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Register a new userLepton Flavour Universality tests in electroweak penguin decays at LHCb
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Carla Marin Benito
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The coupling of the electroweak gauge bosons of the Standard Model (SM) to leptons is flavour universal. Extensions of the SM do not necessarily have this property. Rare decays of heavy flavour are suppressed in the SM and new particles may give sizeable contributions to these processes, therefore, their precise study allows for sensitive tests of lepton flavour universality. Of particular interest are rare b to sll decays that are well accessible at the LHCb experiment. Recent results from LHCb on lepton flavour universality in rare b to sll decays are discussed.
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The direct searches for Beyond Standard Model (BSM) particles have been constraining their mass scale to the extent where it is now becoming consensual that such particles are likely to be above the energy reach of the LHC. Meanwhile, the studies of indirect probes of BSM physics, with all their diversity, have been progressing both in accurracy and in setting up observables with reduced theoretical uncertainties. The observation of flavour anomalies in $b$ hadron decays represents an important part of the program of indirect detection of BSM physics. Several benchmark analyses involving leptonic or semileptonic decays are presented, with an emphasis on intriguing patterns which are systematic in their trend, though not individually significant yet.
We report on recent results on radiative and electroweak penguin B decays at Belle at KEKB accelerator.
Searches for the lepton flavour violating decay $tau^-rightarrow mu^-mu^+mu^-$ and the lepton flavour and baryon number violating decays $tau^-rightarrow bar{p}mu^+mu^-$ and $tau^-rightarrow pmu^-mu^-$ have been carried out using proton-proton collision data, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, taken by the LHCb experiment at $sqrt{s} = 7$ TeV. No evidence has been found for any signal, and limits have been set at 90% confidence level on the branching fractions: $cal B(tau^-rightarrow mu^-mu^+mu^-) < 8.0 times 10^{-8}$, $cal B(tau^-rightarrow bar{p}mu^+mu^-) < 3.3 times 10^{-7}$ and $cal B(tau^-rightarrow pmu^-mu^-) < 4.4 times 10^{-7}$. The results for the $tau^-rightarrow bar{p}mu^+mu^-$ and $tau^-rightarrow pmu^-mu^-$ decay modes represent the first direct experimental limits on these channels.
A precision test of lepton flavour universality has been performed by measuring the ratio RK of kaon leptonic decay rates K+ --> e+nu and K+ --> mu+nu in a sample of 59813 reconstructed K+ --> e+nu candidates with (8.71 +- 0.24)% background contamination. The result RK = (2.487 +- 0.013) * 10^{-5} is in agreement with the Standard Model expectation.
An algorithm is described for tagging the flavour content at production of neutral $B$ mesons in the LHCb experiment. The algorithm exploits the correlation of the flavour of a $B$ meson with the charge of a reconstructed secondary charm hadron from the decay of the other $b$ hadron produced in the proton-proton collision. Charm hadron candidates are identified in a number of fully or partially reconstructed Cabibbo-favoured decay modes. The algorithm is calibrated on the self-tagged decay modes $B^+ to J/psi , K^+$ and $B^0 to J/psi , K^{*0}$ using $3.0mathrm{,fb}^{-1}$ of data collected by the LHCb experiment at $pp$ centre-of-mass energies of $7mathrm{,TeV}$ and $8mathrm{,TeV}$. Its tagging power on these samples of $B to J/psi , X$ decays is $(0.30 pm 0.01 pm 0.01) %$.
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