Abstract
Lepton universality, described in the Standard Model (SM), predicts equal coupling between gauge bosons and the three lepton families. SM extensions give additional interactions, implying in some cases a stronger coupling with the third generation of leptons. Semileptonic decays of b-hadrons provide a sensitive probe to such New Physics effects. The presence of additional charged Higgs bosons, required by such SM extensions, can have significant effect on the semileptonic decay rate of B 0 → D*τν. A probe of new physics effects is the measurement of the quantities: and . The combination of experimental measurements performed by BaBar, Belle and LHCb observing the channel where the τ decays in leptons, gives a deviation from the SM prediction of about 4 σ. It is therefore important to perform additional measurements in this sector in order to improve the precision and confirm or disprove this deviation. Another possibility is to perform this measurement using the channel where the semileptonic τ decays in 3 pions. This in LHCb allows to have e better reconstruction of vertices and other kinematic variables. Results obtained by LHCb on B 0 → D*τν decays, where the τ decays hadronically, are reported.
Highlights
Semileptonic decays of b-hadrons provide a sensitive probe to such New Physics effects
A probe of new physics effects is the measurement of the quantities: R(D∗) =
Vertex inversion The most abundant background source is due to hadronic B decays into D∗3πX. background has a branching fraction that is about 100 times the signal one
Summary
The LHCb detector [1] (see Fig. 1) is a single arm spectrometer covering the rapidity region. There are some analysis challenges in an hadronic enviroment, e.g., it needs to find the correct kinematic variables in order to distinguish between signal and background, suppress the background due to additional charged and/or neutral particles, and find the best normalization channel. These challenges have different levels of importance and difficulty, and different solution between analyses, expecially between muonic and hadronic τ decays. 3. τ leptons with hadronic final state A way to measure R(D∗) is to reconstruct the τ − in the hadronic π−π+π−ντ decay channel. The quantities multiplied by K(D∗) are taken as external inputs
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