arXiv subjects
Publications and source records attributed to E. Oset.
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.
We present an overview of the latest theoretical studies on the antikaon properties in the nuclear medium, in connection with the recent experimental claims of very deeply bound antikaon-nuclear states. We argue that proper many-body formulations using modern realistic antikaon-nucleon interactions are not able to generate such systems. Instead, a simple two-nucleon antikaon absorption mechanism where the remaining nucleus acts as spectator explains the peak in the semi-inclusive proton momentum spectrum, observed on a 4He target at KEK (but later not confirmed in an inclusive experiment) and on a 6Li target at FINUDA. This signal is clearly seen in another FINUDA experiment measuring the invariant mass of Lambda-proton pairs after two-nucleon kaon absorption. We show that another peak of this experiment, seen at lower invariant masses and interpreted as a bound K-pp state, is simply generated by the same two-nucleon absorption mechanism followed by final-state interactions of the produced particles with the residual nucleus. Our conclusion is that all the experimental claims for the formation of very deeply bound antikaonic nuclear systems receive an alternative explanation in terms of conventional nuclear processes.
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.
We briefly review the experiments of KEK and FINUDA, that claim evidence for deeply bound kaon states, from the perspective of recent theoretical papers and experiments that provide an alternative explanation of the peaks seen. At the same time we show that recent criticisms raised by Akaishi and Yamazaki, and exposed by Akaishi in this Conference, have no base.
We critically revise the recent claims of narrow deeply bound kaonic states and show that at present there is no convincing experimental evidence for their existence. In particular, we discuss in details the claim of K- pp deeply bound state associated to a peak seen in the Lambda p invariant mass spectrum from K- nuclear absorption reactions by the FINUDA collaboration. An explicit theoretical simulation shows that the peak is simply generated from a two-nucleon absorption process, like K- pp --> Lambda p, followed by final-state interactions of the produced particles with the residual nucleus.
We perform calculations for ωproduction in nuclei by means of the (γ,p) reaction for photon energies and proton angles suited to running and future experiments in present Laboratories. For some cases of possible ωoptical potentials we find clear peaks which could be observable provided a good resolution in the ωenergy is available. We also study the inclusive production of π^0 γin nuclei around the ωenergy and find a double hump structure for the energy spectra, with a peak around a π^0 γenergy of m_ω-100 MeV, which could easily be misidentified by a signal of a bound ωstate in nuclei, while it is due to a different scaling of the uncorrelated π^0 γproduction and ωproduction with subsequent π^0 γdecay.
With the aim of extracting information on the shift of the $ω$ mass in the nuclear medium we analyze data obtained at ELSA from where claims for evidence of a mass shift of the $ω$ have been made. We develop a Monte Carlo simulation code which takes into account the possible reactions in the experimental set up of $(γ~ A \to π^0 γ~X)$ in the vicinity of the $ω$ production region with subsequent $ω\to π^0 γ$ decay. We compare our results with experiment for the distribution of $π^0 γ$ invariant masses and conclude that the distribution is compatible with an enlarged $ω$ width of about 90 MeV at nuclear matter density and no shift in the mass. This change in the width would be compatible with the preliminary results obtained from the transparency ratio in the A dependence of $ω$ production. The discrepancy of the present conclusions with former claims of an evidence for a shift of the $ω$ mass stem from a different choice of background which is discussed in the paper.
We briefly review the situation around the claimed deeply bound K^- states in different recent experiments and concentrate particularly on the state K^- pp advocated by the FINUDA collaboration in nuclear K^- absorption. We perform a theoretical simulation of the process and show that the peak in the Lambda p spectrum that was interpreted as a deep K^- pp bound state corresponds mostly to the process K^- p p --> Lambda p followed by final state interactions of the produced particles with the daughter nucleus.
We critically discuss the theoretical developments that led to predictions of very deeply bound kaon states and then revise the data of the KEK experiment from where claims for evidence of deeply bound kaon atoms in nuclei were made. We conclude that the peaks seen in the experiment correspond to the absorption of kaons by a pair of nucleons leading to $Σp$ and $Λp$ and leaving the daughter nucleus as a spectator. These conclusions have been reconfirmed by a recent experiment at FINUDA.
We study the central part of Lambda N and Lambda Lambda potential by considering the correlated and uncorrelated two-meson exchange besides the omega exchange contribution. The correlated two-meson is evaluated in a chiral unitary approach. We find that a short range repulsion is generated by the correlated two-meson potential which also produces an attraction in the intermediate distance region. The uncorrelated two-meson exchange produces a sizeable attraction in all cases which is counterbalanced by omega exchange contribution.
