bottom quark

bottom quark. The bottom quark is a third-generation fermion and a fundamental constituent of matter described by the Standard Model of particle physics. With a charge of –1/3 e, it is the second-heaviest of the six known quark flavors. Its discovery was pivotal in completing the quark model and provided crucial evidence for the Cabibbo–Kobayashi–Maskawa matrix, a framework explaining CP violation.

Discovery and properties

The existence of the bottom quark was independently theorized in 1973 by Makoto Kobayashi and Toshihide Maskawa as part of their extension to the Cabibbo angle mechanism. This work, which later earned them the Nobel Prize in Physics, required a third generation of quarks to incorporate CP violation into the Standard Model. Experimental confirmation came in 1977 from a team led by Leon M. Lederman working at the Fermilab using a fixed-target experiment with proton beams colliding with a beryllium or copper target. They observed the Upsilon meson, a bound state of a bottom quark and its antiquark, through its decay into muon pairs. The bottom quark has a mass of approximately 4.18 GeV/c², making it significantly heavier than the charm quark but lighter than the top quark. It carries a color charge and participates in all fundamental interactions except the purely leptonic weak force transitions governed by the GIM mechanism.

Production and decay

Bottom quarks are not found in ordinary matter and must be produced in high-energy collisions. They are copiously generated in hadron colliders like the Large Hadron Collider at CERN and formerly at the Tevatron at Fermilab, primarily through strong interaction processes such as gluon fusion. They are also produced in electron–positron colliders like KEK in Japan and CESR at Cornell University. Once created, the bottom quark undergoes weak decay almost exclusively, with a mean lifetime around 1.5 picoseconds. It primarily decays to a charm quark or, less frequently, an up quark, via the emission of a W boson. This decay produces a jet of hadrons, often including a B meson, and can yield final states with leptons like the electron or tau lepton. The study of these decays is central to experiments at the LHCb experiment and the Belle experiment.

Role in particle physics

The bottom quark plays a critical role in testing the Standard Model and probing for physics beyond the Standard Model. Its relatively long lifetime allows the formation of distinct hadron states, such as B mesons and bottomonium, which are ideal laboratories for precision measurements. Studies of B meson oscillations and decays provide stringent tests of the Cabibbo–Kobayashi–Maskawa matrix and are the primary means to investigate CP violation in the quark sector. Furthermore, rare decays of bottom quarks, which are highly suppressed in the Standard Model, are sensitive probes for new particles predicted by theories like supersymmetry. The B factories at SLAC National Accelerator Laboratory and KEK were specifically built to produce large numbers of B mesons for this purpose.

Experimental evidence and studies

The initial discovery of the Upsilon meson at Fermilab was followed by detailed spectroscopy at facilities like CESR, which mapped the bottomonium spectrum. The definitive confirmation of B meson production and the observation of B meson mixing came from experiments at DESY in Germany and later at the Tevatron. The ARGUS experiment at DESY first observed B meson oscillations in 1987. In the 2000s, the Belle experiment at KEK and the BaBar experiment at SLAC National Accelerator Laboratory made precise measurements of CP violation in the B meson system, validating the Kobayashi-Maskawa theory. Currently, the LHCb experiment at CERN, along with general-purpose detectors like ATLAS experiment and CMS experiment, study bottom quark production and decay in unprecedented detail, searching for anomalies that could indicate new physics.

Theoretical significance

The bottom quark is integral to the theoretical structure of the Standard Model. Its existence and properties were essential for the Kobayashi-Maskawa mechanism, which successfully explained the observed CP violation in the kaon system and predicted three generations of quarks. The subsequent discovery of the top quark at Fermilab completed this picture. The bottom quark's mass and couplings are key parameters in quantum chromodynamics and the electroweak interaction, influencing calculations of fundamental quantities like the Higgs boson coupling. Furthermore, the precise study of B meson decays imposes tight constraints on models of flavor physics and is a critical component in global fits of the Cabibbo–Kobayashi–Maskawa matrix parameters, testing the unitarity of this fundamental framework.

Category:Quarks Category:Elementary particles Category:Subatomic particles