arXiv subjects

J. A. Oller

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

At least 73 records · Page 4Linked to original sources

arXiv2012

Chiral dynamics in form factors, spectral-function sum rules, meson-meson scattering and semilocal duality

In this work, we perform the one-loop calculation of the scalar and pseudoscalar form factors in the framework of U(3) chiral perturbation theory with explicit tree level exchanges of resonances. The meson-meson scattering calculation from Ref.[1] is extended as well. The spectral functions of the nonet scalar-scalar (SS) and pseudoscalar-pseudoscalar (PP) correlators are constructed by using the corresponding form factors. After fitting the unknown parameters to the scattering data, we discuss the resonance content of the resulting scattering amplitudes. We also study spectral-function sum rules in the SS-SS, PP-PP and SS-PP sectors as well as semi-local duality from scattering. The former relate the scalar and pseudoscalar spectra between themselves while the latter mainly connects the scalar spectrum with the vector one. Finally we investigate these items as a function of Nc for Nc > 3. All these results pose strong constraints on the scalar dynamics and spectroscopy that are discussed. They are successfully fulfilled by our meson-meson scattering amplitudes and spectral functions.

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arXiv2012

Recent developments in U(3) ChPT: meson-meson scattering and finite energy sum rules

We discuss the meson-meson scattering and finite energy sum rule(FESR), based on the one-loop calculation within U(3) chiral perturbation theory. First we obtain the pertinent resonance spectroscopy from the unitarized partial wave scattering amplitudes. Then we investigate how well the FESR can be satisfied in the physical situation at Nc=3. Further discussions on the extrapolation of Nc are also given.

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arXiv2012

Developments in Chiral effective Field Theory for Nuclear Matter

We review on a chiral power counting scheme for in-medium chiral perturbation theory with nucleons and pions as degrees of freedom \cite{ref}. It allows for a systematic expansion taking into account local as well as pion-mediated inter-nucleon interactions. Based on this power counting, one can identify classes of non-perturbative diagrams that require a resummation. We then calculate the nuclear matter energy density for the symmetric and purely neutron matter cases up-to-and-including next-to-leading order (NLO), in good agreement with sophisticated many-body calculations. Next, the neutron matter equation of state is applied to calculate the upper limit for neutron stars, with an upper bound around 2.3 solar masses, large enough to accommodate the most massive neutron star observed until now. We also apply our equation state to constraint $G_N$ in exceptionally large gravitational fields.

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arXiv2012

On the size of the sigma meson and its nature

In this work the nature of the σor f_0(600) resonance is discussed by evaluating its quadratic scalar radius, \la r^2\ra_s^σ. This allows one to have a quantitative estimate for the size of this resonance. We obtain that the σresonance is a compact object with $\la r^2\ra_s^σ=(0.19\pm0.02)-i(0.06\pm0.02)$ fm^2. Within our approach, employing unitary chiral perturbation theory, the σis a dynamically generated resonance that stems from the pion-pion interactions. Given its small size we conclude that the two pions inside the resonance are merged. A four-quark picture is then more appropriate. However, when the pion mass increases, for pion masses somewhat above 400 MeV, the picture of a two-pion molecule is the appropriate one. The σis then a spread ππbound state. These results are connected with other recent works that support a non standard nature of the σas well, while fulfilling strong QCD constraints, as well as with lattice QCD. We offer a detailed study of the low-energy S-wave ππscattering data from where we extract our values for the threshold parameters of S-wave ππphase shifts, the {\cal O}(p^4) chiral perturbation theory low energy constants as well as the σpole position. From the comparison with other accurate determinations in the literature we obtain the average values for the isospin 0 S-wave ππthreshold parameters, $a_0^0=0.220\pm0.003$, $b_0^0=0.279\pm 0.003 M_π^{-2}$, and for the real and imaginary parts of the σpole position in \sqrt{s}, $458\pm14-i(261\pm17)$ MeV. The quark mass dependence of the size of the σ, its mass and width are considered too. The latter agree accurately with a previous lattice QCD calculation. The fact that the mass of this resonance tends to follow the threshold of two pions is a clear indication that the σis a dynamically generated meson-meson resonance.

