The NUMEN project: Heavy-Ion Double Charge Exchange reactions towards 0νββ NME determination

Neutrinoless double beta (0νββ) decayhasfundamentalimplications on particlephysics, cosmology and fundamentalphysics. Ifobserved, itisconsideredone of the mostpromising ways to probe the Majorana or Dirac nature of neutrino and to haveaccess to itseffective mass. Furthermore, the observation of 0νββ wouldsignalthat the totalleptonnumberisnotconserved.

Since the 0νββ decayprocessinvolves nuclei, itsanalysisnecessarilyimpliesnuclearstructureelements. The 0νββ decay rate can be expressedas a product of threeindependentfactors: the phase-spacefactor, the nuclearmatrixelement (NME) and a termcontaining the effective neutrino masses. Thus, evenif the decay rate will be measured, the knowledge of the NME ismandatory to extract information on the neutrino masses. From an updatedcomparison of the main NME calculations, obtained with variousnuclearstructureframeworks [1]-[5], there are stillsignificantdifferences. In addition some assumption common to differentcompetingcalculation, like the unavoidabletruncation of the many body wave-function, could cause overallsystematicuncertainties.

To access quantitative information, relevant for 0νββdecay NME, the NUMEN projectproposes to use HI-DCE reactionsas a tool [6], [7], [8]. Thesereactions are characterized by the transfer of twochargeunits, leaving the mass numberunchanged, and can proceed by a sequential multi nucleon transfer mechanism or by a double mesonexchange.

Despite 0νββ decay and HI-DCE reactions are mediated by different interactions, theypresent a number of similarities. Amongthat, a keyaspectisthat the initial and finalnuclearstates.Moreover, the transitionoperators are similar, in bothcases Fermi, Gamow-Teller and rank-twotensorcomponents are present; a large linear momentum (~100 MeV/c) isavailable in the virtual intermediate channel; the twoprocesses are non-local and are characterized by two vertices localized in a pair of valencenucleons; they take placein the samenuclear medium; a relevant off-shell propagation through virtual intermediate channels is present.

In thispicture, first pioneering experimental results obtained at the INFN-Laboratori Nazionali del Sud in Catania,using the MAGNEX large acceptance magnetic spectrometer [9], for the ⁴⁰Ca(¹⁸O,¹⁸Ne)⁴⁰Ar reaction at 270 MeV, give encouraging indication on the capability of the proposed technique to access relevant quantitative information. In this wayNUMENhasstartedan experimentalcampaignfocused on DCE reactionsinvolving the nuclei candidatesfor 0νββ decay.

At INFN-LNS weperform the DCE reaction⁴⁰Ca(¹⁸O,¹⁸Ne)⁴⁰Ar at 270 MeV, with the aim to measureaccurately the cross sectionat zero degrees [10]. For thisreasonwehavechosen a particularlyadvantageoussystem, using a beam of ¹⁸O and a double magic target as⁴⁰Ca, choosing the bombardingenergy in such a way to mismatch the competing transfer reactionsleading to the samefinalstate [11]. Crucial for the mainexperimentalchallengesinvolvedhasbeen the use of the CS beamsdeliveredat LNS and the use MAGNEX, a modern high resolution and large acceptancemagneticspectrometer with high resolution in energy, mass and angle [12]. Thisfacilityhasbeenproven to be veryeffective for accurate nuclearstructure and dynamicsstudies [13],[14],[15],[16],[17],[18],[19]. In this "pilotexperiment" wehaveshown [10], for the first time, high resolution and statisticallysignificantexperimental data on heavy-ion double chargeexchangereactions in a wide range of transferredmomenta and thatpreciousinformationstowards NME determinationcould be atourreach.

