Search for black holes and sphalerons using novel machine learning techniques at CMS

TL;DR

This study tackles the search for microscopic black holes and electroweak sphalerons using CMS data at $\sqrt{s}=13$ TeV with an integrated luminosity of $138~\text{fb}^{-1}$. It introduces a novel phase-space distance representation on the manifold $\\Pi_N$ and leverages a distance-based SVM to distinguish BH- and sphaleron-like events from QCD multijets, complemented by a data-driven background estimation. The results exclude semiclassical BHs with masses around $9$–$11.4$ TeV in several large extra dimension scenarios and set the strongest to-date limits on the sphaleron pre-exponential factor across multiple $p(N_{CS})$ hypotheses, including an upper bound of $0.0025$ on the fraction of high-energy quark-quark interactions that produce sphaleron transitions. Collectively, the work demonstrates a powerful, interpretable ML-augmented framework for high-multiplicity, isotropic final states in hadronic collisions, pushing the frontier of beyond-Standard-Model searches at the LHC.

Abstract

A comprehensive search for microscopic black holes and electroweak sphalerons is presented, using proton-proton collision data collected by the CMS detector during 2016-2018, corresponding to an integrated luminosity of $138~\mathrm{~fb}^{-1}$. A novel tool has been developed to identify collider events with distinct kinematic features, based on the phase-space distance between events. Model-independent limits are set on the cross section of new physics signals producing multiple jets and leptons, which are further interpreted as constraints on black hole and sphaleron production. In the context of models with large extra dimensions, semiclassical black holes with masses below 9.0-11.4 TeV are excluded by this search, significantly extending previous sensitivity. Additionally, a dedicated search for electroweak sphaleron transitions has been performed. An upper limit of 0.0025 is set at 95% confidence level on the fraction of quark-quark interactions with center-of-mass energy above the nominal threshold of 9 TeV that result in sphaleron transitions.

Figures (10)

• Figure 1: Event display of the final products from a simulated microscopic black hole evaporation at the LHC. The red lines represent muons, the orange cones are jets, the yellow lines are tracks, the green line is an electron, and the blue block is for a photon interaction in the electric calorimeter.
• Figure 2: Event display of the final products from a simulated sphaleron process at the LHC.
• Figure 3: Pairwise distance between 10,000 events in each category of signal (BH with mixture mass points) and background (QCD multijets).
• Figure 4: SVM score distributions for simulated QCD multijets and selected black hole (with $n = 2$) and sphaleron models.
• Figure 5: The SVM score vs. the $S_{T}$ distributions for simulated background (left) and a selected black hole signal model (right).