Search results for "perturbation theory"
showing 10 items of 584 documents
Generalized Polarizabilities and the Chiral Structure of the Nucleon
1997
We are studying the electron scattering process e p to e' p' gamma in order to obtain information on the genuine virtual Compton scattering (VCS) process gamma^* N to gamma N. In addition to the two kinematical variables of real Compton scattering, e.g. the scattering angle theta and the energy omega' of the outgoing photon, the invariant amplitude for VCS depends on a third kinematical variable, which we choose as the absolute value of the three-momentum transfer to the nucleon. The structure-dependent coefficients in the VCS amplitude therefore acquire a momentum dependence and are termed ``generalized polarizabilities'' of the nucleon in analogy to real Compton scattering. Utilizing the …
Properties of the ground-state baryons in chiral perturbation theory
2010
15th High Energy Physics International Conference on Quantum Chromodynamics. Montpellier (FRANCE). JUN 28-JUL 02, 2010
Higher order forward spin polarizability
2010
As a guideline for future experiments to extract the four (leading) spin polarizabilities of the nucleon, we have constructed the forward amplitude for polarized Compton scattering by dispersion integrals. These integrals have been saturated by recently measured helicity-dependent photoabsorption cross sections as well as predictions for pion photoproduction multipoles from several phenomenological descriptions and chiral perturbation theory. The comparison of these results corroborates the strategy to extract the spin polarizabilities by fitting them to polarized Compton data and fixing all higher order spin effects by dispersion relations based on pion photoproduction multipoles.
Measurement of the radiative K-e3 branching ratio
2005
We present a measurement of the relative branching ratio of the decay KL -> pi e nu gamma (Ke3gamma) with respect to KL-> pi e nu (gamma) (Ke3+Ke3gamma) decay. The result is based on observation of 19 000 Ke3gamma and 5.6 x 10^6 Ke3 decays. The value of the branching ratio is Br(Ke3gamma, Egamma^*>30 MeV,theta(e,gamma)^*>20^o)/Br(Ke3)= (0.964+-0.008+0.011-0.009)%. This result agrees with theoretical predictions but is at variance with a recently published result.
Unitarized Chiral Perturbation Theory in a finite volume: scalar meson sector
2011
We develop a scheme for the extraction of the properties of the scalar mesons f0(600), f0(980), and a0(980) from lattice QCD data. This scheme is based on a two-channel chiral unitary approach with fully relativistic propagators in a finite volume. In order to discuss the feasibility of finding the mass and width of the scalar resonances, we analyze synthetic lattice data with a fixed error assigned, and show that the framework can be indeed used for an accurate determination of resonance pole positions in the multi-channel scattering.
Vacuum correlators at short distances from lattice QCD
2021
Non-perturbatively computing the hadronic vacuum polarization at large photon virtualities and making contact with perturbation theory enables a precision determination of the electromagnetic coupling at the $Z$ pole, which enters global electroweak fits. In order to achieve this goal ab initio using lattice QCD, one faces the challenge that, at the short distances which dominate the observable, discretization errors are hard to control. Here we address challenges of this type with the help of static screening correlators in the high-temperature phase of QCD, yet without incurring any bias. The idea is motivated by the observations that (a) the cost of high-temperature simulations is typica…
The electronic field shift factor of the 4s2S1/2?4p2PJ transitions in CaII
1993
The isotope shifts (IS) of the resonance lines 4s2S1/2−4p2PJin CaII (seven isotopes) have been measured by fast ion beam collinear laser spectroscopy (FIBCLS) with non-optical detection of the resonant laser-ion interaction. The electronic field shift factorF was derived from the experimental IS via a King plot procedure using the mean square nuclear charge radii from muon spectroscopy andelectron scattering. TheJ-independent resultF=−283(6) Mhz/fm2 was obtained. TheF-value resulting from extensive many body perturbation theory (MBPT) calculations, recently performed by Martensson-Pendrill et al., is in excellent agreement with the experimental value, confirming the reliability of the MBPT …
A strategy to study the role of the charm quark in explaining the Delta{I}=1/2 rule
2004
We present a strategy designed to separate several possible origins of the well-known enhancement of the Delta{I}=1/2 amplitude in non-leptonic kaon decays. In particular, we seek to disentangle the contribution of physics at the typical QCD scale (soft-gluon exchange) from the effects at the scale of the charm quark mass. This is achieved by considering QCD with an unphysically light charm quark, so that the theory possesses an approximate SU(4)_L x SU(4)_R chiral symmetry. By computing the relevant operator matrix elements and monitoring their values as the charm quark mass departs from the SU(4)-symmetric situation, the role of the charm quark can be assessed. We study the influence of t…
Pion and kaon vector form factors
2001
We develop a unitarity approach to consider the final state interaction corrections to the tree level graphs calculated from Chiral Perturbation Theory ($\chi PT$) allowing the inclusion of explicit resonance fields. The method is discussed considering the coupled channel pion and kaon vector form factors. These form factors are then matched with the one loop $\chi PT$ results. A very good description of experimental data is accomplished for the vector form factors and for the $\pi\pi$ P-wave phase shifts up to $\sqrt{s}\lesssim 1.2$ GeV, beyond which multiparticle states play a non negligible role. In particular the low and resonance energy regions are discussed in detail and for the forme…
High-precision determination of the $K_{e3}$ radiative corrections
2021
We report a high-precision calculation of the Standard Model electroweak radiative corrections in the $K\to \pi e^+\nu(\gamma)$ decay as a part of the combined theory effort to understand the existing anomaly in the determinations of $V_{us}$. Our new analysis features a chiral resummation of the large infrared-singular terms in the radiative corrections and a well-under-control strong interaction uncertainty based on the most recent lattice QCD inputs. While being consistent with the current state-of-the-art results obtained from chiral perturbation theory, we reduce the existing theory uncertainty from $10^{-3}$ to $10^{-4}$. Our result suggests that the Standard Model electroweak effects…