The smallest kind of matter is typically considered to be elementary particles. Elementary particles are fundamental particles that are not made up of smaller constituents. They are the building blocks of matter and cannot be broken down into simpler components.
According to the Standard Model of particle physics, which is the prevailing theory describing the fundamental particles and their interactions, there are several types of elementary particles. These include:
Quarks: Quarks are the basic building blocks of particles called hadrons, which include protons and neutrons. Quarks come in six different flavors: up, down, charm, strange, top, and bottom.
Leptons: Leptons are another type of elementary particle. They include electrons, muons, and taus, along with their associated neutrinos. Electrons are commonly known as the negatively charged particles that orbit around the nucleus of atoms.
Gauge Bosons: Gauge bosons are particles that mediate the fundamental forces in nature. They include the photon (electromagnetic force), W and Z bosons (weak nuclear force), and gluons (strong nuclear force).
Higgs Boson: The Higgs boson is an elementary particle associated with the Higgs field, which gives other particles mass.
These elementary particles are extremely small, with sizes on the order of 10^-18 meters or smaller. However, it's important to note that elementary particles are point-like in the sense that they do not have a well-defined size or structure. They are described by their properties such as mass, charge, and spin.
It's worth mentioning that there are ongoing investigations in particle physics to understand the nature of elementary particles further, including the possibility of even smaller substructure or undiscovered particles beyond the Standard Model. However, elementary particles are considered the smallest known form of matter.