SCIENCE NEWS

Neutrinos weigh in on dark matter

For the first time, physicists have confirmed that certain subatomic particles have mass and that they could account for a large proportion of matter in the universe, the so-called dark matter that astrophysicists know is there but that cannot be observed by conventional means.

The finding concerns the behavior of neutrinos, ghostlike particles that travel at the speed of light. Neutrinos interact with matter so weakly that they can travel through the entire Earth with the ease of a light beam traveling through a windowpane. Physicists generally don't see neutrinos. Instead, they observe the debris left behind on the very rare occasions when a neutrino strikes an atom head-on. They now know that there are three types of neutrino: electron, muon and tau, each named for the particle that is produced in the collision.

In research announced last week, physicists for the first time captured a muon neutrino in the process of transforming into a tau neutrino. Physicists from CERN (the European Organization for Nuclear Research) in Geneva and the Italian National Institute of Nuclear Physics's Gran Sasso National Laboratory were involved.

The new finding is important because in the theories now used to explain the behavior of fundamental particles, called the standard model, neutrinos have no mass. But if they have no mass, they cannot oscillate between muon and tau forms. The fact that they do oscillate indicates that they have mass and that the fundamentals of the standard model need some reworking, at the very least.

The discovery comes from the infinitely patient and creative researchers in an experiment known as OPERA, for Oscillation Project with Emulsion-tRacking Apparatus. The project's source of neutrinos is an accelerator at CERN that slams protons into a target, producing particles that quickly decay into muon neutrinos.

The accelerator is pointed directly at detectors under Gran Sasso, a mountain 453 miles away. When neutrinos are produced, they continue in the same direction as the proton beam, arriving at Gran Sasso in 2.4 milliseconds.

The detector at Gran Sasso is a massive apparatus made up of 150,000 "bricks" of photographic film interleaved with lead sheets. OPERA has sent a vast number of muon neutrinos to Gran Sasso in the past three years. But the interaction of the neutrinos with the lead is so weak that the first tau neutrino has only just been observed.

Astrophysicists have inferred the existence of dark matter from their observations that the total amount of visible matter is insufficient to account for gravitational effects. It is estimated that dark matter accounts for 80 percent of the mass of the universe and visible matter only 20 percent.

-- Los Angeles Times