[ φs = 0.03 ± 0.16 ± 0.07 ]
LHCb physicists have presented today the most precise measurement of φs (the Bs mixing phase, for experts) at the Lepton Photon conference in Mumbai (India). The value of φs is precisely predicted in the Standard Model and sets the scale for the difference between properties of matter and antimatter for Bs mesons, known to physicists as CP violation. The predicted value is small and therefore the effects of new physics could change its value significantly – see the analogy in the 8 April news.
The decay of the strange-beauty particle B0s, composed of a beauty antiquark (b) bound with a strange quark (s), into a J/ψ meson and a φ meson was used for this measurement. The J/ψ meson decays in turn into a μ+μ– pair, and the φ decays to K+K– pair. In order to make this difficult measurement LHCb physicists had to analyse the Bs decay particles in 3 dimensions as well as to measure precisely the fast oscillations of strange beauty – see 15 March news.
The “artist’s view” below shows the result of the φs measurement in a plane together with the correlated measurement of another value, ΔΓs. The results of the measurement favour two regions, one of which is located around φs = -0.036 ± 0.002 rad, the Standard Model prediction. The LHCb measurement is in agreement with the Standard Model prediction but the shaded region representing the LHCb result indicates that there is still room for a new physics contribution. The hints for a larger contribution of new physics suggested by the CDF and D0 experiments at Fermilab, also shown in the figure, are not confirmed.
click the image for higher resolution
In February the LHCb Collaboration made first observation of the Bs decay into J/ψ f0(980) – see 27 February news. This decay contributed to the φs measurement.
Using both Bs decays LHCb physicists have obtained the value φs = 0.03 ± 0.16 ± 0.07.
Read also CERN Courier article.