arXiv subjects

M. J. Vicente Vacas

Publications and source records attributed to M. J. Vicente Vacas.

At least 145 records · Page 8Linked to original sources

arXiv2002

Chiral unitary approach to hadron spectroscopy

The s-wave meson-baryon interaction in the $S = -1$, $S= 0$ and $S= -2$ sectors is studied by means of coupled channels, using the lowest-order chiral Lagrangian and the N/D method or equivalently the Bethe-Salpeter equation to implement unitarity. This chiral approach leads to the dynamical generation of the $Λ(1405)$, $Λ(1670)$ and $Σ(1620)$ states for $S = -1$, the $N^*(1535)$ for $S= 0$ and the $Ξ(1620)$ for $S= -2$. We look for poles in the complex plane and extract the couplings of the resonances to the different final states. This allows identifying the $Λ(1405)$ and the $Λ(1670)$ resonances with $\bar{K}N$ and $KΞ$ quasibound states, respectively. Our results are found to be incompatible with the measured properties of the $Ξ(1690)$ resonance, thus ruling this state out as the remaining member of this octet of dynamically generated resonances. We therefore assign $1/2^-$ for the spin and parity of the $Ξ(1620)$ resonance as the $S=-2$ member of the lowest-lying $1/2^-$ octet.

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arXiv2002

The $σ$ meson in a nuclear medium through two pion photoproduction

We present theoretical results for $(γ, π^0 π^0)$ production on nucleons and nuclei in the kinematical region where the scalar isoscalar $ππ$ amplitude is influenced by the $σ$ pole. The final state interaction of the pions modified by the nuclear medium produces a spectacular shift of strength of the two pion invariant mass distribution induced by the moving of the $σ$ pole to lower masses and widths as the nuclear density increases.

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arXiv2002

The sigma meson in a nuclear medium through two pion photoproduction

We present theoretical results for (gamma,pi0 pi0) and (gamma,pi+- pi0) production on nucleons and nuclei in the kinematical region where the scalar isoscalar pi pi amplitude is influenced by the sigma pole. The final state interaction of the pions modified by the nuclear medium produces a spectacular shift of strength of the pi0 pi0 invariant mass distribution induced by the moving of the sigma pole to lower masses and widths as the nuclear density increases.

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arXiv2002

Chiral approach to the rho meson in nuclear matter

In this work, the properties of the $ρ$ meson at rest in cold symmetric nuclear matter are studied. We make use of a chiral unitary approach to pion-pion scattering in the vector-isovector channel, calculated from the lowest order Chiral Perturbation Theory ($χPT$) lagrangian including explicit resonance fields. Low energy chiral constraints are considered by matching our expressions to those of one loop $χPT$. To account for the medium corrections, the $ρ$ couples to $ππ$ pairs which are properly renormalized in the nuclear medium, accounting for both $p-h$ and $Δ-h$ excitations. The terms where the $ρ$ couples directly to the hadrons in the $p-h$ or $Δ-h$ excitations are also accounted for. In addition, the $ρ$ is also allowed to couple to $N^{*}(1520)-h$ components.

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arXiv2001

Nuclear medium effects on the sigma mass and width

The meson meson interaction in the scalar isoscalar channel at finite baryonic density is studied in the framework of a chiral unitary approach which generates the f0 and sigma resonances and reproduces well the meson meson phase shifts in vacuum. We investigate the sigma spectral function, its mass and its width as a function of the baryon density and discuss possible experimental signatures in the (pi,2pi) and the (gamma,2pi) reactions.

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arXiv2001

Two pion photoproduction on nucleons and nuclei in the rho and sigma regions

In this talk we report on two physical processes, the photoproduction of $ρ$ mesons in nuclei and the $π^0 π^0$ photoproduction in nuclei at low energies. In the first case the aim is to observe experimentally the theoretically predicted changes on the $ρ$ properties in the nuclear medium. In the second case one wishes to investigate the modifications of the $ππ$ interaction in the nuclear medium in the region of the $σ$ meson. In the first case we observe that it is quite difficult to see large effects due to the $ρ$ modification in the medium because the detection of the $ρ$ is done through the two pions which are produced in the $Δ$ region and are largely distorted in the medium. In the second case, when the I=0 part of the $ππ$ interaction is substituted by the in medium $ππ$ amplitude, we observe a very large shift of strength in the invariant mass distribution to small values of the mass in $^{12}C$ and $^{208}Pb$ with respect to the distribution of the elementary reaction. This spectacular shift appears to be corroborated by recent experiments at Mainz reported in this same Workshop.

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arXiv2001

Chiral dynamics and pion-nucleon scattering around the N*(1535) resonance

We study the S-wave interaction of mesons with baryons in the strangeness S=0 sector in a coupled channel unitary approach. The basic dynamics is drawn from the lowest order meson baryon chiral Lagrangians. Small modifications inspired by models with explicit vector meson exchange in the t-channel are also considered. In addition the pi pi N channel is included and shown to have an important repercussion in the results, particularly in the isospin 3/2 sector.

hep-ph↗

arXiv2001

Chiral unitary approach to S-wave meson baryon scattering in the strangeness S=0 sector

hep-ph↗

arXiv2001

The role of Delta(1700) excitation and rho production in double pion photoproduction

Recent information on invariant mass distributions of the $γp\toπ^+π^0 n$ reaction, where previous theoretical models had shown deficiencies, have made more evident the need for new mechanisms, so far neglected or inaccurately included. We have updated a previous model to include new necessary mechanisms. We find that the production of the $ρ$ meson, and the $Δ(1700)$ excitation, through interference with the dominant terms, are important mechanisms that solve the puzzle of the $γp\toπ^+π^0 n$ reaction without spoiling the early agreement with the $γp\toπ^+π^- p$ and $γp\toπ^0π^0 p$ reactions.

