From Higgs to pions and Back -- the Unbearable Lightness of a Composite Scalar Boson at 125 GeV in Purely Vectorial Theories

TL;DR

The paper argues that a 125 GeV Higgs is unlikely to be a fermion-antifermion composite in vectorial gauge theories, and offers new insights into the nature of light pseudoscalars and scalars in such theories and QCD.

Contribution

It provides a theoretical analysis showing the unlikelihood of the Higgs being a composite in vectorial theories and offers a novel perspective on pseudoscalar and scalar mass generation.

Findings

Lightest pseudoscalars are massless Nambu-Goldstone bosons in confining theories.

Scalar mesons are predicted to be very massive in vectorial gauge theories.

Massless pseudoscalars arise from specific path contributions in two-point functions.

Abstract

We argue that the $125 GeV$ "Higgs" particle is unlikely to arise as a fermion- antifermion composite if the underlying dynamics is a vectorial gauge theory. The reason is that the lightest scalar in such theories is heavier than the lightest pseudo-scalar with the mass difference being fixed by the scale of the theory. LHC searches suggest that the scale of any new physics, including that of a putative new theory dynamically generating the 125 GeV "Higgs" particle, is relatively high $\sim{(1/2TeV-1TeV)}$. Also the LHC analysis suggests that it is {\it scalar} namely $J^P = 0^+$ rather than pseudo-scalar. Thus it is unlikely that the "Higgs" could arise as a composite in such theories- though it will arise in special cases when the underlying binding gauge group is real as a fermion-fermion bound state. The direct considerations of the various two point functions in the large $N_c$ limit presented below- suggest that massless pseudo-scalars, but not any other anomalously light meson, arise as composites of massless fermions say the massless u and $\bar{d}$ quarks in QCD. These massless pions manifest the spontaneous breaking of the global axial symmetry in QCD with the pions being (pseudo) Nambu Goldstone Bosons. This offers a different insight into SXSB in QCD and most other confining non-abelian gauge vectorial gauge theory. Specifically we consider the euclidean two point functions $F_I|x-y|$ for asymptotic $|x-y|$ expressed as a sum over fermionic paths. We conjecture that for the pseudo-scalar two point function - and for that case only- self retracing paths and closely related paths make in this limit a positive, coherent and dominant contribution, a contribution which evades the generic asymptotic exponential fall-off and allows the lightest pseudoscalars to be massless. The same arguments imply that the scalars are very massive.