Search for natural supersymmetry in 13 TeV collisions with the CMS detector

Search for natural supersymmetry in 13 TeV collisions with the CMS detector For five years, particle physicists at the Large Hadron Collider (LHC) at CERN have been analyzing collisions of protons that are are collided at 99.9999968% of the speed of light or, equivalently, at an energy of 8 TeV. In these high energetic reactions, a new particle was discovered that had properties just as predicted for the long sought Higgs boson. Measurements of decay properties confirmed this suspicion and the two major experimental collaborations, ATLAS and CMS, proudly announced that milestone on July 4th. , 2012. Theoretically, the Higgs boson is a challenge because of the special way it interacts with the other particles. This famous Higgs meachanism provides masses to the fun- damental particles, but when the relations are turned around, the Higgs boson receives large corrections to its mass from those particles in turn. The net sum of these cor- rections needs to be stabilized according to widespread belief, or otherwise the Higgs boson would be almost infinitely heavy. Interestingly, the corrections have different sign for force carrier particles, such as the photon, which is the particle of visible light, and matter particles, such as the electron. If a property of nature ensured that there was the same number of particles for each of those species, then the corrections cancelled themselves. Supersymmetry, a theoretical framework invented in the 1970s, provides ex- actly this mechanism. It not only stabilizes the Higgs boson mass, it also predicts that there is a stable particle that does hardly interact with other matter. Those particles would therefore fill the universe, interact only gravitationally and leave almost no trace otherwise. Is it a coincidence that our universe seems to contain 63% of so called dark matter which exactly fits this prescription? If this is a hint for supersymmetry, how can we uncover it in collision data? This proposal addresses these questions by analyzing LHC collision data taken by the CMS experiment until 2018. By then, enough data will have been accumulated to decide either way. The proposal is a joint effort by the University of Ghent (UGent, Prof. D. Dobur) and the Institute for High Energy Physics in Austria (HEPHY, R. Schofbeck) who combine their strengths in analyzing collisions where also electrons or muons, collectively called leptons, are created. Event samples with exactly one lepton or a pair of leptons with the same charge have very different properties. Hence, different analysis strategies are required for the two cases. Combining these channels, however, promises a tremendous gain in sensitivity. Therefore, we plan a joint analysis strategy comprising same-charge lepton pairs led by UGent and the single-lepton channel led by HEPHY. In this way, we will either uncover supersymmetry as a fundamental property of nature or constrain it so severely that it loses most of its theoretical appeal.

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 10^16 by the end of the year 2013. 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 incomplete. Indeed, the dynamics of these large scale structures indicates that the majority of matter is "dark matter" - by lack of a better name for something unknown. 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 at the LHC. 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 requires 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 provides us with the predictions for the LHC collisions. 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, leaving us with highly imbalanced events. 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.