To movetowards nuclei candidates for 0νββ decayoneneeds to overcome some experimentallimitsasitisproposed in the NUMEN project. The challengeis to measure rare events under a very high flux of heavyions. Weconsiderthat:

a) The Q-value for DCE reactions on nuclei of interest for 0νββ isnormally more negative than in the case of ⁴⁰Ca explored in ref. [10]. Thiscouldstrongly reduce the cross sectionatveryforwardangles.

b) The (¹⁸O,¹⁸Ne) reactionisparticularlyadvantageous, due to the large value of the B(GT) strengths. However, thisreactionis of β+β+ kind, whilemost of the research on 0νββ is in the opposite side. None of the reactions of β-β- kindlookslikeasfavourableas the (¹⁸O,¹⁸Ne). For example, the (¹⁸Ne,¹⁸O) requires a radioactivebeam, whichcannot be available with comparableintensity. The proposed (²⁰Ne,²⁰O) hassmaller B(GT), so a sensiblereduction of the yieldisexpected;

d) In some cases gas or implanted targets are necessary, e.g. ¹³⁶Xe or ¹³⁰Xe, which are normallymuchthinnerthansolid state ones, with a consequentreduction of the collectedyield;

e) In some cases the energyresolution (abouthalfMeV) isnotenough to separate the ground from the excitedstates in the finalnucleus. Thus, the coincidentdetection of γ-rays from the de-excitation of the populatedstatesismandatory, butat the price of the collectedyield.

In order to start a systematicexploration of all the nuclei of interest for 0νββ decay, an upgraded set-up, able to work with atleasttwoorders of magnitude more luminositythan the present, isnecessary. This goal can be achieved by a substantialchange in the technologiesimplemented in the beamextraction [20], in the control of the beaminducedradioactivity, in the detection of the ejectiles [21-25] and in the powerdissipation of the thin targets [26]. In addition, the projectdemands for an enhancement of the maximum acceptedmagneticrigidity, preserving the geometry and fielduniformity of the magneticfield [27] in order to keep the high-precision of the presenttrajectoryreconstruction. We are alsoinvestigating the possible link between the theoreticaldescription of the 0νββ decay and DCE reactions.

Neverthless the presentlimits of beampower (~100 W) for the CS accelerator and acceptable rate for the MAGNEX focalplane detector (few kHz) allowus to concentrate on somefewcases. In thisframework, wealreadyperform some testsand measurementsboth with the (¹⁸O,¹⁸Ne) reactionas a probe for the β ⁺ β ⁺ liketransitions and for the first time alsothe (²⁰Ne,²⁰O) as a probe for β ^- β ^- .

Asexample, in the reaction test: ¹¹⁶Sn + ¹⁸O at 15 MeV/A wehavemeasuredat 0° <θ_lab< 10° DCEX reaction¹¹⁶Sn(¹⁸O,¹⁸Ne)¹¹⁶Cd; CEX reaction¹¹⁶Sn(¹⁸O,¹⁸F)¹¹⁶In; 2p-transfer ¹¹⁶Sn(¹⁸O,²⁰Ne)¹¹⁴Cd; 1p-transfer ¹¹⁶Sn(¹⁸O,¹⁹F)¹¹⁵In.

In the reaction¹¹⁶Cd + ²⁰Ne at 15 MeV/A wehavemeasuredat 0° <θ_lab< 8°: DCEX reaction¹¹⁶Cd(²⁰Ne,²⁰O)¹¹⁶Sn; CEX reaction¹¹⁶Cd(²⁰Ne,²⁰F)¹¹⁶In; 2p-transfer ¹¹⁶Cd(²⁰Ne,¹⁸O)¹¹⁸Sn; 2n-transfer ¹¹⁶Cd(²⁰Ne,²²Ne)¹¹⁴Cd; 1p-transfer ¹¹⁶Cd(²⁰Ne,¹⁹F)¹¹⁷In; 1n-transfer ¹¹⁶Cd(²⁰Ne,²¹Ne)¹¹⁵Cd.

For most of the reactionsstudieddata reductionis in progress and for the reaction¹¹⁶Cd + ²⁰Ne at 15 MeV/Athe analysis are almostcompleted and theresultswill be published in the nextfuture.

Wehaveshownthat high resolution and statisticallysignificantexperimental data can be measured for DCE processes and thatprecious information towards NME determinationcould be atourreach.

On the basis of theseground-breaking achievement, NUMEN aims to go deepinsight in the HI-DCE studies on nuclei of interest in 0νββ decay, both from the theoretical and the experimentalpoint of view, lookingforwardat the 0νββ NME dermination, although a simple relation between DCE cross sections and ββ- decayhalf-livesisnottrivial and needs to be explored.