Pion Double Charge Exchange Research Articles

Nuclear fragmentation and charge-exchange reactions induced by pions in the Δ-resonance region

Dynamics of the nuclear fragmentations and the charge exchange reactions in pion-nucleus collisions near the $\Delta$(1232) resonance energies has been investigated within the Lanzhou quantum molecular dynamics (LQMD) transport model. An isospin, momentum and density-dependent pion-nucleon potential is implemented in the model, which influences the pion dynamics, in particular the kinetic energy spectra, but weakly impacts the fragmentation mechanism. The absorption process in pion-nucleon collisions to form the $\Delta$(1232) resonance dominates the heating mechanism of target nucleus. The excitation energy transferred to the target nucleus increases with the pion kinetic energy and is similar for both $\pi^{-}$ and $\pi^{+}$ induced reactions. The magnitude of fragmentation of target nucleus weakly depends on the pion energy. The isospin ratio in the pion double charge exchange is influenced by the isospin ingredient of target nucleus.

Inclusive pion double charge exchange in lightp-shell nuclei

We report the results of a series of measurements of the differential cross sections for inclusive pion double charge exchange in $^{6,7}\mathrm{Li}$, $^{9}\mathrm{Be}$, and $^{12}\mathrm{C}$ for positive and negative incident pions of energies 120, 180, and 240 MeV. The data are compared with the predictions of an intranuclear cascade model and a model based on two sequential single charge exchange processes.

Meson exchange currents in pion double charge exchange reaction

It is shown that in the Born approximation the forward scattering pion double charge exchange (DCX) amplitude for the meson exchange current (MEC) mechanism does not depend on the energy. The unitarization of the amplidute as well as the contributions of the ρ, σ, and f0 mesons into MEC mechanism based on the composite meson model increase the cross section and lead to its energy dependence.

Composite-meson model and meson exchange currents

We study the meson exchange currents (MEC) mechanism for the pion double-charge exchange (DCX) reaction in a composite-meson model. The model assumes that the mesons are two-quark systems and can interact with each other only through quark loops. The contributions of the ρ, σ, and f 0 mesons, the four-quark box diagram as well as a contact diagram has been taken into account. It is shown that the contribution of the ρ, σ, and f 0 mesons increases the forward scattering cross-section in an average by 25% and decreases with energy.

Pion double charge exchange in a composite-meson model

The pion double charge exchange amplitude is evaluated in a composite-meson model based on the four-quark interaction. The model assumes that the mesons are two-quark systems and can interact with each other only through quark loops. To evaluate the meson exchange current contribution, the form factors of the two-pion decay modes of the $\ensuremath{\rho},\ensuremath{\sigma}$, and ${f}_{0}$ mesons have been used in the calculations. The contribution of the four-quark box diagram has been taken into account as well as a contact diagram. The contributions of the $\ensuremath{\rho},\ensuremath{\sigma}$, and ${f}_{0}$ mesons increase the forward scattering cross section, which depends weakly on the energy.

Inclusive pion double charge exchange inHe4at intermediate energies

A systematic experimental study of inclusive pion double charge exchange in $^{4}\mathrm{He}$ has been undertaken. The reaction $^{4}\mathrm{He}$$({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{\ensuremath{-}})4p$ was observed at incident energies of 120, 150, 180, 240, and 270 MeV; the $^{4}\mathrm{He}$$({\ensuremath{\pi}}^{\ensuremath{-}},{\ensuremath{\pi}}^{+})4n$ reaction was observed at incident energies of 180 and 240 MeV. At each incident energy, the doubly differential cross section was measured at three to five outgoing pion laboratory angles between ${25}^{\ifmmode^\circ\else\textdegree\fi{}}$ and ${130}^{\ifmmode^\circ\else\textdegree\fi{}}$. At each angle, cross sections were measured over the range of outgoing pion energies from 10 MeV to the kinematic limit for the reaction in which the final state consists of the oppositely charged pion plus four free nucleons. The spectra of outgoing pions are strikingly different from those observed for the inclusive double charge exchange reaction in heavier nuclei but resemble those observed in the $({\ensuremath{\pi}}^{\ensuremath{-}},{\ensuremath{\pi}}^{+})$ reaction in $^{3}\mathrm{He}$. The forward-angle spectra in the $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$ reactions exhibit a prominent peak at high outgoing pion energies. Interpretation of the peaks in $^{3}\mathrm{He}$ ($^{4}\mathrm{He}$) as a three-nucleon (four-nucleon) resonance is ruled out by kinematic analysis. The results of a calculation, wherein the double charge exchange reaction is assumed to proceed as two sequential single charge exchange interactions, suggest that the high-energy peak is naturally explained by this double scattering mechanism. Nonstatic treatment of the $\ensuremath{\pi}N$ interactions and the inclusion of nuclear binding effects appear to be important in reproducing the shape of the energy spectra at forward angles.

