Journal of High Energy Physics 2015.2 (2015): 043 reproduced by permission of Scuola Internazionale Superiore di Studi Avanzati (SISSA)We describe a new set of gauge configurations generated within the CLS effort. These ensembles have N f = 2 + 1 flavors of non-perturbatively improved Wilson fermions in the sea with the L ̈uscher-Weisz action used for the gluons. Open boundary conditions in time are used to address the problem of topological freezing at small lattice spacings and twisted-mass reweighting for improved stability of the simulations. We give the bare parameters at which the ensembles have been generated and how these parameters have been chosen. Details of the algorithmic setup and its performance are presented as well as measurements of the pion and kaon masses alongside the scale parameter t 0M.B., P.K., T.K. and S.S. are supported by the Deutsche Forschungsgemeinschaft (DFG) in the SFB/TR 09 “Computational Particle Physics”. G.P.E. acknowledges partial support by the MIUR-PRIN contract 20093BMNNPR and G.H. acknowledges support by the the Spanish MINECO through the Ram ́on y Cajal Programme and through the project FPA2012-31686 and by the Centro de excelencia Severo Ochoa Program SEV- 2012-0249. G.H. and H.H. acknowledge the support from the DFG in the SFB 1044. M.P. acknowledges partial support by the MIUR-PRIN contract 2010YJ2NYW and by the INFN SUMA project. E.E.S, J.S., and W.S. are supported by the SFB/TRR-55 “Hadron Physics from Lattice QCD” by the DFG. E.E.S. also acknowledges support from the EU grant PIRG07-GA-2010-26836

We present results for the nucleon electromagnetic form factors, including the momentum transfer dependence and derived quantities (charge radii and magnetic moment). The analysis is performed using O(a) improved Wilson fermions in N f = 2 QCD measured on the CLS ensembles. Particular focus is placed on a systematic evaluation of the influence of excited states in three-point correlation functions, which lead to a biased evaluation, if not accounted for correctly. We argue that the use of summed operator insertions and fit ansätze including excited states allow us to suppress and control this effect. We employ a novel method to perform joint chiral and continuum extrapolations, by fitting the form factors directly to the expressions of covariant baryonic chiral effective field theory. The final results for the charge radii and magnetic moment from our lattice calculations include, for the first time, a full error budget. We find that our estimates are compatible with experimental results within their overall uncertainties.

Quantum electrodynamics for ρ mesons is considered. It is shown that, at tree level, the value of the gyromagnetic ratio of the ρ + is fixed to 2 in a self-consistent effective quantum field theory. Further, the mixing parameter of the photon and the neutral vector meson is equal to the ratio of electromagnetic and strong couplings, leading to the mass difference M ρ 0 − M ρ ± ∼ 1 MeV at tree order.PACS numbers: 11.10. Gh, 12.39.Fe The question of the intrinsic magnetic moment of (elementary) particles of arbitrary spin s has been discussed controversially in the literature and is still of great interest. On the one hand, low-energy theorems and the optical theorem require that the gyromagnetic ratio g ≃ 2 for a particle with arbitrary spin s different from zero (at least for particles which do not participate in strong interactions) [1]. On the other hand, general arguments have been given that the minimal coupling leads to 1/s for this quantity [2]. Finally, the investigations of Ref. [3] regarding the theory of charged vector mesons interacting with the electromagnetic field suggested that the gyromagnetic ratio depends on a free parameter, thus allowing it to take any value. Below, we will address this question from the point of view of effective field theory (EFT).In Ref.[4] we have shown how the universal coupling of the ρ meson and the Kawarabayashi-Suzuki-RiadzuddinFayyazuddin (KSRF) relation [5,6] follow from the requirement that chiral perturbation theory of pions, nucleons, and ρ mesons is a consistent EFT. Although EFTs are non-renormalizable in the traditional sense, the general principles of EFT [7] require that all ultraviolet divergences can be absorbed into the redefinition of fields and parameters of the most general Lagrangian [8]. Imposing the renormalizability in this sense one finds that not all parameters of the most general Lagrangian are free but satisfy consistency conditions [9]. In this Letter we will use similar arguments for the effective Lagrangian including, in addition, the interaction with photons to show that the gyromagnetic ratio is fixed to g = 2 at tree level. Furthermore, the mixing parameter of the photon and the neutral vector meson is also fixed and leads to M ρ 0 − M ρ ± ∼ 1 MeV at tree order.We start with the chirally invariant effective Lagrangian including vector mesons in the form given by Weinberg [10], containing all interaction terms which respect Lorentz invariance, the discrete symmetries, and chiral symmetry. The electromagnetic interaction is introduced by adding all terms with photon fields which are allowed by U(1) gauge invariance,Here, B µ is a U(1) gauge vector field, V a µ (a = 1, 2, 3) denote the Cartesian components of an isospin triplet of vector fields, and Ψ is an isospin doublet of nucleon fields with mass m 0 [11]. Furthermore,All fields and coupling constants in Eq. (1) are bare quantities. From the point of view of EFT it is not consistent to consider a minimal coupling only (see, e.g., Ref. [12]). Using symmetry arguments only, c 0 and κ 0...

