Rencontres de Moriond EW 2012: Addressing symmetry breaking and mass hierarchy

Last Friday at the Moriond conference in La Thuile in Italy, Lisa Randall from Harvard University reminded the audience how all fields are related: electroweak symmetry breaking must take into account flavour physics for example. Every good model should address this intrinsic connection.


Despite many expectations, no signs for supersymmetry (SUSY) of any type has been found to date. So Lisa Randall worked with Csaba Csaki and John Terning to explore alternatives and developed a version of supersymmetry built on the Minimal Composite Supersymmetry Standard Model (MCSSM) that Csaki, Shirman, and Terning had developed, incorporating a strongly interacting theory with compositeness that addresses among other things the fact that the top quark is so much heavier than all other quarks. Randall and collaborators showed that this model, when supersymmetry is incorporated, naturally accommodates both a Higgs boson around 125 GeV and a light stop, the supersymmetric partner to the top quark.

“The top quark is really different,” she stressed. In SUSY, this introduces the reasonable possibility that the stop should be much lighter than the other SUSY particles. Their model predicts two hierarchies, meaning SUSY particles will also appear with very different masses just like the Standard Model particles.

Lisa Randall showed four different scenarios, some leading to very similar masses for the top quark and stop squark. The top quark could also be a composite object but this effect could be so suppressed that we might not be able to detect it experimentally.

Will this new model turn out to be the right one? No one knows but as Lisa Randall stressed: “It is important to build new models that address all those problems.” Already, many SUSY models seem to be excluded by the latest LHC data so theorists now have the hard task to conciliate all these stringent constraints. In the end, experimental results will point us in the right direction.

by Pauline Gagnon