arXiv subjects

L. Roca

Publications and source records attributed to L. Roca.

At least 73 records · Page 4Linked to original sources

arXiv2007

Quantum loops in radiative decays of the $a_1$ and $b_1$ axial-vector mesons

A previous model where the low-lying axial-vector mesons are dynamically generated, implementing unitarity in coupled channels in the vector-pseudoscalar ($VP$) meson interaction, is applied to evaluate the decay widths of the a_1(1260)$ and $b_1(1235)$ axial-vector mesons into $πγ$. Unlike the case of the $a_1$, the $b_1$ radiative decay is systematically underestimated at tree level. In this work we evaluate for the first time the loop contribution coming from an initial $VP$ 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 very well with the experimental values within uncertainties, and show that the loop contribution is crucial in the $b_1$ case and also important for the $a_1$ case.

hep-ph↗

arXiv2007

Recent developments in chiral dynamics of hadrons and hadrons in nuclei

In this talk I present recent developments in the field of hadronic physics and hadrons in the nuclear medium. I review the unitary chiral approach to meson baryon interaction and address the topics of the two dynamically generated $Λ(1405)$ resonances, with experiments testing it, the $Λ(1520)$ and $Δ(1700)$ resonances, plus the $Λ(1520)$, $Σ(1385)$ and $ω$ in the nuclear medium.

nucl-th↗

arXiv2007

Clues for the existence of two $K_1(1270)$ resonances

The axial vector meson $K_1(1270)$ was studied within the chiral unitary approach, where it was shown that it has a two-pole structure. We reanalyze the high-statistics WA3 experiment $K^- p\to K^-π^+π^- p$ at 63 GeV, which established the existence of both $K_1(1270)$ and $K_1(1400)$, and we show that it clearly favors our two-pole interpretation. We also reanalyze the traditional K-matrix interpretation of the WA3 data and find that the good fit of the data obtained there comes from large cancellations of terms of unclear physical interpretation.

hep-ph↗

arXiv2006

Recent developments in chiral dynamics of hadrons and hadrons in a nuclear medium

In this talk I present recent developments in chiral dynamics of hadrons and hadrons in a medium addressing the following points: interaction of the octet of pseudoscalar mesons with the octet of baryons of the nucleon, showing recent experimental evidence on the existence of two $Λ(1405)$ states, the interaction of the octet of pseudoscalar mesons with the decuplet of baryons of the $Δ$, with particular emphasis on the $Λ(1520)$ resonance, dynamically generated by this interaction. Then I review the interaction of kaons in a nuclear medium and briefly discuss the situation around the claims of deeply bound states in nuclei. The large renormalization of the $Λ(1520)$ in the nuclear medium is shown as another example of successful application of the chiral unitary techniques.

nucl-th↗

arXiv2006

Unitary coupled channel analysis of the $Λ(1520)$ resonance

We study the $Λ(1520)$ resonance in a coupled channel approach involving the $πΣ(1385)$, $KΞ(1530)$, $\bar{K}N$ and $πΣ$ channels. Implementing unitarity in coupled channels, we make an analysis of the relative importance of the different mechanisms which contribute to the dynamical structure of this resonance. From experimental information on some partial wave amplitudes and constraints imposed by unitarity, we get a comprehensive description of the amplitudes and hence the couplings to the different channels. We test these amplitudes in different reactions like $K^-p\toΛππ$, $γp\to K^+K^-p$, $γp\to K^+π^0π^0Λ$ and $π^- p\to K^0 K^-p$ and find a fair agreement with the experimental data.

hep-ph↗

arXiv2006

Testing the nature of the $Λ(1520)$ resonance in proton-induced production

The $Λ(1520)$ resonance has been recently studied in a unitarized coupled channel formalism with $πΣ(1385)$, $KΞ(1530)$, $\bar{K}N$ and $πΣ$ as constituents blocks. We provide a theoretical study of the predictions of this model in physical observables of the $pp\to pK^+K^-p$ and $pp\to pK^+π^0π^0Λ$ reactions. In particular, we show that the ratio between the $π^0π^0Λ$ and $K^-p$ mass distributions can provide valuable information on the ratio of the couplings of the $Λ(1520)$ resonance to $πΣ(1385)$ and $\bar{K}N$ that the theory predicts. Calculations are done for energies which are accessible in an experimental facility like COSY at Jülich or the developing CSR facility at Lanzhou.

