Neutrino experiments sow seeds of possible revolution

Nearly massless particles could turn physics on its ear

Neutrinos are the big nothings of subatomic physics. Nearly massless and lacking an electric charge, these ghostly particles interact so weakly with other types of matter that more than 50 trillion of them pass unimpeded through a person’s body each second.

UNSETTLING DETECTION One of the two detectors in the MINOS neutrino experiment sits in the Soudan Underground Laboratory in Minnesota. A recent analysis of MINOS data hints that neutrinos and antineutrinos might not weigh the same, suggesting either the presence of an unknown force or a violation of Einstein’s theory of special relativity. Fermilab

Yet recent preliminary findings from two experiments hint that neutrinos may be opening a window on a hidden world of subatomic particles and forces.

The findings from both experiments have relatively large margins of error, so they could end up being statistical flukes. But so far the results, announced June 14 at the Neutrino 2010 conference in Athens, indicate that neutrinos and their antiparticle counterparts, antineutrinos, are not the nearly exact mirror images of each other that current physics supposes them to be.

If confirmed, those conclusions “indicate a fundamentally new direction in our thinking” about subatomic particles and the origin of matter in the universe, says theorist Rabindra Mohapatra of the University of Maryland in College Park.

The new results may help explain a long-standing puzzle — how the universe, believed to have begun with such a perfect balance of matter and antimatter that the two would have destroyed each other upon contact, became dominated by matter. That imbalance has led to the evolution of galaxies, planets and life

The new findings “could even signal a tiny breakdown of Einstein’s theory of special relativity,” Mohapatra adds. “This could completely alter the way we are doing physics now.”

Current theories of particle physics are based on two assumptions, notes Mohapatra. All known forces arise from interactions with neighboring particles and they all obey Einstein’s special relativity theory, which holds that the speed of light and the laws of physics are always the same regardless of a particle’s speed or rotation. For that to hold true, particles and antiparticles—-including neutrinos and their antipartners — must have the same mass, he says.

But new measurements from an experiment called MINOS (for Main Injector Neutrino Oscillation Search) seem to contradict that notion. The three known types of neutrinos —electron, muon and tau — act like chameleons, transforming from one type into another as they travel.

MINOS found that during a 735-kilometer journey from Fermilab to the Soudan Underground Laboratory in Minnesota, about 37 percent of muon antineutrinos disappeared — presumably morphing into one of the other neutrino types — compared with just 19 percent of muon neutrinos, reports MINOS spokesman Robert Plunkett of Fermilab.

That difference in transformation rates suggests a difference in mass between antineutrinos and neutrinos — although more data will have to accumulate to confirm the observation. With the amount of data collected so far, there’s a 5 percent probability that the two types of particles weigh the same.

“One thing is clear — if the masses are different for neutrinos and antineutrinos, then the most sacred symmetry of quantum field theory, CPT (for charge, parity and time), is broken in the neutrino sector,” says Tom Weiler of Vanderbilt University in Nashville.

If the interactions of particles are thought of as a movie, CPT symmetry requires that whatever physics occurs during the show must be the same whether the film is run forward or backward (time), viewed through a mirror (parity) and repopulated with each particle being replaced by an antiparticle (charge).

If CPT is broken, then a cornerstone of Einstein’s special relativity is also violated, Weiler notes.

To save CPT and Einstein’s theory — assuming they need saving — Ann Nelson of the University of Washington in Seattle favors the introduction of a new force. “It’s a less radical idea” than throwing out Einstein’s theory of special relativity, she notes. The force Nelson envisions would endow matter with a new kind of charge that would allow it to interact differently with neutrinos than antineutrinos.

In a smaller-scale study based at Fermilab, an experiment called MiniBooNE found a different kind of asymmetry between the particles and the antiparticles. Over a distance of about half a kilometer, muon antineutrinos morphed into electron antineutrinos more often than muon neutrinos transformed into electron neutrinos. That result would also require a difference in mass between neutrinos and antineutrinos, Mohapatra says, although others disagree.

There’s about a 3 percent chance the MiniBooNE finding is a fluke. However, it matches findings, earlier refuted, from the Liquid Scintillator Neutrino Detector experiment, which operated at the Los Alamos National Laboratory in New Mexico during the 1990s.

The kind of asymmetry between particles and antiparticles indicated by MiniBooNE occurs in the standard model of particle physics, but it isn’t large enough to account for the new results, notes Boris Kayser of Fermilab. If the results are confirmed, the findings may require a fourth, previously unknown neutrino type, dubbed sterile because it would interact with matter even more weakly than the other three.

Because neutrinos played a key role in forging the elements in the early universe and govern how supernovas explode, a new type of neutrino could have a profound effect on cosmology and astrophysics, says Nelson. With current experiments gathering more data and newer experiments about to come online, it may be only a few years before physicists know if the MiniBooNE and MINOS results are the beginning of a revolution.