Quarks and Hadrons
PROTONS and NEUTRONS are not fundamental either. They are made of QUARKS — even smaller particles that come in 6 "flavors" (up, down, charm, strange, top, bottom). A PROTON has 2 up quarks + 1 down quark. A NEUTRON has 1 up + 2 down. The quarks are bound together by the STRONG NUCLEAR FORCE, mediated by GLUONS. The strong force is so powerful that quarks can NEVER be isolated — they are always in groups (HADRONS).
Hadrons. The general term for quark-containing particles. BARYONS contain 3 quarks (proton, neutron, others). MESONS contain a quark and an antiquark (pions, kaons). New, exotic hadrons keep being discovered — pentaquarks (5 quarks), tetraquarks (4 quarks). Most hadrons are unstable and decay quickly. Protons might be the only truly stable hadron (though some theories predict they decay over 10^34 years — far longer than the universe's age).
Why have we never seen a SINGLE QUARK by itself in experiments?
Glimpses of quarks. We can probe quark structure indirectly — by smashing protons together at high energy in particle accelerators (LHC). The patterns of debris reveal the underlying quark-gluon dynamics. QCD (quantum chromodynamics) is the theory of the strong force — mathematically beautiful but very hard to compute. Lattice QCD (massive supercomputer simulations) has confirmed predictions and helped fill in the picture.
Reflect
You are made of ATOMS. Atoms are mostly EMPTY SPACE with tiny NUCLEI. Nuclei are made of PROTONS AND NEUTRONS. These are made of QUARKS held together by the strong force. Below quarks? We do not know yet. The chain goes deeper than we can currently see.
Quarks are nature's deepest revealed building blocks. The strong force binding them is one of physics' most fascinating phenomena.
Want to keep learning?
Sign up for free to access the full curriculum — all subjects, all ages.
Start Learning Free