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Flavio Archilli
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Rare lepton decays of the B(s), D and K mesons are sensitive probes of New Physics. In particular, the search for the decays $B^0_(s) -> mu^+ mu^-$ provides information on the presence of new (pseudo-)scalar particles. LHCb is well suited for these analyses due to its large acceptance and trigger efficiency, as well as its excellent invariant mass resolution and lepton identification capabilities. The status of these analyses with $sim1$ fb$^{-1}$ of pp collisions collected by LHCb in 2011 at $sqrt{s}=7$ TeV is reviewed.
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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.
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.
Rare decays involving leptons or photons in the final states are studied using 1.0 fb^{-1} of pp collisions at a centre-of-mass energy of sqrt{s}=7TeV collected by the LHCb experiment in 2011. We present results of measurements of branching ratios, angular distributions, and isospin asymmetries obtained using this data sample.
The rare B decays Bs(d)-->mumu, B-->K*mumu and Bs-->phigamma are studied using up to sim 0.41 fb^{-1} of pp collisions at sqrt{s} = 7 TeV collected by the lhcb experiment in 2010 and 2011. A search for the decays Bs(d)-->mumu, is performed with 0.41 fb^{-1} . The absence of significant signal leads to BR(Bs-->mumu<1.4 x 10^{-8} and BR(Bd-->mumu<3.2 x 10^{-9} at 95% confidence level. The forward-backward asymmetry, fraction of longitudinal polarization and differential branching fraction of B-->K*mumu, as a function of dimuon invariant mass, are measured in 0.31 fb^{-1}. The ratio of branching ratios of the radiative B decays B--> K*gamma and Bs-->phigamma h as been measured using 0.34fb^{-1}. The obtained value for the ratio is 1.52 pm 0.14 (stat) pm 0.10 (syst) pm 0.12 (f_s/f_d). Using the HFAG value for BR(Bd --> K^*gamma), BR(Bs-->phigamma) has been found to be (2.8 pm 0.5)times 10^{-5}.
We report new measurements of the branching fraction $cal B(D_s^+to ell^+ u)$, where $ell^+$ is either $mu^+$ or $tau^+(topi^+bar{ u}_tau)$, based on $6.32$ fb$^{-1}$ of electron-positron annihilation data collected by the BESIII experiment at six center-of-mass energy points between $4.178$ GeV and $4.226$ GeV. Simultaneously floating the $D_s^+tomu^+ u_mu$ and $D_s^+totau^+ u_tau$ components yields $cal B(D_s^+to tau^+ u_tau) = (5.21pm0.25pm0.17)times10^{-2}$, $cal B(D_s^+to mu^+ u_mu) = (5.35pm0.13pm0.16)times10^{-3}$, and the ratio of decay widths $R=frac{Gamma(D_s^+to tau^+ u_tau)}{Gamma(D_s^+to mu^+ u_mu)} = 9.73^{+0.61}_{-0.58}pm 0.36$, where the first uncertainties are statistical and the second systematic. No evidence of $CP$ asymmetry is observed in the decay rates $D_s^pmtomu^pm u_mu$ and $D_s^pmtotau^pm u_tau$: $A_{CP}(mu^pm u) = (-1.2pm2.7)%$ and $A_{CP}(tau^pm u) = (+2.9pm4.9)%$. Constraining our measurement to the Standard Model expectation of lepton universality ($R=9.75$), we find the more precise results $cal B(D_s^+to tau^+ u_tau) = (5.22pm0.10pm 0.14)times10^{-2}$ and $A_{CP}(tau^pm u_tau) = (-0.1pm2.1)%$. Combining our results with inputs external to our analysis, we determine the $cto s$ quark mixing matrix element, $D_s^+$ decay constant, and ratio of the decay constants: $|V_{cs}| = 0.973pm0.009pm0.014$, $f_{D^+_s} = 249.9pm2.4pm3.5~text{MeV}$, and $f_{D^+_s}/f_{D^+} = 1.232pm0.035$.
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