2:1 for Naturalness at the LHC?

TL;DR

The paper investigates whether a large diphoton enhancement at the LHC can be explained by new charged fermions, finding that such explanations imply new physics at accessible energies or challenge existing theories.

Contribution

It demonstrates that explaining a 2x diphoton enhancement with only new fermions leads to vacuum instability, and explores implications for new particles and theories beyond the Standard Model.

Findings

A 2x enhancement requires large Yukawa couplings causing vacuum instability.

A 1.5x enhancement is possible with fermions lighter than 150 GeV.

If no light charged fermions are found, new bosons must exist below 10 TeV.

Abstract

A large enhancement of a factor of 1.5 - 2 in Higgs production and decay in the diphoton channel, with little deviation in the ZZ channel, can only plausibly arise from a loop of new charged particles with large couplings to the Higgs. We show that, allowing only new fermions with marginal interactions at the weak scale, the required Yukawa couplings for a factor of 2 enhancement are so large that the Higgs quartic coupling is pushed to large negative values in the UV, triggering an unacceptable vacuum instability far beneath the 10 TeV scale. An enhancement by a factor of 1.5 can be accommodated if the charged fermions are lighter than 150 GeV, within reach of discovery in almost all cases in the 8 TeV run at the LHC, and in even the most difficult cases at 14 TeV. Thus if the diphoton enhancement survives further scrutiny, and no charged fermions beneath 150 GeV are found, there must be new bosons far beneath the 10 TeV scale. This would unambiguously rule out a large class of fine-tuned theories for physics beyond the Standard Model, including split SUSY and many of its variants, and provide strong circumstantial evidence for a natural theory of electroweak symmetry breaking at the TeV scale. Alternately, theories with only a single fine-tuned Higgs and new fermions at the weak scale, with no additional scalars or gauge bosons up to a cutoff much larger than the 10 TeV scale, unambiguously predict that the hints for a large diphoton enhancement in the current data will disappear.