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2011-11-16 07:40:51
15 November 2011 Last updated at 12:18 GMT
By Jason Palmer Science and technology reporter, BBC News
Large Hadron Collider researchers have shown off what may be the facility's
first "new physics" outside our current understanding of the Universe.
Particles called D-mesons seem to decay slightly differently from their
antiparticles, LHCb physicist Matthew Charles told the HCP 2011 meeting on
Monday.
The result may help explain why we see so much more matter than antimatter.
The team stresses that further analysis will be needed to shore up the result.
At the moment, they are claiming a statistical certainty of "3.5 sigma" -
suggesting that there is less than a 0.05% chance that the result they see is
down to chance.
The team has nearly double the amount of data that they have analysed so far,
so time will tell whether the result reaches the "five-sigma" level that
qualifies it for a formal discovery.
It matters
The LHCb detector was designed to examine particles containing so-called beauty
quarks, watching them decay through time after high-energy collisions of other
fundamental particles.
The LHCb Collaboration was looking at decays of particles called D-mesons,
which contain what are known as charmed-quarks, which can in turn decay into
kaons and pions.
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 standard deviations, or 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
LHCb, one of the six separate experiments at the Large Hadron Collider, is
particularly suited for examining what is called "charge-parity violation" -
slight differences in behaviour if a given particle is swapped for its
antimatter counterpart (changing its charge) and turned around one of its axes
(changing its parity).
Our best understanding of physics so far, called the Standard Model, suggests
that 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.
Other experiments, notably at the Fermi National Accelerator facility in the
US, have not definitively found a notable difference between the two kinds of
decay of D-mesons.
But the LHCb team is reporting a difference of about 0.8% - a significant
difference that, if true, could herald the first "new physics" to be found at
the LHC.
"Our result is more significant because our precision is improved - somewhat
more precise than all of the previous results put together," Dr Charles told
BBC News.
Spotting such a difference in the behaviour of matter and antimatter particles
may also finally help explain why our Universe is overwhelmingly made of
matter.
"Certainly this kind of effect, a new source of CP violation, could be a
manifestation of the physics which drives the matter - antimatter asymmetry,"
Dr Charles explained.
However, he stressed there are "many steps in the chain" between confirming the
collaboration's experimental result, and resolving the theory to accommodate
it.
"This result is a hint of something interesting and if it bears out, it will
mean that, at a minimum, our current theoretical understanding needs
improving," Dr Charles said.
"It's exactly the sort of thing for which the LHC was originally built."