Lepto-producción de pares v-v y el número de Generaciones

Large-N(C) properties of the rho and f(0)(600) mesons from unitary resonance chiral dynamics

We construct pi-pi amplitudes that fulfill exact elastic unitarity, account for one loop Chiral Perturbation Theory contributions and include all 1/Nc leading terms, with the only limitation of considering just the lowest-lying nonet of exchanged resonances. Within such scheme, the Nc dependence of sigma and rho masses and widths is discussed. Robust conclusions are drawn in the case of the rho resonance, confirming that it is a stable meson in the limit of a large number of QCD colors, Nc. Less definitive conclusions are reached in the scalar-isoscalar sector. With the present quality of data, we cannot firmly conclude whether or not the Nc=3 f0(600) resonance completely disappears at larg…

The tau-->nu tauetapi process in and beyond QCD

We estimate various sources of the tau-->nu tauetapi process by using QCD-duality sum rule information on the decay constants of standard and exotic mesons. We use the pi--> lnu l decay in order to constrain the effects of new interactions. We conclude from our analysis that in al cases, the tau-->nu tauetapi branching ration is small. We also point out that the present data on the tau-->nupi process already provide an upper limit of 0.1% on the etapi mode while the tau-->nu tauKK¯ data imply an upper limit of 0.5% if the a 0 (980)- meson is a four-quark state.

K 0 --> pi 0 gammagamma decays in chiral perturbation theory

The decays K L,S --> pi 0gammagamma are calculated within the framework of chiral perturbation theory. The amplitude for K L --> pi 0gammagamma is found to be finite at the one-loop level yielding a branching ratio of 6.8 × 10 -7. The decay spectra of both decays are very characteristic and provide good tests of the effective chiral symmetry realization of the standard model.

Spectral distribution for the decay tau -> v(tau)K pi

With the newly available data sets on hadronic tau decays from the B-factories BaBar and BELLE, and future data from BESIII, precise information on the decay distributions will soon become available. This calls for an improvement of the decay spectra also on the theoretical side. In this work, the distribution function for the decay tau -> v(tau)K pi will be presented with the relevant K pi vector and scalar forth factors being calculated in the framework of the resonance chiral theory, also taking into account additional constraints from dispersion relations and short-distances. As a by-product the slope and curvature of the vector form factor F-+(K pi)(s) are determined to be lambda'(+) =…

pi(0)-pi mixing and CP violation

We discuss pi(0)-eta mixing and its implication for epsilon'/epsilon to next-to-leading order in the low-energy expansion. The big effect due to eta-eta' mixing is shown to be largely cancelled by other contributions occurring at the same order in the chiral expansion.

Long-distance contributions to the K-L ->mu(+)mu(-) decay width

The dispersive two-photon contribution to the K-L --> mu(+)mu(-) decay amplitude is analyzed, using chiral perturbation theory techniques and large-N-C considerations. A consistent description of the decays pi(0) --> e(+)e(-), eta --> mu(+)mu(-), and K-L --> mu(+)mu(-) is obtained. As a by-product, one predicts B(eta --> e(+)e(-)) = (5.8 +/- 0.2) X 10(-9) and B(K-L --> e(+)e(-)) = (9.0 +/- 0.4) X 10(-12).

B(bar) -> X_s gamma rate and CP asymmetry within the aligned two-Higgs-doublet model

In the two-Higgs-doublet model the alignment of the Yukawa matrices in flavour space guarantees the absence of flavour-changing neutral currents at tree level, while introducing new sources for CP violation parametrized in a very economical way. This implies potentially large influence in a number of processes, b -> s gamma being a prominent example where rather high experimental and theoretical precision meet. We analyze the CP rate asymmetry in this inclusive decay and determine the resulting constraints on the model parameters. We demonstrate the compatibility with previously obtained limits. Moreover we extend the phenomenological analysis of the branching ratio, and examine the influen…

Green function in the resonance region

11 páginas, 2 figuras.-- arXiv:hep-ph/0404004v2

Top quark mass from radiative corrections to the Z ---> b anti-b decay

The one-loop correction to the ZbbBAR vertex presents a quadratic dependence on the top-quark mass, which has its origin in the spontaneous symmetry breaking mechanism of the Standard Model. We study the possibility of fixing the top mass by comparing LEP measurements with theoretical predictions. Using the MSBAR renormalization scheme, we calculate the top mass dependence of the ZbbBAR vertex. For all Z-widths, we give simple approximate formulae which work at the 0.05% level. It is found that if some branching ratios involving the b-quark are measured at the 0.5% level the top mass will be determined with an error of 30 GeV using only LEP1 experiments.

