The lightest known quark is the up quark (u). Quarks are elementary particles that are the building blocks of hadrons, such as protons and neutrons. There are six known types of quarks, often referred to as flavors: up (u), down (d), charm (c), strange (s), top (t), and bottom (b).
Among these quarks, the up quark is the lightest in terms of mass. It carries a fractional electric charge of +2/3 and is commonly found in the protons and neutrons that make up atomic nuclei. The mass of the up quark is estimated to be around 2.2 to 2.8 MeV/c² (megaelectron volts divided by the speed of light squared).
It's worth noting that quarks are never observed in isolation due to a phenomenon called color confinement. They are always bound together to form composite particles called hadrons. The up quark combines with the down quark to form the proton (uud) or with two up quarks and one down quark to form the neutron (udd).