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arXiv2001

Test of phi renormalization in nuclei through phi photoproduction

We propose an experimental procedure to find out the medium modifications of the $ϕ$ meson. The reaction is inclusive $ϕ$ photoproduction in nuclei, looking for $K^+ K^-$ pairs from the $ϕ$ decay with total momentum smaller than 100-150 $MeV/c$, which are made possible at energies of present laboratories from center of mass $ϕ$ backward production and the help of Fermi motion. We have conducted a many body calculation of the mass distribution of the $ϕ$ adapted to the experimental set up of a recent JLAB experiment where the backwards $ϕ$ photoproduction has been measured. Using recent results for the in medium properties of the $ϕ$, we find that the width of the invariant mass distribution for photoproduction on medium to heavy nuclei is larger than the free $ϕ$ width by a factor of two or more.

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arXiv2001

Chiral unitary theory: application to nuclear problems

In this talk we briefly describe some basic elements of chiral perturbation theory, $χPT$, and how the implementation of unitarity and other novel elements lead to a better expansion of the $T$ matrix for meson meson and meson baryon interactions. Applications are then done to the $ ππ$ interaction in nuclear matter in the scalar and vector channels, antikaons in nuclei and $K^-$ atoms, and how the $ϕ$ meson properties are changed in a nuclear medium.

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arXiv2000

The rho meson in a nuclear medium

In this work, propagation properties of the rho meson in symmetric nuclear matter are studied. We make use of a coupled channel unitary approach to meson-meson scattering, calculated from the lowest order Chiral Perturbation Theory lagrangian including explicit resonance fields. Low energy chiral constraints are considered by matching our expressions to those of one loop Ch.P.T. To account for the medium corrections, the rho couples to pion-pion and kaon-antikaon pairs which are properly renormalized in the nuclear medium.

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arXiv2000

Chiral Unitary Approach to the Kaon Nucleus Interaction and Kaonic Atoms

We review recent work on various topics related to the modification of kaon properties in nuclei. After a brief exposition of the kaon-nucleon and kaon-nucleus interaction, results from the application to kaonic atoms, renormalization of the f0 and a0 scalar resonances in nuclei and phi decay in the nucleus are shown.

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arXiv2000

Renormalization of the f0(980) and a0(980) scalar resonances in a nuclear medium

The meson meson interaction in the scalar sector in the presence of a nuclear medium is studied with particular attention to the change of the properties of the f0(980) and a0(980) resonances. By using a chiral unitary approach which generates the f0 and a0 resonances and reproduces their free properties, we find that the position of the resonances in the medium is barely changed but their widths are considerably broadened. New many body corrections generating from higher orders in the chiral Lagrangian plus the contribution from N*h excitations, not considered before in connection with the pion-pion interaction in the nuclear medium, are also investigated.

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arXiv2000

S=I=0 Pion Pairs in the A(pi,pi pi)X Reaction

Recent experimental results show a large enhancement of the 2 pion emission on the scalar isoscalar channel, which had been predicted by some theoretical estimates. We present here a detailed calculation of the A(pi, 2pi)X process incorporating a microscopic model for the elementary reaction in vacuum, pion-pion final state interaction in the nuclear medium, and other nuclear medium effects.

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arXiv1999

Charged current weak production of the Delta resonance

The charge changing weak production of Delta in nucleons and nuclei is studied. The reactions e^{-} p -> Delta^{0} ν_{e} and e^{+} p -> Delta^{++} \barν_{e} are considered as a possible source of information about the weak N-Delta transition form factors. The low q^2 BNL data on neutrino production of Delta are used to extract the axial vector N-Delta coupling, taking into account the deuteron structure and the Delta width. Finally, pion production induced by neutrinos in ^{16}O in the Delta region, relevant to atmospheric neutrino experiments, is investigated.

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arXiv1999

Scalar Isoscalar pion pairs in nuclei and the A(pi,pipi)X reaction

The reaction A(pi,pi pi)X has been studied at low energies, paying particular attention to the interaction of the two final pions in the scalar isoscalar (I=J=0) channel. We have developed a microscopic model for the pion production, and then implemented the two pion final state interaction by using the results of a non-perturbative unitary coupled-channels method based in the standard chiral Lagrangians. The resulting model, describes well the reaction on the nucleon for all different isospin channels. Finally, we have considered the reaction in nuclei. Our calculation takes into account Fermi motion, Pauli blocking, pion absorption, and also the strong modification of the pi-pi interaction in the nuclear medium.

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arXiv1998

A coupled channel unitary chiral approach to the meson-meson interaction and pion-pion scattering in a nuclear medium

A new tool which combines chiral perturbation theory and unitarity in coupled channels is applied with success to the study of the meson-meson interaction, extending the theoretical predictions up to sqrt{s} = 1.2 GeV, hence improving considerably the convergence radius of conventional chiral perturbation theory. The method is applied to obtain the meson-meson phase shifts and inelasticities as well as the isoscalar pion-pion scattering amplitude inside a nuclear medium.

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