LHCb

Abstract:

A binned Dalitz plot analysis of the decays B 0 → DK ∗0, with D → K 0 S π + π − and D → K 0 S K + K −, is performed to measure the observables x ± and y ±, which are related to the CKM angle γ and the hadronic parameters of the decays. The D decay strong phase variation over the Dalitz plot is taken from measurements performed at the CLEO-c experiment, making the analysis independent of the D decay model. With a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb−1, collected by the LHCb experiment, the values of the CP violation parameters are found to be x + = 0.05 ± 0.35 ± 0.02, x − = −0.31 ± 0.20 ± 0.04, y + = −0.81 ± 0.28 ± 0.06 and y − = 0.31 ± 0.21 ± 0.05, where the first uncertainties are statistical and the second systematic. These observables correspond to values γ = (71 ± 20)°, and . The parameters and are the magnitude ratio and strong phase difference between the suppressed and favoured B 0 decay amplitudes, and have been measured in a region of ±50 MeV/c 2 around the K ∗(892)0 mass and with the magnitude of the cosine of the K ∗(892)0 helicity angle larger than 0.4.

Abstract:

The TORCH time-of-flight (TOF) detector is being developed to provide particle identification up to a momentum of 10 GeV/c over a flight distance of 10 m. It has a DIRC-like construction with thick synthetic amorphous fused-silica plates as a Cherenkov radiator. Photons propagate by total internal reflection to the plate periphery where they are focused onto an array of customised position-sensitive micro-channel plate (MCP) detectors. The goal is to achieve a 15 ps time-of-flight resolution per incident particle by combining arrival times from multiple photons. The MCPs have pixels of effective size 0.4 mm×6.6 mm2 in the vertical and horizontal directions, respectively, by incorporating a novel charge-sharing technique to improve the spatial resolution to better than the pitch of the readout anodes. Prototype photon detectors and readout electronics have been tested and calibrated in the laboratory. Preliminary results from testbeam measurements of a prototype TORCH detector are also presented.

Abstract:

A study of the decay $D^{0}\rightarrow K^{-}\pi^{+}\mu^{+}\mu^{-}$ is performed using data collected by the LHCb detector in proton-proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 2.0 fb$^{-1}$. Decay candidates with muon pairs that have an invariant mass in the range 675--875 MeV$/c^2$ are considered. This region is dominated by the $\rho^{0}$ and $\omega$ resonances. The branching fraction in this range is measured to be ${\cal B}$($D^{0}\rightarrow K^{-}\pi^{+}\mu^{+}\mu^{-}$ ) = $( 4.17 \pm 0.12(stat) \pm 0.40(syst) )\times10^{-6}$. This is the first observation of the decay $D^{0}\rightarrow K^{-}\pi^{+}\mu^{+}\mu^{-}$. Its branching fraction is consistent with the value expected in the Standard Model.

Abstract:

© 2016 The Authors.Measurements of the coherence factor RK3π, the average strong-phase difference δDK3π and mean amplitude ratio rDK3π for the decay D→K-π+π+π- are presented. These parameters are important inputs to the determination of the unitarity triangle angle γ in B-→DK- decays, where D designates a superposition of D0 and D-0 mesons decaying to a common final state. The results are based on a combined fit to observables obtained from a re-analysis of the CLEO-c ψ(3770) data set and those measured in a D0D-0 mixing study performed by the LHCb Collaboration.

Abstract:

Charm meson oscillations are observed in a time-dependent analysis of the ratio of D⁰ → K⁺π⁻π⁺π⁻ to D⁰ → K⁻π⁺π⁻π⁺ decay rates, using data corresponding to an integrated luminosity of 3.0  fb^⁻1 recorded by the LHCb experiment. The measurements presented are sensitive to the phase-space averaged ratio of doubly Cabibbo-suppressed to Cabibbo-favored amplitudes rDK3π and the product of the coherence factor RDK3π and a charm mixing parameter y'K3π. The constraints measured are rDK3π=(5.67±0.12)×10^-2, which is the most precise determination to date, and RDK3π . y'K3π=(0.3 ± 1.8) × 10^-3, which provides useful input for determinations of the CP-violating phase γ in B^± → DK^±, D → K∓π^±π^∓π^± decays. The analysis also gives the most precise measurement of the D⁰ → K⁺π⁻π⁺π⁻ branching fraction, and the first observation of D0 - D0 oscillations in this decay mode, with a significance of 8.2 standard deviations.