Protons and neutrons are the primary constituents of atomic nuclei, but these bits of matter can be further divided into quarks.  Quark come in six types, or "flavors:" up, down, strange, charm, top and bottom (or, if you prefer, truth and beauty.) Quarks are never observed by themselves, but are instead found in groups of two (mesons) or three (baryons). Nuclear particles are baryons -- neutrons are composed of two down quarks and an up quark (ddu), while protons are two up quarks and a down quark (uud).  The other four types of quarks are less stable, and tend to be observed only as components of high-energy, short-lived particles inside supercolliders, bubble chambers, and the like.\n\nProtons and neutrons are thus the only directly observable particles within the nucleus.  However, they are bound together by another type of particle, not directly detectable that appropriately enough is known as a gluon.  Gluons, like photons, are force-carrier particles, and they are the intermediary for the transmission of the weak nuclear force, which binds the positively charged protons and electrically neutral neutrons.
What are some of the particles in the nucleus of an atom other than the proton and neutron?