nucl-th↗

arXiv2006

$A$ dependence of the $γ$- and $p$-induced production of the $Λ(1520)$ from nuclei

Using results of a recent calculation of the $Λ(1520)$ in the nuclear medium, which show that the medium width is about five times the free width, we study the A dependence of the $Λ(1520)$ production cross section in the reactions $γ~A \to K^+ Λ(1520) A^\prime$ and $p~ A \to p~ K^+ Λ(1520) A^\prime$. We find a sizable A dependence in the ratio of the nuclear cross sections for heavy nuclei with respect to a light one due to the large value of the $Λ(1520)$ width in the medium, showing that devoted experiments, easily within reach in present facilities, can provide good information on that magnitude by measuring the cross sections studied here.

nucl-th↗

arXiv2005

Chiral dynamics of the $Λ(1520)$ in coupled channels tested in the $K^-p\toππΛ$ reaction

The $Λ(1520)$ resonance is generated dynamically in a unitary coupled channel framework with the $πΣ^*$ and $KΞ^*$ channels in s-wave and $πΣ$ and $\bar K N$ channels in d-wave. The dynamics of this resonance close to and above threshold is then tested through the reactions $K^-p\toππΛ$ and a good agreement with the experimentally measured cross section is observed.

nucl-th↗

arXiv2005

Dynamical generation of $J^P=3/2-$ resonances and the $Λ(1520)$ resonance

The lowest order chiral Lagrangian is used to study s-wave interactions of the baryon decuplet with the octet of pseudoscalar mesons. The coupled channel Bethe Salpeter equation is used to obtain dynamically generated $\tfrac{3}{2}^-$ resonances in the partial wave amplitudes which provide a reasonable description to a number of 3 and 4-star resonances like $Δ(1700)$, $Λ(1520)$, $Σ(1670)$, $Σ(1940)$, $Ξ(1820)$, $Ω(2250)$ etc. The phenomenological introduction of d-wave channels in the coupled channel scheme along with the existing s-wave channels is shown to provide a much improved description of the $Λ(1520)$ resonance. The prediction of the absolute strength of the cross section in the reactions $K^-p\toΛπ^0π^0$ and $K^-p\toΛπ^+π^-$ provided by this scheme shows a good agreement with existing data.

nucl-th↗

arXiv2005

Discussion of the eta to pi^0 gamma gamma decay within a chiral unitary approach

We improve the calculations of the $η\to π^0 γγ$ decay within the context of meson chiral lagrangians. We use a chiral unitary approach for the meson-meson interaction, thus generating the $a_0(980)$ resonance and fixing the longstanding sign ambiguity on its contribution. This also allows us to calculate the loops with one vector meson exchange, thus removing a former source of uncertainty. In addition we ensure the consistency of the approach with other processes. First, by using vector meson dominance couplings normalized to agree with radiative vector meson decays. And, second, by checking the consistency of the calculations with the related $γγ\to π^0 η$ reaction. We find an $η\to π^0 γγ$ decay width of $0.47\pm 0.10$ eV, in clear disagreement with published data but in remarkable agreement with the most recent measurement.

hep-ph↗

arXiv2005

Low lying axial-vector mesons as dynamically generated resonances

We make a theoretical study of the s-wave interaction of the nonet of vector mesons with the octet of pseudoscalar mesons starting from a chiral invariant Lagrangian and implementing unitarity in coupled channels. By looking for poles in the unphysical Riemann sheets of the unitarized scattering amplitudes, we get two octets and one singlet of axial-vector dynamically generated resonances. The poles found can be associated to most of the low lying axial-vector resonances quoted in the Particle Data Book: $b_1(1235)$, $h_1(1170)$, $h_1(1380)$, $a_1(1260)$, $f_1(1285)$ and two poles to the $K_1(1270)$ resonance. We evaluate the couplings of the resonances to the $VP$ states and the partial decay widths in order to reinforce the arguments in the discussion.

