Search for the nucleon and di-nucleon decay into invisible channels

Data collected deep underground at the Gran Sasso National Laboratory of INFN by using the similar or equal to 6.5 kg DAMA liquid Xenon scintillator have been analysed to search for nucleon and di-nucleon decays into invisible channels (decay to nu (i)<(<nu>)over bar>(i) or to nu (i)<(<nu>)over bar>(i)nu (i) or to 5 nu (i), etc., with i = e, mu, tau) or disappearance. The considered statistics has been 2257.7 kg x day. The new obtained limits are: tau (p) = 1.9 x 10(24) yr, tau (nn), = 1.2 x 10(25) yr and tau (pp) = 5.5 X 10(23) yr at 90% C.L. The latter values are the same or better than those previously established by the Frejus Collaboration; in particular, the limits for the NN decay into nu (tau)<(<nu>)over bar>(tau) are set for the first time. (C) 2000 Elsevier Science B.V. All rights reserved.