• PDF / 651,150 Bytes
• 11 Pages / 612 x 792 pts (letter) Page_size
• 82 Downloads / 193 Views

DOWNLOAD

REPORT

---

CLEI Theory

Delta-Isobar Configurations in

12

C and

16

O Nuclei

I. V. Glavanakov* , Yu. F. Krechetov, and A. N. Tabachenko Institute of Physics and Technology, National Research Tomsk Polytechnic University, pr. Lenina 30, Tomsk, 634050 Russia Received November 6, 2013

Abstract—The differential yield of the reaction 16 O(γ, π + p) at the maximum photon energy of 450 MeV was measured in the region of high momentum transfers to the residual nuclear system. The experimental results obtained for the 16 O nucleus and the cross section measured earlier for the reaction 12 C(γ, π + p) were analyzed on the basis of a model that takes into account the admixture of isobar states in the nuclear wave function. The probabilities for the delta-isobar configurations in the ground states of the carbon and oxygen nuclei per nucleon were estimated empirically at 0.012 ± 0.003 ± 0.002 and 0.019 ± 0.003 ± 0.003, respectively. DOI: 10.1134/S1063778814060064

1. INTRODUCTION The nontrivial structure of the nucleon suggests the existence of its excited states in nuclei in the form of nucleon resonance (isobars). One possible manifestation of isobar degrees of freedom in nuclei is associated with isobar configurations in their ground states (for an overview, see [1, 2]). Modern nuclear models admit that, with a probability PΔ of a few tenths of a percent to a few percent, some bound nucleons may go over via N N → ΔN or N N → ΔΔ processes to a virtual delta-isobar state. In describing both static properties of nuclei and nuclear reactions, it is important to take into account probabilities for such transitions. Nuclear reactions that cannot be explained within a model that assumes a single interaction of a projectile particle with bound intranuclear nucleons is an efficient tool for experimentally studying isobar degrees of freedom in ground states of nuclei. As an example, we can indicate (π + , π − p) reactions, where the charged state of a scattered particle changes by 2e, or (p, p π + p) and (γ, π − n) reactions accompanied by the production of particles whose total electric charge is +2 or –1. Since decay to pions and a nucleon is a dominant mode of decay of nonstrange nucleon resonances, the processes listed immediately above exhibit the highest sensitivity to manifestations of isobar degrees of freedom. This circumstance was used in [3–8] to estimate the isobar-configuration probability PΔ . Experimental data on such reactions are usually interpreted in terms of the mechanism of direct isobar knockout by a high-energy particle. *

E-mail: [email protected]

As background reaction mechanisms, one considers two-step processes involving charge-exchange particle rescattering in the intermediate state. Nuclear models involving various versions of baryon–baryon interaction are used to estimate theoretically the probabilities for isobar configurations, but this leads to a considerable scatter of PΔ . The majority of theoretical estimates were obtained for extremely light nuclei [1, 2, 9–11]. In particular, the most recent estimates for the d