0000000000002357
AUTHOR
M.j. Vicente-vacas
Double pion photoproduction in nuclei
The inclusive A(gamma,pi+ pi-)X reaction is studied theoretically. A sizeable enhancement of the cross section is found, in comparison with the scaling of the deuteron cross section (sigma_deuteron * A/2). This enhancement is due to the modifications in the nuclear medium of the gamma N ----> pi pi N amplitude and the pion dispersion relation. The enhancement is found to be bigger than the one already observed in the (pi,pi pi) reaction in nuclei.
Non-mesonic weak decay of A hypernuclei
Abstract The non-mesonic decay modes of Λ hypernuclei are examined. The one-nucleon induced decay (ΛN → NN) is studied in a shell-model framework using a meson-exchange picture. The total decay rates and parity-violating asymmetries reproduce the available experimental data. However, the model does not yet explain the partial rates and the resulting neutron- to proton-induced ratio Γ n /Γ p . The two-nucleon-induced decay rate (ΛNN -> NNN) is evaluated using the propagator formalism and the local-density approximation and is found to contribute about 15% to the total decay width of medium to heavy hypernuclei. The influence of this channel on the experimental determination of the ratio Γ n …
Recoilles and resonant production of double-pion atoms
Abstract We exploit two ideas found successful in the direct production of pionic atoms: recoilless condition and resonance production, in order to produce atomic systems with two bound π − . As an example of recoilless reaction we study π − + A X → p ( A−1 X , π − , π − ) , and as an example of resonant reaction we study π − X → ( π − π − X′) → π − X. In both cases the signals are found to be only a small fraction of the background indicating the big experimental difficulties that one is facing in producing these states.
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.
Neutron and proton spectra from the decay ofΛhypernuclei
We have determined the spectra of neutrons and protons following the decay of {Lambda} hypernuclei through the one- and two-nucleon-induced mechanisms. The momentum distributions of the primary nucleons are calculated and a Monte Carlo simulation is used to account for final state interactions. The shape of the proton spectrum is sensitive to the value of {Gamma}{sub n}/{Gamma}{sub p} the ratio of neutron- to proton-induced decay, and the available experimental information favors larger values than those predicted by the one-pion-exchange model. From the spectra we calculate the number of neutrons (N{sub n}) and protons (N{sub p}) per {Lambda} decay and show how the measurement of these qua…
Pion induced pion production on deuterium: A Quasifree process
Abstract A detailed experimental analysis of the π+d → π+π−pp in-plane coincidence data first presented by Rui et al. is compared to an expanded version of the Oset and Vicente-Vacas model for pion-induced pion production on a free nucleon. This extended model averages over Fermi motion to describe the assumed quasifree nature of the process occurring on the deuteron and includes nine additional diagrams to account for the N ∗ → N (ππ) p-wave reaction channels. Experimental effects such as pion energy loss in the target and in the detectors, pion decay and muon detection are investigated and incorporated into the comparison of experimental data and theory. Inclusion of Fermi motion was foun…
PION DOUBLE CHARGE EXCHANGE REACTIONS LEADING TO DOUBLE PIONIC ATOMS
We study theoretically pion double charge exchange reactions leading to double pionic atoms. The reaction cross-sections with two pions in the deeper bound pionic orbits in 208Pb are calculated with realistic pionic atom wave functions and distortion effects. The cross-sections are found to be d2σ/dEdΩ~10−3−10−4 µ b/srMeV , which are only a small fraction of the double charge exchange background.
π+π− coincidence measurement in the 4He(π+, π+π−) reaction at Tπ+ = 280 MeV
Abstract The pion induced pion production (π, 2π) reaction on deuterium has been studied at an incident pion energy of 280 MeV. The outgoing pions were detected in coincidence and the measured four-fold cross sections were compared with a πN → ππ N microscopic model, and, upon integration, with the available experimental total cross sections from the same reaction on H and 2 H. Finally, the results were directly compared with recent data from the 16 O(π + , π + π − ) reaction at the same incident energy.
Coherent pion production induced by protons and light ions
Abstract We study coherent pion production by means of the (p,n) and (He,t) reactions on different nuclei and at different energies of the projectile. Energy and angular distributions are calculated. The angular distributions are rather narrow along the direction of the momentum transfer, particularly in heavy nuclei. The reaction is sensitive to the longitudinal part of the elementary NN→NΔ interaction. It also provides a new tool to obtain information on the pion-nuclear interaction, complementary to that offered by reactions with real pions.
Meson exchange currents in kaon scattering on the lightest nuclei
The K^+ scattering on the lightest nuclei, d, 3He and 4He is studied in the framework of multiple scattering theory. Effects from MEC tied to the K^+N-->KNpi reaction are evaluated. We found that at momentum transfers Q^2<0.5 (GeV/c)^2 contributions from MEC are much smaller than kaon rescattering corrections. This makes the conventional multiple scattering picture a reliable tool to study these reactions in this kinematical domain and to extract the K^+n scattering amplitude from the K^+d data. At larger transferred momentum MEC can become more relevant.
Meson exchange currents in pion double charge exchange at high energies
In this letter we study the high energy behavior of the forward differential cross section for the $^{18}O(\pi^+,\pi^-)^{18}Ne$ double charge exchange reaction. We have evaluated the sequential and the meson exchange current mechanisms. The meson exchange current contribution shows a very weak energy dependence and becomes relevant at incident pion kinetic energies above 600 MeV.
Coherent, semicoherent and incoherent pion production in heavy-ion reactions
Abstract Two types of coherent pion production in heavy-ion collisions are studied and calculations are performed for 12C against 12C collisions. The first one, doubly coherent production, produces a pion in one nucleus leaving it in its ground state, while the second nucleus is excited to the 1+, T = 1 state. The second process, semicoherent production, also leaves in its ground state the nucleus where the pion is produced while the second one is broken. The cross sections for forward pions are compared to the standard incoherent production process. We observe that at energies below 130 MeV/A the semicoherent process dominates the cross section for forward pion angles while at large energi…
Nuclear effects in neutrino production of Δ at intermediate energies
Abstract The neutrino production of Δ in 16 O has been studied at intermediate energies relevant to atmospheric neutrino experiments. The Δ production cross sections with electrons or muons in the final state is found to be reduced due to nuclear effects. This reduction is even larger for the accompanying one pion production cross sections. The ratio of electron to muon production cross sections, however, does not show any significant dependence on nuclear medium effects.
Coherent π0 electroproduction
Abstract We analyze the coherent electroproduction of neutral pions in nuclei. We find that this process does not contribute to the longitudinal response function for spin saturated nuclei and the dependence of the cross section on the azimuthal pion angle is particularly simple and given by 1− ϵ cos 2 o π . This angular dependence could be useful to isolate the coherent contribution in future experiments, offering an advantage over coherent photoproduction. The Δ-peak is considerably moved to lower pion energies and appreciably modified due to medium corrections.