  Unknown isotope 26S, expected to decay by two-proton (2p) emission, was
studied theoretically and was searched experimentally. The structure of this
nucleus was examined within the relativistic mean field (RMF) approach. A
method for taking into account the many-body structure in the three-body decay
calculations was developed. The results of the RMF calculations were used as an
input for the three-cluster decay model worked out to study a possible 2p decay
branch of this nucleus. The experimental search for 26S was performed in
fragmentation reactions of a 50.3 A MeV 32S beam. No events of 26S or 25P (a
presumably proton-unstable subsystem of 26S) were observed. Based on the
obtained production systematics an upper half-life limit of T_{1/2}<79 ns was
established from the time-of-flight through the fragment separator. Together
with the theoretical lifetime estimates for two-proton decay this gives a decay
energy limit of Q_{2p}>640 keV for 26S. Analogous limits for 25P are found as
T_{1/2}<38 ns and Q_{p}>110 keV. In the case that the one-proton emission is
the main branch of the 26S decay a limit Q_{2p}>230 keV would follow for this
nucleus. It is likely that 26S resides in the picosecond lifetime range and the
further search for this isotope is prospective for the decay-in-flight
technique.
