[ assl = (-0.24 ± 0.54 ± 0.33)% ]

Physicists from the LHCb experiment have today released results that help to shed light on one of the most significant experimental discrepancies with the Standard Model of particle physics. In 2010, and with an update in 2011, the D0 experiment, analyzing data taken at the proton-antiproton collider Tevatron at Fermilab, 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, see 2010 Fermilab Press Release. The observed difference was close to 1%, measured with almost the full D0 data sample of 9 fb-1. Like-signed dimuons can be produced from the decay of particles containing the b quark, which can mix between their particle and antiparticle states. A difference between the number of positive and negative dimuons would be an indication of CP violation. The D0 result differs by 3.9σ from the tiny value predicted in the framework of the Standard Model and could indicate the presence of a new physics contribution. This difference can be expressed as an asymmetry, Absl, where the label “b” indicates decay of particles containing b-quarks and “sl” (semileptonic) indicates that the decay involves leptons, in this case muons. The decaying matter (antimatter) B particles are composed of b-antiquarks(quarks) and d- or s-quarks(antiquarks). D0 physicists could not distinguish which type of decaying particles is at the origin of the measured asymmetry, therefore they present the measured asymmetry Absl in the image below as an inclined band across the plane of individual asymmetries in the decays of Bd and Bs mesons, labelled adsl and assl, respectively. The vertical band shows the measurement of the adsl asymmetry by the BaBar and Belle collaborations working at the Υ(4S) resonance, which is in agreement with the Standard Model calculations shown as the SM point in the image.

click the image for higher resolution

LHCb physicists have presented today at the ICHEP2012 Conference in Melbourne the most precise determination to date of the corresponding asymmetry for the Bs meson, assl. The LHCb result assl = (-0.24 ± 0.54 ± 0.33)% is shown as the horizontal blue band in the image above. The B0s semileptonic decays into D±sμ final states were studied to obtain this result. The charmed D±s mesons were reconstructed in the φπ± mode. More details can be found in the LHCb presentation in Melbourne and in the LHCb Conference Contribution here. The LHCb result is consistent with the Standard Model prediction and does not confirm the deviation from the Standard Model reported by the D0 experiment. The D0 experiment previously published a result using Dsμ events, shown as the horizontal yellow band.

The full 2011 LHCb data sample was used to obtain this result. LHCb physicists expect to more than triple their data sample this year.

Read also the CERN Bulletin article in English and French, in the CERN Courier article and the CERN Quantum Diaries.

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