The smallest particles known to modern science are subatomic particles, which include fundamental particles such as quarks, leptons, and gauge bosons. Quarks and leptons are considered elementary particles, meaning they have no known substructure or internal constituents. They are often referred to as point-like particles, but it's important to note that they are not physical points in the traditional sense. Instead, they are described by quantum field theories as excitations of their respective quantum fields.
However, it is still an open question whether there could be smaller particles or substructure beyond what we currently understand. The search for even smaller constituents of matter is an active area of research in theoretical physics. Various speculative theories propose the existence of sub-quark or sub-lepton particles, sometimes called preons, but there is no direct experimental evidence to support these ideas.
Regarding the concept of "infinitely small," it is a challenging concept to grasp. In physics, the idea of infinitesimally small quantities often arises in mathematical descriptions, such as in calculus and quantum mechanics. However, it's important to note that the concept of infinity is not directly applicable to physical reality. The infinite small is a mathematical abstraction used to describe extremely small quantities that approach zero but never actually reach it. In reality, there are limits to how small a physical quantity can be, such as the Planck length, which is believed to be the smallest meaningful scale in physics.
It's worth mentioning that our understanding of these topics is based on current scientific knowledge, and ongoing research and discoveries can potentially refine or expand our understanding of the fundamental nature of particles and the concept of smallness.