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Mass-superselection rule (MSR) states that in the non-relativistic quantumtheory superpositions of states with different masses are unphysical. While MSRfeatures even in textbooks, its validity, physical content and consequencesremain debated. Its original derivation is known to be inconsistent, while aconsistent approach does not yield a superselection rule. Yet, superpositionsof masses do not seem to be present in Newtonian physics. Here we offer aresolution of the MSR puzzle. Crucial for the result is the understanding oftwo issues: how the notion of a mass parameter arises from the fundamentallydynamical relativistic notion of mass-energy, and what is the correct Newtonianlimit of relativistic dynamics of composite particles. The result explains thephysical content of the MSR without invoking any non-standard physics andclarifies the relation between MSR and a formalism describing relativisticcomposite particles, developed for studying relativistic effects in table topexperiments.

Puzzling out the mass-superselection rule