Background: The Strong CP Problems
One of the "fake" unsolved problems of physics is why the strong force doesn't have charge parity (CP) violation like the weak force does, even though the equations of quantum chromodynamics that govern the strong force have an obvious place to put such a term, with the experimentally measured parameter of CP violation in the strong force commonly called "theta".
I call it a "fake" problem because the existence of theta that is exactly equal to zero doesn't cause any tensions with observational data or lead to inconsistencies in the equations. It is simply a case of Nature making a different choice about what its laws are than a few presumptuous theoretical physicists think it should have.
For some reason, according to an utterly counterproductive hypothesis called "naturalness" theoretical physicists seem to think that all possible dimensionless parameters of fundamental physics should have values with an order of magnitude similar to 1 and think it is "unnatural" when they don't, unless there is some good reason for this to not be the case. When an experimentally measured parameter has some other value (at least if it is non-zero) this parameter is called "fine-tuned" and is assumed to be a problem that needs to be explained somehow, usually with New Physics.
The introduction to the new paper discussed below recaps the strong CP problem as follows:
There are a couple of ways that people concerned about the "strong CP problem" imagine could resolve it.
One is that the issue would disappear if the up quark had a mass of zero, although experimental data fairly conclusively establish that up quarks have a non-zero mass.
Another is to add a beyond the Standard Model particle called "the axion" which is very, very low in mass (a tiny fraction of an electron-volt).
My "go to" explanation, in contrast, has been that since gluons are massless that they don't experience the passage of time. Thus, the strong force shouldn't have a parameter that is sensitive to the direction of time that its carrier boson does not experience, because CP violation is equivalent to saying that a process behaves differently going forward and backward in time.
Background: Confinement
One of the most characteristic observational aspects of the strong force is that quarks and gluons are always confined in composite structures known as hadrons, except (1) in the cases of top quarks which decay so quickly that they don't have time to hadronize (and are only produced in high energy particle collider environments), and (2) in quark-gluon plasma states which are produced in even more extremely high temperature systems. This observational reality is called "confinement."
Without confinement, our world would be profoundly different than it is today.
As it is, in our post-Big Bang era energies era, strong force phenomena are contained in many dozens of other kinds of hadrons.
In reality, the only hadrons that every arise in our low energy reality, however, because the others take immense energies to create and then decay rapidly, are protons (which are stable), neutrons (which are stable in bound atoms and last about fifteen minutes as free particles), and a few short-lived mesons (quark-antiquark bound structures) like the pion and kaon, which are carriers of the nuclear binding force between protons and neutrons in nuclei, but generally don't escape an individual atomic nucleus before decaying.
These light mesons also come up in a few other sub-collider energy contexts outside nuclear physics, for example, in cosmic rays, that slightly affect the normal workings of the universe in the post-Big Bang energies era of the universe. But, due to confinement, we never directly encounter the unmediated strong force in Nature (this is one of the reasons that it was the last force of nature to be discovered).
A New Solution To The Strong CP "Problem"
A new preprint uses lattice QCD methods to suggest another reason that there is no CP violation in the strong force, without a need to resort to axions, and that such axions should not exist.
It suggests that if theta were non-zero and there was CP violation in the strong force, that confinement wouldn't happen. Therefore, the theta term in the strong force equations must be zero, and the hypothetical axion cannot exist.
Three hard problems!
In this talk I investigate the long-distance properties of quantum chromodynamics in the presence of a topological theta term. This is done on the lattice, using the gradient flow to isolate the long-distance modes in the functional integral measure and tracing it over successive length scales.
It turns out that the color fields produced by quarks and gluons are screened, and confinement is lost, for vacuum angles theta > 0, thus providing a natural solution of the strong CP problem. This solution is compatible with recent lattice calculations of the electric dipole moment of the neutron, while it excludes the axion extension of the Standard Model.
2 comments:
Might be useful to compare this with where (and why) we
do see CP violation. More generally, the strong force
and Electromagnetism are time symmetric, whereas the weak
force (and maybe gravitation) are not.
GR does not violate CP. I have yet to see a quantum gravity theory with a massless graviton that has CP violation. Loop quantum gravity class theories (e.g. dynamical causal sets), are generically causal, but don't have CP violation.
This is what one would expect with a massless graviton that does not experience time in its own reference frame and does not couple to either charge or to parity.
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