David Bacher

Abstract:

The first searches for $B^0\to K^{+}{\pi }^{-}{\tau }^{+}{\tau }^{-}$ and $B_s^0\to K^{+}{K}^{-}{\tau }^{+}{\tau }^{-}$ decays at the LHCb experiment are conducted with $pp$ collision data corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ . The tau leptons are reconstructed using the ${\tau }^{+}\to {\mu }^{+}{\overline{\nu }}_{\tau }{\nu }_{\mu }$ decay and the results are presented in bins of $K^{+}{\pi }^{-}$ or $K^{+}{K}^{-}$ mass. No signal is observed and upper limits are set on the branching fractions. The searches result in the first upper limits for $B^0\to K^{+}{\pi }^{-}{\tau }^{+}{\tau }^{-}$ decays outside the $K^{*}(892{)}^0$ region in $K^{+}{\pi }^{-}$ mass and the first limits for $B_s^0\to K^{+}{K}^{-}{\tau }^{+}{\tau }^{-}$ decays. The searches are recast into limits on the decays $B^0\to K^{*}(892{)}^0{\tau }^{+}{\tau }^{-}$ and $B_s^0\to \varphi (1020){\tau }^{+}{\tau }^{-}$ , yielding $2.8\times {10}^{-4}$ ( $2.5\times {10}^{-4}$ ) and $4.7\times {10}^{-4}$ ( $4.1\times {10}^{-4}$ ) at the 95% (90%) confidence level, respectively. For the decay $B^0\to K^{*}(892{)}^0{\tau }^{+}{\tau }^{-}$ , this result improves on the current best upper limit by an order of magnitude.

Abstract:

The first search for the $B_s^0\to K^{-}{\pi }^{+}\gamma$ decay in the range $796<m(K^{-}{\pi }^{+})<1800\mathrm{MeV}/c^2$ 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 $796<m(K^{-}{\pi }^{+})<996\mathrm{MeV}/c^2$ , the ratio $\mathcal{R}$ between the branching fractions of the signal decay and the favored ${\overline{B}}^0\to K^{-}{\pi }^{+}\gamma$ decay is measured to be $\mathcal{R}=(3.7\pm 1.2\pm 0.4)\times {10}^{-2}$ 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 $996<m(K^{-}{\pi }^{+})<1800\mathrm{MeV}/c^2$ , the ratio $\mathcal{R}=(0.2\pm 2.7\pm 1.3)\times {10}^{-2}$ is measured.

Abstract:

The decay $B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm }$ , 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 $B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm }$ and $B^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm }$ 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 ${\mathcal{A}}^{CP}(B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm })=-0.028\pm 0.009\pm 0.009$ and ${\mathcal{A}}^{CP}(B^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm })=0.118\pm 0.062\pm 0.031$ , and the branching fraction ratio is measured to be $\mathcal{B}(B^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm })/\mathcal{B}(B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm })=0.055\pm 0.004\pm 0.002$ , 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 $B_c^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm }$ is also performed. No significant signal is observed, and the upper limit on the product of the branching fraction ratio $\mathcal{B}(B_c^{\pm }\to K_{\mathrm{S}}^0{K}^{\pm })/\mathcal{B}(B^{\pm }\to K_{\mathrm{S}}^0{\pi }^{\pm })$ 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.

Abstract:

The data acquisition system of the upgraded LHCb experiment includes the fast reconstruction of all hits in the vertex locator (VELO) pixel detector at the beam-crossing rate of 40 MHz, implemented as on-the-fly clustering embedded in the firmware of the readout board FPGAs. The availability of a high rate of reconstructed clusters in real time enables a new fast approach for measuring luminosity and monitoring the LHCb luminous region, directly at the detector readout level. This methodology has been implemented as an array of real-time cluster counters in the VELO readout FPGAs and has been in operation since the start of the 2024 physics run of LHCb. This paper describes the methodology and its features and performance, both on proton-proton and lead-lead collision data. The method shows a statistical resolution better than the percent level, and a sensitivity to variable running conditions of the same level. This is achieved with an intrinsic time granularity better than 100 ms, undersampled to 3 s for analysis purposes. Nonlinear behaviour is compatible with zero in a luminosity range including the LHCb Run 3 operating point.