I’ve been working on a measurement of the ratio of production of W and Z bosons in events that also contain a jet. The W and Z are heavy (80-90 times the proton mass!) cousins of the massless photon, and are responsible for the weak nuclear force. Many theories for new physics beyond the standard model include couplings between the W and Z and the new particles. Often, these couplings lead to signatures involving production of W and/or Z particles accompanied by jets. By measuring the ratio of production of W and Z along with a jet, we are able to achieve cancellation of many sources of systematic uncertainties, enabling a precision test of the physics of jet production, so-called perturbative QCD. We further measure this quantity versus the minimum transverse momentum of the jet, increasing the sensitivity of the comparison with theory. This kind of measurement is also sensitive to deviations in the production rates of either the W or Z, so if we see a discrepancy from prediction, it might be our first hint that there is new physics coupling to the W or Z.

Check out the Cern Courier article in the November issue.

This plot shows the ratio along with several different predictions using different algorithms for comparison. The ratio is plotted versus the minimum jet transverse momentum required.

On April 6th, the CDF collaboration reported an anomaly observed in proton anti-proton collision events which produced a W boson and two sprays of particles called “jets”.  The analysis analyzes events containing one lepton (electron or muon), missing momentum perpendicular to the nominal proton beam axis from an escaping neutrino, and two jets whose energies are measured in the calorimeter. In these events, the two-jet invariant mass shows a small excess of events over a steeply falling background distribution. The challenge with this measurement is understanding the shape of the background very well.  There is a mood of guarded excitement in the community because of the possibility that this excess might be due to a new particle never before observed, and not predicted by the Standard Model of particle physics.

The New York Times article is interesting, as is the post to the arXiv.  The question on everyone’s mind is, “Is this real?” The analysis uses only about half of the Tevatron Run II data, and in principle the other Tevatron detector collaboration, D0, should be able to see this too if it is real, as should the LHC experiments ATLAS and CMS once they have enough data.  There will definately be another chapter to this story once more data is analyzed and D0 and the LHC experiments weigh in.