Prime Time at the LHC

In subatomic physics, the year 2010 will be remembered as an exciting one with the successful commissioning of the Large Hadron Collider (LHC) and first physics results from the LHC experiments.  During that year, the LHC delivered 45 pb^-1 of proton-proton collisions to the ATLAS experiment at a record center of mass energy of 7 TeV.  With two orders of magnitude less data than the CDF and D0 experiments collected in the past 8 years, ATLAS became sensitive to new physics beyond the limits set at the Tevatron, thanks to the unprecedented high-energy collisions at the LHC.

New heavy gauge bosons are required by many theories beyond the Standard Model of particle physics such as grand unification, models of extra dimensions and super-symmetry. A team of ATLAS physicists from TRIUMF and Simon Fraser University were instrumental in performing the first searches for new heavy gauge bosons with ATLAS. Their results on new heavy W’ and Z’ (Wprime and Zprime) were shown this week at an international conference at La Thuile, Italy, and will be submitted to Physics Letters B. The Vancouver team includes Ph.D. student Simon Viel, from TRIUMF and the University of British Columbia, post-doctoral fellow Dominique Fortin and research scientist Oliver Stelzer-Chilton, from TRIUMF, as well as SFU faculty Bernd Stelzer with M.Sc. students Suvayu Ali and Koos van Nieuwkoop.

The Z’ analysis takes advantage of the Standard Model Z boson as a calibration and normalization sample and then proceeds to search, in the previously blinded sample, for a resonance peak at large di-lepton mass. A novel technique for ATLAS is used to scan for a resonance peak using signal templates that can take on any resonance mass.  Although no evidence for new physics was found, three interesting events were observed near 650 GeV in the di-electron spectrum and one event at 770 GeV in the di-muon channel. A statistical analysis was performed to quantify the consistency of the data with the expectation from the Standard Model and no significant deviation could be concluded. Nevertheless, the di-muon event represents the highest di-lepton mass event recorded to date.

The analysis team thus set limits at 95% confidence level on the cross-section for Z’ bosons decaying to di-leptons which yield a lower mass limit of 1.048 TeV (1.084 TeV expected) on the Sequential Standard Model Z’. In this model the couplings of the new gauge boson are assumed to be the same as to the Z boson in the Standard Model.  Lower mass limits are also set on Z’ bosons in a grand unified E6 model that range from 0.738 – 0.900 TeV, which are amongst the world’s most stringent limits to date.

       Di-muon event with a mass of  770 GeV.

      Data and background prediction in the di-muon
      channel. Three hypothetical resonances at different
      masses are also shown for illustration.