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Register a new userObservation of $B^0_s$-$bar{B}^0_s$ mixing and measurement of mixing frequencies using semileptonic B decays
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The $B^0_s$ and $B^0$ mixing frequencies, $Delta m_s$ and $Delta m_d$, are measured using a data sample corresponding to an integrated luminosity of 1.0 fb^{-1} collected by the LHCb experiment in $pp$ collisions at a centre of mass energy of 7 TeV during 2011. Around 1.8x10^6 candidate events are selected of the type $B^0_{(s)} to D^-_{(s)} mu^+$ (+ anything), where about half are from peaking and combinatorial backgrounds. To determine the B decay times, a correction is required for the momentum carried by missing particles, which is performed using a simulation-based statistical method. Associated production of muons or mesons allows us to tag the initial-state flavour and so to resolve oscillations due to mixing. We obtain Delta m_s = (17.93 pm 0.22 (stat) pm 0.15 (syst)) ps^{-1}, Delta m_d = (0.503 pm 0.011 (stat) pm 0.013 (syst)) ps^{-1}. The hypothesis of no oscillations is rejected by the equivalent of 5.8 standard deviations for $B^0_s$ and 13.0 standard deviations for $B^0$. This is the first observation of $B^0_s$ mixing to be made using only semileptonic decays.
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We determine hadronic matrix elements relevant for the mass and width differences, $Delta M_s$ & $Delta Gamma_s$ in the $B^0_s - bar{B^0_s}$ meson system using fully unquenched lattice QCD. We employ the MILC collaboration gauge configurations that include $u$, $d$ and $s$ sea quarks using the improved staggered quark (AsqTad) action and a highly improved gluon action. We implement the valence $s$ quark also with the AsqTad action and use Nonrelativistic QCD for the valence $b$ quark. For the nonperturbative QCD input into the Standard Model expression for $Delta M_s$ we find $f_{B_s} sqrt{hat{B}_{B_s}} = 0.281(21)$GeV. Results for four-fermion operator matrix elements entering Standard Model formulas for $Delta Gamma_s$ are also presented.
The CKM angle $gamma$ is measured for the first time from mixing-induced $CP$ violation between $B^0_s rightarrow D_s^mp K^pm pi^pm pi^mp$ and $bar{B}^0_s rightarrow D_s^pm K^mp pi^mp pi^pm$ decays reconstructed in proton-proton collision data corresponding to an integrated luminosity of 9 ${rm fb}^{-1}$ recorded with the LHCb detector. A time-dependent amplitude analysis is performed to extract the $CP$-violating weak phase $gamma-2beta_s$ and, subsequently, $gamma$ by taking the $B^0_s$-$bar{B}^0_s$ mixing phase $beta_{s}$ as an external input. The measurement yields $gamma = (44 pm 12)^circ$ modulo $180^circ$, where statistical and systematic uncertainties are combined. An alternative model-independent measurement, integrating over the five-dimensional phase space of the decay, yields $gamma = (44^{,+,20}_{,-,13})^circ$ modulo $180^circ$. Moreover, the $B^0_s$-$bar{B}^0_s$ oscillation frequency is measured from the flavour-specific control channel $B^0_s rightarrow D_s^- pi^+ pi^+ pi^-$ to be $Delta m_s = (17.757 pm 0.007 ,({rm stat.}) pm 0.008 ,({rm syst.})) text{ps}^{-1}$, consistent with and more precise than the current world-average value.
The B^0_s-bar{B}^0_s oscillation frequency Delta m_s is measured with 36 pb^{-1} of data collected in pp collisions at sqrt{s} = 7 TeV by the LHCb experiment at the Large Hadron Collider. A total of 1381 B^0_s -> D_s^- pi^+ and B^0_s -> D_s^- pi^+ pi^- pi^+ signal decays are reconstructed, with average decay time resolutions of 44 fs and 36 fs, respectively. An oscillation signal with a statistical significance of 4.6 sigma is observed. The measured oscillation frequency is Delta m_s = 17.63 pm 0.11 (stat) pm 0.02 (syst) ps^{-1}.
A simple analysis of time-dependent $B_sto K^+K^-$ transitions, based on recent results from the LHCb experiment, is presented. The benefits of adopting a fully consistent theoretical description of the $B^0_s$--$bar B^0_s$ mixing are stressed. It is shown that bounds on CPT violation in the $B^0_s$--$bar B^0_s$ system can be consistently obtained and that direct CP violation in $B_sto K^+K^-$ can be robustly established, even in the presence of CPT violation in the mixing.
The first observation of the $B^0_stobar{D}^0 K^0_S$ decay mode and evidence for the $B^0_stobar{D}^{*0} K^0_S$ decay mode are reported. The data sample corresponds to an integrated luminosity of 3.0 $text{fb}^{-1}$ collected in $pp$ collisions by LHCb at center-of-mass energies of 7 and 8 TeV. The branching fractions are measured to be begin{align*} mathcal{B}(B^0_stobar{D}^0 bar{K}^0) &= (4.3pm0.5(text{stat})pm0.3(text{syst})pm0.3(text{frag})pm0.6(text{norm}))times10^{-4}, mathcal{B}(B^0_stobar{D}^{*0} bar{K}^0) &= (2.8pm1.0(text{stat})pm0.3(text{syst})pm0.2(text{frag})pm0.4(text{norm}))times10^{-4}, end{align*} where the uncertainties are due to contributions coming from statistical precision, systematic effects, and the precision of two external inputs, the ratio $f_s/f_d$ and the branching fraction of $B^0tobar{D}^0 K^0_S$, which is used as a calibration channel.
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