Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Experimental Particle Physics

Heavy Quarks

Together with leptons and gauge bosons, quarks are one of the fundamental components of matter.  All hadrons, such as protons, neutrons or pions, are made are of quarks.  The first signs of the existence of quarks was given by the inelastic scattering of protons in the sixties.  After several more experimental hints, the quark model was finally accepted in 1974 when the J/ψ meson was discovered.  This meson consists of charm quarks which had been predicted by the quark model.  Up to now, six sorts (flavours) of quarks are observed: up-, down-, strange, charm, bottom and top-quark.  While the first three flavours have very small masses, the other three quarks are much heavier. Also, there are large differences between their masses.  This hierarchy of masses is - like the mass hierarchy for the leptons - as yet an unsolved problem of particle physics.

The top-quark is the heaviest quark by far.  It has a mass of 171 GeV and thus, is as heavy as the gold atom.  Because of its very short lifetime, 4.2x10-25s, it is not capable of building hadronic bound states which means that there are no hadrons containing top-quarks.  Already in 1977 after the discovery of its partner quark, the bottom-quark, the top-quark was predicted, but it was observed for the first time in 1995.  This was because that high energies are needed to produce the mass of a top-quark.  This was possible by the proton-antiproton collider TevaTron at the Fermi National Laboratory which fort the first time produced more than 1 TeV of energy per collision.

The bottom-quark was discovered in 1977.  Among others, it is part of b mesons and Υ mesons.  Its mass of 4.2 GeV is notably smaller than the mass of the top-quark, but also four times heavier than the charm-quark whose mass is 1.3 GeV.

Heavy quarks are of much interest to modern research.  They can be used to test predictions for the strong interaction (Quantum Chromodynamics) but also to study the weak interactions.  For the latter, the violation of the CP-symmetry can be measured with the help of heavy quarks.

Matter | Dark Matter | Higgs-Particle | 4th Generation | Heavy Quarks