arXiv subjects

J. A. Oller

Publications and source records attributed to J. A. Oller.

At least 109 records · Page 6Linked to original sources

arXiv2003

Chiral Dynamics of the Two Λ(1405) States

Using a chiral unitary approach for the meson--baryon interactions, we show that two octets of J^π=1/2^- baryon states, which are degenerate in the limit of exact SU(3) symmetry, and a singlet are generated dynamically. The SU(3) breaking produces the splitting of the two octets, resulting in the case of strangeness S=-1 in two poles of the scattering matrix close to the nominal Λ(1405) resonance. These poles are combinations of the singlet state and the octets. We show how actual experiments see just one effective resonance shape, but with properties which change from one reaction to another.

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arXiv2001

Pion and Kaon Vector Form Factors

We develop a unitarity approach to consider the final state interaction corrections to the tree level graphs calculated from Chiral Perturbation Theory ($χPT$) 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 $χPT$ results. A very good description of experimental data is accomplished for the vector form factors and for the $ππ$ 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 former a comparison with one and two loop $χPT$ is made showing a remarkable coincidence with the two loop $χPT$ results.

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arXiv2000

Chiral dynamics in the presence of bound states: kaon-nucleon interactions revisited

We study the S-wave kaon-nucleon interactions for strangeness S=-1 in a novel relativistic chiral unitary approach based on coupled channels. Dispersion relations are used to perform the necessary resummation of the lowest order relativistic chiral Lagrangian. A good description of the data in the K^- p, πΣand πΛchannels is obtained. We show how this method can be systematically extended to higher orders, emphasizing its applicability to any scenario of strong self-interactions where the perturbative series diverges even at low energies. Discussions about the differences to existing approaches employing pseudo-potentials in a regulated Lippmann-Schwinger equation are included. Finally, we describe the resonance content of our meson-baryon amplitudes and discuss its nature.

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arXiv2000

Pion and Kaon Vector Form Factors: a Chiral Unitary Approach

The pion and kaon coupled-channel vector form factors are described by making use of the resonance chiral Lagrangian results together with a suitable unitarization method in order to take care of the final state interactions. A very good reproduction of experimental data is accomplished for the vector form factors up to $\sqrt{s}\leq 1.2$ GeV and for the $ππ$ P-wave phase shifts up to $\sqrt{s}\leq 1.5$ GeV.

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arXiv2000

J/Ψ\to ϕππ(K \bar{K}) decays, chiral dynamcis and OZI violation

We have studied the invariant mass distributions of the ππand K \bar{K} systems for invariant masses up to 1.2 GeV from the J/Ψ\to ϕππ(K\bar{K}) decays. The approach exploits the connection between these processes and the ππand K\bar{K} strange and non-strange scalar form factors by considering the ϕmeson as a spectator. The calculated scalar form factors are then matched with the ones from next-to-leading order chiral perturbation theory, including the calculation of the the K\bar{K} scalar form factors. Final state interactions in the J/Ψ\to ϕππ(K\bar{K}) processes are taken into account as rescattering effects in the system of the two pseudoscalar mesons. A very good agreement with the experimental data from DM2 and MARK-III is achieved. Furthermore, making use of SU(3) symmetry, the S-wave contribution to the π^+π^- event distribution in the J/Ψ\to ωπ^+π^- reaction is also given and the data up to energies of about 0.7 GeV are reproduced. These decays of the J/Ψto a vector and a pair of pseudoscalars turn out to be very sensitive to OZI violating physics which we parametrize in terms of a direct OZI violation parameter and the chiral perturbation theory low energy constants L_4^r and L_6^r. These constants all come out very different from zero, lending further credit to the statement that the OZI rule is not operative in the scalar 0^{++} channel.

