29 September 2006

Particle wobble shakes up supersymmetry

Cosmos Online
The barely detectable wobble of a short-lived particle with the unlikely name of Bs meson could challenge current theories about how the universe is constructed, according to an international team of particle physicists.
Particle wobble shakes up supersymmetry

The Tevatron, the particle accelerator used to find the oscillating Bs meson, has huge detectors surrounded by a cylindrical 'tracking chamber', shown here. Credit: Fermilab

SYDNEY: The barely detectable wobble of a short-lived particle with the unlikely name of Bs meson could challenge current theories about how the universe is constructed, according to an international team of particle physicists.

Announcing their result earlier this week, the team – made up of 700 physicists – said the sub-atomic particle, which is pronounced ‘B-sub-s mees-on’, was found to oscillate between matter and anti-matter three trillion times a second before it decayed.

This is significant because the Standard Model of particle physics – the accepted description of the fundamental forces and the fundamental particles that make up the universe – predicts that some sub-atomic particles may oscillate between matter and anti-matter. But measuring that oscillation and the rate at which it occurs has been an all-too-elusive piece in the grand puzzle.

Finally that piece has been found. And it only took 700 scientists from 13 countries with hundreds of millions of dollars’ worth of equipment, incomprehensible amounts of energy and 20 years of labour.

“Scientists have been pursuing this measurement for two decades, but the convergence of capabilities to make it possible has occurred just now,” said Jacobo Konigsberg, from the University of Florida and a spokesperson for the collaboration. “With a process this fast we needed extremely precise detectors and sophisticated analysis tools.”

By accurately measuring such behaviour of sub-atomic particles, scientists can begin to understand why the particles exist, how they interact and eventually how the universe developed the way it did.

Every sub-atomic particle has a corresponding anti-particle. For example, an electron’s anti-particle is the positron, which has the same mass but an opposite charge – rather than being negatively charged it is positively charged. When a particle meets its anti-particle the two annihilate each other, usually transforming into radiation. Matter, which exists with abundance on Earth, is comprised only of particles; anti-matter, which is found rarely on Earth, is comprised only of anti-particles.

The researchers acquired their data between February 2002 and January 2006, on the Tevatron, the world’s highest-energy particle accelerator, located near Chicago in the USA. The Tevatron accelerates protons and anti-protons close to the speed of light and then collides them head-on.

As the particles wipe themselves out, a Bs meson is formed which oscillates between being matter and antimatter. The speed of the oscillations, three trillion times a second, sounds fast but is actually too slow to support some popular theories about how the universe is assembled.

Supersymmetry is a theory that predicts a ‘super’ partner for every known particle (and antiparticle). While supersymmetry is a well-founded theory, the exact mathematical details are disputed among the scientific community.

The more popular models of supersymmetry predict a much higher transition rate than three trillion times per second. This is a serious blow to particle physicists, as these popular models will now have to be reconsidered.

And it’s hard to challenge these new findings: the physicists are 99.99999992 per cent sure that they’re right.

The discovery of this oscillation rate reinforces the validity of the Standard Model, even though many physicists believe the Standard Model needs more work.

The international collaboration continues to seek phenomena not predicted by the Standard Model. Co-spokesman Rob Roser from Fermilab, USA said “while the Bs oscillation discovery was one of the benchmark results we wanted from the Tevatron, we still have more than half the data … waiting to be analysed. We’re looking forward to more results, and we’re always hoping for surprises.”


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