arXiv subjects

G. Lopez Castro

Publications and source records attributed to G. Lopez Castro.

At least 37 records · Page 2Linked to original sources

arXiv2003

Unstable particles, gauge invariance and the Delta^{++} resonance parameters

The elastic and radiative pion(+)-proton scattering are studied in the framework of an effective Lagrangian model for the Delta^{++} resonance and its interactions. The finite width effects of this spin-3/2 resonance are introduced in the scattering amplitudes through a complex mass scheme to respect electromagnetic gauge invariance. The resonant pole (Delta^{++}) and background contributions (rho^0, sigma, Delta and neutron states) are separated according to the principles of the analytic S-matrix theory. The mass and width parameters of the Δ^{++} obtained from a fit to experimental data on the total cross section are in agreement with the results of a model-independent analysis based on the analytic S-matrix approach. The magnetic dipole moment determined from the radiative pion-proton scattering is mu_Delta^{++}=(6.14 +/- 0.51) nuclear magnetons.

hep-ph↗

arXiv2002

Double beta decay of sigma^- hyperons

We compute the strangeness-conserving double beta decay of $Σ^-$ hyperons, which is the only hadronic system that can undergo such decays. We consider both, the lepton number conserving %$Σ^-\go Σ^++2e^-+2 \barν$ $Σ^-\go Σ^+e^-e^-\barν\barν$ ($ββ_{Σ_{2ν}}$) and the lepton number violating $Σ^-\go Σ^+e^-e^-$ ($ββ_{Σ_{0ν}}$) modes. The branching ratios of these $ββ$ decays are suppressed at the level of $10^{-30}$ considering a light neutrino scenario in the case of the $ββ_{Σ_{0ν}}$ channel. The dynamical origin of such low rates and their possible enhancements are briefly discussed. Given its simplicity those decays can be used also for the purposes of illustrating the main features of double beta decays.

nucl-th↗

arXiv2002

Non-leptonic decays of the B_c into tensor mesons

We have computed the branching ratios of the exclusive pseudoscalar (vector) + tensor modes that are allowed in the decays of the B_c meson. The dominant spectator and annihilation contributions in those decays are evaluated using the factorization hypothesis. We find that some of these decay channels, such as B_c --> (ρ^-, D_s^{*-}, D_s^{-})χ_{c2} and B_c^ -->π^- \bar{B_{s2}^{*0}}, have branching ratios of the order of 10^{-4}, which seems to be at the reach of forthcoming experiments at the LHC. The inclusive branching fraction of the two-body B_c decays involving tensor particles is approximately 1.28 x10^{-3}. At the dynamical level, it is interesting to observe that the exclusive decays B_c --> K^-(π^-)\bar{D_2^{*0}}, π^0 D_2^{*-}, η^{'} D_{s2}^{*-} are dominated by the annihilation contributions.

hep-ph↗

arXiv2001

Updating V_us from kaon semileptonic decays

We update the determination of |V_us| using semielectronic and semimuonic decays of K mesons. A modest improvement of 15% with respect to its present value is obtained for the error bar of this matrix element when we combine the four available semileptonic decays. The combined effects of long-distance radiative corrections and nonlinear terms in the vector form factors can decrease the value of |V_us| by up to 1%. Refined measurements of the decay widths and slope form factors in the semimuonic modes and a more accurate calculation of vector form factors at zero momentum transfer can push the determination of |V_us| at a few of percent level.

hep-ph↗

arXiv2001

Beyond the soft photon approximation in radiative production and decay of charged vector mesons

We study the effects of model-dependent contributions and the electric quadrupole moment of vector mesons in the decays $V^- \to P^-P^0γ$ and $τ^- \to νV^-γ$. Their interference with the amplitude originating from the radiation due to electric charges vanishes for photons emitted collinearly to the charged particle in the final state. This brings further support to our claim in previous works, that measurements of the photon energy spectrum for nearly collinear photons in those decays are suitable for a first measurement of the magnetic dipole moment of charged vector mesons.

