Friday, July 26, 2019

Baryon number as R-symmetry, baryogenesis, Dirac gauginos

As in many other cases, the most exciting hep-ph paper today is the first one
A Supersymmetric Theory of Baryogenesis and Sterile Sneutrino Dark Matter from B Mesons.
The degree of excitement is obvious from the fact that the paper was submitted at 18:00:00 UTC, in the first second required to be at the top of the daily list. The authors, Ann Nelson et al. (AáENX), mainly build on their October 2018 paper that was also submitted within the first second of the business day, to make your thinking harder if you think that the timing is a coincidence. ;-)

Phenomenologically, aside from the cosmic inflation, one of the events or processes that are needed but unexplained by the Standard Model is baryogenesis. Matter and antimatter could have been produced in equal amounts and they would have annihilated. But they haven't because you are reading this sentence and most TRF readers are composed of matter. There had to be an asymmetry and it had to be substantial, at least dozens of trillionths. The asymmetric processes that favored matter had to break C, CP, and the baryon number. The last Sakharov condition that had to be obeyed was a violation of the thermal equilibrium.

How do you observe some objects where the CP-violation is obvious? Yes, the experimental particle physicists know an answer: B-mesons. B-workers employed by B-factories look at B-mesons that may oscillate due to the CP-violating terms. You need the third generation bottom quark because the CP-violation in the Standard Model is only visible if all three generations are included.

The B-meson oscillations look like a man-made, anthropomorphic way of looking at the CP-violation. Some engineered bound states, B-mesons, happen to make the CP-violation more visible. But like Ann Nelson, Mother Nature is a feminist and She protests against your suggestion that the B-meson method of observation is man-made. Can't She, a woman, be allowed to be equally associated with this trick of emphasizing the CP-violation?

The clever claim by Nelson et al. is that Mother Nature may copy the men's clever idea of highlighting the CP-violation by B-mesons. So in the early Universe, when the temperature of the Universe was in some \({\rm MeV}\)'s, the Universe was full of B-mesons, they oscillated into their antiparticles etc., and amplified the CP-violation. And then they decayed – into the dark sector. And who knows, an anti-patriarchal heretic could even suggest that Mother Nature may have invented this clever trick before the men did!

If I understand well, the CP-violating phase in the CKM matrix of the Standard Model is the only CP-violating term they need for baryogenesis. But of course they do need new particles, basically superpartners plus a dark sector. The superpartners aren't useless, they play an important role. In particle, long-lived sterile sneutrinos are their dark matter while squarks and the Dirac binos mediate the decay of the B-mesons into the dark sector.

Yes, I said "Dirac binos". It's totally cool because the masses of the gauginos are Dirac masses – these particles' fields are doubled so that their masses are Dirac masses. Majorana masses are supposed to be completely forbidden. I consider Dirac gauginos to be particularly compatible with some general lessons of some stringy vacua, especially the D-braneworlds where the SUSY is enhanced to \(\NNN=2\) in the sectors of gauge multiplets (only broken to lower SUSY by the "matter fields"). Dirac gauginos arguably allow heavier masses to be more natural than Majorana gauginos, too.

And there's one more cool thing – the R-symmetry is identified with the baryon number! So it's exact or almost exact and the proton decay – and neutron-antineutron oscillations – is therefore naturally prevented. It's pretty cool to think that the R-symmetry and the baryon number may be the same thing. I still have to convince myself that this identification doesn't lead to some obvious problems for the model. Also, I can't resist to think about an old question I had – whether the ban on global symmetries in quantum gravity prevents the existence of an R-symmetry. Well, there has to be a gauge field associated with this continuous symmetry but the coupling must be weak. But as we have argued, it can't really be too weak etc. I will look.

It seems like an extremely creative scenario to explain the gaugino masses, baryogenesis, and dark matter in a new, potentially very natural way.

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