Search results for "PERTURBATION"
showing 10 items of 811 documents
Baryon masses and nucleon sigma terms in manifestly Lorentz-invariant baryon chiral perturbation theory
2004
We discuss the masses of the ground state baryon octet and the nucleon sigma terms in the framework of manifestly Lorentz-invariant baryon chiral perturbation theory. In order to obtain a consistent power counting for renormalized diagrams the extended on-mass-shell renormalization scheme is applied.
Thresholdπ0photoproduction in relativistic chiral perturbation theory
2013
We present a calculation of $\pi^0$ photoproduction on the proton in manifestly Lorentz-invariant baryon chiral perturbation theory up to and including chiral order $q^4$. With the results we analyze the latest $\pi^0$ photoproduction data in the threshold region obtained at the Mainz Microtron. In the calculation of observables and the fit of the low-energy constants, we take $S$, $P$, and $D$ waves into account. We compare the results for the multipoles with the corresponding single-energy analysis. Furthermore, we also fit the $O(q^4)$ heavy-baryon chiral perturbation theory calculation and compare both results. We provide predictions for several polarization observables for future exper…
Bethe-Salpeter approach for unitarized chiral perturbation theory
2000
The Bethe-Salpeter equation restores exact elastic unitarity in the $s-$ channel by summing up an infinite set of chiral loops. We use this equation to show how a chiral expansion can be undertaken in the two particle irreducible amplitude and the propagators accomplishing exact elastic unitarity at any step. Renormalizability of the amplitudes can be achieved by allowing for an infinite set of counter-terms as it is the case in ordinary Chiral Perturbation Theory. Crossing constraints can be imposed on the parameters to a given order. Within this framework, we calculate the leading and next-to-leading contributions to the elastic $\pi \pi$ scattering amplitudes, for all isospin channels, a…
Universal Bounds for SU(3) Low Energy Constants
2008
In this paper bounds for L_1, L_2 and L_3 are obtained in Chiral Perturbation Theory with three flavours. At the same time we test the compatibility of this theory with axiomatic principles. Following a recent paper we use dispersion relations to write positivity conditions that translate into bounds for the chiral low energy constants. As a first approach we consider the exact SU(3)_V limit and notice that if a common mass of the order of that of the kaon is adopted for the octet of pseudo-Goldstone bosons the bounds have very large O(p^6) corrections. Once the positivity conditions are adapted to account for different masses, we correct the previous bounds for a physical kaon mass and fin…
Lowest-lying spin-1/2 and spin-3/2 baryon magnetic moments in chiral perturbation theory
2010
5th International Conference on Quarks and Nuclear Physics (QNP09). Inst High Energy Phys Chinese Acad Sci, Beijing, PEOPLES R CHINA, SEP 21-25, 2009
Photoproduction of meson and baryon resonances in a chiral unitary approach
1999
By means of a coupled channel non-perturbative unitary approach, it is possible to extend the strong constrains of Chiral Perturbation Theory to higher energies. In particular, it is possible to reproduce the lowest lying resonances in meson-meson scattering up to 1.2 GeV using the parameters of the O(p^2) and O(p^4) Chiral Lagrangian. The meson baryon sector can also be tackled along similar lines. We report on an update of these results showing some examples of photon induced reactions where the techniques have been recently applied.
Isospin 0 and 1 resonances fromπΣphotoproduction data
2013
Recently we presented a successful strategy to extract the position of the two $\ensuremath{\Lambda}(1405)$ poles from experimental photoproduction data on the $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\pi}}^{0}{\ensuremath{\Sigma}}^{0}$ reaction at Jefferson Lab. Following a similar strategy, we extend the previous method to incorporate also the isospin 1 component which allows us to consider in addition the experimental data on $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\Sigma}}^{\ensuremath{\mp}}$. The idea is based on considering a production mechanism as model independent as possible and imple…
Quantum loops in radiative decays of the a1 and b1 axial-vector mesons
2007
A previous model where the low-lying axial-vector mesons are dynamically generated, implementing unitarity in coupled channels in the vector-pseudoscalar (V P) meson interaction, is applied to evaluate the decay widths of the a1(1260) and b1(1235) axial-vector mesons into �. Unlike the case of the a1, the b1 radiative decay is systematically underestimated at tree level. In this work we evaluate for the first time the loop contribution coming from an initial V P vertex. Despite the large superficial divergence of the loops, the convergence of the relevant loops can be established by using arguments of gauge invariance. The partial decay widths obtained agree with the experimental values wit…
Muon capture by a proton in heavy baryon chiral perturbation theory
1997
The matrix element for muon capture by a proton is calculated to O(p^3) within heavy baryon chiral perturbation theory using the new O(p^3) Lagrangian of Ecker and Mojzis. External nucleon fields are renormalized using the appropriate definition of the wave function renormalization factor Z_N. Our expression for Z_N differs somewhat from that found in existing literature, but is the one which is consistent with the Lagrangian we use and the one which ensures, within our approach, the nonrenormalization of the vector coupling as required by the conserved vector current. Expressions for the standard muon capture form factors are derived and compared to experimental data and we determine three…
Recent progress on the chiral unitary approach to meson meson and meson baryon interactions
1999
We report on recent progress on the chiral unitary approach, analogous to the effective range expansion in Quantum Mechanics, which is shown to have a much larger convergence radius than ordinary chiral perturbation theory, allowing one to reproduce data for meson meson interaction up to 1.2 GeV. Applications to physical processes so far unsuited for a standard chiral perturbative approach are presented. Results for the extension of these ideas to the meson baryon sector are discussed, together with applications to kaons in a nuclear medium and $K^-$ atoms.