Soft-collinear effective theory

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In quantum field theory, soft-collinear effective theory (or SCET) is a theoretical framework for doing calculations in quantum chromodynamics (QCD) that involve interacting particles carrying widely different energies.

The motivation for developing SCET was to control the infrared divergences that occur in QCD calculations that involve particles that are soft -- carrying much lower energy or momentum than other particles in the process -- or collinear -- traveling in the same direction as another particle in the process. SCET is an effective theory for highly energetic quarks interacting with collinear and/or soft gluons. It has been used for calculations of the decays of B mesons (quark-antiquark bound states involving a bottom quark) and the properties of jets (sprays of hadrons that emerge from particle collisions when a quark or gluon is produced).

The new feature of SCET is its ability to handle more than one soft energy scale. For example, processes involving quarks carrying a high energy Q interacting with gluons have two soft scales: the transverse momentum pT of the collinear particles, plus the even softer scale pT²/Q. SCET provides a power-counting formalism for doing perturbation theory in the small parameter Λ_QCD/Q in a sensible way.

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Original papers:

• C.W. Bauer, S. Fleming, D. Pirjol, and I.W. Stewart, "An effective field theory for collinear and soft gluons: heavy to light decays", Phys. Rev. D63, 114020 (2001), arXiv:hep-ph/0011336.
• C.W. Bauer, D. Pirjol, and I.W. Stewart, "Soft-collinear factorization in effective field theory", Phys. Rev. D65, 054022 (2002), arXiv:hep-ph/0109045.
• C.W. Bauer, D. Pirjol, and I.W. Stewart, "Power counting in the soft-collinear effective theory", Phys. Rev. D66, 054005 (2002), arXiv:hep-ph/0205289.