Search for black holes and sphalerons in high-multiplicity final states in proton-proton collisions at 13 TeV

A search for microscopic black holes and sphalerons using proton-proton collisions at 13 TeV recorded by the CMS detector during the 2016-2018 data taking, and corresponding to an integrated luminosity of 138/fb, is presented. Two data-driven search strategies are used. Model independent limits on the cross section of a new physics signal with multiple jets and leptons are set using a method that relies on the shape invariance of the scalar sum of the transverse momenta of all objects in the event. Model dependent limits on black hole and sphaleron production are set using a newly introduced method that has been developed for the identification of collider events with distinct kinematic features by separating them into classes based on phase-space proximity. In the context of models with large extra dimensions, semiclassical black holes with masses below 9.0-11.4 TeV are excluded, significantly extending the reach in comparison to limits from previous searches. At 95% confidence limit, more than 3 extra dimensions are excluded for most models probed. Results of a dedicated search for electroweak sphalerons are used to derive an upper limit of 0.0025 at 95% confidence level on the fraction of all quark-quark interactions above the nominal threshold of 9 TeV energy for the sphaleron transition.