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arXiv2012

Finite volume treatment of pi pi scattering and limits to phase shifts extraction from lattice QCD

We study theoretically the effects of finite volume for pipi scattering in order to extract physical observables for infinite volume from lattice QCD. We compare three different approaches for pipi scattering (lowest order Bethe-Salpeter approach, N/D and inverse amplitude methods) with the aim to study the effects of the finite size of the box in the potential of the different theories, specially the left-hand cut contribution through loops in the crossed t,u-channels. We quantify the error made by neglecting these effects in usual extractions of physical observables from lattice QCD spectra. We conclude that for pipi phase-shifts in the scalar-isoscalar channel up to 800 MeV this effect is negligible for box sizes bigger than 2.5m_pi^-1 and of the order of 5% at around 1.5-2m_pi^-1. For isospin 2 the finite size effects can reach up to 10% for that energy. We also quantify the error made when using the standard Luscher method to extract physical observables from lattice QCD, which is widely used in the literature but is an approximation of the one used in the present work.

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arXiv2012

Chiral dynamics in U(3) unitary chiral perturbation theory

We perform a complete one-loop calculation of meson-meson scattering, and of the scalar and pseudoscalar form factors in U(3) chiral perturbation theory with the inclusion of explicit resonance fields. This effective field theory takes into account the low-energy effects of the QCD U_A(1) anomaly explicitly in the dynamics. The calculations are supplied by non-perturbative unitarization techniques that provide the final results for the meson-meson scattering partial waves and the scalar form factors considered. We present thorough analyses on the scattering data, resonance spectroscopy, spectral functions, Weinberg-like sum rules and semi-local duality. The last two requirements establish relations between the scalar spectrum with the pseudoscalar and vector ones, respectively. The Nc extrapolation of the various quantities is studied as well. The fulfillment of all these non-trivial aspects of the QCD dynamics by our results gives a strong support to the emerging picture for the scalar dynamics and its related spectrum.

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arXiv2012

The chiral representation of the $πN$ scattering amplitude and the pion-nucleon sigma term

We present a novel analysis of the $πN$ scattering amplitude in Lorentz covariant baryon chiral perturbation theory renormalized in the extended-on-mass-shell scheme. This amplitude, valid up to $\mathcal{O}(p^3)$ in the chiral expansion, systematically includes the effects of the $Δ(1232)$ in the $δ$-counting, has the right analytic properties and is renormalization-scale independent. This approach overcomes the limitations that previous chiral analyses of the $πN$ scattering amplitude had, providing an accurate description of the partial wave phase shifts of the Karlsruhe-Helsinki and George-Washington groups up to energies just below the resonance region. We also study the solution of the Matsinos group which focuses on the parameterization of the data at low energies. Once the values of the low-energy constants are determined by adjusting the center-of-mass energy dependence of the amplitude to the scattering data, we obtain predictions on different observables. In particular, we extract an accurate value for the pion-nucleon sigma term, $ σ_{πN}$. This allows us to avoid the usual method of extrapolation to the unphysical region of the amplitude. Our study indicates that the inclusion of modern meson-factory and pionic-atom data favors relatively large values of the sigma term. We report the value $σ_{πN}=59(7)$MeV and comment on implications that this result may have.

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arXiv2011

Relativistic chiral representation of the $πN$ scattering amplitude II: The pion-nucleon sigma term

We present a determination of the pion-nucleon sigma-term based on a novel analysis of the $πN$ scattering amplitude in Lorentz covariant baryon chiral perturbation theory renormalized in the extended-on-mass-shell scheme. This amplitude, valid up-to next-to-next-leading order in the chiral expansion, systematically includes the effects of the $Δ(1232)$, giving a reliable description of the phase shifts of different partial wave analyses up to energies just below the resonance region. We obtain predictions on some observables that are within experimental bounds and phenomenological expectations. In particular, we use the center-of-mass energy dependence of the amplitude adjusted with the data above threshold to extract accurately the value of $σ_{πN}$. Our study indicates that the inclusion of modern meson-factory and pionic-atom data favors relatively large values of the sigma term. We report the value $σ_{πN}=59(7)$ MeV.

