Well...none, in the first approximation. The past decade has been marked by inertia and intellectual masturbation. The last truly novel ideas, like AdS/CFT, Randall-Sundrum, or ADD, were all born back in the 90s. No surprise that the list of 50 top cited articles last year contains only 3 particle theory papers written in the 00s, none of which in a prominent position.
Nevertheless, our understanding of particle theory has progressed, somewhat. Here is my subjective, biased, and utterly unfair summary of the most interesting developments.
- Extra dimensions are strong dynamics
Warped extra dimensions a-la Randall-Sundrum have dominated new physics model building. Perhaps the most interesting aspect of that industry is a qualitative analogy between five-dimensional warped models and purely four-dimensional strongly coupled models. This of course is not completely unexpected given the AdS/CFT conjecture. Nonetheless it is interesting that the correspondence extends to more down-to-Earth and phenomenologically relevant examples, even if in a vulgarized form. And so, 5D Higgsless theories are large N technicolor models in disguise, 5D gravity in a black hole background captures some aspects of heavy ion physics, etc. Even low-energy QCD can, to a certain extent, be modeled this way, and some quantitative predictions for the parameters of the effective chiral lagrangian can be derived.
- Higgs can be stabilized without supersymmetry
The bulk of particle theory is driven by the fact that the Higgs boson mass in the standard model receives large, quadratically divergent corrections at the quantum level. The common expectation, or maybe just wishful thinking, is that new symmetries and new particles appear at the TeV scale to fix that problem. The best known example - supersymmetry - is based on a boson-fermion interplay, for example, quantum corrections from the top quark are canceled by its scalar partners called stops. This is however not the only possibility, and the cancellations can occur between the same-statistics particles, for example top quark contributions can be canceled by another heavy colored fermion. This option has been known since 70s, but only during the last decade it was systematically understood and classified in the framework of little Higgs and gauge-Higgs unification theories. The end results is rather depressing though: all models we have constructed so far are just as good, or rather just as bad, as supersymmetry.
- QCD is boring but it's here to stay
Theorists working on QCD have always been looked down upon by smartasses building new fancy models of the universe. Yet in the past and in the coming decade hadron colliders are the sad reality, and an input from QCD theory is necessary to isolate new physics from mundane background processes. Definitely, tons of good work in that direction has been done. At the most basic level, we have seen heroic computations of higher-order corrections to SM processes like W+jets, Z+jets or ttbar+jets, and so on, without which life at the LHC would be much harder. At a more sophisticated level, new jet algorithms better suited for hadron colliders have been developed, and new ways to search for new physics using jet substructure have been proposed. One should also mention the progress in theoretical handling of QCD, for example the soft-collinear effective theory.
- There is more to dark matter than meets the eye
Models of dark matter are more numerous than stars in the sky, so why bother about another thousand spawned during the last decade? However, some recent proposals are important because they changed the way we search for dark matter. On one hand, models based on KK parity and T-parity prompted us to explore new collider signatures. Even more important was the impact on direct detection experiments. Not so long ago experimenters, brainwashed by MSSM preachers, searched only for spin-independent (coupled to nucleus' mass) or spin-dependent (coupled to nucleus' spin) elastic WIMP scattering. Experimental set-ups as well as data analyses were tailored for these 2 possibilities to the point that less standard dark matter signals would simply be discarded as background. This embarassing situation has been greatly improving in recent years. Thanks in part to inelastic dark matter models, or the recent offensive of light GeV scale elastic dark matter models, experimental analyses are becoming more flexible and developing alternative experimental techniques is being encouraged.
- There are more ways to compute scattering amplitudes
Anybody who ever computed scattering amplitudes in gauge theories can't help the feeling that there is something wrong with the standard way of doing it. In the approach via Feynman diagrams, hundreds of complicated expressions at the end of the day magically combine into something far more simple. It is becoming more and more clear that gauge theories may hide surprising mathematical structures that control scattering amplitudes. During the last decade some of these structures have been uncovered thanks to e.g. BCFW recursion relations, CSW rules, or fancy twistor space techniques. More recently, a new approach based on Grassmannians suggests that the hidden simplicity extends to higher loop levels, at least in the maximally supersymmetric case. But this last one might be more appropriate for my Farewell to the Teenies...
So much for the last decade, now dying to see the new one. Clearly, it can't get much worse :-)