A search for the decays Bs-->mumu and Bd-->mumu is performed with about 37 pb^{-1} of pp collisions at sqrt{s} = 7 TeV collected by the LHCb experiment at the Large Hadron Collider at CERN. The ...observed numbers of events are consistent with the background expectations. The resulting upper limits on the branching ratios are BR(Bs-->mumu) < 5.6 x 10^{-8} and BR(Bd-->mumu) <1.5 x 10^{-8} at 95% confidence level.
The decay asymmetries of the weak radiative hyperon decays
Ξ
0
→
Λ
γ
and
Ξ
0
→
Σ
0
γ
have been measured with high precision using data of the NA48/1 experiment at CERN. From about 52 000
Ξ
0
→
Λ
γ
...and 15 000
Ξ
0
→
Σ
0
γ
decays, we obtain for the decay asymmetries
α
Ξ
0
→
Λ
γ
=
−
0.704
±
0.019
stat
±
0.064
syst
and
α
Ξ
0
→
Σ
0
γ
=
−
0.729
±
0.030
stat
±
0.076
syst
, respectively. These results are in good agreement with previous experiments, but more precise.
With the steady increase in the precision of flavour physics measurements collected during LHC Run 2, the LHCb experiment requires simulated data samples of larger and larger sizes to study the ...detector response in detail. The simulation of the detector response is the main contribution to the time needed to simulate full events. This time scales linearly with the particle multiplicity. Of the dozens of particles present in the simulation only the few participating in the signal decay under study are of interest, while all remaining particles mainly affect the resolutions and efficiencies of the detector. This paper presents a novel development for the LHCb simulation software which re-uses the rest of the event from previously simulated events. This approach achieves an order of magnitude increase in speed and the same quality compared to the nominal simulation.
The weak radiative decay Ξ0→Λe+e− has been detected for the first time. We find 412 candidates in the signal region, with an estimated background of 15±5 events. We determine the branching fraction ...B(Ξ0→Λe+e−)=7.6±0.4(stat)±0.4(syst)±0.2(norm)×10−6, consistent with an internal bremsstrahlung process, and the decay asymmetry parameter αΞΛee=−0.8±0.2, consistent with that of Ξ0→Λγ. The charge conjugate reaction Ξ0¯→Λ¯e+e− has also been observed.
From 56 days of data taking in 2002, the NA48/1 experiment observed 6316 Ξ0→Σ+e−ν¯e candidates (with the subsequent Σ+→pπ0 decay) and 555 Ξ0¯→Σ+¯e+νe candidates with background contamination of ...215±44 and 136±8 events, respectively. From these samples, the branching ratios BR(Ξ0→Σ+e−ν¯e)=(2.51±0.03stat±0.09syst)×10−4 and BR(Ξ0¯→Σ+¯e+νe)=(2.55±0.14stat±0.10syst)×10−4 were measured allowing the determination of the CKM matrix element |Vus|=0.209−0.028+0.023. Using the Particle Data Group average for |Vus| obtained in semileptonic kaon decays, we measured the ratio g1/f1=1.20±0.05 of the axial-vector to vector form factors.
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