Astronomers have probed the Perseus galaxy cluster in search of an as-yet undetected particle that would help support string theory.
Astronomers test string theory using NASA's Chandra X-ray space telescope : Read more
There is a convergence of problems for extensions of our current quantum field theory for particles, such as these. Axions in of itself would solve the so called strong CP problem of why quarks are not part of the particles that give the universe matter, it looks like neutrinos are but why not some of both? And supersymmetry axion-like particles are generic for string extensions of the standard model of particles.
But there are many results that fail to turn up axions or axion-like particles, such as nuclear spin measurements or gamma ray spectra of galaxies [
https://www.forbes.com/sites/briank...rule-out-a-theory-of-everything/#516194464f6a ]. And now X-ray emissions from galaxy clusters.
Here they join experiments of particle collisions in LHC and electron sphericity in ACME in making string theory - as well as axions - much less appetizing. If absence of natural energy scale WIMPs in LHC and ACME count as rejecting strings, absence of natural consequence axion/axion-like particles counts too.
And so it counts as absence of a solution for the strong CP finetuning. Some of the references to the new work describes the strong CP as *very* finetuned if axions or something similar do not exist. (Technically and AFAIU those technicalities, cancellation of non-linearity causing CP asymmetry happens up to order 14 in quark field theory. There is some CP asymmetry among quarks and gluons, but it is virtually zero in comparison to what is needed for having matter.)
Instead of the notion that something must be added to our current quantum field theory for particles, I note that inflation excel in solving such apparent finetuning in the same way that it solves for finetuning the amount of dark energy.
So instead of finetuning cancellations, it is easier to think that when the universe cooled down and putatively the fields froze out by way of non-linear phase transitions, it was a coin toss whether quarks or neutrinos would have the chirality asymmetry that solves the CP asymmetry for having matter. In our universe it was the neutrinos.