Chiral Symmetry and Meson Vertex Operators in QCD Strings

TL;DR

This paper derives meson vertex operators in QCD strings using worldline representation, revealing how chiral symmetry breaking affects meson masses and emphasizing the importance of boundary fermions and possible world sheet supersymmetry.

Contribution

It introduces a novel derivation of pion and sigma vertex operators in QCD strings, highlighting the role of boundary fermions and the connection to chiral symmetry breaking.

Findings

The sigma propagator relates to the pion propagator through a massless ghost state.

The vertex operator V_{0} is linked to the chiral condensate.

Boundary fermions are essential in string models of mesons.

Abstract

The worldline representation of the one loop fermionic effective action is used to obtain the vertex operator for the pion and the sigma in QCD strings. The vertex operator of the scalar sigma is distinguished from that of the pseudo-scalar pion by the presence of an additional operator m V_{0} where m is the current quark mass and V_{0} is a vertex operator that would describe a tachyon in the open bosonic string theory. This leads to a relation between the sigma propagator and the pion propagator, when chiral symmetry is spontaneously broken this relation implies that the propagator constructed from V_{0} must behave like a massless ghost state. The presence of this state ensures that the sigma is massive and no longer degenerate with the pion. The expectation value of V_{0} in a string description is related to the vacuum expectation value of the chiral condensate in QCD. Our analysis emphasizes the need for boundary fermions in any string representation of mesons and is suggestive of world sheet supersymmetry.