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Register a new userMeasurement of phi_s at LHCb
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Stephanie Hansmann-Menzemer
Publication date
2010
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English
Authors
Stephanie Hansmann-Menzemer
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A time dependent angular analysis of the decay mode $B_s rightarrow J/psi phi$ allows for the measurement of the mixing induced CP-violating phase $phi_s$. Within the Standard Model $phi_s$ is theoretically precisely predicted to be very small, however many Standard Model extensions predict sizeable contributions to this phase. The current experimental knowledge of $phi_s$ has very larger uncertainties. However already with the data expected to be delivered within the next year, the LHCb experiment at the Large Hadron Collider at CERN, has the potential to improve significantly existing measurements. In a data set of up to 37.5 pb$^{-1}$ taken in 2010, first physics signals in the LHCb detector are reconstructed and their properties are compared to Monte Carlo predictions. Based on recently published measurements of $bbar{b}$ cross-sections from the LHCb collaboration, the sensitivity on the $CP$ violating phase $phi_s$ in the decay $B_s rightarrow J/psi phi$ is evaluated. Additionally an alternative method to potentially extract complementary information on $phi_s$ from the measurement of the asymmetry in semileptonic final states is presented.
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These proceedings present the current status of measurements of the CP-violating phase $phi_s$ by the LHCb collaboration, reviewing the measurements in channels such as $B_s^0to J/psiphi$, $B_s^0to J/psi pi^+pi^-$ and $B_s^0 to psi(2S)phi$. The observation of the $B_s^0toeta_cphi$ decay mode is presented for the first time, which can be used to measure $phi_s$ with larger data samples that will be collected over the coming years by the LHCb experiment. Finally, the expected increase in precision from LHCb measurements of $phi_s$ over the next decade is presented.
The single electron track-reconstruction efficiency is calibrated using a sample corresponding to $1.3~mathrm{fb}^{-1}$of $pp$ collision data recorded with the LHCb detector in 2017. This measurement exploits $B^+to J/psi (e^+e^-)K^+$ decays, where one of the electrons is fully reconstructed and paired with the kaon, while the other electron is reconstructed using only the information of the vertex detector. Despite this partial reconstruction, kinematic and geometric constraints allow the $B^+$-meson mass to be reconstructed and the signal to be well separated from backgrounds. This in turn allows the electron reconstruction efficiency to be measured by matching the partial track segment found in the vertex detector to tracks found by LHCbs regular reconstruction algorithms. The agreement between data and simulation is evaluated, and corrections are derived for simulated electrons in bins of kinematics. These correction factors allow LHCb to measure branching fractions involving single electrons with a systematic uncertainty below $1%$.
The determination of track reconstruction efficiencies at LHCb using $J/psirightarrowmu^{+}mu^{-}$ decays is presented. Efficiencies above $95%$ are found for the data taking periods in 2010, 2011, and 2012. The ratio of the track reconstruction efficiency of muons in data and simulation is compatible with unity and measured with an uncertainty of $0.8,%$ for data taking in 2010, and at a precision of $0.4,%$ for data taking in 2011 and 2012. For hadrons an additional $1.4,%$ uncertainty due to material interactions is assumed. This result is crucial for accurate cross section and branching fraction measurements in LHCb.
The results on rare decay processes obtained by the LHCb experiment using 1.0 fb^(-1) of pp collisions collected in 2011 at a centre-of-mass energy of sqrt(s)=7 TeV are presented. Branching fractions, angular distributions, CP and isospin asymmetries are investigated to search for new physics effects.
We report on the first searches for lepton flavour violating $tau^-$ decays at a hadron collider. These include searches for the lepton flavour violating decay $tau^-to mu^+mu^-mu^-$ and the lepton flavour and baryon number violating decays $tau^-to bar{p}mu^+mu^-$ and $tau^-to pmu^-mu^-$. Upper limits of ${cal B}(tau^-to mu^+mu^-mu^-) < 4.6 times 10^{-8}$, ${cal B}(tau^-to bar{p}mu^+mu^-) < 3.4 times 10^{-7}$ and ${cal B}(tau^-to pmu^-mu^-) < 4.6 times 10^{-7}$ are set at 90% confidence level. A measurement of the inclusive $Ztotau^+tau^-$ cross-section at 7 TeV is also reported and is found to be consistent with the Standard Model. The ratio of the $Ztotau^+tau^-$ cross-section to the $Ztomu^+mu^-$ cross-section is found to be consistent with lepton universality.
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