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2011-07-04 04:19:48
US particle physicists are inching closer to determining why the Universe
exists in its current form, made overwhelmingly of matter.
Physics suggests equal amounts of matter and antimatter should have been made
in the Big Bang.
In 2010, researchers at the Tevatron accelerator claimed preliminary results
showing a small excess of matter over antimatter as particles decayed.
The team has submitted a paper showing those results are on a firmer footing.
Each of the fundamental particles known has an antimatter cousin, with
identical properties but opposite electric charge.
When a particle encounters its antiparticle, they "annihilate" each other,
disappearing in a high-energy flash of light.
The question remains: why did this not occur in the early Universe with the
equal amounts of matter and antimatter, resulting in a Universe devoid of both?
New physics?
The Tevatron results come from a shower of particles produced at the facility
when smashing protons into their antimatter counterparts, antiprotons.
The proton-antiproton collisions in turn create a number of different
particles, and the team operating the Tevatron's DZero detector first noticed a
discrepancy in the decay of particles called B mesons.
Statistics of a 'discovery'
Two-pence piece
Particle physics has an accepted definition for a "discovery": a five-sigma
level of certainty
The number of sigmas is a measure of how unlikely it is that an experimental
result is simply down to chance rather than 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 more
than eight heads in a row
Five sigma, on the other hand, would correspond to tossing more than 20 in a
row
A five-sigma result is highly unlikely to happen by chance, and thus an
experimental result becomes an accepted discovery
These decayed into pairs of particles called muons alongside pairs of their
antimatter versions, antimuons. But, as the team reported in May 2010 in a
paper published in Physical Review Letters, there was a notable 1% excess of
the matter particles.
However, unpicking important events in the soup of interactions created in
particle physics experiments meant that those measurements were associated with
a level of uncertainty - reflecting the probability that the effect they see is
a random statistical occurrence, rather than new physics.
The researchers now have 50% more data to work with, and have tried to
establish that their earlier result in fact came from the particle decays that
they first proposed.
As they reported this Thursday, they have now reduced the uncertainty in their
experiment to a level of 3.9 sigma - equivalent to a 0.005% probability that
the effect is a fluke.
But particle physics has a strict definition for what may be called a discovery
- the "five sigma" level of certainty, or about a 0.00003% chance that the
effect is not real - which the team must show before they can claim to have
solved the long-standing matter/antimatter mystery.