2,003 research outputs found
Dispersive techniques for low-energy QCD: Pion-pion and pion-kaon scattering
A good understanding of strong interactions at low energies is key in the precision frontier of Particle and Hadron Physics, since most hadronic observables end up in final states involving pions and kaons, the lightest mesons in the Hadron Spectrum. In particular, the pion–pion and pion–kaon scattering lengths are the lowest energy observables for these interactions, and hence a fundamental quantity for understanding hadron interaction at low energies. In this talk we review the current status of their determination. After discussing the predictions expected from chiral symmetry at different orders in the chiral expansion, we review current experimental and lattice determinations. We then focus on the dispersive determination of pion-pion and pion-kaon interactions, based on the analysis of Roy and Roy-Steiner equation, and continue discussing in detail the current tension between the chiral symmetry and dispersive predicition for the pion- kaon scattering lenthgs. We finish this talk providing an explanation of this disagreement
Estudio de las propiedades y naturaleza de las resonancias escalares más ligeras y su relación con la ruptura espontánea de la simetría quiral
Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 01/02/2013. Tesis formato europeo (compendio de artículos)Fac. de Ciencias FísicasTRUEunpu
Nucleon matrix elements of the antisymmetric quark tensor
If physics beyond the Standard Model enters well above the electroweak scale,
its low-energy effects are described by Standard Model Effective Field Theory.
Already at dimension six many operators involve the antisymmetric quark tensor
, whose matrix elements are difficult to constrain
from experiment, Ward identities, or low-energy theorems, in contrast to the
corresponding vector and axial-vector or even scalar and pseudoscalar currents.
However, with normalizations determined from lattice QCD, analyticity and
unitarity often allow one to predict the momentum dependence in a large
kinematic range. Starting from recent results in the meson sector, we extend
this method to the nucleon case and, in combination with pole dominance,
provide a comprehensive assessment of the current status of the nucleon form
factors of the quark tensor.Comment: 7 pages, 3 figures; strangeness input update
Patterns and partners for chiral symmetry restoration
We present and analyze a new set of Ward Identities which shed light on the distinction between different patterns of chiral symmetry restoration in QCD, namely O(4) vs O(4) x U(1)_A. The degeneracy of chiral partners for all scalar and pseudoscalar meson nonet members is studied through their corresponding correlators. Around chiral symmetry degeneration of O(4) partners, our analysis predicts that U(1)_A partners are also degenerated. Our analysis also leads to I = 1/2 scalar-pseudoscalar partner degeneration at exact chiral restoration and supports ideal mixing between the η- η' and the f₀(500)-f₀(980) mesons at O(4) x U(1)_A restoration, with a possible range where the pseudoscalar mixing vanishes if the two transitions are well separated. We test our results with lattice data and provide further relevant observables regarding chiral and U(1)_A restoration for future lattice and model analyses
Extracting the sigma-term from low-energy pion-nucleon scattering
We present an extraction of the pion-nucleon () scattering lengths
from low-energy scattering, by fitting a representation based on
Roy-Steiner equations to the low-energy data base. We show that the resulting
values confirm the scattering-length determination from pionic atoms, and
discuss the stability of the fit results regarding electromagnetic corrections
and experimental normalization uncertainties in detail. Our results provide
further evidence for a large -term, MeV,
in agreement with, albeit less precise than, the determination from pionic
atoms.Comment: 17 pages, 3 figures; journal versio
High-precision determination of the pion-nucleon -term from Roy-Steiner equations
We present a determination of the pion-nucleon () -term
based on the Cheng-Dashen low-energy theorem (LET), taking
advantage of the recent high-precision data from pionic atoms to pin down the
scattering lengths as well as of constraints from analyticity,
unitarity, and crossing symmetry in the form of Roy-Steiner equations to
perform the extrapolation to the Cheng-Dashen point in a reliable manner. With
isospin-violating corrections included both in the scattering lengths and the
LET, we obtain MeV MeV,
where the first error refers to uncertainties in the amplitude and the
second to the LET. Consequences for the scalar nucleon couplings relevant for
the direct detection of dark matter are discussed.Comment: 6 pages, 1 figure; title changed by journal, version to be published
in PR
Matching pion-nucleon Roy-Steiner equations to chiral perturbation theory
We match the results for the subthreshold parameters of pion-nucleon
scattering obtained from a solution of Roy-Steiner equations to chiral
perturbation theory up to next-to-next-to-next-to-leading order, to extract the
pertinent low-energy constants including a comprehensive analysis of systematic
uncertainties and correlations. We study the convergence of the chiral series
by investigating the chiral expansion of threshold parameters up to the same
order and discuss the role of the \Delta(1232) resonance in this context.
Results for the low-energy constants are also presented in the counting scheme
usually applied in chiral nuclear effective field theory, where they serve as
crucial input to determine the long-range part of the nucleon-nucleon potential
as well as three-nucleon forces.Comment: 6 pages, 4 tables; version to appear in PR
Extracting the sigma-term from low-energy pion-nucleon scattering
We present an extraction of the pion-nucleon () scattering lengths
from low-energy scattering, by fitting a representation based on
Roy-Steiner equations to the low-energy data base. We show that the resulting
values confirm the scattering-length determination from pionic atoms, and
discuss the stability of the fit results regarding electromagnetic corrections
and experimental normalization uncertainties in detail. Our results provide
further evidence for a large -term, MeV,
in agreement with, albeit less precise than, the determination from pionic
atoms.Comment: 17 pages, 3 figures; journal versio
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