Physicists unlock mystery of subatomic particle: important step in understanding the universe's dark matter

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European researchers observe for the first time a transformation in neutrinos, evidence that they have mass. It's an important step in understanding the universe's dark matter.

By Thomas H. Maugh II, Los Angeles Times

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, ghost-like particles that travel at the speed of light. In the new experiment, physicists captured a muon neutrino in the process of transforming into a tau neutrino.

Researchers had strongly believed that such transformations occur because they have been able to observe the disappearance of muon neutrinos in a variety of experiments.

But the research announced Monday marks the first time that the appearance of a tau neutrino has been directly observed. Physicists from CERN (the European Organization for Nuclear Research) in Geneva and the Italian National Institute of Nuclear Physics' Gran Sasso National Laboratory were involved.

"This is an important step for neutrino physics," CERN Director-General Rolf Heuer said in a statement. "We're all looking forward to unveiling the new physics this result presages."

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% of the mass of the universe and visible matter only 20%.

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.

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. They have no electrical charge — hence the name, meaning "little neutral one."

Physicists generally don't see neutrinos. Instead, they observe the debris left behind on the 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.

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 a proton accelerator at CERN in Geneva that slams protons into a graphite target, producing particles called pions and kaons that quickly decay into muon neutrinos.

Because the neutrino beam that is created is not affected by electrical or magnetic fields, the proton accelerator must be pointed directly at detectors in the laboratory under Gran Sasso mountain 453 miles away in central Italy, between the towns of L'Aquila and Teramo. When neutrinos are produced, they continue in the same direction of 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. The total mass of the bricks, which are accompanied by electronic detectors and other apparatus, is about 1,300 tons.

OPERA began operating three years ago and has since sent "billions of billions" of muon neutrinos to Gran Sasso. But the interaction of the neutrinos with the lead is so weak that the first tau neutrino has only just been observed.

"We are fully confident that this first event will be followed by others that will fully demonstrate the appearance of neutrino oscillation," said OPERA spokesman Antonio Ereditato.

thomas.maugh@latimes.com

LA Times

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