The ATLAS experiment at the LHC has set the world’s best known limits for the mass of a hypothetical excited quark, q*. The analysis, accepted for publication by Physics Review Letters, represents ATLAS’s first exclusion of physics outside the Standard Model and extends the scientific reach of previous experiments. The existence of such a state would indicate that a quark is a composite particle as opposed to an elementary one as the Standard Model assumes.
The search was based on a sample of 315 nb–1 of proton–proton collision data collected at 7 TeV in the centre-of-mass. Looking at the mass distribution of measured dijets – events with two jets – the analysis used six different model-independent statistical tests to hunt for narrow resonances that could indicated the production of new heavy particles. The lack of evidence for such resonances allows the collaboration to set limits on the existence of the hypothesized q*, in particular, because predictions indicated a chance that it could be observed in the first samples of data at the LHC.
The results exclude at the 95% confidence level the existence of a q* with a mass in the range 0.40–1.26 TeV. With further data ATLAS will continue its searches to exclude or discover hypothesized particles such as the q* over greater ranges in mass.
