Flipping particle could explain missing antimatter
Curtesy Space Daily

          

It is one the biggest mysteries in physics - where did all the antimatter go? Now a team of physicists claims to have found the first ever hint of an answer in experimental data. The findings could signal a major crack in the standard model, the theoretical edifice that describes nature's fundamental particles and forces.

In its early days, the cosmos was a cauldron of radiation and equal amounts of matter and antimatter. As it cooled, all the antimatter annihilated in collisions with matter - but for some reason the proportions ended up lopsided, leaving some of the matter intact.

Physicists think the explanation for this lies with the weak nuclear force, which differs from the other fundamental forces in that it does not act equally on matter and antimatter. This asymmetry, called CP violation, could have allowed the matter to survive to form the elements, stars and galaxies we see today.

The standard model, our best effort to describe the universe's structure, fails to fully explain CP violation. Many alternative theories claim to have the answer, such as those incorporating supersymmetry, extra dimensions and hitherto unseen forces. However, they often invoke new particles, and experiments have yet to turn up evidence of these.

Particle physicists have long thought that they might find such evidence in a particle called the Bs meson, which comprises a bottom antiquark bound to a strange quark. The Bs is one of a handful of mesons that transforms into its own antiparticle and back again 3 trillion times per second before decaying into other particles (see Diagram). These oscillations between matter and antimatter make it a good place to look for evidence that CP violation goes beyond the standard model.

At the Tevatron particle accelerator at Fermilab in Batavia, Illinois, two groups of scientists running the rival CDF and D-Zero experiments have been studying several properties of Bs mesons and their oscillations by picking through the debris created when protons and antiprotons collide. While each experiment on its own has found faint hints of CP violation above and beyond the standard model, the experimental uncertainties have been too large to make a definitive claim, says Giovanni Punzi, a physicist at the University of Pisa in Italy and one of the leaders of the B meson physics group at CDF.

Now Luca Silvestrini at Italy's National Institute of Nuclear Physics (INFN) in Rome and colleagues in Italy, France and Switzerland have managed to reduce these uncertainties. By combining the published results of the CDF and D-Zero teams, they have shown there seems to be much more CP violation than the standard model permits. "We can say with greater than 99.7 per cent probability that CP violation is there," says Silvestrini (www.arxiv.org/abs/08030659). In other words, new physics is at work in the oscillations. His group cannot yet say what kind of new physics - that will require others to test whether existing theories explain the data.

"It is tantalisingly interesting at the moment," says Val Gibson, an expert on B meson physics at the University of Cambridge. "If it is true, it is earth-shattering."

Jacobo Konigsberg, who leads the CDF collaboration, says that Tevatron researchers are "cautiously excited" about the analysis. He points out that more data needs to be analysed to rule out a statistical fluke, which has happened several times before in particle physics.

The real proof could come later this year when the Large Hadron Collider switches on at CERN, near Geneva, Switzerland. The LHC-b experiment has been designed specifically to study mesons containing bottom quarks. "LHC-b will make an unambiguous measurement within two months," says Gibson.