Inclusive pion double charge exchange onO16above the Δ resonance

The forward inclusive pion double charge exchange reaction, ^{16}O(pi^-,pi^+)X, at T_0 = 0.50 and 0.75 GeV has been studied in the kinematical region where an additional pion production is forbidden by energy-momentum conservation. The experiment was performed with the SKS spectrometer at KEK PS. The measured ratio of double charge exchange cross-section for these energies dsigma(0.50 GeV)/dOmega / dsigma(0.75 GeV)/dOmega = 1.7 +/- 0.2, disagrees with the value of 7.2 predicted within the conventional sequential single charge exchange mechanism. Possible reasons for the disagreement are discussed in connection with the Glauber inelastic rescatterings.

A dependence of inclusive pion double charge exchange

The A dependence of the forward cross section for inclusive pion double charge exchange on nine target nuclei from 6Li to 209Bi at T0 = 0.59 GeV, as well as the cross section for 6Li, 7Li, and 12C nuclei at T0 = 0.59, 0.75, and 1.1 GeV, was measured with the 3-m magnetic spectrometer of the Institute of Theoretical and Experimental Physics (ITEP, Moscow). The resulting A dependence is well described within the model involving two sequential single charge exchanges and taking into account the renormalization of the amplitude for pion single charge exchange in a nucleus. A relatively weak energy dependence of the cross section for the 6Li, 7Li, and 12C nuclei agrees with the analogous dependence obtained previously for the 16O nucleus, but it contradicts the predicted sharp decrease in the cross section within the model involving two sequential single charge exchanges. This result provides an additional piece of evidence that the contribution from the mechanism of inelastic Glauber rescattering is significant at T0 ≳ 0.6 GeV.

Measurement of inclusive pion double charge exchange on 16O at T 0 ≥ 0.5 GeV with the SKS spectrometer at KEK

The inclusive cross section for pion double charge exchange on 16O at T0 = 0.5 and 0.75 GeV was measured with the superconducting kaon spectrometer (SKS) at KEK in a joint ITEP/KEK experiment. The result shows a relatively weak energy dependence of the measured cross section, which is in contradiction with its rapid drop predicted within the conventional model of two sequential single charge exchanges. The data of this experiment agree with the results that were obtained previously from similar measurements at ITEP and which are indicative of a significant contribution from the mechanism of inelastic Glauber rescatterings for T0 ≳ 0.6 GeV.

Double charge exchange and configuration mixing

The energy dependence of forward pion double charge exchange reactions on light nuclei is studied for both the Ground State transition and the Double-Isobaric-Analog-State transitions. A common characteristic of these double reactions is a resonance-like peak around 50 MeV pion lab energy. This peak arises naturally in a two-step process in the conventional pion-nucleon system with proper handling of nuclear structure and pion distortion. A comparison among the results of different nuclear structure models demonstrates the effects of configuration mixing. The angular distribution is used to fix the single particle wave function.

Experiments on the search for the NN-decoupled $NN\pi$ -resonance d’

The status on the search for N N-decoupled N Nπ-resonances is reviewed with regard to recent experimental searches in the pionic double-charge exchange in nuclei, the two-pion production in nucleon-nucleon collisions, the photo pion-production on the deuteron and the electro pion-production in nuclei.

Study of baryon and search for dibaryon resonances by the $pp \rightarrow pp\pi^ + \pi^-$ reaction

Exclusive measurements of the two-pion production channel $pp \rightarrow pp\pi^ + \pi^-$ have been carried out near threshold at CELSIUS with the PROMICE/WASA detector. They reveal $pp \rightarrow pp^*(1440)\rightarrow pp\sigma \rightarrow pp(\pi^ + \pi^-)_{I = \ell = 0}$ as the dominant process at these energies, however, the data exhibit also significant contributions from $p^*(1440) \rightarrow \Delta \pi \rightarrow p(\pi^ + \pi^-)_{I = \ell = 0}$ . From the observed interference of these Roper decay routes their relative branching ratio is derived. The status on the search for NN-decoupled $NN\pi$ -resonances is reviewed with regard to recent experimental searches in the pionic double charge exchange in nuclei, the two-pion production in nucleon-nucleon collisions, the photo pion-production on the deuteron and the electro pion-production in nuclei.

