The asymmetry between matter and anti-matter observed in the Universe is one of the greatest unresolved mysteries in modern particle physics.
We, physicists, imagine that during the Big Bang equal amounts of matter and anti-matter were present, but experimental evidence shows that the present Universe contains matter and not anti-matter.
In order to explain this asymmetry it is necessary that the fundamental forces are able to distinguish between matter and anti-matter. Speaking with a physicists words we say that there is CP violation.
Fifty years ago physicist were astonished when measuring experimentally this CP violation, even though extremely small, for a kind of particles called kaons. Subsequently it was explained theoretically that this is possible when there are at least three generations of elementary particles, that is of quarks and leptons.
Until now CP violation had been measured experimentally for particles containing strange and bottom quarks. However, the measured effect is too small to explain the matter – anti-matter asymmetry in the Universe. The CP violation in the leptonic sector, in particular for neutrinos, has not yet been shown experimentally.
Today, thanks to the LHCb experiment, a new piece was added to this puzzle at the root of the very existence of the Universe: the first observation of CP violation in the decay of particles containing charm quarks.
In this case the observed matter – anti-matter asymmetry is of about one per mille and the measurement
was made possible thanks to the huge number of D0 e D0 bar particles (containing charm quark and up anti-quark and charm anti-quark and up quark) produced in proton-proton collisions at the LHC. These particles, that have a lifetime smaller than one millionth of a millionth of a second, were identified through their decay products with extremely sophisticated techniques, with an experiment specifically designed to perform this kind of measurements and located in the LHC tunnel at CERN: LHCb.
Among the more than 1000 physicists from all over the world that participate in the LHCb collaboration there are also researchers from the Padova University and the INFN Padova division.
“This discovery opens a new field of investigation of the matter – anti-matter asymmetry, including for the first time the particles containing charm quarks to those already used for these studies” says Donatella Lucchesi, coordinator of the Padova research group involved in LHCb. “It also confirms what we had seen up to now, that is that the CP violation for particles containing quarks can be explained entirely by the so-called Standard Model of particle physics, but it is too small to explain the asymmetry between matter and anti-matter observed in the Universe. Therefore nature still has something in reserve that goes beyond our actual knowledge on elementary particles, to explain the very fact of our existence.”
The measurements that will be performed in the next data-taking campaign at the LHC, scheduled to start
in 2021, will employ even more sensitive instrumentation and an even greater amount of data, with the goal of observing any deviation from the Standard Model in order to make another step towards the completion of the theory.
The LHCb research group in Padova participates actively in these studies, preparing new strategies for the data analysis and working on a new detector that will increase the particle identification performance in the
next data-taking run at the LHC.