Tuesday, 15 November 2011

LHC reveals hints of 'new physics' in particle decays

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 more often into one kind of particle rather than another, 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 less than a 0.5% 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 called mesons, 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 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 "CP violation" - slight differences in behaviour if a given particle is swapped for its antimatter counterpart.
Our best understanding of physics so far, called the Standard Model, suggests that the complicated cascades of decay of matter particles 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 found a CP violation of about 0.1%, but with an uncertainty in their measurement that meant the result might just fit within the Standard Model.
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 firstly because it comes out with a [greater difference] and secondly 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."


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