The decelerating particles will return to the initial distance and beyond into infinity, or stop and repeat the collision (oscillation takes place).
This shows that the system, which loses no energy, does not combine (bind) into a solid object, parts of which oscillate at short distances.
Therefore, to bind the particles, the kinetic energy gained due to the attraction must be dissipated by resistive force.
Complex objects in collision ordinarily undergo inelastic collision, transforming some kinetic energy into internal energy (heat content, which is atomic movement), which is further radiated in the form of photons – the light and heat.
Once the energy to escape the gravity is dissipated in the collision, the parts will oscillate at a closer, possibly atomic, distance, thus looking like one solid object.
No mass deficit can appear, in theory, until this radiation or this energy has been emitted and is no longer part of the system.
This mass change must be released as various types of photon or other particle energy as above, according to the relation E = mc2.
It represents energy that must be resupplied from the environment for the nucleus to be broken up into individual nucleons.
For example, an atom of deuterium has a mass defect of 0.0023884 Da, and its binding energy is nearly equal to 2.23 MeV.