Bottom Quark

The bottom quark or b quark (from its symbol, b), also known as the beauty quark, is a third-generation quark with a charge of −1⁄₃ e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large bare mass (around 4,200 MeV/c2, a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements V_ub and V_cb, gives it a distinctive signature that makes it relatively easy to identify experimentally (using a technique called B-tagging). Because three generations of quark are required for CP violation (see CKM matrix), mesons containing the bottom quark are the easiest particles to use to investigate the phenomenon; such experiments are being performed at the BaBar and Belle experiments. The bottom quark is also notable because it is a product in almost all top quark decays, and would be a frequent decay product for the Higgs boson if it is sufficiently light.

The bottom quark was theorized in 1973 by physicists Makoto Kobayashi and Toshihide Maskawa to explain CP violation. The name "bottom" was introduced in 1975 by Haim Harari. The bottom quark was discovered in 1977 by the Fermilab E288 experiment team led by Leon M. Lederman, when collisions produced bottomonium. Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for their explanation of CP-violation. On its discovery, there were efforts to name the bottom quark "beauty", but "bottom" became the predominant usage.

The bottom quark can decay into either an up or charm quark via the weak interaction. Both these decays are suppressed by the CKM matrix, making lifetimes of most bottom particles (~10−12 s) somewhat higher than those of charmed particles (~10−13 s), but lower than those of strange particles (from ~10−10 to ~10−8 s).