R. Linares

The NUMEN project: NUclear Matrix Elements for Neutrinoless double beta decay

Abstract.The article describes the main achievements of the NUMEN project together with an updated and detailed overview of the related R&D activities and theoretical developments. NUMEN proposes an innovative technique to access the nuclear matrix elements entering the expression of the lifetime of the double beta decay by cross section measurements of heavy-ion induced Double Charge Exchange (DCE) reactions. Despite the fact that the two processes, namely neutrinoless double beta decay and DCE reactions, are triggered by the weak and strong interaction respectively, important analogies are suggested. The basic point is the coincidence of the initial and final state many-body wave functions in the two types of processes and the formal similarity of the transition operators. First experimental results obtained at the INFN-LNS laboratory for the 40Ca(18O,18Ne)40Ar reaction at 270MeV give an encouraging indication on the capability of the proposed technique to access relevant quantitative information. The main experimental tools for this project are the K800 Superconducting Cyclotron and MAGNEX spectrometer. The former is used for the acceleration of the required high resolution and low emittance heavy-ion beams and the latter is the large acceptance magnetic spectrometer for the detection of the ejectiles. The use of the high-order trajectory reconstruction technique, implemented in MAGNEX, allows to reach the experimental resolution and sensitivity required for the accurate measurement of the DCE cross sections at forward angles. However, the tiny values of such cross sections and the resolution requirements demand beam intensities much larger than those manageable with the present facility. The on-going upgrade of the INFN-LNS facilities in this perspective is part of the NUMEN project and will be discussed in the article.

Why the complete fusion of weakly bound nuclei is enhanced at sub-barrier energies and suppressed above the barrier?

In this paper we explain the reasons for the systematic behaviour of enhancement of the complete fusion cross section of weakly bound systems at sub-barrier energies and its suppression above the barrier, when compared with predictions from calculations which do not take into account breakup and transfer channels. The methodology used is the study of dynamic polarization potentials. At above barrier energies the net polarization potential due to direct breakup is dominant and repulsive, consequently fusion is suppressed. At sub-barrier energies, the enhancement of fusion cross section is consistent with recent experimental observations that nucleon transfer leading to breakup is dominant compared to direct breakup and produces attractive polarization potential.

Important role of projectile excitation in

The elastic scattering angular distribution of the

^{16}

^{16}

O+

O

^{60}

+^{60}

Ni and

Ni system at

^{16}

260

O+

MeV was measured in the range of the Rutherford cross section down to

^{27}

7