Inclusive pion double charge exchange on 16O at 0.6–1.1 GeV

The inclusive pion double charge exchange (DCX) on oxygen nuclei has been measured in the region where additional pion production is kinematically forbidden. The experiment was performed at the ITEP PS at incident π − kinetic energies T 0=0.59, 0.75 and 1.1 GeV. The integrated forward differential cross section was found to decrease with energy slowly. At 1.1 GeV it exceeds the theoretical prediction within the conventional sequential single charge exchange mechanism with a neutral pion in the intermediate state (Glauber elastic rescattering) by about a factor of five. The sequential mechanism with two pions in the intermediate state (Glauber inelastic rescatterings), which was proposed recently, seems to be able to explain the observed energy dependence and allows to predict the DCX cross section at higher energies.

Pion double charge exchange on4He

The doubly differential cross sections for the $^4$He$(\pi^+,\pi^-) 4p$ reaction were calculated using both a two-nucleon sequential single charge exchange model and an intranuclear cascade code. Final state interactions between the two final protons which were the initial neutrons were included in both methods. At incident pion energies of 240 and 270 MeV the low-energy peak observed experimentally in the energy spectrum of the final pions can be understood only if the contribution of pion production is included. The calculated cross sections are compared with data.

Implications of inelastic rescatterings for pion doublecharge exchange on nuclei

Different aspects of the Gribov-Glauber approach for the calculation ofinclusive pion double charge exchange (DCX) cross sections on nuclei areinvestigated. Recently, we have shown that inelastic rescatterings (IRs)with two (and more) pions in the intermediate states give an importantcontribution to the process of DCX at energies above ~ 0.6 GeV. Inthis paper we use the one-pion exchange model to study in detail theamplitude of two-pion production. This allows us to verify theoreticalassumptions made in the previous paper and to predict the forwardinclusive pion DCX cross section at energies ≳1 GeV, where IRsdominate over the conventional DCX mechanism of two sequentialsingle-charge exchanges.

Systematics of the low-energy pionic double charge exchange in nuclei

The experimental results for the $({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{\ensuremath{-}})$ reaction on nuclei obtained in recent years reveal clear systematic features of this reaction. New data on ${}^{7}\mathrm{Li},$ ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O},$ and ${}^{56}\mathrm{Fe}$ supplementing the existing data base are presented. The data on ${}^{12}\mathrm{C}$ are partly at variance with previous results. The dependence of the cross sections on incident energy, scattering angle, and on the target mass is discussed for transitions leading to the ground state of the final nucleus or to the double isobaric analog state.

Inclusive measurements of pionic double charge exchange on 3He at low energies

Using the CHAOS spectrometer at TRIUMF pion momentum spectra and angular distributions were measured for the 3He( π −, π +) reaction at incident energies from 65 to 120 MeV and total cross sections were deduced. The data are compared to Monte Carlo simulations for the conventional reaction process as well as to predictions for the production of the hypothetical πNN resonance d′ with and without collision damping.

Contribution of the σ meson exchange to pionic double charge exchange reaction

The contribution of the σ meson exchange to the pionic double charge exchange (DCX) reaction is investigated. A concrete calculation on the forward excitation function of the low energy DCX reaction 14C(π+, π−)14O has been performed. It shows that the contribution of the σ meson exchange can reproduce well the resonance like excitation function of low energy DCX reaction.

Energy dependence of pion double charge exchange

The energy dependence of forward angle pion double charge exchange is calculated in the energy range of 0--250 MeV. The most striking feature is a peak around 40 MeV which is in excellent agreement with the data when distorted waves obtained from a realistic optical model are used. Two possible short-range corrections to the reaction mechanism are addressed.

Pauli principle and pion distortion in the p-wave pionic double charge exchange reaction

We propose an approach which takes the Pauli principle and pion distortion into account to investigate the p-wave absorption/emission mechanism of the double charge exchange reaction. The ground state to ground-state processes and are discussed in the new approach. Excitation functions with a maximum energy distribution at have been obtained. The calculated differential cross sections at with and their ratio agree well with experimental data and depend only slightly on the particle-particle interaction parameter . Results indicate that the Pauli principle lowers the cross section and stabilizes its dependence on evidently while pion distortion reduces the cross section in the energy region above the resonance.