Comments on Unparticles

TL;DR

This paper critically examines unparticle physics, correcting previous results on operator dimensions and propagators, and discusses the implications of contact interactions and unitarity constraints on experimental signatures.

Contribution

It corrects and clarifies unitarity bounds and propagator forms for unparticles, highlighting the impact of contact interactions on experimental signals.

Findings

Unitarity bounds for vector unparticles are corrected to d ≥ 3.

Contact interactions induced by coupling to the Standard Model can dominate unparticle effects.

Many previous unparticle studies need revision based on these corrections.

Abstract

We comment on several points concerning unparticles which have been overlooked in the literature. One regards Mack's unitarity constraint lower bounds on CFT operator dimensions,e.g. d\geq 3 for primary, gauge invariant, vector unparticle operators. We correct the results in the literature to account for this, and also for a needed correction in the form of the propagator for vector and tensor unparticles. We show that the unitarity constraints can be directly related to unitarity requirements on scattering amplitudes of particles, e.g. those of the standard model, coupled to the CFT operators. We also stress the existence of explicit standard model contact terms, which are generically induced by the coupling to the CFT (or any other hidden sector), and are subject to LEP bounds. Barring an unknown mechanism to tune away these contact interactions, they can swamp interference effects generated by the CFT. We illustrate these points in the context of a weakly coupled CFT example. A significant amount of the unparticle literature should be reconsidered in light of the observations in this note.