Bounds in 4D Conformal Field Theories with Global Symmetry

TL;DR

This paper derives bounds on scalar operator dimensions in 4D conformal field theories with global symmetry, using crossing symmetry constraints and analyzing specific group cases, with implications for electroweak symmetry breaking models.

Contribution

It introduces vectorial sum rules for conformal blocks in 4D CFTs with global symmetry and derives bounds on scalar operator dimensions, extending previous bootstrap methods.

Findings

Derived explicit vectorial sum rules for SO(N) and SU(N) groups.

Proved an upper bound on the lowest singlet scalar dimension depending on operator dimension.

Analyzed the bound's behavior as the scalar dimension approaches 1.

Abstract

We explore the constraining power of OPE associativity in 4D Conformal Field Theory with a continuous global symmetry group. We give a general analysis of crossing symmetry constraints in the 4-point function <Phi Phi Phi* Phi*>, where Phi is a primary scalar operator in a given representation R. These constraints take the form of 'vectorial sum rules' for conformal blocks of operators whose representations appear in R x R and R x Rbar. The coefficients in these sum rules are related to the Fierz transformation matrices for the R x R x Rbar x Rbar invariant tensors. We show that the number of equations is always equal to the number of symmetry channels to be constrained. We also analyze in detail two cases - the fundamental of SO(N) and the fundamental of SU(N). We derive the vectorial sum rules explicitly, and use them to study the dimension of the lowest singlet scalar in the Phi x Phi* OPE. We prove the existence of an upper bound on the dimension of this scalar. The bound depends on the conformal dimension of Phi and approaches 2 in the limit dim(Phi)-->1. For several small groups, we compute the behavior of the bound at dim(Phi)>1. We discuss implications of our bound for the Conformal Technicolor scenario of electroweak symmetry breaking.