According to our current understanding of physics, the smallest theoretically possible mass is the Planck mass. The Planck mass, denoted by "m_p", is derived from fundamental physical constants and has a value of approximately 2.18 × 10^(-8) kilograms.
The Planck mass is defined in the context of Planck units, a system of natural units where fundamental constants such as the speed of light, the gravitational constant, and the reduced Planck constant are set to unity. In Planck units, the Planck mass is the natural unit of mass.
It is important to note that the Planck mass represents a scale where both quantum mechanics and gravity become significant. At scales below the Planck mass, our current understanding of physics breaks down, and a theory of quantum gravity is needed to describe the fundamental nature of matter and spacetime. However, since a complete theory of quantum gravity is still elusive, the behavior and properties of mass at scales near or below the Planck mass remain a subject of active research and speculation.