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arXiv2000

The phi to pi+ pi- decay within a chiral unitary approach

Starting from the Chiral Perturbation Theory Lagrangian, but keeping different masses for the charged and neutral mesons, and using a previously developed non-perturbative unitary scheme that generates the lightest meson-meson resonances, we construct K Kbar to K Kbar and K Kbar to pi+ pi- in the vector channel. This allows us to obtain the kaon-loop contribution to the phi-rho mixing and study the phi to pi+pi- decay. The dominant contribution to this decay comes from the phi to gamma to pi+pi- process. However, there can be large interferences with the subdominant contributions coming from phi-rho and phi-omega mixing, or of these two contributions among themselves. As a consequence, a reliable measurement of phi to pi+pi- decay could be used to differentiate between some phi-omega mixing scenarios proposed in the literature.

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arXiv2000

Scalar Mesons and Chiral Symmetry

It is the purpose of the present manuscript to emphasize those aspects that make the scalar sector with vacuum quantum numbers rather unique. Chiral symmetry is the basic tool for our study together with a resummation of Chiral Perturbation Theory (CHPT) that stresses the role of unitarity but also allows one to include explicit resonance fields and to match with the CHPT expansion at low energies.

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arXiv2000

Chiral Unitary approach to meson-meson and meson-baryon interactions and nuclear applications

We report on recent nonperturbative techniques that combine the information of chiral Lagrangians (with and without resonances) with unitarity in coupled channels and other requirements of the S-matrix theory of the strong interactions. As a result, the region of applicability of such techniques is much larger than the one of Chiral Perturbation Theory allowing one to study also resonance physics. Applications to meson-meson and meson-baryon scattering, as well as to problems where pairs of mesons or a meson and a baryon appear in the initial or final state are shown. Implications in several nuclear problems are also discussed.

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arXiv2000

The phi --> pi^+ pi^- and phi radiative decays within a chiral unitary approach

We report on recent results on the decay of the phi into pi^+ pi^- and phi radiative decays into pi^0 pi^0 gamma and pi^0 eta gamma, which require the consideration of the final state interaction of a pair of mesons in a region inaccessible to Chiral Perturbation Theory. By using nonperturbative chiral unitary methods for the meson meson interaction we can obtain the corresponding decay widths and the results are compared with recent experimental data.

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arXiv1999

Chiral unitary meson-baryon dynamics in the presence of resonances: Elastic pion-nucleon scattering

We develop a novel approach to chiral meson-baryon dynamics incorporating unitarity constraints and explicit resonance fields. It is based on the most general structure of any pion-nucleon partial wave amplitude neglecting the unphysical cuts as derived from the N/D method. This amplitude is then matched to the one-loop heavy baryon chiral perturbation theory result at third order and to tree level exchanges of baryon- and meson states in the s,t and u channels. This generates the left-hand cuts. The unitarization procedure does not involve form factors or regulator functions. The resonance parameters are determined from fits to the S- and P-wave pion-nucleon partial wave amplitudes for energies up to 1.3 GeV. In particular, the $Δ(1232)$ is accurately reproduced whereas scalar and vector meson couplings are less precisely pinned down. We also obtain a satisfactory description of the $π$N threshold parameters. Further extensions of this method to coupled channels and the three-flavor case are briefly discussed.

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arXiv1999

Photoproduction of meson and baryon resonances in a chiral unitary approach

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.

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arXiv1999

The Case of a WW Dynamical Scalar Resonance within a Chiral Effective Description of the Strongly Interacting Higgs Sector

We have studied the strongly interacting $W_L W_L \to W_L W_L$ I=L=0 partial wave amplitude making use of effective chiral Lagrangians. The Higgs boson is explicitly included and the N/D method is used to unitarize the amplitude. We recover the chiral perturbative expansion at next to leading order for low energies. The cases $m_H<< 4πv$ and $m_H>> 4 πv$ are considered in detail. It is shown that in the latter situation a state appears with a mass $\lesssim 1$ TeV. This state is dynamically generated through the strong interactions between the $W_L$ and is not responsible for the spontaneous electroweak symmetry breaking. However, its shape can be very similar to that of the $m_H\lesssim 1$ TeV case, which corresponds to a conventional heavy Higgs boson.

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arXiv1999

Resonances, Chiral Symmetry, Coupled Channel Unitarity and Effective Lagrangians

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. We report on an update of these results examining their possible relevance for meson spectroscopy.

