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Abstract:

The first search for the $B_s^0→K^{−}{\mathrm{Ï€}}^{+}\mathrm{γ}$ decay in the range $796<m(K^{−}{\mathrm{Ï€}}^{+})<1800\mathrm{MeV}/c^2$ is performed using data from proton-proton collisions collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{−1}$ . The photons are reconstructed through their conversion into an electron-positron pair, which significantly improves the mass resolution of the reconstructed decays with respect to decays with an unconverted photon. A signal excess with a significance of 3.5 standard deviations is measured, constituting the first experimental evidence for this decay. In the range $796<m(K^{−}{\mathrm{Ï€}}^{+})<996\mathrm{MeV}/c^2$ , the ratio $\mathcal{R}$ between the branching fractions of the signal decay and the favored ${\stackrel{¯}{B}}^0→K^{−}{\mathrm{Ï€}}^{+}\mathrm{γ}$ decay is measured to be $\mathcal{R}=(3.7Â\pm 1.2Â\pm 0.4)×{10}^{−2}$ where the first uncertainty is statistical and the second is systematic. This measurement is consistent with the value predicted in the Standard Model. In the range $996<m(K^{−}{\mathrm{Ï€}}^{+})<1800\mathrm{MeV}/c^2$ , the ratio $\mathcal{R}=(0.2Â\pm 2.7Â\pm 1.3)×{10}^{−2}$ is measured.

Abstract:

The decay $B^{Â\pm }→K_{\mathrm{S}}^0{\mathrm{Ï€}}^{Â\pm }$ , with a $CP$ asymmetry expected to be close to zero in the standard model, is theoretically clean and sensitive to potential new physics. An analysis of the decays $B^{Â\pm }→K_{\mathrm{S}}^0{\mathrm{Ï€}}^{Â\pm }$ and $B^{Â\pm }→K_{\mathrm{S}}^0{K}^{Â\pm }$ is performed using proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{−1}$ . The $CP$ asymmetries are determined to be ${\mathcal{A}}^{CP}(B^{Â\pm }→K_{\mathrm{S}}^0{\mathrm{Ï€}}^{Â\pm })=−0.028Â\pm 0.009Â\pm 0.009$ and ${\mathcal{A}}^{CP}(B^{Â\pm }→K_{\mathrm{S}}^0{K}^{Â\pm })=0.118Â\pm 0.062Â\pm 0.031$ , and the branching fraction ratio is measured to be $\mathcal{B}(B^{Â\pm }→K_{\mathrm{S}}^0{K}^{Â\pm })/\mathcal{B}(B^{Â\pm }→K_{\mathrm{S}}^0{\mathrm{Ï€}}^{Â\pm })=0.055Â\pm 0.004Â\pm 0.002$ , where the first uncertainties are statistical and the second are systematic. These results are the most precise measurements of these quantities to date. A search for the rare decay $B_c^{Â\pm }→K_{\mathrm{S}}^0{K}^{Â\pm }$ is also performed. No significant signal is observed, and the upper limit on the product of the branching fraction ratio $\mathcal{B}(B_c^{Â\pm }→K_{\mathrm{S}}^0{K}^{Â\pm })/\mathcal{B}(B^{Â\pm }→K_{\mathrm{S}}^0{\mathrm{Ï€}}^{Â\pm })$ and the fragmentation-fraction ratio $f_c/f_u$ is set to be 0.015 (0.016) at the 90% (95%) confidence level.

Abstract:

The normalised decay rate of B+ → J/ψ(→ μ+μ−)K+ is measured as a function of the lepton helicity angle using a data sample corresponding to an integrated luminosity of 1.1 fb−1 collected during October 2024 with the upgraded (Upgrade I) LHCb detector. This angular distribution can be parameterised by two coefficients, the forward-backward asymmetry, AFB, and the flatness parameter, FH, whose values are constrained by conservation of angular momentum. These coefficients are measured both integrated and differentially across various kinematic and detector-response variables, and the results are found to be in good agreement with expectations. These measurements show that the detector response of the LHCb Upgrade I experiment is understood to the precision required to reliably extract the angular coefficients associated with rare b → sμ+μ− and b → dμ+μ− transitions, which are particularly sensitive to physics beyond the Standard Model.