Study of nuclear effects in the computation of the 0νββ decay matrix elements

We analyze the effects that different nuclear structure approximations associated with the short range correlations (SRC), finite nucleon size (FNS), higher order terms in the nucleon currents (HOC) and with some nuclear input parameters, have on the values of the nuclear matrix elements (NMEs) for the neutrinoless double beta (0 nu beta beta) decay. The calculations are performed with a new shell model (ShM) code which allows a fast computation of the two-body matrix elements of the transition operators. The treatment of SRC, FNS and HOC and the use of quenched or unquenched values for the axial vector coupling constant produce the most important effects on the NMEs values. Equivalent effects of some of these approximations are also possible, which may lead (accidentally) to close final results. We found that the cumulative effect of all these nuclear ingredients on the calculated NMEs is significant. Since the NME values are often obtained with different approximations and/or with different input parameters, a convergent viewpoint on their inclusion/neglection and a uniformization of the calculations are needed, in order to enter into an era of precision concerning the computation of the NMEs for double beta decay.