Towards a determination of the tau lepton dipole moments

The tau anomalous magnetic moment (a_tau) and electric dipole moment (d_tau) have not yet been observed. The present bounds on their values are of order 10^-2 and 10^-17 e*cm, respectively. We propose to measure a_tau with a precision of O(10^-3) or better and improve the existing limits on d_tau using precise tau- -> l- nu_tau \bar{nu}_l gamma (l=e or mu) data from high-luminosity B factories. A detailed feasibility study of this method is underway.Comment: 4 pages, presented at the 12th International Workshop on Tau Lepton Physics, Nagoya, Japan, 17-21 September 201

VcbV_{cb} determination from inclusive b→cb \to c decays: an alternative method

The determination of $V_{cb}$ relies on the Heavy-Quark Expansion and the extraction of the non-perturbative matrix elements from inclusive $b\to c$ decays. The proliferation of these matrix elements complicates their extraction at $1/m_b^4$ and higher, thereby limiting the $V_{cb}$ extraction. Reparametrization invariance links different operators in the Heavy-Quark expansion thus reducing the number of independent operators at $1/m_b^4$ to eight for the total rate. We show that this reduction also holds for spectral moments as long as they are defined by reparametrization invariant weight-functions. This is valid in particular for the leptonic invariant mass spectrum ($q^2$), i.e. the differential rate and its moments. Currently, $V_{cb}$ is determined by fitting the energy and hadronic mass moments, which do not manifest this parameter reduction and depend on the full set of 13 matrix elements up to $1/m_b^4$. In light of this, we propose an experimental analysis of the $q^2$ moments to open the possibility of a model-independent $V_{cb}$ extraction from semileptonic decays including the $1/m_b^4$ terms in a fully data-driven way.Comment: 16 pages, 2 figures. v2: version published in JHEP, references added plus minor change

B physics Beyond the Standard Model at One Loop: Complete Renormalization Group Evolution below the Electroweak Scale

General analyses of $B$-physics processes beyond the Standard Model require accounting for operator mixing in the renormalization-group evolution from the matching scale down to the typical scale of $B$ physics. For this purpose the anomalous dimensions of the full set of local dimension-six operators beyond the Standard Model are needed. We present here for the first time a complete and non-redundant set of dimension-six operators relevant for $B$-meson mixing and decay, together with the complete one-loop anomalous dimensions in QCD and QED. These results are an important step towards the automation of general New Physics analyses.Comment: 37 pages, 3 figures + ancillary mathematica package. Version published in JHEP, with a corrected list of lepton-number-violating operator

On the decays B→K(∗)+B \to K^{(*)} + leptonium

We determine the rates of the $B$ meson decays into a $K^{(*)}$ and an $\ell^+ \ell^-$ bound state, the leptonium, where $\ell = e,\mu,\tau$. The two spin states of the leptonium, the spin singlet and the spin triplet, couple to the axial current and to the vector current, respectively, thus probing different helicity structures of the underlying $b\to s \ell^+\ell^-$ effective Hamiltonian. Since ortho- and para-leptonia have different decay modes, a distinction between the two is relatively easy and these decays may become a cross check for the results of lepton-flavour-violation searches obtained with free leptons. We find that some of the decays involving muon and tau have a branching ratio of the order of $10^{-13}$ and they may become accessible at the LHCb with 50 fb$^{-1}$ of integrated luminosity. In addition, since the tau-pair threshold lies right between the $J/\psi$ and the $\psi(2S)$ resonances, we estimate the charm-loop contribution to the decays $B \to K^{(*)} +$tauonium.Comment: 17 page

Electromagnetic dipole moments of fermions

The electric (EDM) and magnetic (g-2) dipole moments are static properties sensitive to quantum corrections induced by the virtual particles that populate the vacuum. Indeed, they are well suited to test the Standard Model of Elementary of particle physics and to unveil unknown New Physics (NP) hidden at high energy. The electron and muon g-2 have been measured with the wonderful precision of 0.24 ppb and 0.54 ppm, respectively, and thus they represent one of the strongest confirmation of the SM and greatest achievement in Quantum Field Theory. Nonetheless the SM deficiencies, the explanation of dark matter and dark energy, cosmological inflaton, neutrino oscillations and masses, the strong CP problem and the origin of matter-antimatter asymmetry, call for new physics beyond the SM. Since NP contribution to the dipole moments of a fermion f is expected to be proportional to m_f^2, dipole moments of heavy fermions, such as the top quark or the tau lepton, are much more sensitive to NP effects than the electron or muon ones. However the very short lifetime of these unstables particle makes it impossible to directly measure their electromagnetic properties. Therefore, indirect information must be obtain by precisely measuring cross sections and decay rates in processes involving the emission of a real photon by the heavy fermion. In this thesis, we investigate the possibility to measure the anomalous magnetic moment and the electric dipole moment of the top quark at the LHC and tau lepton at future high luminosity B-factories