D. Bongiovanni

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.

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.

Challenges for high rate signal processing for the NUMEN experiment

The objective of the research and development activities, regarding the upgrade of the MAGNEX focal plane detector (FPD) and the development of the γ detector array for the NUMEN project, is the construction of new detectors capable to fulfil the requirements of high event rate, radiation tolerance and data acquisition and transmission bandwidth deriving from the upgrade of the Superconducting Cyclotron at INFN Laboratori Nazionali del Sud. The design of the front-end (FE) and read-out (RO) electronics has been performed in parallel with that of the new tracker. The design of the new segmented anode and the architecture of the front-end and read-out electronics are presented.

The read-out and data transmission for the MAGNEX focal plane detector for the NUMEN project

The main task of the read-out electronics for the new NUMEN focal plane detector (FPD) is the real-time data collection from the front-end ASICs and the high bandwidth transmission to data acquisition, in addition to the slow control of the front-end electronics and the synchronization of the whole detector. The read-out electronics architecture, designed to be modular and expandable to the final size of the detectors, is based on System On Module (SOM). This very versatile device couples high performance FPGA to powerful processor architecture and allows a graphical approach to the programming and interfacing. A detailed description of the architecture and the specifications of the read-out electronics are presented.

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.

The Front-end for the new focal plane detector for the NUMEN project

The design of the front-end electronics for the new tracker of the MAGNEX Focal Plane Detector (FPD) for the NUMEN project is presented. The front-end is based on the VMM chip, developed for ATLAS experiment at CERN. The architecture of the front-end (FE) electronics is designed to be modular and scalable to the final detector. The segmented anode board is designed in order to take advantage of the unique performances of the VMM chip, allowing a digital reconstruction of the track at high event rate. This anode board is connected to front-end by mean of Micro Coax Cable Assembly and does not make use of vacuum connectors. The front-end boards will be placed in air, facilitating in this way the heat dissipation and the connection to the read-out (RO) electronics. An innovative anode read-out strategy allows the reduction of the total number of channels to about 1400 and the measurement of the track at different depth in the detector with 750 µm spatial resolution.

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).

Focal plane detector optical readout

A preliminary study of an alternative solution for the optical detection and tracking of the ions in the MAGNEX focal plane detector for the NUMEN project is presented. The tracks of the ions are sampled by means of the light that they produce through a scintillating gas, which is collected by arrays of Silicon Photo Multiplier suitably arranged. A complete Geant4 simulation is under development in order to correlate detector geometry, characteristics of the gas and light collection efficiency. Fast timing performance could be achieved through a completely digital (on-off) read-out system of the light sensors and a position reconstruction algorithm.