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Register a new userA new algorithm for identifying the flavour of $B_s^0$ mesons at LHCb
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A new algorithm for the determination of the initial flavour of $B_s^0$ mesons is presented. The algorithm is based on two neural networks and exploits the $b$ hadron production mechanism at a hadron collider. The first network is trained to select charged kaons produced in association with the $B_s^0$ meson. The second network combines the kaon charges to assign the $B_s^0$ flavour and estimates the probability of a wrong assignment. The algorithm is calibrated using data corresponding to an integrated luminosity of 3 fb$^{-1}$ collected by the LHCb experiment in proton-proton collisions at 7 and 8 TeV centre-of-mass energies. The calibration is performed in two ways: by resolving the $B_s^0$-$bar{B}_s^0$ flavour oscillations in $B_s^0 to D_s^- pi^+$ decays, and by analysing flavour-specific $B_{s 2}^{*}(5840)^0 to B^+ K^-$ decays. The tagging power measured in $B_s^0 to D_s^- pi^+$ decays is found to be $(1.80 pm 0.19({rm stat}) pm 0.18({rm syst}))$%, which is an improvement of about 50% compared to a similar algorithm previously used in the LHCb experiment.
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Two new algorithms for use in the analysis of $pp$ collision are developed to identify the flavour of $B^0$ mesons at production using pions and protons from the hadronization process. The algorithms are optimized and calibrated on data, using $B^0 rightarrow D^- pi^+$ decays from $pp$ collision data collected by LHCb at centre-of-mass energies of 7 and 8 TeV. The tagging power of the new pion algorithm is 60 % greater than the previously available one; the algorithm using protons to identify the flavour of a $B^0$ meson is the first of its kind.
Latest LHCb measurements of $CP$ violation in the interference between mixing and decay are presented based on $pp$ collision data collected during LHC Run I, corresponding to an integrated luminosity of $3.0, {rm fb}^{-1}$. Approximately $27, 000$ $B_s^0to J/psi pi^+pi^-$ signal events are used to make what is at the moment the most precise single measurement of the $CP$-violating phase in $bto cbar{c}s$ transitions, $phi_s^{cbar{c}s}=0.070pm0.068{rm (stat.)}pm0.008{rm (syst.)}$ rad. The most accurate measurement of the $CP$-violating phase in $bto sbar{s}s$ transitions, $phi_s^{sbar{s}s}$, is found from approximately $4, 000$ $B_s^0to phiphi$ signal events to be $phi_s^{sbar{s}s}=-0.17pm{rm (stat.)}0.15pm0.03{rm (syst.)}$ rad.
The calibration and performance of the opposite-side flavour tagging algorithms used for the measurements of time-dependent asymmetries at the LHCb experiment are described. The algorithms have been developed using simulated events and optimized and calibrated with B+ -> J/psi K+, B0 -> J/psi K*0 and B0 -> D*- mu+ nu_mu decay modes with 0.37 fb^-1 of data collected in pp collisions at sqrt(s) = 7 TeV during the 2011 physics run. The opposite-side tagging power is determined in the B+ -> J/psi K+ channel to be (2.10 +- 0.08 +- 0.24) %, where the first uncertainty is statistical and the second is systematic.
The LHCb experiment has the potential, during the 2010-11 run, to observe the rare decay $B^0_sto mu^+mu^-$ or improve significantly its exclusion limits. This study will provide very sensitive probes of New Physics (NP) effects. High sensitivity to NP contributions is also achieved by measuring photon polarization by performing a time dependent analysis of $B^0_s to phigamma$, and by an angular study of the decay $B^0_d to K^{*0}mu^+mu^-$. Preparations for these analyses are presented and studies shown of how existing data, for example prompt $J/psi$ events, can be used to validate the analysis strategy.
A structure is observed in the $B^+K^-$ mass spectrum in a sample of proton--proton collisions at centre-of-mass energies of 7, 8, and 13 TeV, collected with the LHCb detector and corresponding to a total integrated luminosity of 9 fb${}^-1$. The structure is interpreted as the result of overlapping excited $B_s^0$ states. With high significance, a two-peak hypothesis provides a better description of the data than a single resonance. Under this hypothesis the masses and widths of the two states, assuming they decay directly to $B^+K^-$, are determined to be $m_1 = 6063.5 pm 1.2 text{ (stat)} pm 0.8text{ (syst) MeV},$ $Gamma_1 = 26 pm 4 text{ (stat)} pm 4text{ (syst) MeV},$ $m_2 = 6114 pm 3 text{ (stat)} pm 5text{ (syst) MeV},$ $Gamma_2 = 66 pm 18 text{ (stat)} pm 21text{ (syst) MeV}.$ Alternative values assuming a decay through $B^{*+}K^-$, with a missing photon from the $B^{*+} rightarrow B^+gamma$ decay, which are shifted by approximately 45 MeV are also determined. The possibility of a single state decaying in both channels is also considered. The ratio of the total production cross-section times branching fraction of the new states relative to the previously observed $B_{s2}^{*0}$ state is determined to be $0.87 pm 0.15 text{ (stat)} pm 0.19 text{ (syst)}$.
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