196,191 research outputs found
Hadronic corrections to μ-e scattering at NNLO with space-like data
No full textAbstract The Standard Model prediction for μ-e scattering at Next-to-Next-to-Leading Order (NNLO) contains non-perturbative QCD contributions given by diagrams with a hadronic vacuum polarization insertion in the photon propagator. By taking advantage of the hyperspherical integration method, we show that the subset of hadronic NNLO corrections where the vacuum polarization appears inside a loop, the irreducible diagrams, can be calculated employing the hadronic vacuum polarization in the space-like region, without making use of the R ratio and time-like data. We present the analytic expressions of the kernels necessary to evaluate numerically the two types of irreducible diagrams: the two-loop vertex and box corrections. As a cross check, we evaluate these corrections numerically and we compare them with the results given by the traditional dispersive approach and with analytic two-loop vertex results in QED
Radiative μ and τ leptonic decays at NLO
No full textWe present the differential rates and branching ratios of the radiative decays tau -> l nu nu gamma, with l = e or mu, and mu -> e nu nu gamma in the Standard Model at next-to-leading order. Radiative corrections are computed taking into account the full depencence on the mass m l of the final charged leptons, which is necessary for the correct determination of the branching ratios. Only partial agreement is found with previous calculations performed in the m(l) -> 0 limit. Our results agree with the measurements of the branching ratios B (mu -> e (nu) over bar nu gamma) and B (tau -> mu(nu) over bar nu gamma) for a minimum photon energy of 10MeV in the mu and tau rest frames, respectively. BABAR's recent precise measurement of the branching ratio B (tau -> e (nu) over bar nu gamma), for the same photon energy threshold, differs from our prediction by 3.5 standard deviations
On the decays B → K(⁎)+ leptonium
No full textWe determine the rates of the B meson decays into a K(⁎) and an ℓ+ℓ− bound state, the leptonium, where ℓ=e,μ,τ. 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→sℓ+ℓ− 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-flavor-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/ψ and the ψ(2S) resonances, we estimate the charm-loop contribution to the decays B→K(⁎)+tauonium
Muon-Electron Scattering at Next-To-Next-To-Leading Order: The Hadronic Corrections
No full textThe standard model prediction for muon-electron scattering beyond leading order requires the inclusion of QCD contributions which cannot be computed perturbatively. At next-to- and next-to-next-to-leading order, they arise from one- and two-loop diagrams with hadronic vacuum polarization insertions in the photon propagator. We present their evaluation using the dispersive approach with hadronic e(+) e(-) annihilation data and estimate their uncertainty. We find that these corrections are crucial for the analysis of future high-precision muon-electron scattering data, like those of the recently proposed MUonE experiment at CERN
Next-to-leading order prediction for the decay mu -> e (e(+)e(-))nu(nu)over-bar
No full textWe present the di ff erential decay rates and the branching ratios of the muon decay with internal conversion, mu -> e (e(+)e(-)) nu(nu) over bar, in the Standard Model at next-to-leading order (NLO) in the on-shell scheme. This rare decay mode of the muon is among the main sources of background to the search for mu -> eee decay. We found that in the phase space region where the neutrino energies are small, and the three-electron momenta have a similar signature as in the mu -> eee decay, the NLO corrections decrease the leading-order prediction by about 10 - 20% depending on the applied cut
The heavy quark expansion for inclusive semileptonic charm decays revisited
No full textThe Heavy Quark Expansion (HQE) has become an extremely powerful tool in flavor physics. For charm decays, where the expansion parameters alpha (s)(m(c)) and Lambda (QCD)/m(c) are bigger than for bottom decays, it remains to be seen if the HQE can be applied with similar success. Nevertheless, to make optimal use of the plethora of data already available and coming in the near future, a better understanding of HQE for charm decays is crucial. This paper discusses in detail how the HQE for charm decays is set up, what is the role of four-quark (weak annihilation) operators and how this compares to the well understood bottom decays. Subtleties concerning radiative corrections and the charm mass scheme are briefly discussed. An experimental study of the relevant HQE hadronic matrix elements will then show if the HQE expansion for charm converges well enough. Besides serving as an important cross check for inclusive B decays, in the end, this study might open the road for inclusive |V-cs| and |V-cd| extractions
tau -> mu mu mu at a rate of one out of 10^14 tau decays?
No full textWe present in a full analytic form the partial widths for the lepton flavour violating decays mu +/--> e +/- e+e- and tau +/--> l +/- l '+l '-, with l,l '=mu,e, mediated by neutrino oscillations in the one-loop diagrams. Compared to the first result by Petcov (Sov J Nucl Phys 25:340, 1977), obtained in the zero momentum limit PMUCH LESS-THANm nu MUCH LESS-THANMW, we retain full dependence on P, the momenta and masses of external particles, and we determine the branching ratios in the physical limit m nu MUCH LESS-THANPMUCH LESS-THANMW. We show that the claim presented in Pham (Eur Phys J C8:513, 1999) that the tau -> ll ' l ' branching ratios could be as large as 10(-14), as a consequence of keeping the P dependence, is flawed. We find rates of order 10(-55), even smaller than those obtained in the zero momentum limit, as the latter prediction contains an unphysical logarithmic enhancement
Charm-quark mass effects in NRQCD matching coefficients and the leptonic decay of the Upsilon(1S) meson
No full textWe compute two-loop corrections to the vector current matching coefficient involving two heavy quark masses. The result is applied to the computation of the Υ(1S) decay width into an electron or muon pair. We complement the next-to-next-to-next-to-leading order corrections of M. Beneke, et al. [Phys. Rev. Lett. 112, 151801 (2014)] by charm quark mass effects up to second order in perturbation theory. Furthermore, we apply the formalism to Γ(J/Ψ→l+l−) and compare to the experimental data
Third order corrections to the semileptonic b -> c and the muon decays
No full textWe compute corrections of order alpha(3)(s) to the decay b -> b -> cl ( nu) over bar taking into account massive charm quarks. In the on-shell scheme large three-loop corrections are found. However, in the kinetic scheme the three-loop corrections are below 1% and thus perturbation theory is well under control. We furthermore provide results for the order alpha(3)(s) corrections to b -> cl (nu ) over bar and the third-order QED corrections to the muon decay which will be important input for reducing the uncertainty of the Fermi coupling constant G(F)
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