hep-ph↗

arXiv2001

Determination of the Delta++ magnetic dipole moment

We study the elastic and radiative pi+ p scattering within a full dynamical model which incorporates the finite width effects of the Delta++. The scattering amplitudes are invariant under contact transformations of the spin 3/2 field and gauge-invariance is fulfilled for the radiative case. The pole parameters of the Delta++ obtained from the elastic cross section are m_Delta = 1211.2 \pm 0.4 MeV and Gamma_Delta = 88.2 \pm 0.4 MeV. From a fit to the most sensitive observables in radiative pi+ p scattering, we obtain mu_Delta = 6.14 \pm 0.51 e/2m_p for the magnetic dipole moment of the Delta++.

nucl-th↗

arXiv2001

Elastic and radiative pi^+p scattering and properties of the Delta^{++} resonance

We study the Delta^{++} contributions to elastic and radiative π^+p scattering within an effective Lagrangian model which incorporates the Delta, N, rho and sigma meson degrees of freedom. This model provides a description of the Delta resonance and its interactions that respects electromagnetic gauge invariance and invariance under contact transformations when finite width effects are incorporated. Following recent developments in the description of unstable gauge bosons, we use a complex mass scheme to introduce the finite width of the Delta^{++} without spoiling gauge invariance. The total cross section of elastic π^+p scattering, whose amplitude exhibits the resonant plus background structure of S-matrix theory, is used to fix the mass, width and strong coupling of the Δresonance. The differential cross section of elastic scattering is found in very good agreement with experimental data. The magnetic dipole moment of the Delta^{++}, μ_{Delta}, is left as the only adjustable parameter in radiative π^+p scattering. From a fit to the most sensitive configurations for photon emission in this process, we obtain μ_Δ = (6.14 +-0.51)e/2m_p, in agreement with predictions based on the SU(6) quark model.

nucl-th↗

arXiv2000

Neutron beta decay and the current determination of Vud

Measurements of neutron beta decay observables required to determine $|V_{ud}|$ are reaching the 0.1% accuracy. In this paper we review the calculation of the decay rate of this process, discuss its relevant uncertainties, and obtain an expression that is precise at the $10^{-4}$ level. Our analysis clearly shows the necessity of measurements of $λ$, the ratio of axial/vector couplings. The current situation in neutron beta decay is that one cannot yet quote a single consistent value for $|V_{ud}|$ from it. We also discuss the region of parameter space in the $|V_{ud}|$--$λ$ plane where new physics effects should lie, if they contribute to neutron beta decay.

nucl-th↗

arXiv2000

Parapositronium Decay and Dispersion Relations

Positronium decay rates are computed at the one-loop level, using convolution-type factorized amplitudes. The dynamics of this factorization is probed with dispersion relations, showing that unallowed approximations are usually made, and some ordre alpha^2 corrections missed. Further, we discuss the relevance of the Schrodinger wavefunction as the basis for perturbative calculations. Finally, we apply our formalism to the parapositronium two-photon decay.

hep-ph↗

arXiv2000

Analytical Behaviour of Positronium Decay Amplitudes

Positronium annihilation amplitudes that are computed by assuming a factorization approximation with on-shell intermediate leptons do not exhibit good analytical behaviour. Using dispersion techniques, we find new contributions that interfere with the known results to restore analytical properties. Those new amplitudes which cannot be obtained using standard factorized amplitude formalism, contribute at order alpha^2. Therefore they have to be evaluated before any theoretical conclusion can be drawn upon the orthopositronium lifetime puzzle.

hep-ph↗

arXiv2000

Binding energy corrections in positronium decays

Positronium annihilation amplitudes that are computed by assuming a factorization approximation with on-shell intermediate leptons, do not exhibit good analytical behavior. We propose an ansatz which allows to include binding energy corrections and obtain the correct analytical and gauge invariance behavior of these QED amplitudes. As a consequence of these non-perturbative corrections, the parapositronium and orthopositronium decay rates receive corrections of order alpha^4 and alpha^2, respectively. These new corrections for orthopositronium are relevant in view of a precise comparison between recent theoretical and experimental developments. Implications are pointed out for analogous decays of quarkonia .

