Muons are particles that can be identified and measured with high precision by the CMS detector at the LHC. CMS can therefore be used to study the invariant mass spectrum of di-muon pairs and search for high-mass unstable particles (resonances) in a yet unexplored high mass range. High-mass resonances decaying into muon pairs are predicted in a number of models beyond the Standard Model of the fundamental interactions. Notable examples are heavy neutral gauge bosons predicted by grand unification theories, as well as gravitons arising in the Randall-Sundrum model of extra dimensions. The first search for high mass resonances was published in JHEP by CMS using the data acquired in 2010. Updated results were produced using part of the 2011 dataset in Summer 2011. By combining di-electron and di-muon data, CMS has excluded the existence of resonances predicted by a number of theoretical models with masses below about 2 TeV. These limits are the most stringent to date. S. Basegmez, G. Bruno and D. Pagano of the UCL CP3 group have contributed to this analysis by being one of the three teams of the CMS Collaboration that has regularly analyzed new data to produce the updated invariant mass spectrum, by setting up the technique for identifying isolated muons and by computing the significance, including the look-elsewhere effect, of an excess observed at 120 GeV in both the di-electron and the di-muon channels. In addition, a new technique for measuring high energy muons, which is the fundamental ingredient of the entire analysis, has been developed in the past year by the team. This technique, which has been proved to be the most robust against the catastrophic energy losses that can be experienced by muons, is expected to be adopted in the search starting from the 2012 run.
The matrix element reweighting method attempts to compute the full likelihood of an observed event given a theoretical model. The method therefore measures the degree of compatibility of the event with the given model using as much information as available. MadWeight is a tool that fully automatize the computation of the event likelihood for any model implemented in MadGraph, by performing phase-space integration and providing a framework for taking into account the experimental resolution on the observed final state objects. This project aims at validating the matrix element reweighting technique implemented in MadWeight on a number of benchmark searches. In some cases, the final goal is the efficient identification of background events. The final states that are being considered are: Zbb, single top, ttbar resonances and dimuon resonances.
