Matter-antimatter puzzle: LHCb improves resolution

In 2010, Fermilab’s DØ experiment reported a one percent difference in the properties of matter and antimatter in decays of B mesons (that is, particles containing beauty quarks) to muons. Saturday, at the ICHEP Conference in Melbourne, the LHCb experiment at CERN presents new results, which do not confirm this anomaly and are consistent with the Standard Model predictions. The same experiment has also presented the first evidence of asymmetry arising in other decays of the same family of mesons. The image becomes clearer but the puzzle has not yet been solved.


Inside the LHCb detector.

The matter-antimatter imbalance in the Universe is a very hot topic in physics. The conundrum arises from the fact that, although objects made of antimatter are not observed in the Universe, theory predicts that matter and antimatter be created equally in particle interactions and in the Big Bang. Only small deviations from this very symmetric behaviour are incorporated in the theory. Experiments in the laboratory also confirm that the deviations are small and not enough to explain the total imbalance observed in the Universe.

The difference of about 1% reported by the DØ experiment was big enough to potentially indicate the effects of new physics. However, studying the particles produced in the high-energy collisions at the LHC, LHCb physicists have analysed the decay of the B0s particles into final states containing positive and negative muons that are used to tag matter and antimatter. The difference between the number of positive and negative muons can be expressed as an asymmetry parameter, assl. The value of this parameter found by the LHCb collaboration is the most precise to date and is found to be: assl = (-0.24 ± 0.54 ± 0.33)%.

This result is consistent with the Standard Model prediction and does not confirm the deviation from the Standard Model reported by the DØ experiment. However, the LHCb and DØ results are still compatible within 2 standard deviations, so more data will be needed to definitively resolve this issue

At the same conference, LHCb physicists also presented the first evidence that the matter-antimatter asymmetry arises in the B±→K±K+K- decay. LHCb physicists are planning further studies to fully understand this decay, which could help to shed light on the whole matter-antimatter picture.

The full 2011 LHCb data sample was used to obtain both results. Given the excellent performance of the LHC, LHCb physicists expect to more than triple their data sample this year.

More details can be found in the LHCb presentation given at ICHEP in Melbourne and in the LHCb conference contribution.

For further information, please read the LHCb public webpage.

by Antonella Del Rosso