Testing natural supersymmetry at the high energy proton-proton run at the LHC

During the 2011/2012 LHC run, the CMS and ATLAS experiments each recorded proton-proton collisions amounting to an integrated luminosity of 25 fb-1 at a centre-of- mass energy of 7 and 8 TeV and discovered a new boson in this dataset, most probably a scalar particle. Measurements of the decay branchings are compatible with the predictions for the Higgs boson of the Standard Model (SM). Another salient feature of the data is the absence of any supersymmetric (SUSY) signature, resulting in lower limits for the masses of many strongly-interacting SUSY particles of the order of 1 TeV. Can SUSY still provide the stabilization of the quantum corrections to the Higgs boson mass according to the long advertised scheme, where fermionic and bosonic loops cancel? And is the discovered scalar particle not the SM Higgs boson, but in fact, the lightest SUSY Higgs particle? The past LHC runs left open one important possibility, natural SUSY, which condenses the idea that this new symmetry of nature preserves the quantum Higgs boson mass. It has become a leading priority of the experimental collaborations and a central pillar of the general idea of SUSY as a theory of fundamental interactions. A distinct prediction of natural SUSY is the existence of gluinos, new particles with masses in the reach of the next LHC run at 13 - 14 TeV starting in 2015. We therefore propose to settle the question by searching for gluinos in CMS data in events with exactly one muon or electron. This channel retains a large efficiency for signal events in combination with high suppression of backgrounds from multijet production, filing it among the discovery channels with the highest sensitivity. The analysis focuses on the most promising signature involving up to four top quarks, but is sensitive to a wide range of natural SUSY final states. Technically, we plan to develop a novel multi-variate analysis (MVA) strategy, never before tried in searches for gluinos, in order to significantly extend sensitivity to SUSY mass configurations where jet energies, missing transverse energy and lepton momentum are not all necessarily large. In this way, we will be able to leap into unknown SUSY territory at LHC design energy already with 4 fb-1. With a dataset of 20 fb-1, presumably accumulated during the first year of running, we will be sensitive to most natural SUSY mass configurations and either deprive SUSY of a central attractive feature or establish a novel property of nature. Combining the MVA strategy with improved reconstruction methods for missing transverse energy and leptons, which incorporate the experience gained during three years of data taking, maximizes the discovery potential and distinguishes this proposal from other ongoing projects.

When after the 2011/2012 data taking period at the Large Hadron collider (LHC), the Higgs boson discovery had been announced by the CMS and ATLAS experiments, the standard model of particle physics seemed complete. For sure, it could explain the physical processes of the vast majority of the proton-proton collisions which amounted to the astounding number of 1016 by the end of the year 2013 and which all had happened in the underground collider tunnel. And yet, if we take a look in the opposite direction to distant galaxies and galaxy clusters, we find strong evidence that our picture is in- complete. Indeed, the dynamics of these large scale structures, as well as the evolution of our universe as a whole since it originated from the big bang, indicate that the majority of matter is dark - by lack of a better name, because we do not know how that elusive form of matter is related to the ordinary matter around us. Bridging this gap was one of the main questions of the project. Because if dark matter was produced during the big bang, then we can reasonably hope to do so again under the similar and rather extreme conditions of LHC proton-proton collisions.There is moreover an awkward theoretical feature in the description of the Higgs boson in the standard model that requires the fine tuning of its quantum corrections. While this is a technical term, it was long thought that a full description of our universe re- quires a new symmetry that controls this tuning to a large extent. One example to this end is supersymmetry. It predicts partners for all the known particles, and interestingly, in many cases one of them just fits the bill of a dark matter candidate. This, in turn, provides us with the predictions for the LHC collisions. We now know what we can look for. The phenomenology of supersymmetry is a rich field, and we decided to search for events with a single muon or electron; both are well-known standard model particles. These extra particles appear sufficiently rare in the standard model such that backgrounds from well-known processes are largely reduced. Moreover, we can exploit a central feature of dark matter: It can interact with the ordinary matter of the detector material weakly at the utmost, thus leave no detectable trace in the reconstructed events. This leaves us with highly imbalanced events, because we record no signal in the direction of the dark matter particles. The term missing energy intuitively describes that property.The main part of the work under this project was to develop strategies to control the indirect measurement of missing energy as well as the other experimental observables to an extent that allows sensitive measurements under the hostile LHC operational conditions. We were successful in devising suitable analysis strategies and control the uncertainties to a level that allowed sensitive tests of supersymmetric predictions. We fully exploited the large 13 TeV dataset of the 2016 LHC run period and published the result through the CMS collaboration. For our search channels we set new and the most stringent limits on supersymmetry to date.