hep-ph↗

arXiv1999

Gauge invariance and finite width effects in radiative two-pion tau lepton decay

The contribution of the rho^{\pm} vector meson to the tau -> pi pi nu gamma decay is considered as a potential source for the determination of the magnetic dipole moment of this light vector meson. In order to keep gauge-invariance of the whole decay amplitude, a procedure similar to the fermion loop-scheme for charged gauge bosons is implemented to incorporate the finite width effects of the rho^{\pm} vector meson. The absorptive pieces of the one-loop corrections to the propagators and electromagnetic vertices of the rho^{\pm} meson and W^{\pm} gauge boson have identical forms in the limit of massless particles in the loops, suggesting this to be a universal feature of spin-one unstable particles. Model-dependent contributions to the tau -> pi pi nu gamma decay are suppressed by fixing the two-pion invariant mass distribution at the rho meson mass value. The resulting photon energy and angular distribution is relatively sensitive to the effects of the rho magnetic dipole moment.

hep-ph↗

arXiv1999

Vector-meson magnetic dipole moment effects in radiative tau decays

We study the possibility that the magnetic dipole moment of light charged vector mesons could be measured from their effects in τ^- --> V^-ν_τγdecays. We conclude that the energy spectrum and angular distribution of photons emitted at small angles with respect to vector mesons is sensitive the effects of the magnetic dipole moment. Model-dependent contributions and photon radiation off other electromagnetic multipoles are small in this region. We also compute the effects of the magnetic dipole moment on the integrated rates and photon energy spectrum of these $τ$ lepton decays.

hep-ph↗

arXiv1999

Tests of flavor symmetry in J/psi decays

We use SU(3) flavor symmetry to analyze the $PP', VP$ and baryon-antibaryon decays of $J/ψ$. Both, the SU(3)-invariant and -violating contributions are considered. Particular attention is paid to the interference of the electromagnetic and strong amplitudes.

hep-ph↗

arXiv1998

Cabibbo-suppressed non-leptonic B- and D-decays involving tensor mesons

The Cabibbo-suppressed non-leptonic decays of B (and D) mesons to final states involving tensor mesons are computed using the non-relativistic quark model of Isgur-Scora-Grinstein-Wise with the factorization hypothesis. We find that some of these B decay modes, as B --> (K^*, D^*)D^*_2, can have branching ratios as large as 6 x 10^{-5} which seems to be at the reach of future B factories.

hep-ph↗

arXiv1998

Mixing of two-level unstable systems

Unstable particles can be consistently described in the framework of quantum field theory. Starting from the full S-matrix amplitudes of B^+ --> (2 pi, 3 pi) l nu decays as examples in the energy region where the rho-omega resonances are dominating, we propose a prescription for the mixing of two quasi `physical' unstable states that differs from the one obtained from the diagonalization of the M -i*Gamma/2 non-hermitian hamiltonian. We discuss some important consequences for CP violation in the K_L - K_S system.

hep-ph↗

arXiv1998

FCNC in leptonic and semileptonic decays of D mesons in a general two-Higgs doublet model

Large long-distance standard model effects in FCNC semileptonic D decays can make observable these processes in future measurements. Eventual disagreements in this sector and/or the observation of lepton family violating (LFV) D decays would require an explanatio beyond the SM framework. In this paper we confront present experimental data on leptonic and semileptonic FCNC and LFV D meson decays with a version of the two-Higgs doublet model that allows these effects to occur at tree- level. The stringent bounds on the parameters of the model are obtained from D^0 --> l^+l'^- and D --> pi l^+l'^- decays. The consistency of the model requires that the branching fractions of D --> V l^+l'^- decays should be below the 10^{-9} level.

hep-ph↗

arXiv1998

Arithmetic and the standard electroweak theory

We propose the relations 1/e - e =3 and tan(2theta_W)=3/2, where e is the positron charge and theta_W is the weak angle. Present experimental data support these relations to a very high accuracy. We suggest that some duality relates the weak isospin and hypercharge gauge groups of the standard electroweak theory.

hep-ph↗