Beyond the Standard Model and Collider Phenomenology
My page on the Higgs-boson tells you about the Standard Model (SM), but mostly in the context of the higgs boson and asking the question how do we know what we have seen is the higgs boson. Well, let's be more open-minded, in spite of all it successes, how do we know the Standard Model is correct?
There are theoretical arguments pointing to the incompleteness of the SM, like the “hierarchy problem” and the “strong CP problem”, which however do not technically require modifying the model. More relevant are observations pointing to the incompleteness of the SM. We know, for example, that the universe is filled with transparent matter, inaptly called “dark-matter,” which however cannot be made of normal stuff, where by “normal stuff” I mean matter made of the particles described by the SM. The SM is certainly incomplete, but not necessarily incorrect. The dark-matter could well be stuff that interacts with normal stuff only gravitationally, so that it does not really require modification of the SM.
But once in a while there is an anomaly, an experiment design to check yet another aspect of the SM that does not give the SM-expected result. One such case is the anomalously large top-quark Forward-Backward asymmetry observed by two experiments, CDF and D0, at Fermilab. If you want to know what this ominous sounding “top-quark Forward-Backward asymmetry” is, and how can it arise, here is my awesome attempt at an explanation for non-experts.
The figure is from a paper with colleagues Kagan and Zupan from Cincinnati and Trott from CERN, and shows that in explaining the observed Fermilab data the new physics (“NP”) contribution to the cross section in the backward direction has to be negative while its contribution in the forward direction has to vanish, or at best be slightly positive. A contribution to a cross section can be negative if the interference between the NP and SM contributions is negative. But how can that be truein the backwards dierection and not in the forward direction? That puts a tough constraint on which can of NP can explain this. We went on to explore models that were predicated on Minimal Flavor Violation, a scheme that automatically avoided, or largely avoided, additional tough constraints on the new models from such esoteric things as the rate of neutral kaon and Beauty meson mixing (see also here).
Incidentally, the explanation on Forward-Backward asymmetry was prepared in connection with a paper with my student, Chris Murphy, in which we studied the asymmetry but now for bottom -quarks rather than top-quarks. Bottom quarks are called beauty quarks in Europe, they always have had different taste than americans! Chris realized that beauty quarks would be sensitive probes of theNP that had been proposed to explain the observed the anomaly in the top quark FB-asymmetry. This is specially the case since the bottom quark is so much lighter than the top quark that resonant production of bottom-quark pairs occurs for energies comparable to the rest energy of the Z-particle (one of the mediators of the weak interactions). When this happens, the interference between the Z and other production mechanisms results in large amplification of the other mechanisms, including those hypothesized in NP models. We are still waiting for the experimental result!!