hep-ph↗

arXiv2005

The $ϕ$ meson width in the medium from proton induced $ϕ$ production in nuclei

We perform calculations for the production of $ϕ$ mesons in nuclei at energies just above threshold and study the $A$ dependence of the cross section. We use results for the $ϕ$ selfenergy in the medium obtained within a chiral unitary approach. We find a strong $A$ dependence which is tied to the distortion of the incident proton and to the absorption of the $ϕ$ in its way out of the nucleus. The effect of this latter process reduces the cross section in about a factor two in heavy nuclei proving that the $A$ dependence of the cross section bears valuable information on the $ϕ$ width in the nuclear medium. Calculations are done for energies which are accessible in an experimental facility like COSY.

nucl-th↗

arXiv2005

Chiral dynamics of baryon resonances and hadrons in a nuclear medium

In these lectures I make an introduction to chiral unitary theory applied to the meson baryon interaction and show how several well known resonances are dynamically generated, and others are predicted. Two very recent experiments are analyzed, one of them showing the existence of two $Λ(1405)$ states and the other one providing support for the $Λ(1520)$ resonance as a quasibound state of $Σ(1385) π$. The use of chiral Lagrangians to account for the hadronic interaction at the elementary level introduces a new approach to deal with the modification of meson and baryon properties in a nuclear medium. Examples of it for $\bar{K}$, $η$ and $ϕ$ modification in the nuclear medium are presented.

nucl-th↗

arXiv2005

Chiral dynamics of hadrons in nuclei

In this talk I report on selected topics of hadron modification in the nuclear medium using the chiral unitary approach to describe the dynamics of the problems. I shall mention how antikaons, $η$, and $ϕ$ are modified in the medium and will report upon different experiments done or planned to measure the $ϕ$ width in the medium.

nucl-th↗

arXiv2004

Measuring the $ϕ$ meson width in the medium from $p$ induced $ϕ$ production in nuclei

We study the $A$ dependence of the $ϕ$ meson production cross section in proton nucleus reactios at energies just above threshold, which are accessible in an experimental facility like COSY. This $A$ dependence has two sources: the distortion of the incident proton and the absorption of the $ϕ$ in its way out of the nucleus. This second process reduces the cross section in about a factor two in heavy nuclei. Thus we show that the $A$ dependence of the cross section contains valuable information on the $ϕ$ width in the nuclear medium.

nucl-th↗

arXiv2004

Testing the nature of the $Λ(1520)$ resonance through photoproduction

Recent studies within the framework of chiral unitary theory produce the $Λ(1520)$ resonance, among several others, as a dynamically generated resonance from the interaction of the baryon decuplet and the meson octet. The $Λ(1520)$ qualifies as a quasibound state of $πΣ(1385)$ and this has repercussion in some observable quantities. In the present work we show that the $γp\to K^+K^-p$ reaction has a sizeable cross section for invariant $K^-p$ masses above the $Λ(1520)$ mass. On the other hand, we also find that the $γp\to K^+πΣ(1385)$ reaction has a sizeable cross section in that energy region as a consequence of the strong coupling of the $Λ(1520)$ to $πΣ(1385)$, and then we make predictions for the ratio of this cross section to that of the $γp\to K^+ K^-p $ reaction.

hep-ph↗

arXiv2004

Helicity asymmetries in double pion photoproduction on the proton

Based on a prior model on double pion photoproduction on the proton, successfully tested in total cross sections and invariant mass distributions, we make a theoretical study of the angular dependence of helicity asymmetries from the interaction of circularly polarized photons with unpolarized protons. We show that this observable is sensitive to details of the internal mechanisms and, thus, represents a complementary test of the theoretical model.

nucl-th↗

arXiv2004

Radiative vector meson decay

We study the radiative $ρ$, $ω$ and $ϕ$ decay into $π^0 π^0 γ$ and $π^0 ηγ$ taking into account mechanisms in which there are two sequential vector-vector-pseudoscalar or axial-vector--vector--pseudoscalar steps followed by the coupling of a vector meson to the photon, considering the final state interaction of the two mesons. Other mechanisms in which two kaons are produced through the same sequential mechanisms or from vector meson decay into two kaons which undergo final state interaction leading to the final pair of pions or $π^0 η$, are also considered. The results of the parameter free theory, together with the theoretical uncertainties, are compared with the latest experimental results at Frascati and Novosibirsk.

hep-ph↗