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arXiv1999

SU(3) Chiral approach to meson and baryon dynamics

We report on recent progress on the chiral unitary approach, 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 and meson baryon interaction up to the first baryonic resonances. Applications to physical processes so far unsuited for a standard chiral perturbative approach are presented, concretely the K^- p\toΛ(1405)γreaction and the N^\ast (1535)N^\ast(1535)πand ηcouplings.

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arXiv1999

Two Meson Scattering Amplitudes and their Resonances from Chiral Symmetry and the N/D Method

We study the vector and scalar meson-meson amplitudes up to \sqrt{s}\lesssim 1.4 GeV and their associated spectroscopy. The study has been done considering jointly the N/D method, Chiral Symmetry and implications from large N_c QCD. The N/D method provides us with the way to unitarize the tree level amplitudes constructed in agreement with Chiral Symmetry and its breaking (explicit and spontaneous). These amplitudes are calculated making use of the lowest order Chiral Perturbation Theory (\chiPT) Lagrangians and the exchanges of resonances compatible with Chiral Symmetry as given in. On the other hand the large N_c considerations allow us to distinguish between elementary (as elementary as the pions, for instance) and compound (meson-meson) states. Making use of this formalism one observes that the σ, κand a_0(980) resonances are meson-meson states originating from the unitarization of the lowest order \chiPT amplitudes. On the other hand, the f_0(980) is a combination of a strong S-wave meson-meson unitarity effect and of a preexisting singlet resonance with a mass around 1 GeV.

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arXiv1999

A Non-Perturbative Chiral Approach for Meson-Meson Interactions

A non-perturbative method which combines constraints from chiral symmetry breaking and coupled channel unitarity is used to describe meson-meson interactions up to \sqrt{s}\lesssim 1.2 GeV, extending in this way the range of applicability of the information contained in Chiral Perturbation Theory (χPT), since this perturbative series is typically restricted to \sqrt{s}\lesssim 500 MeV. The approach uses the O(p^2) and O(p^4) \chiPT Lagrangians. The seven free parameters resulting from the O(p^4) Lagrangian are fitted to the experimental data. The approach makes use of the expansion of T^{-1} instead of the amplitude itself as done in \chiPT. The former expansion is suggested by analogy with the effective range approximation in Quantum Mechanics and it appears to be very useful. The results, in fact, are in good agreement with a vast amount of experimental analyses.

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arXiv1999

N/D Description of Two Meson Amplitudes and Chiral Symmetry

The most general structure of an elastic partial wave amplitude when the unphysical cuts are neglected is deduced in terms of the N/D method. This result is then matched to lowest order, ${\mathcal{O}}(p^2)$, Chiral Perturbation Theory($χ$PT) and to the exchange (consistent with chiral symmetry) of resonances in the s-channel. The extension of the method to coupled channels is also given. Making use of the former formalism, the $ππ$ and $Kπ$(I=1/2) P-wave scattering amplitudes are described without free parameters when taking into account relations coming from the 1/$N_c$ expansion and unitarity. Next, the scalar sector is studied and good agreement with experiment up to $\sqrt{s}=1.4$ GeV is found. It is observed that the $a_0(980)$, $σ$ and $κ(900)$ resonances are meson-meson states originating from the unitarization of the ${\mathcal{O}}(p^2)$ $χ$PT amplitudes. On the other hand, the $f_0(980)$ is a combination of a strong S-wave meson-meson unitarity effect and of a preexisting singlet resonance with a mass around 1 GeV. We have also studied the size of the contributions of the unphysical cuts to the $ππ$(I=0) and $Kπ$(I=1/2) elastic S-wave amplitudes from $χ$PT and the exchange of resonances in crossed channels up to $\sqrt{s}\approx 800$ MeV. The loops are calculated as in $χ$PT at next to leading order. We find a small correction from the unphysical cuts to our calculated partial waves.

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arXiv1999

Recent progress on the chiral unitary approach to meson meson and meson baryon interactions

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.

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