<P> Each member of a generation has greater mass than the corresponding particles of lower generations . The first - generation charged particles do not decay, hence all ordinary (baryonic) matter is made of such particles . Specifically, all atoms consist of electrons orbiting around atomic nuclei, ultimately constituted of up and down quarks . Second - and third - generation charged particles, on the other hand, decay with very short half - lives and are observed only in very high - energy environments . Neutrinos of all generations also do not decay and pervade the universe, but rarely interact with baryonic matter . </P> <P> In the Standard Model, gauge bosons are defined as force carriers that mediate the strong, weak, and electromagnetic fundamental interactions . </P> <P> Interactions in physics are the ways that particles influence other particles . At a macroscopic level, electromagnetism allows particles to interact with one another via electric and magnetic fields, and gravitation allows particles with mass to attract one another in accordance with Einstein's theory of general relativity . The Standard Model explains such forces as resulting from matter particles exchanging other particles, generally referred to as force mediating particles . When a force - mediating particle is exchanged, at a macroscopic level the effect is equivalent to a force influencing both of them, and the particle is therefore said to have mediated (i.e., been the agent of) that force . The Feynman diagram calculations, which are a graphical representation of the perturbation theory approximation, invoke "force mediating particles", and when applied to analyze high - energy scattering experiments are in reasonable agreement with the data . However, perturbation theory (and with it the concept of a "force - mediating particle") fails in other situations . These include low - energy quantum chromodynamics, bound states, and solitons . </P> <P> The gauge bosons of the Standard Model all have spin (as do matter particles). The value of the spin is 1, making them bosons . As a result, they do not follow the Pauli exclusion principle that constrains fermions: thus bosons (e.g. photons) do not have a theoretical limit on their spatial density (number per volume). The different types of gauge bosons are described below . </P>

State the four key assumptions of the particle model for matter