LHCb in CERN reaches a milestone in exploring the asymmetry between matter and antimatter

A proton collision event in the LHCb. Image courtesy CERN.

The LHCb experiment in CERN - in which the School takes a leading role - is exploring processes important to the understanding of what happened fourteen billion years ago, when the Universe began.

Crammed within an infinitely small space, equal quantities of matter and antimatter were created. But its composition changed as the Universe cooled and expanded: just one second after the Big Bang antimatter had all but disappeared, leaving only matter to form everything that we see around us.

This matter-antimatter asymmetry relies upon a phenomenon known as CP violation. It is predicted by the Standard Model of Physics but at a much lower level than we need to explain the early Universe.

The LHCb experiment measures the decay of elementary particles known as B mesons, and in this case a quantity known as φs. Experimental results which deviate from the Standard Model's predictions would suggest new physics sources of CP violation. Fermilab in the USA has previously measured φs, but the LHCb detector at the Large Hadron Collider has improved the precision by a factor of two with preliminary results and will soon improve this by a factor of ten. These new results, which were eagerly awaited, were announced at the 'Lepton Photon' conference in Mumbai in August.

"This is a milestone for the LHCb experiment. It is a flagship result for LHCb, where we have now made the World's most precise measurement of this quantity." Peter Clarke, Institute for Particle and Nuclear Physics

Peter Clarke, of the School's Institute for Particle and Nuclear Physics, said: "This is a milestone for the LHCb experiment. It is is a flagship result for LHCb, where we have now made the World's most precise measurement of this quantity. A team of seven physicists and PhD students from the University of Edinburgh was one of the groups leading the measurement of φs by analysing decays of Bs mesons into J/psi and phi mesons.

"Measuring φs is all about understanding the origin of difference in behaviour of matter and antimatter. This difference is a key ingredient in the evolution of the early Universe. Without it we would have no 'matter' left today and we wouldn't be writing this article.

"We know that the matter anti-matter difference in our Standard Model is insufficient to explain the Universe. So we are looking to find evidence of new sources of this effect, ie discover new physics phenomenon. This is the big motivation for us all."

The Standard Model, quarks and the missing anti-matter

Matter and antimatter are thought to have existed in equal amounts at the beginning of the Universe, but as the Universe expanded and cooled, an asymmetry developed between them, leaving a universe that appears to be composed entirely of matter. Heavy quarks provide a good place to investigate this phenomenon because the heavier the quark, the more ways it can decay, and all of these decays are described by the Standard Model.

The Standard Model predicts matter-antimatter asymmetry, but at a level which is too small to explain the observed asymmetry in the Universe. Deviations from the predictions would bring an indication of new physics. b-quarks are produced copiously at the LHC, which makes them the particle of choice for studying matter-antimatter asymmetry in the laboratory. Quarks are never produced alone, but always travel in company: they are accompanied by another quark giving rise to the family of particles called B mesons. It is these that LHCb studies.