Citation: Baer, H.; Barger, V.; Tata, X.; Zhang, K. Detecting Heavy Neutral SUSY Higgs Bosons Decaying to Sparticles at the High-Luminosity LHC. Symmetry 2023, 15, 548. https://doi.org/10.3390/ sym15020548 Academic Editors: Tianjun Li and Jun-Jie Cao Received: 26 January 2023 Revised: 09 February 2023 Accepted: 15 February 2023 Published: 18 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). symmetry S S Article Detecting Heavy Neutral SUSY Higgs Bosons Decaying to Sparticles at the High-Luminosity LHC Howard Baer 1,2, * , Vernon Barger 2 , Xerxes Tata 3 and Kairui Zhang 2 1 Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019, USA 2 Department of Physics, University of Wisconsin, Madison, WI 53706, USA 3 Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 53706, USA * Correspondence: baer@ou.edu Abstract: In supersymmetry (SUSY) models with low electroweak naturalness (natSUSY), which have been suggested to be the most likely version of SUSY to emerge from the string landscape, higgsinos are expected at the few hundred GeV scale, whilst electroweak gauginos inhabit the TeV scale. For TeV-scale heavy neutral SUSY Higgs bosons H and A, as currently required by LHC searches, the dominant decay modes of H, A are gaugino plus higgsino provided these decays are kinematically open. The light higgsinos decay to soft particles, so are largely invisible, whilst the gauginos decay to W, Z or h plus missing transverse energy ( E T ). Thus, we examine the viability of H, A → W+  E T , Z+  E T and h+  E T signatures at the high luminosity LHC (HL-LHC) in light of large standard model (SM) backgrounds from (mainly) t ¯ t, VV and Vh production (where V = W, Z). We also examine whether these signal channels can be enhanced over backgrounds by requiring the presence of an additional soft lepton from the decays of the light higgsinos. We find significant regions in the vicinity of m A ∼ 1 − 2 TeV of the m A vs. tan β plane, which can be probed at the high luminosity LHC, using these dominant signatures by HL-LHC at 5σ and at the 95% confidence level (CL). Keywords: Higgs bosons; LHC; supersymmetry; naturalness 1. Introduction An advantage to searching for (R-parity conserving) supersymmetry [1–3] (SUSY) via heavy Higgs boson production at the CERN Large Hadron Collider (LHC) is that, instead of having to pair produce new states of matter, one may singly produce some of the new R-even states directly via s-channel resonances. In the minimal supersymmetric standard model (MSSM), this means direct production of the heavy scalar and pseudoscalar Higgs bosons, H and A, respectively. Indeed, LHC measurements of the properties of the light Higgs boson h so far have shown it to be nearly standard model (SM)-like [4]. This situation is expected in the decoupling regime, where the heavy SUSY Higgs bosons, and possibly also many sparticles, are well beyond the current LHC reach. (A very SM-like light Higgs boson can also be obtained in the alignment regime [5–7], where the new Higgs bosons, H and A, need not be so heavy.) The most stringent LHC Run 2 limits on heavy Higgs bosons have been obtained by the ATLAS [8] and CMS [9] collaborations by searching for H, A → τ ¯ τ with ∼139 fb −1 of integrated luminosity. These heavy Higgs search limits are presented in the m A vs. tan β plane within the so-called m 125 h scenario as proposed by Bagnaschi et al. in Ref. [10]. In the m 125 h benchmark scenario, most SUSY particles are taken to be at or around the 2 TeV scale, with a SUSY μ parameter at μ = 1 TeV. This ensures that SUSY particles only slightly affect the heavy Higgs searches, and that the dominant H and A decay modes are into SM particles. The ATLAS exclusion contour (which is shown later in this manuscript) shows that the Higgs decoupling limit with a heavy SUSY spectrum is now a likely possibility, particularly since LHC Run 2 limits with ∼139 fb −1 of integrated luminosity seem to require Symmetry 2023, 15, 548. https://doi.org/10.3390/sym15020548 https://www.mdpi.com/journal/symmetry