We present a lattice calculation of the nucleon isovector axial and induced pseudoscalar form factors on the CLS ensembles using [Formula: see text] dynamical flavors of nonperturbatively [Formula: see text]-improved Wilson fermions and an [Formula: see text]-improved axial current together with the pseudoscalar density. Excited-state effects in the extraction of the form factors are treated using a variety of methods, with a detailed discussion of their respective merits. The chiral and continuum extrapolation of the results is performed both using formulae inspired by Heavy Baryon Chiral Perturbation Theory (HBChPT) and a global approach to the form factors based on a chiral effective field theory (EFT) including axial vector mesons. Our results indicate that careful treatment of excited-state effects is important in order to obtain reliable results for the axial form factors of the nucleon, and that the main remaining error stems from the systematic uncertainties of the chiral extrapolation. As final results, we quote [Formula: see text], [Formula: see text], and [Formula: see text] for the axial charge, axial charge radius and induced pseudoscalar charge, respectively, where the first error is statistical and the second is systematic.

We consider a low-energy effective field theory of vector mesons and Goldstone bosons using the complex-mass renormalization. As an application we calculate the mass and the width of the $\rho$ meson.Comment: 7 pages, 1 fugure, REVTeX

It is shown that both the universal coupling of the ρ meson and the Kawarabayashi-SuzukiRiadzuddin-Fayyazuddin expression for the magnitude of its coupling constant follow from the requirement that chiral perturbation theory of pions, nucleons, and ρ mesons is a consistent effective field theory. The prerequisite of the derivation is that all ultraviolet divergences can be absorbed in the redefinition of fields and the available parameters of the most general effective Lagrangian.PACS numbers: 11.10. Gh, 12.39.Fe. Already in the 1960s, vector mesons were discussed in the framework of phenomenological low-energy chiral Lagrangians [1,2,3]. For the details of the construction of chirally invariant effective Lagrangians describing the interaction of vector mesons with pseudoscalars and baryons see, e.g., Refs. [4,5,6,7,8,9,10,11,12]. In Ref. [12], different formulations of vector-meson effective theories were shown to be equivalent. In the massive Yang-Mills approach (for a review see, e.g., Ref.[4]) vector mesons are treated as gauge bosons of local chiral symmetry (with symmetry breaking mass terms added by hand). This scenario implies that the ρ meson couples universally, i.e. with the same strength, to fermions and pseudoscalars. In the so-called hidden chiral symmetry approach (see, e.g., Refs. [4, 5, 6]) ρ-coupling universality is obtained only with a specific choice of a free parameter of the Lagrangian. Although the hypotheses of dynamical bosons of both approximate and hidden local chiral symmetry are attractive, there is, as was emphasized in Ref. [8], no proof for the existence of such gauge bosons of local chiral symmetry in QCD.Besides the construction of the effective Lagrangian, a consistent effective field theory (EFT) program requires a systematic power counting which allows one to organize the perturbation series. For example, within the extended on-mass-shell renormalization scheme of Ref. [13] it is possible to consistently include virtual (axial-) vector mesons in a manifestly Lorentz-invariant formulation of the EFT [14], provided they appear only as internal lines in Feynman diagrams involving soft external pions and nucleons with small three-momenta. Moreover, a consistent power counting also exists within the reformulated version [15] of the infrared renormalization of Becher and Leutwyler [16].In this letter we consider the effective Lagrangian of Ref.[3] describing the interaction among ρ mesons, pions, and nucleons. In principle, the Lagrangian contains all interaction terms which respect Lorentz invariance, the discrete symmetries, and chiral symmetry. As was stressed in Ref. [3], the equality of the ρππ and the ρN N coupling constants does not follow as a consequence of the symmetries of the Lagrangian. Below we perform a one-loop order analysis of the nucleon and ρ-meson self-energies as well as the ρρρ and ρN N vertex functions. In accordance with the general principles of effective field theory [17], we require that all ultra-violet (UV) divergences can be absorbed into the redef...

We introduce a new formulation of baryon chiral perturbation theory which improves the ultraviolet behavior of propagators and can be interpreted as a smooth cutoff regularization scheme. It is equivalent to the standard approach, preserves all symmetries and therefore satisfies the Ward identities. Our formulation is equally well defined in the vacuum, one- and few-nucleon sectors of the theory. The equations (Bethe-Salpeter, Lippmann-Schwinger, etc.) for the scattering amplitudes of the few-nucleon sector are free of divergences in the new approach. Unlike the usual cutoff regularization, our 'cutoffs' are parameters of the Lagrangian and do not have to be removed.Comment: 19 pages, 3 figures, REVTeX 4; version to be published in Phys. Rev. D, additional section on issues of renormalization in few-body sector include

No abstract