Operator counting and soft blocks in chiral perturbation theory

Lin Dai*, Ian Low, Thomas Mehen, Abhishek Mohapatra

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Chiral perturbation theory (ChPT) is a low-energy effective field theory of QCD and also a nonlinear sigma model based on the symmetry breaking pattern SU(Nf) × SU(Nf) → SU(Nf). In the limit of massless Nf quarks, we enumerate the independent operators without external sources in ChPT using an on-shell method, by counting and presenting the soft blocks at each order in the derivative expansion, up to O(p10). Given the massless on-shell condition and total momentum conservation, soft blocks are homogeneous polynomials of kinematic invariants exhibiting the Adler’s zero when any external momentum becomes soft and vanishing. In addition, soft blocks are seeds for recursively generating all tree amplitudes of Nambu-Goldstone bosons without recourse to ChPT, and in one-to-one correspondence with the”low energy constants” which are the Wilson coefficients. Relations among operators, such as those arising from equations of motion, integration-by-parts, hermiticity, and symmetry structure, manifest themselves in the soft blocks in simple ways. We find agreements with the existing results up to NNNLO, and make a prediction at N4LO.

Original languageEnglish (US)
JournalUnknown Journal
StatePublished - Sep 3 2020

ASJC Scopus subject areas

  • General

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