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Anna Phan | USLHC | USA

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Measuring matter-antimatter asymmetries…

I’ve mentioned before that measuring CP violation is important in understanding why we have a matter dominated universe. So far, CP violation has been observed in the decay and mixing of neutral mesons containing strange, charm and bottom quarks and most measurements have been consistent with theory.

However, there is one measurement which has found evidence for significant CP violation in the decays of neutral B mesons, beyond what is expected from theory. In 2010, with an update in 2011, reported an interesting observation: that the number of events containing two positively charged muons is lower than the number of events containing two negatively charged muons. Like-sign dimuons can be produced from the decays of pairs of neutral B mesons, since they can mix between their particle and antiparticle states. A difference between the number of positive and negative dimuons is an indication of CP violation. The observed difference was close to 1% and 3.9σ away from the theory prediction. The analysis could not distinguish between the two different neutral B mesons, \(B^0_d\) and \(B^0_s\), so the difference had to be expressed in terms of two asymmetries: \(a^d_{sl}\), the semileptonic asymmetry of \(B^0_d\) mesons, and \(a^s_{sl}\), the semileptonic asymmetry of \(B^0_s\) mesons.

 
At ICHEP, DØ presented direct measurements of \(a^d_{sl}\) and \(a^s_{sl}\), by looking at the decays, \(B^0_d \rightarrow D^{(*)\pm}\mu^\mp X\) and \(B^0_s \rightarrow D_s^\pm\mu^\mp X\).

On the left, I have made a plot of these three results, comparing them to the Standard Model predictions. You can see that all three results are somewhat inconsistent with the prediction, which could indicate a contribution from new physics.

But of course, DØ isn’t the only experiment that is able to measure these asymmetries…

 
 

\(a^d_{sl}\) has been previously measured by both Belle and BaBar using \(B^0_d\) meson pairs produced by the decay of the \(\Upsilon(4S)\) meson and the results combined by the Heavy Flavour Averaging Group (HFAG).

And… LHCb released a preliminary result for ICHEP, measuring \(a^s_{sl}\) using \(B^0_s \rightarrow D_s^\pm\mu^\mp X\) decays.

On the right, I’ve added these results to the DØ ones, and now you can see that the situation now isn’t as compelling for new physics, with the BaBar, Belle and LHCb results all being compatible with the theory.

 
However, all experimental results are still compatible within two standard deviations, so new results are needed to definitively resolve the issue… Stay tuned to see if this is where evidence of new physics is found!

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