Double Charge Exchange Research Articles

Isotensor double giant resonance from charge-exchange reactions

Giant resonances are collective excitations of the nucleus in which a large fraction of the component nucleons are set into coherent modes of vibrations. Exotic excitations like the double giant dipole were predicted for many years but not observed experimentally until recently. Several experiments have been carried out at Los Alamos National Laboratory to search for these new collective modes of the nucleus. The results discover two previously unobserved types of double giant resonances. This work presents the recent pion double charge exchange data and the analysis that support the existence of two such exotic vibrational nuclear modes. Multiple excitations of the giant dipole resonance using relativistic heavy ion excitation is under investigation. The results from pion double charge exchange excitation will be compared with the relativistic heavy ion scattering.

On the electromagnetic production of the proposed πNN resonance d′

Abstract We examine the electromagnetic decay rates as well as the photo- and electroproduction cross sections of the proposed πNN resonance d ′, for the existence of which evidence has been claimed in the pionic double charge exchange (DCX) on nuclei as well as in the 2π-production in pp -collisions. Based on the d ′ parameters deduced from DCX, we estimate the d ′-production on the deuteron in the reactions γd → NNπ and ed → e ′ NNπ . Comparing to the non-resonant cross sections at the relevant energies and momentum transfers, we discuss the feasibility of a search for a d ′ signal both in γd → npπ 0 and in ed → e ′ NNπ .

The d′-dibaryon in the nonrelativistic quark model

The narrow peak found in various pionic double charge exchange (DCX) cross sections can be explained by the assumption of a universal J P = 0 − resonance at 2065 MeV, called d′. We calculate the mass of a six-quark system with J P = 0 −, T = 0 quantum numbers employing a cluster model and a shell model basis to diagonalize the nonrelativistic quark model Hamiltonian.

Search for narrow NN-decoupled resonances in the πNN-system — an overview

The status of the search for NN-decoupled resonances in the πNN-system is reviewed. After a short survey on the history of dibaryon searches reactions suitable for detecting such resonances are discussed. Special emphasis is put on the pionic double charge exchange, where recently evidence for the signature of a narrow πNN resonance, called d′, with I( J P ) = even (0 −) and m = 2.06 GeV has been quoted. Further reactions discussed in this context are the pion photoproduction on the deuteron and the two-pion production in nucleon-nucleon collisions.

A Jπ = 0 − resonance for πNN → πNN in the Skyrme model

The scattering of pions on a dibaryon configuration is analyzed within the SU(2) Skyrme model. It is shown that this model leads to a low-lying ( J p , I) = (0 −, 2) resonance. The possibility that this resonance corresponds to one proposed recently in the context of double charge exchange pion scattering on nuclei is discussed. Given the setup used in those experiments we also show that a resonance with isospin assignment I = 0 cannot be excited according to the description presented here.

Search for double Gamow-Teller strength by heavy-ion double charge exchange

We have carried out a search for double Gamow-Teller excitations, employing the 24Mg( 18O, 18Ne) 24Ne reaction at 100 and 76 MeV/nucleon at NSCL-MSU and GANIL, respectively. The cross sections for low-lying excitations are typically a few nb/sr, providing evidence for a strong suppression of double Gamow-Teller excitations in heavy-ion double charge exchange, compared to pion-induced reactions. This result does not give good hope that heavy ions at intermediate energies could be used to probe double Gamow-Teller strength.

Constituent quark model calculation for a possible JP = 0−, T = 0 dibaryon

There exists experimental evidence that a dibaryon resonance d' with quantum numbers J^P=0^-,T=0 and mass 2065 MeV could be the origin of the narrow peak in the (\pi^+ ,\pi^- ) double charge exchange cross--sections on nuclei. We investigate the six--quark system with these quantum--numbers within the constituent quark model, with linear confinement, effective one--gluon exchange at short range and chiral interactions between quarks (\pi and \sigma exchange). We classify all possible six quark states with J^P=0^-,T=0, and with N=1 and N=3 harmonic oscillator excitations, using different reduction chains. The six--quark Hamiltonian is diagonalized in the basis including the unique N=1 state and the 10 most important states from the N=3 shell. We find, that with most of the possible sets of parameters, the mass of such a "dibaryon" lies above the N(939)+N^\ast(1535) threshold. The only possibility to describe the supposed d'(2065) in the present context is to reduce the confinement strength to very small values, however at the expense of describing the negative parity resonances N^\ast. We also analyze the J^P=0^-,T=2,N=1 six--quark state.

Comment on "Three-body resonances in 6He, 6Li, and 6Be, and the soft dipole mode problem of neutron halo nuclei"

In a recent publication Cs\'ot\'o suggests that the sequential decay of $^{11}\mathrm{Li}$ through resonances in $^{10}\mathrm{Li}$ could explain the peak seen at 1.2 MeV of excitation in the $^{11}\mathrm{B}$(${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$,${\mathrm{\ensuremath{\pi}}}^{+}$) reaction. We point out that this is in conflict with widely accepted notions of probe and target interactions in the measurement of inclusive excitation functions via nuclear reactions ranging from inelastic scattering and heavy-ion transfer reactions to pion double-charge exchange. We also show that the suggested mechanism leads to incorrect predictions in other nuclei, and so we conclude that the apparent success of the mechanism in explaining the observed energy of excited states in the $^{11}\mathrm{Li}$ and $^{6}\mathrm{He}$ systems is accidental.

