Does Chiral EM Asymmetry Imply Matter-Antimatter Asymmetry?

[ohwilleke]

A Science Daily report discusses observations of cosmic gamma rays that suggest that magnetic fields in the universe disproportionately have left handed rather than right handed helicity. It goes on to suggest that under a 2001 theory of another physicist that this could explain matter-antimatter asymmetry in the universe.

http://www.sciencedaily.com/releases/2015/05/150514085655.htm

Is there anything in the Standard Model that would suggest this? (The weak force has chiral asymmetry, but I've never known that there was something similar in electromagnetism.) By what mechanism would one lead to the other?

 

[fzero]

The 2001 paper being referred to is astro-ph/0101261. The proposal there is roughly the following.

One possible explanation of the baryon asymmetry is called electroweak baryogenesis (EWBG). This mechanism relies on topological solutions to the Yang-Mills equations called sphalerons. The idea is that the W and Z fields, along with the Higgs can take topologically nontrivial classical configurations. As a topological configuration, there is an associated topological charge that can be related to baryon number $B$ and lepton number $L$. In the case of the sphaleron, the configuration is unstable and has some lifetime to decay. The idea of EWBG is that as the temperature of the universe cools through the electroweak scale, we have a phase transition associated with electroweak symmetry breaking. During the phase transition, sphalerons can be produced, which then rapidly decay. These events can mediate baryon and lepton number violating processes that satisfy $\Delta B = \Delta L$ and conserve $B-L$. For example, an antiquark can be changed into two quarks and 3 antileptons. So these processes open the possibility that antiquarks were converted into quarks, creating a baryon asymmetry resembling what we observe. As I understand it, it is still an open question whether EWBG can explain the observed baryon asymmetry, but there are various technical reasons to be pessimistic.

What Vachaspati notes is that there is another interpretation of the sphaleron as a magnetic monopole connected to an antimonopole connected by something called a Z-string. By a "string" here we mean another kind of topologically classical configuration that has an interpretation as an extended object with a finite energy per unity length. It is called a Z-string because is corresponds to a configuration for the Z field (and the Higgs). In a certain sense, the Weinberg angle is a relationship between the Z field and the EM field. Vachaspati argues that the magnetic field associated with the monopoles is what leads to the generation of a helical magnetic field during the sphaleron decay process. He ties the change in EM helicity to the change in baryon number.

I think the Science Daily article to some degree implies that the magnetic field asymmetry is responsible for the baryon asymmetry in Vachaspati's picture. However, it is the case that the sphaleron process generates both of the asymmetries simultaneously. What is interesting if the claimed observation of magnetic field asymmetry is correct, is that it would hopefully give us a way to estimate the amount of baryon asymmetry due to EWBG. Then we would have a better idea of whether it can explain the total observed baryon asymmetry or if new physics is definitely needed.

 

[ohwilleke]

Thanks. That's a really clear explanation.