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arXiv2011

Relativistic chiral representation of the $πN$ scattering amplitude I: The Goldberger-Treiman relation

In this work we study the $πN$ scattering process within the Baryon Chiral Perturbation Theory framework in the covariant scheme of Extended-On-Mass-Shell (EOMS). We compare the description obtained in this scheme with the previously obtained using the Infrared Regularization scheme and show that EOMS accomplishes the best convergence, being able to extract from partial wave analyses reliable values of important quantities as the Goldberger-Treiman deviation. In regard to the latter, we solve the long-standing problem concerning to the extraction of the Goldberger-Treiman deviation with covariant ChPT that jeopardized the applicability of ChPT to the $πN$ system. We also show the potential of the unitarization techniques applied to the perturbative calculation in the EOMS scheme, that allow us to increase the range of validity of our description up to $\approx 200$ MeV in $\sqrt{s}$.

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arXiv2011

Nucleon-nucleon interactions from dispersion relations: Elastic partial waves

We consider nucleon-nucleon (NN) interactions from chiral effective field theory. In this work we restrict ourselves to the elastic NN scattering. We apply the N/D method to calculate the NN partial waves taking as input the one-pion exchange discontinuity along the left-hand cut. This discontinuity is amenable to a chiral power counting as discussed by Lacour, Oller and Meissner [Ann. Phys. (NY) 326, 241 (2011)], with one-pion exchange as its leading order contribution. The resulting linear integral equation for a partial wave with orbital angular momentum L>=2 is solved in the presence of L-1 constraints, so as to guarantee the right behavior of the D and higher partial waves near threshold. The calculated NN partial waves are based on dispersion relations and are independent of regulator. This method can also be applied to higher orders in the calculation of the discontinuity along the left-hand cut and extended to triplet coupled partial waves.

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arXiv2011

Exploration of resonance properties in chiral perturbation theory with explicit U_A(1) anomaly

We study the resonance properties within chiral perturbation theory by explicitly taking into account the U_A(1) anomaly effect. This assures we have the appropriate degrees of freedom of low energy QCD in the large Nc limit. We calculate the various resonance properties, such as mass, width and residues, for the physical case, i.e. Nc=3. Then we extrapolate the values of Nc to study the trajectories of resonance poles.

hep-ph↗

arXiv2011

Relativistic chiral representation of the πN scattering amplitude

We have analyzed pion-nucleon scattering using the manifestly relativistic covariant frameworks of Infrared Regularization (IR) and Extended-On-Mass-Shell (EOMS) up to O(q^3) in the chiral expansion, where q is a generic small momentum. We describe the low-energy phase shifts with a similar quality as previously achieved with Heavy Baryon Chiral Perturbation Theory, being the EOMS description better than the IR one. The Goldberger-Treiman discrepancy is extracted from data of partial wave analyses using both schemes, obtaining an unacceptable large value for the case of IR due to the loop contribution. On the other hand, EOMS gives small values compatible with other phenomenological approaches. Finally, we have unitarized the amplitudes provided by both schemes to extend the range of our description obtaining a good agreement with the data up to energies of \sqrt{s}\approx 1.3 GeV for the EOMS scheme while IR can not go beyond energies of \sqrt{s}\approx 1.25 GeV due to the unphysical cut that this scheme introduces.