Active Electrons in the CHe 4+ Quasimolecule

Low energy C4++ He collisions are known to be unusual in the sense that double-electron transfer dominates single capture. A basic question is whether one, two or more active electrons play a role in these processes. To study this problem, results are reported for the molecular energies and dynamical couplings of the CHe4+ system, modified with a common translation factor. The interactions that are responsible for the capture processes are discussed in detail, and as recently proposed in the literature, double capture is shown to occur simultaneously through essentially single-electron interactions. Some illustrative results of single- and double-charge exchange cross sections are also presented.

Pion double charge exchange on the even selenium isotopes.

Pion-induced double charge exchange (${\mathrm{\ensuremath{\pi}}}^{+}$,${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$) on $^{76,78,80,82}\mathrm{Se}$, leading to the double isobaric analog states (DIAS) and the ground states of $^{76,78,80,82}\mathrm{Kr}$, has been studied at a laboratory angle of ${5}^{0}$ and incident pion kinetic energy of 293.2 MeV. Cross sections for these transitions have been extracted, and those for the DIAS are compared to two simple models of pion double charge exchange.

Radiative double electron capture in heavy-ion atom collisions

Total cross sections for radiative double electron capture (RDEC) in collisions of fast highly charged ions with light target atoms were estimated. The applied method is based on the principle of detailed balance using the cross sections for the double photoionization process. Preliminary results of a dedicated RDEC experiment aiming at the observation of photons with twice the single REC photon energy, associated with double charge exchange, are discussed. Additionally, results of complementary measurements of double capture (without photon registration) by fully stripped U ions in collisions with C atoms at 295 MeV/u are presented. Here, a surprisingly enhanced cross section for double electron capture is found in accordance with observations reported in the literature.

Single and double charge exchange in fast ion-atom collisions

A novel theoretical framework, which includes multiple scattering effects, is devised for consistently treating charge exchange at intermediate and high impact energies. From this versatile and general formalism, which obeys the asymptotic convergence of total scattering states, as the most essential criterium for the validity of collision theory, a number of particular methods can be derived in a systematic manner. In such a way, several proper versions of the impulse approximation are introduced, fulfilling the correct boundary conditions in both channels and using only mathematically justified solutions of the basic dynamic equations, associated with a twofold Coulomb wave of intermediate states. It is precisely the lack of these two basic features, which invalidates the standard impulse approximation and all its previous extensions in the field of atomic collisions.

Production of Λ-hypernuclei with a large neutron excess and a neutron halo

We discuss the strangeness and double charge exchange reaction ( K −, π +) as a source of neutron rich Λ-hypernuclei.

Pion cloud contribution to the s-wave repulsion in pionic atoms

Abstract The nuclear pion cloud contribution to the pion self-energy for pionic atoms is evaluated and one finds large cancellations between terms involving the ππ amplitude and other terms originating from the chiral lagrangian partners. While the individual terms depend strongly on the off-shell extrapolation of the ππ amplitude, the sum is model independent within the Olson and Turner family of chiral lagrangians keeping ξ + 4 η constant, as previously found for the πN → ππN and pion double-charge exchange in nuclei, and vanishes in the limit of m π → 0. One finds a small net repulsion which is however too small to account for the “missing” s-wave repulsion. A revision of the present status of the problem of the “missing” s-wave repulsion is done at the end.

Multiphonon excitations in nuclei built with giant resonances: status and trends

Abstract The concept of multiphonon excitation is introduced and the various methods to excite these states in nuclei are presented and discussed. The present status of the field of multiphonon excitations is reviewed with emphasis on the most recent and exciting developments obtained with inelastic scattering of heavy ions at intermediate and relativistic energy and with the double charge exchange reactions. The perspectives of the field are then summarized.

Nonanalog 40,44Ca( pi +, pi -)40,44Ti(g.s.) reactions at low energy.

We present results of measurements of the forward-angle cross sections for pion double charge exchange on [sup 40,44]Ca to the nonanalog ground states of [sup 40,44]Ti at incident pion energies from 35 to 80 MeV. The data indicate that a substantial contribution to these cross sections arises from core polarization and/or cross-shell transitions.

Constituent quark model calculation for a possible J P=0 −, T=0 dibaryon

The assumption of a narrow (Γ ∼ 5 MeV) dibaryon-resonance with quantum numbers J P =0 −, T=0 and a mass of 2065 MeV allowed recently to describe all pionic double charge exchange cross-sections for various nuclei [1]. We investigate the mass of a 6-quark state with these quantum numbers in the framework of the nonrelativistic constituent quark model. Our shell-model diagonalisation including a large basis of excited states results in masses in the range of 2500 MeV. Since this result depends strongly on the explicit form of the confining potential, we cannot exclude the existence of the proposed dibaryon. For example the usual assumption of a quark-quark pair-confinement mechanism might not be valid in the 6q-system.

Isospin dependence of double analog cross sections at T pi =400-500 MeV.

At energies in the range 400\char21{}500 MeV, pion-induced double charge exchange cross sections leading to the double analog state appear to behave differently for even and odd isospin.

QCD Interaction in Pion-Nucleus DCX Reaction11The project supported by National Natural Science Foundation of China and grant LWTZ 1298 of the Chinese Academy of Sciences. One of the authors (Y.X. LIU) thanks also the support of the China Post-Doctoral Science Foundation.

The pion-nucleus double charge exchange (DCX) transition matrix element is under consideration in the framework of QCD. The interactions including the perturbative effect and nonperturbative contributions of gluon condensate and quark condensate in the DCX process are discussed. The interaction operators are given.

On the production of a narrow πNN resonance in low energy pionic DCX on He isotopes

We estimate the cross section for the production of a narrow isoscalar 0−πNN resonance (d′) in the pionic double charge exchange (DCX) on 3He and 4He. The resonance parameters used are those obtained from a recent analysis of DCX data on light and medium-heavy nuclei. The calculations agree very favourably with presently available low-energy data for 4He.