upquarks

An up quark is a fundamental fermion described by the Standard Model. It is a first-generation quark with electric charge +2/3e and spin 1/2. Along with the down quark it forms the building blocks of hadrons and participates in all three fundamental interactions (strong, weak, and electromagnetic), though its most distinctive property is its role in baryon and meson structure. The up quark carries color charge and interacts via gluons in quantum chromodynamics; its current mass is about 2.2 MeV/c^2, although constituent masses in bound states are effective and larger.

Within baryons, protons are composed of two up quarks and one down quark, while neutrons have one up and two down. Mesons such as the positively charged pi meson (pi+) are made of an up quark and an anti-down quark. Up quarks participate in weak interactions and can change flavor to down (or with CKM-suppressed transitions to strange or heavier quarks) via charged-current processes. These flavor changes are a central feature of quark mixing.

Quarks have not been observed in isolation; confinement prevents free quarks. Free quarks are studied indirectly through high-energy scattering and the jets they produce in particle accelerators. Historically, the up quark was proposed as part of the quark model in the 1960s and established through deep inelastic scattering experiments at SLAC, which revealed pointlike constituents within protons.