New twist in antimatter mystery

CDF (Fermilab) CDF was one of two multi-purpose experiments at the US Tevatron accelerator near Chicago

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Physicists have taken a step forward in their efforts to understand why the Universe is dominated by matter, and not its shadowy opposite antimatter.

A US experiment has confirmed previous findings that hinted at phenomena outside our understanding of physics.

The results show that certain matter particles decay differently from their antimatter counterparts.

Such differences could potentially help explain why there is so much more matter in the cosmos than antimatter.

The findings from scientists working on the CDF experiment have been presented at a particle physics meeting in La Thuile, Italy.

CDF was one of two multi-purpose experiments at the now-defunct Tevatron particle smasher in Illinois.

Physicists think the intense heat of the Big Bang should have forged equal amounts of matter and its "mirror image" antimatter. Yet today we live in a Universe composed overwhelmingly of matter.

Antimatter is relatively uncommon, being produced at particle accelerators, in nuclear reactions or by cosmic rays. Getting to the bottom of where all this antimatter went remains one of the great endeavours of particle physics.

The latest results support findings from the LHCb experiment at the Large Hadron Collider, which were announced in November 2011.

Both CDF and LHCb have been looking at the process by which sub-atomic particles called D-mesons decay - or transform - into other ones. For example, D mesons are made up of particles known as charm quarks, and can decay into kaons and pions.

Our best understanding of physics so far, known as the Standard Model, suggests the complicated cascades of decay of D-mesons into other particles should be very nearly the same - within less than 0.1% - as a similar chain of antimatter decays.

But the LHCb team reported a difference of about 0.8%, and the team from CDF have now presented data showing a difference of 0.62%.

lhcb LHCb is an enormous detector designed to examine CP violation

Getting such a similar measurement as LHCb was "a bit of a surprise", according to CDF's spokesperson Giovanni Punzi, because it is a "very unusual result".

He told BBC News: "That two separate experiments have found this using different methods - different environments - is very interesting."

Prof Punzi, from the University of Pisa and Italy's National Nuclear Physics Institute (INFN), said this was likely to "change the minds of many people about this being just one of those effects, to something that will be considered a confirmed observation - because of this independent result".

Statistics of a 'discovery'

  • Particle physics has an accepted definition for a "discovery": a five-sigma level of certainty
  • The number of standard deviations, or sigmas, is a measure of how unlikely it is that an experimental result is simply down to chance, in the absence of a real effect
  • Similarly, tossing a coin and getting a number of heads in a row may just be chance, rather than a sign of a "loaded" coin
  • The "three sigma" level represents about the same likelihood of tossing nine heads in a row
  • Five sigma, on the other hand, would correspond to tossing more than 21 in a row
  • Unlikely results are more probable when several experiments are carried out at once - equivalent to several people flipping coins at the same time
  • With independent confirmation by other experiments, five-sigma findings become accepted discoveries

He explained that when the results from CDF and LHCb are combined, the statistical significance almost reaches the four sigma level of certainty. This equates to roughly a one in 16,000 chance that the observation is down to some statistical quirk in the data.

Dr Tara Shears, a particle physicist from Liverpool University, who works on the LHCb experiment, told BBC News: "We don't know yet if we are seeing the first signs of new physics, or are starting to understand the Standard Model better.

"What we've seen is a hint that's worth looking into. And the fact that CDF see the same effect as LHCb is confirmation that this is really worth doing."

These views were echoed by Giovanni Punzi: "This effect is definitely much larger than anything that had been predicted. So there will be discussions between the theoreticians, asking: 'Is this really new physics, or did we get our calculations wrong?'"

The dominance of matter in the Universe is possible only if there are differences in the behaviour of particles and anti-particles.

Physicists had already seen such differences - known as called "CP violation". But these known differences are much too small to explain why the Universe appears to prefer matter over anti-matter.

One other experiment has shown a significant "asymmetry" of matter over antimatter. In June 2010, physicists working on the Tevatron's DZero experiment reported seeing a 1% difference in the production of pairs of muon (matter) particles and pairs of anti-muons (antimatter).

The Tevatron was shut down in September last year, after the American government rejected a proposal to fund it until 2014, but scientists continue to analyse data gathered up to the end of operations.

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