Neutrinos slower than light, but continue to befuddle physicists

The neutrino was originally thought to be massless. If that was true then, according to special relativity, it would have to travel at the speed of light. In 1987, however, a supernova originating in the Large Magellanic Cloud (LMC) produced a burst of neutrinos that registered at three separate detectors with different amounts of energy. This has a pretty cut-and-dried explanation. Neutrinos would only have different energies if they traveled at different speeds. This implied that they had mass, albeit an extremely small one. [Editor's note: An earlier version mischaracterized the data from the supernova.]

There's also different types of neutrinos, which physicists call flavors. Pauli’s indirect inference that led to the first observation of the electron neutrino was also used in 1962 at CERN and Brookhaven labs to infer the existence of another type of neutrino. Except this time the energy imbalance involved a muon – a type of subatomic particle similar to an electron  – and this way the existence of the muon neutrino was inferred.  It happened again in 1978 at the Stanford Linear Accelerator, resulting in the inference of the tau neutrino.

Our own sun helped scientists answer another mystery of the neutrino. The Solar Neutrino Problem was a late 20th century discrepancy in the expected amount of electron neutrinos emitted from our sun. If the solar model was trusted, the expected number of detections could be computed with some pretty straightforward math. The problem arose when various detection facilities received only a rough third of what that calculation would have them expect. This phenomenon was inexplicable until about 2002. The theory that accounted for the discrepancy did so by stipulating that neutrinos aren’t invariably locked into their respective flavors. They switch between them. This phenomenon is called neutrino oscillation.

A few problems for the Standard Model

When the Standard Model of particle physics was established in the mid 1970s, it made some basic assumptions about the neutrino. Yet the evidence physicists have recently discovered seems to suggest otherwise.

The original model assumes that neutrinos are massless. Yet the accepted solution to the Solar Neutrino Problem and the observations from the 1987 Large Magellanic Cloud supernova, indicate that the particles have mass.

The Standard Model also assumes that mirror images of particles look and behave the same way. This is intimately related to the conservation of energy. Yet many particles that interact through the weak force, such as neutrinos, have long been known to violate this.

To preserve the Standard Model, physicists have proposed a new symmetry, called CP symmetry (a product of charge and parity symmetry). CP symmetry states that the laws of physics should be the same for a particle that is made into its mirror image, and interchanged with its antiparticle. The addition of the antiparticle clause allowed the conservation of energy to hold. But neutrinos were soon discovered to violate CP symmetry, as well.

All the recent evidence seems to suggest that the neutrino will have to settle for light speed. After all, it has other ways of creating hassles for physicists.

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