Abstract
In the Standard Model of particle physics the three charged leptons are identical copies of each other, apart from mass differences, and the electroweak coupling of the gauge bosons to leptons is independent of the lepton flavour. This prediction is called lepton flavour universality (LFU) and is well tested. In tree level decays, any violation of LFU would be a clear sign of physics beyond the Standard Model. Experimental tests of LFU in semileptonic decays of b-hadrons or rare b decays are highly sensitive to New Physics particles which preferentially couple to the 2nd and 3rd generations of leptons. Recent results from LHCb on lepton flavour universality in transitions and rare decays are discussed. The results are based on 3 fb−1 of proton-proton collisions collected at centre of mass energies and 8 TeV.
Highlights
In the Standard Model of particle physics (SM), the electroweak gauge bosons Z and W ± have identical couplings to all three lepton flavours. This means that branching fractions of decays involving different lepton families do not depend on lepton flavour but differ only by phase space and helicity-suppressed contributions
Any experimental evidence of Lepton Flavour Non-Universality would be a clear sign of physics beyond the SM (BSM)
Many models extending the SM contain additional interactions that could violate lepton flavour universality (LFU). These are for example BSM theories involving leptoquarks [1, 2] or Z′ [3, 4] particles
Summary
In the Standard Model of particle physics (SM), the electroweak gauge bosons Z and W ± have identical couplings to all three lepton flavours. Many models extending the SM contain additional interactions that could violate LFU These are for example BSM theories involving leptoquarks [1, 2] or Z′ [3, 4] particles. Processes with third generation of quarks and leptons (B and tau) are well suited to search for LFU violation since many BSM theories with LFU violation predict stronger couplings to the third generation and experimental results have lower precision. One such example is an extended Higgs Sector, which could have a large effect on semitauonic decay rates through the coupling to new charged Higgs scalars [5]. Systematic uncertainties are dominated by the size of the simulated and control samples
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