55 research outputs found
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
Predicting the tau strange branching ratios and implications for V-us
No full textHadronic τ decays provide several ways to extract the Cabbibo-Kobashi-Maskawa (CKM) matrix element Vus. The most precise determination involves using inclusive τ decays and requires as input the total branching ratio into strange final states.
Recent results from B-factories have led to a discrepancy of about 3.4σ from the value of Vus implied by CKM unitarity and direct determination from Kaon semi-leptonic modes. In this paper we predict the three leading strange τ branching ratios, using dispersive parameterizations of the hadronic form factors and taking as experimental input the measured Kaon decay rates and the τ → Kπντ decay spectrum. We then use our results to reevaluate Vus , for which we find |Vus| = 0.2207 ± 0.0027, in better agreement with CKM unitarity
Dispersive representation and shape of K_l3 form factors
No full text6 pages, 4 figures. Talk given at the KAON07 International Conference, May 21-25 2007, Frascati, ItalyThe Callan-Treiman low-energy theorem offers an opportunity to test electroweak couplings of light quarks to the gauge boson W. To that aim, we introduce a model-independent and accurate dispersive parametrization of the two K_{l3} form factors. We then discuss three applications to the analysis of K_{e3} and K_{mu3} measurements: the prediction of the ratios Gamma(K_{mu3})/Gamma(K_{e3}), the extraction of |f_+(0)V_{us}| and finally the possible measurement of m_u - m_d induced isospin breaking asymmetry
Spectral functions of nucleon form factors: Three-pion continua at low energies
No full textWe study the imaginary parts of the isoscalar electromagnetic and isovector axial form factors of the nucleon close to the -threshold in covariant baryon chiral perturbation theory. At the two-loop level, the contributions arising from leading and next-to-leading order chiral -vertices, as well as pion-induced excitations of virtual -isobars, are calculated. It is found that the heavy baryon treatment overestimates substantially these -continua. From a phenomenological analysis, that includes the narrow -resonance or the broad -resonance, one can recognize small windows near threshold, where chiral -dynamics prevails. However, in the case of the isoscalar electromagnetic form factors , the radiative correction provided by the -intermediate state turns out to be of similar size
Dispersive representation and shape of the Kl3 form factors: robustness
No full textAn accurate low-energy dispersive parametrization of the scalar Kpi form factor was constructed some time ago in terms of a single parameter guided by the Callan-Treiman low-energy theorem. A similar twice-subtracted dispersive parametrization for the vector Kpi form factor will be investigated here. The robustness of the parametrization of these two form factors will be studied in great detail. In particular the cutoff dependence, the isospin breaking effects, and the possible, though not highly probable, presence of zeros in the form factors will be discussed. Interesting constraints in the latter case will be obtained from the soft-kaon analog of the Callan-Treiman theorem and a comparison with the recent tau-->Kpinutau data
An evaluation of broken vertical bar V-us broken vertical bar and precise tests of the Standard Model from world data on leptonic and semileptonic kaon decays
No full textWe present a global analysis of leptonic and semileptonic kaon decay data, including all recent results published by the BNL-E865, KLOE, KTeV, ISTRA+ and NA48 experiments. This analysis, in conjunction with precise lattice calculations of the hadronic matrix elements now available, leads to a very precise determination of broken vertical bar V-us broken vertical bar and allows us to perform several stringent tests of the Standard Model.The authors thank P. Franzini and all other members of
the FlaviaNet Kaon Working Group, as well as V. Lubicz
and A. Jüttner of the FlaviaNet Lattice Averaging Group
(FLAG), for useful discussion, comments, and suggestions.
This work is supported by the European Union Sixth
Framework Programme under contract MTRN-CT-2006-
035482, FlaviaNet. E.P. acknowledges financial support
from MEC (Spain) under grant FPA2007-60323. F.M.
acknowledges financial support from projects FPA2007-
66665, 2009SGR502. E.P. and F.M. acknowledge support
from the Spanish Consolider-Ingenio 2010 Programme
CPAN (CSD2007-00042). G.I. acknowledges support from
the Technische Universität München- Institute for Advanced
Study, funded by the German Excellence Initiative.Peer reviewe
Some aspects of low-energy QCD at the precision frontier
No full textTitle: Some aspects of low-energy QCD at the precision frontier Author: Tomáš Husek Department/Institute: Institute of Particle and Nuclear Physics Supervisor of the doctoral thesis: doc. RNDr. Karol Kampf, Ph.D. Abstract: This thesis concentrates on some low-energy aspects of QCD, namely on those which are connected to the electromagnetic decays of lightest neutral pseudoscalar mesons. Calculations of radiative corrections to neutral pion decays (the Dalitz decay and the rare decay) and a novel model for the pion electro- magnetic transition form factor are subjects discussed in the attached papers, which this work is based on. The associated theoretical aspects including Chiral Perturbation Theory or the large-Nc limit are introduced. We also discuss the complications which arise when the calculations of radiative corrections for η(′) Dalitz decays are performed. Some details about the collaboration with experi- ments which incorporate the calculation of the published corrections are provided. Last but not least, some techniques related to loop integrals are shown. Keywords: Chiral Perturbation Theory, large-Nc limit, radiative corrections, pion electromagnetic transition form factor
τ→ μμμ at a rate of one out of 10¹⁴ tau decays?
No full textWe present in a full analytic form the partial widths for the lepton flavour violating decays μ±→e±e+e− and τ±→ℓ±ℓ′+ℓ′−, with ℓ,ℓ′=μ,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 P≪mν≪MW, we retain full dependence on P, the momenta and masses of external particles, and we determine the branching ratios in the physical limit mν≪P≪MW. We show that the claim presented in Pham (Eur Phys J C8:513, 1999) that the τ→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
Spectral functions of nucleon form factors: Three-pion continua at low energies
No full textWe study the imaginary parts of the isoscalar electromagnetic and isovector axial form factors of the nucleon close to the -threshold in covariant baryon chiral perturbation theory. At the two-loop level, the contributions arising from leading and next-to-leading order chiral -vertices, as well as pion-induced excitations of virtual -isobars, are calculated. It is found that the heavy baryon treatment overestimates substantially these -continua. From a phenomenological analysis, that includes the narrow -resonance or the broad -resonance, one can recognize small windows near threshold, where chiral -dynamics prevails. However, in the case of the isoscalar electromagnetic form factors , the radiative correction provided by the -intermediate state turns out to be of similar size
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