The Standard Model of particle physics does not stipulate that all particles are massless. In fact, many particles in the Standard Model, such as quarks, leptons (e.g., electrons and neutrinos), and the W and Z bosons, are known to have non-zero masses.
The masses of elementary particles in the Standard Model arise from interactions with the Higgs field. The Higgs field permeates all of space, and particles acquire mass by interacting with this field. The Higgs mechanism, proposed in the 1960s and later confirmed by the discovery of the Higgs boson at the Large Hadron Collider in 2012, explains how certain particles acquire mass while others, such as photons (particles of light) and gluons (particles that mediate the strong nuclear force), remain massless.
According to the Standard Model, particles that are not affected by the Higgs mechanism, and thus remain massless, are described as "gauge bosons" or "force carriers." These include the photon, which mediates the electromagnetic force, and the gluons, which mediate the strong nuclear force. These particles are associated with symmetries in the theory that prevent them from acquiring mass.
However, it's important to note that the Higgs mechanism and the masses of particles are not directly derived from an equation within the Standard Model itself. The masses of particles are determined experimentally through various measurements and are inputs to the model, not outcomes of an equation.