale, The Three Neutrinos, have its own D'Artagnan -- a fourth neutrino-in-waiting, ready to emerge as the focus of the story?

The answer might come from Fermilab's next generation of fixed-target experiments.

"The clearest part of our future is the neutrino program," lab director Michael Witherell said during the June 2 fixed-target symposium. "We're in midst of building a new round of Fermilab neutrino experiments. MINOS is pushing towards starting to see neutrinos early in 2003. And MiniBooNE, using the booster beam, is on a very fast track to start taking data in 2001."

The Standard Model includes three types or "flavors" of neutrino: electron, muon and tau. Quantum mechanics declares that neutrinos which change or "oscillate" from one flavor to another must have mass, and experimenters have thus far seen indications of neutrino oscillations in three ranges of mass differences: in solar neutrinos, in atmospheric neutrinos, and in an accelerator experiment at Los Alamos National Laboratory, the Liquid Scintillator Neutrino Detector, which has shown signs of the largest mass difference (noted as "delta-m-squared") among the three by far.

As with any physics issue, answers raise further questions.

"Which forms are oscillations, and what are the oscillation parameters?" asked Witherell. "Are they all really neutrino oscillations? If so, then we need a fourth neutrino that doesn't fit in [the Standard Model]."

Why a fourth neutrino? It's a matter of mathematics.

"There are only three neutrinos that we know, and we say each corresponds to a different mass," said Associate Director for Research (and veteran neutrino experimenter) Mike Shaevitz. "With three numbers, you can't generate three different delta-m-squared results. We can't explain three results with three neutrinos. We need a fourth neutrino."

The fourth neutrino is postulated as the "sterile" neutrino. Or maybe that's only the start; maybe "sterile" is just a figure of speech, and the neutrinos -in-waiting just beget more and more of them.

"There is no reason to think there is only one sterile neutrino. I would argue that the most natural number is three, one for each family," said Janet Conrad, cospokesperson of the MiniBooNE experiment, a critical element in the next fixed-target run.

"There is a quip going around in neutrino circles," Conrad continued, "and I don't know who said it first, and I'm not sure I want to know. But the quip goes: 'Sterile neutrinos are like cockroaches. Once you get one in your theory, there's no stopping them.' Theorists either love or hate the sterile neutrino. There is nothing in between."

But there's plenty at stake.

MiniBooNE will use beam from the Booster accelerator for a short-baseline fixed-target experiment, expected to begin taking data late in 2001 via a 12-meter sphere filled with mineral oil and photomultiplier tubes. The goal: confirm with thousands of events the LSND indications of the appearance of electron neutrinos in an initially pure beam of muon neutrinos. Operating in the range of largest mass-difference, MiniBooNE can utilize a short time of flight for possible neutrino oscillations that might confirm the critical LSND possibility.

"At this point," Conrad said, "we cannot resolve the picture without knowing 'yes' or 'no' on LSND."

Even a 'no' contains further possibilities, but a 'yes' raises the stakes immeasurably.

"Confirming the LSND experiment is the result that would most change our picture of neutrinos and change where we're going in the future to study neutrinos," Witherell said at the symposium. "Experimentally, we could be wining the lottery...because there would then be so many things can do."

The lottery prize of a MiniBooNE confirmation would include the fourth neutrino, with all the associated theoretical and experimental explorations; and a new generation of accelerator-based neutrino experiments, using short pathways like MiniBooNE -- opening another pathway to "new physics" beyond the Standard Model.

Neutrinos easily make their own pathways through just about anything, including the earth -- even the 450 miles of earth between Batavia and a mineshaft a half-mile below the surface in Soudan, Minnesota.

The Main Injector Neutrino Oscillation Search leads the next generation of fixed-target experiments using the high-intensity, 120 GeV proton beam from the new Main Injector. Part of the NuMI project (Neutrinos at the Main Injector), the long-baseline MINOS experiment will explore the very small mass-difference range of oscillations from muon neutrinos to tau neutrinos. The $30-million civil construction project to dig the 1500-meter tunnel at the lab for MINOS had its groundbreaking on May 31.

The Main Injector's intense proton beam would also drive two proposed fixed-target experiments in CP violation, the exploration of asymmetries between matter and anti-matter.

The Charged Kaons at the Main Injector (CKM, honoring the Cabbibo-Kobayashi-Masakawa mixing matrix) intends to measure the branching ratio of the rare charged kaon decay producing charged pions, neutrinos and antineutrinos. The Kaons at the Main Injector experiment intends to investigate the ultra-rare decay of neutral kaons producing neutral pions, neutrinos and antineutrinos; KAMI would use the former Kaons at the Tevatron (KTeV) detector, which has already produced direct evidence of CP violation in neutral kaon decays.

Witherell was clear that the lab's priority centers on Collider Run II of the Tevatron. But he voiced a one-for-all and-all-for-one level of support for the lab's "superb program in the fast developing field of neutrino physics."

"This is an area that has been hot for a while, and will continue to be for some years," Witherell said. "We're going to know a lot more in five or six years than we know now."