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arXiv2011

Resonances from meson-meson scattering in U(3) CHPT

In this work, the complete one loop calculation of meson-meson scattering amplitudes within U(3)\otimes U(3) chiral perturbation theory with explicit resonance states is carried out for the first time. Partial waves are unitarized from the perturbative calculation employing a non-perturbative approach based on the N/D method. Once experimental data are reproduced in a satisfactory way we then study the resonance properties, such as the pole positions, corresponding residues and their N_C behaviors. The resulting N_C dependence is the first one in the literature that takes into account the fact that the η_1 becomes the ninth Goldstone boson in the chiral limit for large N_C. Within this scheme the vector resonances studied, ρ(770), K^*(892) and ϕ(1020), follow an N_C trajectory in agreement with their standard \bar{q}q interpretation. The scalars f_0(1370), a_0(1450) and K^*(1430) also have for large N_C a \bar{q}q pole position trajectory and all of them tend to a bare octet of scalar resonances around 1.4 GeV. The f_0(980) tends asymptotically to the bare pole position of a singlet scalar resonance around 1 GeV. The σ, κand a_0(980) scalar resonances have a very different N_C behavior. The case of the σresonance is analyzed with special detail.

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arXiv2011

πN scattering in relativistic baryon chiral perturbation theory revisited

We have analyzed pion-nucleon scattering using the manifestly relativistic covariant framework of Infrared Regularization up to {\cal O}(q^3) in the chiral expansion, where q is a generic small momentum. We describe the low-energy phase shifts with a similar quality as previously achieved with Heavy Baryon Chiral Perturbation Theory, \sqrt{s}\lesssim1.14 GeV. New values are provided for the {\cal O}(q^2) and {\cal O}(q^3) low-energy constants, which are compared with previous determinations. This is also the case for the scattering lengths and volumes. Finally, we have unitarized the previous amplitudes and as a result the energy range where data are reproduced increases significantly.

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arXiv2010

Scalar-Pseudoscalar scattering and pseudoscalar resonances

The interactions between the f_0(980) and a_0(980) scalar resonances and the lightest pseudoscalar mesons are studied. We first obtain the interacting kernels, without including any ad hoc free parameter, because the lightest scalar resonances are dynamically generated. These kernels are unitarized, giving the final amplitudes, which generate pseudoscalar resonances, associated with the K(1460), π(1300), π(1800), η(1475) and X(1835). We also consider the exotic channels with I=3/2 and I^G=1^+ quantum numbers. The former could be also resonant in agreement with a previous prediction.

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arXiv2010

Hadron resonances generated from the dynamics of the lightest scalar ones

We have studied the interactions of the scalar resonances f_0(980) and a_0(980) with the vector resonance ϕ(1020) and with the lightest pseudoscalars π, K, ηand η'. We first obtain the interaction kernels without including any new free parameter. Afterwards, the interaction kernels are unitarized and the final S-wave amplitudes result. We find that these interactions are very rich and generate a large amount of pseudoscalar resonances including the K(1460), π(1300), π(1800), η(1475) and X(1835) resonances. The f_0(980)ϕ(1020) self-interactions give rise to the ϕ(2170) resonance. For realistic choices of the parameters we also obtain an isovector companion in the same mass region from the a_0(980) ϕ(1020) interactions.

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arXiv2010

Dynamical generation of pseudoscalar resonances

We study the interactions between the f_0(980) and a_0(980) scalar resonances and the lightest pseudoscalar mesons. We first obtain the elementary interaction amplitudes, or interacting kernels, without including any ad hoc free parameter. This is achieved by using previous results on the nature of the lightest scalar resonances as dynamically generated from the rescattering of S-wave two-meson pairs. Afterwards, the interaction kernels are unitarized and the final S-wave amplitudes result. We find that these interactions are very rich and generate a large amount of pseudoscalar resonances that could be associated with the K(1460), π(1300), π(1800), η(1475) and X(1835). We also consider the exotic channels with isospin 3/2 and 1, having the latter positive G-parity. The former could be also resonant in agreement with a previous prediction.

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arXiv2010

Chiral effective field theory for nuclear matter

We report on the recent developments of a new effective field theory for nuclear matter [1,2,3]. We present first the nuclear matter chiral power counting that takes into account both short-- and long--range inter-nucleon interactions. It also identifies non-perturbative strings of diagrams, related to the iteration of nucleon-nucleon interactions, which have to be re-summed. The methods of unitary chiral perturbation theory has been shown to be a useful tool in order to perform those resummations. Results up to next-to-leading order for the ground state energy per particle of nuclear matter, the in-medium chiral quark condensate and pion self-energy are discussed.

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