Hadron structure in tau -> KK pi nu_tau decays

We analyze the hadronization structure of both vector and axial-vector currents leading to tau -> KK pi nu(tau) decays. At leading order in the 1/N-C expansion, and considering only the contribution of the lightest resonances, we work out, within the framework of the resonance chiral Lagrangian, the structure of the local vertices involved in those processes. The couplings in the resonance theory are constrained by imposing the asymptotic behavior of vector and axial-vector spectral functions ruled by QCD. In this way we predict the hadron spectra and conclude that, contrary to previous assertions, the vector contribution dominates by far over the axial-vector one in all KK pi charge channe…

Determination of m(s) and vertical bar V-us vertical bar from hadronic tau decays

The mass of the strange quark is determined from SU(3)-breaking effects in the tau hadronic width. Compared to previous analyses, the contributions from scalar and pseudoscalar spectral functions, which suffer from large perturbative corrections, are replaced by phenomenological parametrisations. This leads to a sizeable reduction of the uncertainties in the strange mass from tau decays. Nevertheless, the resulting m(s) value is still rather sensitive to the moment of the invariant mass distribution which is used for the determination, as well as the size of the quark-mixing matrix element \V-us\. Imposing the unitarity fit for the CKM matrix, we obtain m(s)(2 GeV) = 117 +/- 17 MeV, whereas…

Physics at BES_III

There has recently been a dramatic renewal of interest in the subjects of hadron spectroscopy and charm physics. This renaissance has been driven in part by experimental reports of D-0(D) over bar (0) mixing and the discovery of narrow D-sJ states and a plethora of charmonium-like XY Z states at the B factories, and the observation of an intriguing proton-antiproton threshold enhancement and the possibly related X(1835) meson state at BESII. At the same time, lattice QCD is now coming of age, and we are entering a new era when precise, quantitative predictions from lattice QCD can be tested against experimental measurements. For example, the High Precision QCD (HPQCD) and United Kingdom QCD…

Unitarity and K$_{L}$ --> $\pi^{0}\gamma\gamma$

We discuss the rare decay K(L) --> pi0gammagamma. Although the recently measured 2gamma-invariant-mass distribution is in quite good agreement with the lowest-order chiral perturbation theory prediction, there seems to be a discrepancy with the calculated branching ratio. We extend the previous computations to next-to-leading order, O(p6). The two dominant mechanisms at this order are emphasized: the two-pion intermediate state and vector meson exchange.

tau ->pi pi pi nu(tau) decays in the resonance effective theory

tau-->pipipinu(tau) decays are analyzed within the framework of the resonance effective theory of QCD. We work out the relevant Lagrangian that describes the axial-vector current hadronization contributing to these processes, in particular the local a(1)(1260)-rho(770)-Goldstone interactions. The new coupling constants are constrained by imposing the asymptotic behavior of the corresponding spectral function within QCD. Hence we compare the theoretical framework with the experimental data, obtaining a good quality fit from the ALEPH spectral function and branching ratio. We also get values for the mass and on-shell width of the a(1)(1260) resonance. In this way we are able to provide the st…

QCD duality analysis of B0-B0 mixing

5 páginas, 4 figuras.-- CERN-TH-4951-88

The Physics of the B Factories

This work is on the Physics of the B Factories. Part A of this book contains a brief description of the SLAC and KEK B Factories as well as their detectors, BaBar and Belle, and data taking related issues. Part B discusses tools and methods used by the experiments in order to obtain results. The results themselves can be found in Part C.

What can be learned from the Belle spectrum for the decay tau(-) -> nu(t)K(S)pi(-)

A theoretical description of the differential decay spectrum for the decay tau(-) -> nu(tau)K(S)pi(-), which is based on the contributing K pi vector and scalar form factors F(+)(K pi)(s) and F(0)(K pi)(s) being calculated in the framework of resonance chiral theory (R chi T), additionally imposing constraints from dispersion relations as well as short distance QCD, provides a good representation of a recent measurement of the spectrum by the Belle Collaboration. Our fit allows to deduce the total branching fraction B[tau(-) -> v(tau)K(S)pi(-)] = 0.427 +/- 0.024% by integrating the spectrum, as well as the K* resonance parameters M(K)* = 895.3 +/- 0.2 MeV and Gamma(K)* = 47.5 +/- 0.4 MeV, w…