This is true that the mass of the individual atoms in the BEC is a positive scalar and we only observe a collective behavior that acts like an object with negat

This is true that the mass of the individual atoms in the BEC is a positive scalar and we only observe a collective behavior that acts like an object with negative mass. But, that engineered object (the wavepacket) is behaving like a negative mass. If we assume that everything that we measure in physics is collective (a subsystem of another - there is no such thing as an isolated system), we are not so wrong assigning that 1-D subsystem a negative mass. Let's close with this question: How are we so sure that a "positive" mass of an object is not a collective interaction between the object itself and the rest of the universe?