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.
The properties of the antikaons in nuclear matter are investigated from a chiral unitary approach which incorporates the s- and p-waves of the ${\bar K}N$ interaction. To obtain the in-medium meson-baryon amplitudes we include, in a self-consistent way, Pauli blocking effects, meson self-energies corrected by nuclear short-range correlations and baryon binding potentials. We pay special attention to investigating the validity of the on-shell factorization, showing that it cannot be applied in the evaluation of the in-medium corrections to the p-wave amplitudes. In nuclear matter at saturation energy, the $Λ$ and $Σ$ develop an attractive potential of about -30 MeV, while the $Σ^*$ pole remains at the free space value although its width gets sensibly increased to about 80 MeV. The antikaon also develops a moderate attraction that does not support the existence of very deep and narrow bound states, confirming the findings of previous self-consistent calculations.
We make a study of the (pi^- p --> K^0 pi^0 Lambda), (pi^+ p --> K^+ pi^+ Lambda), (K^+\bar{K}^0 p), (K^+ pi^+ Sigma^0), (K^+ pi^0 Sigma^+), and (eta pi^+ p) reactions, in which the basic dynamics is given by the excitation of the Delta^*(1700) resonance which subsequently decays into (K Sigma^*(1385)) or (Delta(1232) eta). In a similar way we also study the (gamma p --> K^0 pi^+ Lambda), (K^+ pi^- Sigma^+), (K^+ pi^+ Sigma^-), (K^0 pi^0 Sigma^+), and (eta pi^0 p) related reactions. The cross sections are proportional to the square of the coupling of Delta^*(1700) to (Sigma^*K), (Delta eta) for which there is no experimental information but which is provided in the context of coupled channels chiral unitary theory where the Delta^*(1700) is dynamically generated. Within present theoretical and experimental uncertainties one can claim a global qualitative agreement between theory and experiment. We provide a list of items which need to be improved in order to make further progress along these lines.
A recently developed non-perturbative chiral approach to dynamically generate the (3/2^-) baryon resonances has been extended to investigate the radiative decays Lambda^*(1520) --> gamma Lambda(1116) and Lambda^*(1520) --> gamma Sigma^0(1193). We show that the Lambda^*(1520) decay into gamma Lambda is an ideal test for the need of extra components of the resonance beyond those provided by the chiral approach since the largest meson-baryon components give no contribution to this decay. The case is different for gamma Sigma decay where the theory agrees with experiment, though the large uncertainties of these data call for more precise measurements.
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.
Using a chiral unitary approach for meson-baryon scattering in the strangeness zero sector, where the $N^*(1535)$ resonance is dynamically generated, we study the reactions $γp \to π^0 ηp$ and $γp \to π^0 K^0 Σ^+$ at photon energies at which the final states are produced close to threshold. Among several reaction mechanisms, we find the most important is the excitation of the $Δ^*(1700)$ state which subsequently decays into a pseudoscalar meson and a baryon belonging to the $Δ(1232)$ decuplet. Hence, the reaction provides useful information with which to test current theories of the dynamical generation of the low-lying $3/2^-$ states. The first reaction is shown to lead to sizable cross sections and the $N^*(1535)$ resonance shape is seen clearly in the $ηp$ invariant mass distribution. The same dynamical model is shown to lead to much smaller cross sections at low energies in the second reaction. Predictions are made for cross sections and invariant mass distributions which can be compared with forthcoming experiments at ELSA.
We study the coupling of the Lambda(1520)= Lambda* resonance to the \bar K* vector meson and nucleon. This coupling is not directly measured from the resonance decay, but is expected to be important in hyperon production reactions, in particular for the exotic Theta+ production. We compute the coupling in two different schemes, one in the chiral unitary model where the Lambda* is dominated by the quasibound state of mesons and baryons, and the other in the quark model where the resonance is a p-wave excitation in the three valence quarks. Although it is possible to construct both models such that they reproduce the \bar K N and pi Sigma decays, there is a significant difference between the Lambda* \bar K* N couplings in the two models. In the chiral unitary model $|g_{Λ^*\bar{K}^* N}| \sim 1.5$, while in the quark model $|g_{Λ^*\bar{K}^* N}| \sim 10$. The difference of the results stems from the different structure of the Lambda* in both models, and hence, an experimental determination of this coupling would shed light on the nature of the resonance.
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.