A Particle Physics First: Researchers Watch Neutrinos Change Flavors

By Joseph Calamia | June 1, 2010 4:28 pm

detectorFor the first time, physicists say they have witnessed a subatomic particle change its “flavor.” Physicists at OPERA, run by Italy’s national nuclear physics institute, announced yesterday that they have observed one neutrino change its type, or flavor, spontaneously. The experiment solves a 50-year-old physics mystery, and may uncover some of the universe’s hidden mass.

The Mystery

Neutrinos, which come in three different flavors, can have fairly violent births: they can come into the world via nuclear reactions in the sun, particle decay, or collisions in particle accelerators. But, once formed, they seem to ignore almost everything around them, including magnetic fields, electric fields, and matter. In fact, there are trillions of them zipping through each of us every second; they go right through our bodies and keep on moving through the planet itself.

The mystery of “neutrino oscillations” began with the number of neutrinos that should be coming from the sun. Theory predicted a certain number of various flavors to arrive, but observation showed much less:

The neutrino puzzle began with a pioneering and ultimately Nobel Prize winning experiment conducted by US scientist Ray Davis beginning in the 1960s. He observed far fewer neutrinos arriving at the Earth from the Sun than solar models predicted: either solar models were wrong, or something was happening to the neutrinos on their way. [CERN]

In 1969, Bruno Pontecorvo and Vladimir Gribov theorized that the neutrinos weren’t disappearing, they were changing their flavors mid-journey. Though physicists were looking for one type, they weren’t finding what they ordered.

The Experiment

For the past three years, CERN has fired a beam of muon neutrinos (one of the kinds that fall out of decaying muons) from Geneva, Switzerland to the OPERA experiment in a laboratory near Italy’s Gran Sasso mountain. After three years and billions of billions of muon neutrinos in and muon neutrinos out, for the first time, the OPERA physicists saw something else arrive in Italy. After the 450-mile, 2.4-millisecond voyage, a muon neutrino appears to have transformed into a tau neutrino. Researchers are 98 percent sure that this is true neutrino presto-chango:

“You have to be sensitive to even one single neutrino that has been transformed,” says OPERA spokesman Antonio Ereditato at the University of Bern in Switzerland…. “It’s like a murder. You have the murder scene, but now we have found the first part of the body.” [New Scientist]

The Meaning

If this is true neutrino oscillation, this experiment does more than confirms Pontecorvo and Gribov’s theory. It means changes for the organizing theory of particle physics, called the Standard Model. The nitty-gritty of neutrino oscillation theory requires that neutrinos have mass, but the Standard Model assumes that they have none. If neutrinos do have mass, this could explain some of what cosmologists call dark matter, the 25 percent of the universe’s mass that we believe should exist, but can’t seem to find.

“This will be the long-awaited proof of this process. It was a missing piece of the puzzle,” said Antonio Ereditato, a researcher at the Institute and spokesman for the OPERA group that carried out the study. “If true, it means that new physics will be required to explain this fact,” he said by phone…. “Whatever exists in the infinitely small always has repercussions in the infinitely big.” [AFP]

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Image: The Detector OPERA

  • Brian

    Am I missing something? Neutrino oscillations were confirmed almost a decade ago at Sudbury Neutrino Observatory. Since then, there have been a host of other measurements from experiments at SuperK, KamLAND, MINOS, etc. Is there something unique about this result that suggests something different about the Standard Model than the picture painted by these other experimental results?

  • matt

    Yes its weird they even thought neutrinos had no mass, since mass and energy are intertwined. Unless Einstein was flat wrong.

  • matt

    light itself is supposed to be the fastest moving thing and hence the lightest thing in the universe. However light is trapped by blackholes..hence it has a mass, and if light has a mass then everything has a mass.

  • Brian

    @matt: It’s entirely possible for something to have energy and not mass. Einstein’s famous relation is a conversion of sorts — it says that a mass m can be converted to an amount of energy E/c^2. Light is an example — it has no mass, but has energy on account of its vibration. Light is trapped by black holes because gravity doesn’t just act on mass, it acts on anything with energy (even massless objects, like photons).

  • Old Gringo Stan

    ….and now we can manuacture a better toohpaste…..not…….subatomic physics, the most expensive hobby in the world…..but I guess it keeps some physicists out of the bars

  • Ben

    Yeah, its not like past theoretical physics experiments have provided us with anything useful. I mean, who uses electricity or the internet? Sub atomic physics is used routinely in hospitals as part of diagnostics. A PET scan uses antimatter to look at the inside of your brain – prime example of physics which was on the fringe of theoretical research 60 years ago now being used daily. A bit like lasers, discovered to “keep some physicists out of the bars” and now I have 4 within reach from where I’m sitting.

    Come back in 60-70 years and see if the research being done at CERN is still as useless as ever. You don’t get better for money long term investments that pure science research.

    Back to the topic, the real relativistic equation concerning energy and mass is E^2 = p^2 c^2 + m^2 c^4.
    So you can still have energy and no mass as long as you have momentum.

  • FRED


  • Francois

    Brian and Ben, you’re right when answering Matt’s question. Your approach, here, is this of Einstein’s first theory of Special Relativity.

    Now, put even simpler by Einstein’s second theory called General Relativity: gravity interacts with light because gravity curves space-time. So any presence of matter with a mass in the universe bends rays of light.

    Black holes generate such a strong gravitational field that the bending of rays of lights in their proximity is enough to trap them with no possible escape.

  • dadster

    i would like to know whether any physicist has investigated the energies associated with a quantity of “mass” that lie beyond the greater- than- mc^2 region? It would appear to me that such energies are not of the electromagnetic denomination at all. The familiar “e”that is equal to mc^2 is the maximum extractable electromagnetic energy available in a quantity of mass”m”. It could be because of this reason that varieties of energies associated with a quantity of mass”m” are not all detectable by any electromagnetic based instruments of physics. The only other type of non-electromagnetic energy associated with mass”m” that is known to physics is “Gravitational energy” which is indirectly measurable with changes in shape, weight and velocity of moving mass. Gravity according to Einstein is just the geometry of the four dimensional space-time continuum and not a force or even energy. All this may mean that there are energies, that resides in a quantity of mass other than the all too familiar electromagnetic energy, which may generate velocities greater than the speed of light “c”and transparent to electromagnetic measuring instruments ; energies such as the Dark matter or the Dark energy or even “life-energy” which is not an emergent entity but could be a basic root entity originated along with and parallel to the big bang entities like “mass”and “radiation”and, may be even have existed much before a hydrogen nucleus. I feel its time that we seriously investigate the energies that lie beyond the mc^2 region .

  • sam rosenblatt

    i was just saying the other day how we could find an analogy between attributing dark matter to mass and how high school students account significant sources of error to reaction time and air friction


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