M. Fisichella

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.

A NEW COOLING TECHNIQUE FOR TARGETS OPERATING UNDER VERY INTENSE BEAMS

The NUMEN experiment aims to study Double Charge Exchange nuclear reactions at low energy as complementary information of the Neutrino-less Double Beta Decay half-life, for the evaluation of the Majorana effective neutrino mass. Special targets of Sn, Cd, Te, Ge and Se are needed, and their thickness must be in the range between 200 and 500nm, in order to satisfy the energy resolution requirements of the produced ions. The very low cross-section requires high intensity of the beams; thus, the energy loss becomes a constant heat source in the target region illuminated by the beam spot. To avoid the increase of the target temperature up to the melting point, some way must be found to dissipate the heat outside the target. The solution adopted in previous experiments at much lower intensity (thin films of isotopes supported by metal frames) is not suitable in this case. Solving the heat equation in steady state condition and cylindrical coordinates gives temperatures higher than the materials melting point, due to the low internal conductivity of the targets. Therefore, we designed a new cooling technique, which takes advantage of the electron stripper located beyond the target. We changed the stripper material (usually Mylar) in pyrolytic graphite of equivalent thickness (≈10µm), which becomes the substrate of the isotope deposition. Due to the presence of discontinuities in the material and heat source geometry, the temperature equation has been numerically solved. Thanks to the high internal conductivity of the graphite, the results are promising. Concerning the deposition, the goal is twofold: to obtain an isotope film with uniform density, and to create a good degree of adhesion to the graphite substrate. After the first trials with Sn, heating up the substrate during the deposition seems to provide reliable and satisfactory results.

Data reduction for experimental measurements within the NUMEN project

Within the NUMEN project, we present experimental issues and data reduction for the Cd(Ne,F)In single charge exchange, Cd(Ne,O)In two-proton transfer and Cd(Ne,F)In one-proton transfer reactions at 15 AMeV incident energy.

Experimental challenges for the measurement of the 116Cd(20Ne,20O)116Sn double charge exchange reaction at 15 AMeV

The knowledge of the nuclear matrix elements (NME) entering in the expression of the half-life of the neutrinoless double beta decay is fundamental for neutrino physics. Information on the nuclear matrix elements can be obtained by measuring the absolute cross section of double charge exchange nuclear reactions. The two processes present some similarities, the initial and final-state wave functions are the same and the transition operators are similar. The experimental measurements of double charge exchange reactions induced by heavy ions present a number of challenging aspects, since such reactions are characterized by very low cross sections. Such difficulties are discussed for the measurement of the 116Cd(20Ne,20O)116Sn reaction at 15 AMeV.

Project of Thin Targets for the NUMEN Experiment

The goal of the NUMEN collaboration is the measurement of the cross sections of Double Charge Exchange reactions for several couple of ion projectile-target, in order to provide helpful data to study the nuclear matrix elements of the neutrino-less double β-decay. The need of big statistics and high precision in the measurements require the use of high intensity beams and very thin targets. This creates some problems to the design of the target frame and to the dissipation of the heat generated by the beam. The present paper reports a possible solution for the cooling system and the production technique of a tin target, together with the results of the preliminary tests of heat dissipation.

Heavy–ion particle identification for the transfer reaction channels for the system 18O + 116Sn under the NUMEN Project

The current work is a part of the NUMEN project, which aims to deduce the nuclear matrix elements (NME) of neutrinoless double beta decay by measuring the cross sections in heavy-ion induced Double Charge Exchange (DCE) reactions. The particle identification for the competing transfer reaction channels has been studied for the 18O + 116Sn system at 270 MeV.

Measuring nuclear reaction cross sections to extract information on neutrinoless double beta decay

Neutrinoless double beta decay (0vb{eta}b{eta}) is considered the best potential resource to access the absolute neutrino mass scale. Moreover, if observed, it will signal that neutrinos are their own anti-particles (Majorana particles). Presently, this physics case is one of the most important research beyond Standard Model and might guide the way towards a Grand Unified Theory of fundamental interactions. nSince the 0vb{eta}b{eta} decay process involves nuclei, its analysis necessarily implies nuclear structure issues. In the NURE project, supported by a Starting Grant of the European Research Council (ERC), nuclear reactions of double charge-exchange (DCE) are used as a tool to extract information on the 0vb{eta}b{eta} Nuclear Matrix Elements. In DCE reactions and b{eta}b{eta} decay indeed the initial and final nuclear states are the same and the transition operators have similar structure. Thus the measurement of the DCE absolute cross-sections can give crucial information on b{eta}b{eta} matrix elements. In a wider view, the NUMEN international collaboration plans a major upgrade of the INFN-LNS facilities in the next years in order to increase the experimental production of nuclei of at least two orders of magnitude, thus making feasible a systematic study of all the cases of interest as candidates for 0vb{eta}b{eta}.

Post-stripper study for the (20Ne, 20O) double charge exchange reaction at zero degrees with the MAGNEX spectrometer

A study of different post-stripper materials for the (20Ne, 20O) double charge exchange and (20Ne, 20F) single charge exchange reactions at zero degrees using the MAGNEX spectrometer at 22 and 15 AMeV is presented. All these experiments belongs to the experimental campaign planned in the NUMEN project (NUclear Matrix Elements for Neutrinoless double beta decay).

NURE: An ERC project to study nuclear reactions for neutrinoless double beta decay

Neutrinoless double beta decay (0νββ) is considered the best potential resource to determine the absolute neutrino mass scale. Moreover, if observed, it will signal that the total lepton number is not conserved and neutrinos are their own anti-particles. Presently, this physics case is one of the most important research “beyond Standard Model” and might guide the way towards a Grand Unified Theory of fundamental interactions. nSince the ββ decay process involves nuclei, its analysis necessarily implies nuclear structure issues. The 0νββ decay rate can be expressed as a product of independent factors: the phase-space factors, the nuclear matrix elements (NME) and a function of the masses of the neutrino species. Thus the knowledge of the NME can give information on the neutrino mass scale, if the 0νββ decay rate is measured. nIn the NURE project, supported by a Starting Grant of the European Research Council, nuclear reactions of double charge-exchange (DCE) will be used as a tool to extract information on the ββ NME. In DCE reactions and ββ decay, the initial and final nuclear states are the same and the transition operators have similar structure. Thus the measurement of the DCE absolute cross-sections can give crucial information on ββ matrix elements.

The NUMEN project @ LNS: Status and perspectives

The NUMEN project aims at accessing experimentally driven information on Nuclear Matrix Elements (NME) involved in the half-life of the neutrinoless double beta decay (0νββ), by high-accuracy measurements of the cross sections of Heavy Ion (HI) induced Double Charge Exchange (DCE) reactions. Particular attention is given to the (18O,18Ne) and (20Ne,20O) reactions as tools for β+β+ and β-β-decays, respectively. First evidence about the possibility to get quantitative information about NME from experiments is found for both kind of reactions. In the experiments, performed at INFN - Laboratory Nazionali del Sud (LNS) in Catania, the beams are accelerated by the Superconducting Cyclotron (CS) and the reaction products are detected the MAGNEX magnetic spectrometer. The measured cross sections are challengingly low, limiting the present exploration to few selected isotopes of interest in the context of typically low-yield experimental runs. A major upgrade of the LNS facility is foreseen in order to increase the experimental yield of at least two orders of magnitude, thus making feasible a systematic study of all the cases of interest. Frontiers technologies are going to be developed, to this purpose, for the accelerator and the detection systems. In parallel, advanced theoretical models will be developed in order to extract the nuclear structure information from the measured cross sections.