Friday, 15 July 2011

Tevatron finds top/anti-top asymmetry sign of axi-gluons?

Fermilabs Tevatron particle accelerator has found a mysterious asymmetry in high energy collisions that produce pairs of top and anti-top quark. It seems that the at high energies there is a preference for which direction the top quark quark is produced travelling towards. The standard model predicts no or little such asymmetry, if the result continues to be confirmed, new physics and possibly new particle will be required to explain it. The measurement has a been around for over a year, having already grown from a 2.6 sigma signal a year ago to a 4 sigma signal in June 2011. Physicists normally like to have a 5 sigma signal to confirm a result, unfortunately the Tevatron is at the end of its life, and since the LHC collides proton against protons, and not protons against anti-protons, it will be difficult to get more data to confirm the result.

This hasn't stopped a large crop of papers looking to explain the result with new physics as summarised in ArXiv:1107.0841. A recent model I was particular fascinated by is one where the asymmetry is caused by an axigluon, as described most recently by Tavares and Schmaltz. Gluons are the gauge boson holding quarks together and normally act vectorially, identically to left and right handed particles, however a axigluon that acts exactly oppositely on left and right handed particles is also possible. To explain the top anti-top forward backward asymmetry an octet of axigluons with mass of around 450 GeV is introduced. They then also required doubling the number of quark, adding vector like pairs of quarks, that then decay rapidly by axigluon exchange. Their model contains exactly the extra quarks, I introduced to cancel the anomalies in adding a neutrino interacting axi-photon to the standard model, and in fact looks similar to the E6 model I described below. A axigluon explanation of the top and anti-top asymmetry, would be then very supporting of axial force theory, which is perhaps why I've biasedly am attracted to it. The LHC should however soon be able to confirm or disprove such an axigluon model. So time will tell.