Determined by the vibration of strings.
From String Theory we know that the smallest mass is the mass of a string. The vibration of the string determines its mass.
In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. It describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string.
Wikipedia, String Theory, 2019
So the mass is determined by the vibration of the string. However this was portrayed in the Quran 1400 years before it was discovered. The Quran portrays the smallest particle as a wick.
[Quran4.49] Have you not considered those who claim purity for themselves? Rather, Allah purifies whom He wills, and they will not be wronged by a wick.
Fateel فَتِيلً means wick. The smallest thing is a wick.
The wick in the Quran looks exactly like the string in String Theory.
From String Theory we know that the smallest mass is the mass of a string. The vibration of the string determines its mass. However this was portrayed in the Quran 1400 years before it was discovered. The Quran says that the smallest mass is a pluck.
[Quran4.124] But whoever works righteousness, whether male or female, and is a believer, those will enter Paradise, and will not be wronged by a pluck.
Nakara نقر means pluck. Pluck the string of a musical instrument means نقر أوتار العود. Nakir نقير means vibrations of a string. In this verse Nakiran نَقِيرًا means a single vibration of a string. So in the Quran the smallest mass is the single vibration of a string.
In this verse the smallest mass is a pluck and in the previous verse the smallest particle is a wick. Both verses are describing the same smallest particle. Today we know what it is, this is a vibrating string.