High-energy collisions with final states that include leptons (electrons, muons or taus) have played major roles in the history of physics. Several particles – such as J/ψ, W, and Z – as well as productions of pairs of Top quarks were discovered in their leptonic decay modes. In general, leptons provide a clean signature in the detectors.
CMS has performed searches for physics Beyond the Standard Model (BSM) using decays that require at least two leptons in the final states. These searches have been performed in channels in which the leptons have opposite charge (e+e− e+μ−, e−μ+ and μ+μ−) as well as those in which the leptons have the same charge (ee, μμ, ττ, eμ, eτ, μτ), in association with jets and missing transverse energy. These inclusive searches are sensitive to several new physics scenarios, such as supersymmetry (SUSY).
The results described here for di-leptons with opposite charge and same charge use ~1 fb−1 of integrated luminosity collected by the CMS detector from proton collisions at the LHC in the first half of 2011. The papers using 2010 collision data [same charge; opposite charge] have been published in the Journal of High Energy Physics.
Within the framework of the Standard Model (SM), decays involving same-charge di-leptons are extremely rare, whereas di-leptons with opposite charge are more naturally produced. With an additional requirement of significant jet production in association with large missing transverse momentum, both of these final states should be sensitive to SUSY. In many models, heavy SUSY particles carrying strong charge are produced in pairs. The decays of these particles would give rise to several hadronic jets, as well as to invisible weakly-interacting massive particles (WIMPs) which escape detection and result in missing transverse momentum. These WIMPs are candidates for the observed dark matter in the universe.
In these studies, several data-driven background techniques were implemented in order to estimate almost all of the SM background. The search strategy involves validation of these techniques in the background-dominated regions and the expected prediction of the background in the regions in which new physics models predict a large enhancement in the event rates. Any deviation in the rates of production of the standard processes would hint at the possibility of new physics. The results show the background expectation is consistent with the observation in all regions. These studies are compared with the expectations from the Constrained Minimal Supersymmetric extension of the Standard model (CMSSM).
Exclusion region in the CMSSM framework using same charge di-leptons. The result of the previous analysis is shown to illustrate the improvement since.
CMS found no excess in the number of events predicted by the SM. An upper limit of 95% CL has been placed on the non-SM contributions. In the context of CMSSM, the excluded regions in the parameter space exceed those set by the previous experiments such as Tevatron.
With the current wealth of data from all of 2011 collisions, CMS will be able either to find excesses (if the underlying strongly-produced squarks or gluinos in SUSY decays are within the LHC reach) or will exclude masses at the TeV scale. In either case, this information will be vital in understanding the fundamentals of the symmetry between matter and the forces.
— Submitted by Sanjay Padhi and Didar Dobur