Quark

:For other uses of this term, see: Quark (disambiguation)

Quark masses

Although one speaks of quark mass in the same way as the mass of any other particle, the notion of mass for quarks is complicated by the fact that quarks cannot be found free in nature. As a result, the notion of a quark mass is a theoretical construct, which makes sense only when one specifies exactly the procedure used to define it.

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Current quark mass

The approximate chiral symmetry of QCD, for example, allows one to define the ratio between various (up, down and strange) quark masses through combinations of the masses of the pseudo-scalar meson octet in the quark model through chiral perturbation theory, giving

Related Topics:
Chiral symmetry - Quark model - Chiral perturbation theory

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::rac{m_u}{m_d}=0.56qquad{ m and}qquadrac{m_s}{m_d}=20.1.

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The fact that mu  ?  0 is important, since there would be no strong CP problem if mu were to vanish. The absolute values of the masses are currently determined from QCD sum rules (also called spectral function sum rules) and lattice QCD. Masses determined in this manner are called current quark masses. The connection between different definitions of the current quark masses needs the full machinery of renormalization for its specification.

Related Topics:
Strong CP problem - QCD sum rules - Lattice QCD - Renormalization

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Valence quark mass

Another, older, method of specifying the quark masses was to use the Gell-Mann-Nishijima mass formula in the quark model, which connect hadron masses to quark masses. The masses so determined are called constituent quark masses, and are significantly different from the current quark masses defined above. The constituent masses do not have any further dynamical meaning.

Related Topics:
Gell-Mann-Nishijima mass formula - Quark model - Hadron

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Heavy quark masses

The masses of the heavy charm and bottom quarks are obtained from the masses of hadrons containing a single heavy quark (and one light antiquark or two light quarks) and from the analysis of quarkonia. Lattice QCD computations using the heavy quark effective theory (HQET) or non-relativistic quantum chromodynamics (NRQCD) are currently used to determine these quark masses.

Related Topics:
Quarkonia - Lattice QCD - Heavy quark effective theory - Non-relativistic quantum chromodynamics

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The top quark is sufficiently heavy that perturbative QCD can be used to determine its mass. Before its discovery in 1995, the best theoretical estimates of the top quark mass are obtained from global analysis of precision tests of the Standard Model. The top quark, however, is unique amongst quarks in that it decays before having a chance to hadronize. Thus, its mass can be directly measured from the resulting decay products. This can only be done at the Tevatron which is the only particle accelerator energetic enough to produce top quarks in abundance.

Related Topics:
Perturbative QCD - 1995 - Standard Model - Tevatron - Particle accelerator

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• Top quark mass from the Tevatron Electroweak Working Group
• Other quark masses from Particle Data Group. Quark masses are given in the MS-bar scheme.
• The quantum numbers of the top and bottom quarks are sometimes known as truth and beauty respectively.