61 research outputs found
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
determination from inclusive decays: an alternative method
The determination of relies on the Heavy-Quark Expansion and the
extraction of the non-perturbative matrix elements from inclusive
decays. The proliferation of these matrix elements complicates their extraction
at and higher, thereby limiting the extraction.
Reparametrization invariance links different operators in the Heavy-Quark
expansion thus reducing the number of independent operators at 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 (), i.e. the differential rate and its moments. Currently,
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 . In light of this, we propose an experimental
analysis of the moments to open the possibility of a model-independent
extraction from semileptonic decays including the 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 -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 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 -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 leptonium
We determine the rates of the meson decays into a and an
bound state, the leptonium, where . 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 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 and they may become accessible at
the LHCb with 50 fb of integrated luminosity. In addition, since the
tau-pair threshold lies right between the and the
resonances, we estimate the charm-loop contribution to the decays 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
Exact results for and with two mass scales and up to three loops
We consider the on-shell mass and wave function renormalization constants and up to three-loop order allowing for a second non-zero quark mass. We obtain analytic results in terms of harmonic polylogarithms and iterated integrals with the additional letters and which extends the findings from ref. [1] where only numerical expressions are presented. Furthermore, we provide terms of order (ϵ2) and (ϵ) at two- and three-loop order which are crucial ingredients for a future four-loop calculation. Compact results for the expansions around the zero-mass, equal-mass and large-mass cases allow for a fast high-precision numerical evaluation
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