The axion mass from 5D small instantons

Abstract

We calculate a new contribution to the axion mass that arises from gluons propagating in a 5th dimension at high energies. By uplifting the 4D instanton solution to five dimensions, the positive frequency modes of the Kaluza-Klein states generate a power-law term in the effective action that inversely grows with the instanton size. This causes 5D small instantons to enhance the axion mass in a way that does not spoil the axion solution to the strong CP problem. Moreover this enhancement can be much larger than the usual QCD contribution from large instantons, although it requires the 5D gauge theory to be near the non-perturbative limit. Thus our result suggests that the mass range of axions (or axion-like particles), which is important for ongoing experimental searches, can depend sensitively on the UV modification of QCD.

A composite Higgs with a heavy composite axion

“…[6][7][8], and more recently in refs. [9][10][11][12]. A particularly interesting possibility is that associated with enlarging the color gauge group, where new nonperturbative contributions to the axion mass are generated [13,14].…”

Leading logs in QCD axion effective field theory

“…2 See [46][47][48][49] for some recent work motivating the possibility that QCD axions could be heavier than expected.…”

The QCD axion at finite density

“…figure 4 we plot the region (blue) in the {n, n p /n} plane where θ = 0, namely where the system is in the kaoncondensed phase. The evolution for given {n, n p /n} can be understood as follows: for a fixed proton fraction n p /n, as n increases the amount of positive charge due to the protons increases as well, and more leptons are required to satisfy the neutrality condition, leading 12 It is illustrative to also consider the pion effective in-medium mass,…”

“…This prediction seems however to be dependent on the UV dynamics of QCD, in particular on the size of the so-called small instanton contributions, see e.g [10][11][12]…”