Conformal anomaly

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Conformal anomaly is an anomaly i.e. a quantum phenomenon that breaks the conformal symmetry of the classical theory.

In string theory, conformal symmetry on the worldsheet is a local Weyl symmetry and must therefore cancel if the theory is to be consistent. The required cancellation implies that the spacetime dimensionality must be equal to the critical dimension which is either 26 in the case of bosonic string theory or 10 in the case of superstring theory.

In quantum chromodynamics in the chiral limit, the classical theory has no mass scale so there is a conformal symmetry, but this is broken by a conformal anomaly. This introduces a scale, which is the scale at which colour confinement occurs. This determines the sizes and masses of hadrons, including protons and neutrons. Hence this effect is responsible for most of the mass of ordinary matter. (In fact the quarks have non-zero masses, so the classical theory does have a mass scale. However, the masses are small so it is still nearly conformal, so there is still a conformal anomaly. The mass due to the conformal anomaly is much greater than the quark masses, so it has a much greater effect on the masses of hadrons.)

See also dimensional transmutation.