In my last post, I discussed the puzzle posed for cosmologists and particle physicists by the observation of the baryon asymmetry of the universe (BAU) – the fact that the universe is composed almost entirely of matter, with a negligible amount of antimatter. In this post I’ll to go into a little more detail about one popular idea about how the BAU might be generated. Although I’ll be a little more technical here than usual, if people are interested in even more detail, they could read this review article, or this one.
The precise question that concerns us is; as the universe cooled from early times, at which one would expect equal amounts of matter and antimatter, to today, what processes, both particle physics and cosmological, were responsible for the generation of the BAU? In 1967, Andrei Sakharov established that any scenario for achieving this must satisfy the following three criteria;
- Violation of the baryon number (B) symmetry
- Violation of the discrete symmetries C (charge conjugation) and CP (the composition of parity and C)
- A departure from thermal equilibrium.
In recent years, perhaps the most widely studied scenario for generating the BAU has been electroweak baryogenesis. In the standard electroweak theory baryon number is an exact global symmetry. However, baryon number is violated at the quantum level through nonperturbative processes – it is an anomalous symmetry. This feature is closely related to the nontrivial vacuum structure of the electroweak theory, as I explained here some time ago. At zero temperature, baryon number violating events are exponentially suppressed (this is most certainly a good thing, since we would like the protons making up our bodies to remain stable). However, at temperatures above or comparable to the critical temperature of the electroweak phase transition – the temperature at which the expectation value of the Higgs field becomes nonzero as the universe cools – B-violating vacuum transitions may occur frequently due to thermal activation.
Fermions in the electroweak theory are chirally coupled to the gauge fields. In terms of the discrete symmetries of the theory, these chiral couplings result in the electroweak theory being maximally C-violating. However, the issue of CP-violation is more complex. CP is known not to be an exact symmetry of the weak interactions (this is observed experimentally in the neutral Kaon system). However, the relevant effects are parametrized by a dimensionless constant which is no larger than 10-20. This appears to be much too small to account for the observed BAU and so it is usual to turn to extensions of the minimal theory.
The question of the order of the electroweak phase transition is central to electroweak baryogenesis. Since the equilibrium description of particle phenomena is extremely accurate at electroweak temperatures, baryogenesis cannot typically occur at such low scales without the aid of phase transitions. For a continuous transition, the associated departure from equilibrium is still insufficient to lead to relevant baryon number production. However, for a first order transition, at a critical temperature the nucleation of bubbles of the true vacuum in the sea of false begins, and at a particular temperature below this, bubbles just large enough to grow nucleate. These are termed critical bubbles, and they expand, eventually filling all of space and completing the transition. As the bubble walls pass each point in space there is a significant departure from thermal equilibrium so that, if the phase transition is strongly enough first order, it is possible to satisfy the third Sakharov criterion.
There is a further criterion to be satisfied. As the wall passes a point in space, the Higgs fields evolve rapidly and both CP violation and the departure from equilibrium occur. Afterwards, the point is in the true vacuum, baryogenesis has ended, and baryon number violation is suppressed. Since baryogenesis is now over, it is imperative that baryon number violation be small enough at this temperature in the broken phase, otherwise any baryonic excess generated will be equilibrated to zero. Such an effect is known as washout of the asymmetry and the criterion for this not to happen translates into, among other things, a bound on the mass of the lightest Higgs particle in the theory. In the minimal standard model, current experimental bounds on the Higgs mass imply that this criterion is not satisfied. This is therefore a second reason to turn to extensions of the minimal model.
One thing that I find fascinating about the baryon asymmetry problem is that it alone is evidence, from cosmology, of physics beyond the standard model. An important example of such physics, in which these requirements can be met, is the Minimal Supersymmetric Standard Model (MSSM). In addition to allowing a light enough Higgs particle, this theory can also contain light stops (the superpartners of the top quark), which can help to achieve a strongly first order phase transition. For those of you who care about the numbers, according to relatively current calculations, baryogenesis is possible if the lightest Higgs particle has a mass less than 120 GeV, and the lightest stop has a mass less than the top quark mass.
What would it take to have confidence that electroweak baryogenesis within a particular SUSY model actually occurred? First, there are some general predictions: if the Higgs is found, the next test will come from the search for the lightest stop, and important supporting evidence will come from CP-violating effects which may be observable in experiments involving B-mesons. However, to establish a complete model, what are really necessary are precision measurements of the spectrum, masses, couplings and branching ratios to compare with theoretical requirements for a sufficient BAU. Such a convincing case would require both the Large Hadron Collider (LHC) and perhaps ultimately an International Linear Collider (ILC), in order to establish that this is truly how nature works.
We certainly wouldn’t build a collider just for the chance of illuminating the problem of the baryon asymmetry. But it is a wonderful thing that, along with probing the origin of electroweak symmetry breaking, mapping the fundamental symmetries of nature, and showing us what lies around the corner for particle physics, the imminent turn-on of the LHC may hold the key to some of the problems that our telescopes have revealed.