Spotting hidden sectors with Higgs binoculars

Published for SISSA by Springer Received: January 29, 2019 Revised: March 28, 2019 Accepted: April 22, 2019 Published: April 29, 2019 Spotting hidden sectors with Higgs binoculars a Institute for Nuclear Physics (IKP), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany b Institute for Theoretical Particle Physics (TTP), Karlsruhe Institute of Technology, Engesserstraße 7, D-76128 Karlsruhe, Germany c Institute for Theoretical Physics (ITP), Heidelberg University, Philosophenweg 12, D-69120 Heidelberg, Germany E-mail: , , , , Abstract: We explore signals of new physics with two Higgs bosons and large missing transverse energy at the LHC. Such a signature is characteristic of models for dark matter or other secluded particles that couple to the standard model through an extended scalar sector. Our goal is to provide search strategies and an interpretation framework for this new signature that are applicable to a large class of models. To this end, we define simplified models of hidden sectors leading to two different event topologies: symmetric decay, i.e., pair-produced mediators decaying each into a Higgs plus invisible final state; and di-Higgs resonance, i.e., resonant Higgs-pair production recoiling against a pair of invisible particles. For both scenarios, we optimize the discovery potential by performing a multi-variate analysis of final states with four bottom quarks and missing energy, employing state-of-theart machine learning algorithms for signal-background discrimination. We determine the parameter space that the LHC can test in both scenarios, thus facilitating an interpretation of our results in terms of complete models. Di-Higgs production with missing energy is competitive with other missing energy searches and thus provides a new opportunity to find hidden particles at the LHC. Keywords: Beyond Standard Model, Higgs Physics ArXiv ePrint: 1901.07558 Open Access, c The Authors. Article funded by SCOAP3 . https://doi.org/10.1007/JHEP04(2019)160 JHEP04(2019)160 Monika Blanke,a,b Simon Kast,b,c Jennifer M. Thompson,c Susanne Westhoffc and José Zuritaa,b Contents 1 2 Simplified models of a hidden scalar sector 2.1 Symmetric topology 2.2 Resonant topology 3 4 5 3 Higgs-pair production with missing energy at the LHC 3.1 Kinematics and benchmarks 3.2 Jet substructure technique 3.3 Backgrounds and cutflow analysis 6 7 10 12 4 Multi-variate analysis and results 13 5 Dark matter 19 6 Conclusions 20 A UV completing the Sgg coupling 22 1 Introduction Postulating a hidden sector that interacts primarily with the Higgs boson is tempting for good reasons. Higgs couplings to new scalar standard-model (SM) singlets are renormalizable and secluded from visible matter [1, 2]. An extended scalar sector can thus serve as a portal to a hidden sector [3–6]. At the LHC, the Higgs interaction with a hidden sector is best probed in signatures with one or two Higgs bosons. Searches for invisibly decaying Higgs bosons or for mono-Higgs production in association with missing transverse energy / T are well-established parts of the LHC program. Invisible Higgs decays probe hidden E sectors with particles significantly lighter than the Higgs boson. Mono-Higgs signals are often predicted in models that can also be probed in other channels, such as mono-jet production, mono-Z production, or signatures with missing energy and several leptons and/or jets. For a review of missing energy searches at the LHC, we refer the reader to refs. [7, 8] and references therein. A signal of two Higgs bosons and missing energy is naturally predicted in the context of supersymmetry (SUSY), for instance from Goldstino production in models with gauge mediated SUSY breaking [9, 10], or from chain decays of superpartners into Higgs bosons and neutralinos in the minimal supersymmetric standard model (MSSM) and its extensions [11–13]. More generally, di-Higgs plus missing energy is a signature of models with extra scalars [14], such as a pseudo-scalar portal to a dark sector [15], axion-like –1– JHEP04(2019)160 1 Introduction pp → B → AA → (hχ)(hχ). (1.1) In the second model, referred to as resonant topology, each of the pair-produced scalars A decays into either two Higgs bosons or invisibly. The corresponding production chain is pp → B → AA → (hh)(χχ). (1.2) Such a topology is typical in scalar portal models. Since the definition of the two simplified models is based solely on the kinematic properties of the final state, LHC searches for these simple topologies can easily be recasted in terms of concrete models. Our analysis focuses on the Higgs decay into bottom quarks, h → bb̄, which maximizes the event rates. The signal thus consists of four b-jets and a large amount of missing transverse energy. To reconstruct the two Higgs bosons from the four b-jets, we will make ample use of the mature analysis techniques for di-Higgs searches without associated missing energy. Due to its sensitivity to the Higgs self-interaction, Higgs pair production in the SM (see refs. [21, 22] for the latest experimental prospects) and beyond (for a review see e.g. ref. [23] and references therein) is a key target of the LHC program and proposed future colliders [24]. The prospects to observe a signal of Higgs pairs has evolved from “seemingly impossible” [25] to a detailed investigation of the final states bb̄ τ + τ − [26], bb̄ W W ∗ [27] and bb̄ bb̄ [28]. This tremendous progress was triggered by exploiting novel techniques such as jet substructure and shower deconstruction [29–31]. Today these techniques are applied by the ATLAS and CMS collaborations in experimental analyses of Higgs pair production [32, 33]. In our search for hhχχ production with four b-jets and missing energy, we will combine jet substructure techniques with a state-of-the-art multivariate analysis to optimize the sensitivity to our signal. For Higgs pair production in the SM, the channel with four b-jets –2– JHEP04(2019)160 particles [16], massive right-handed neutrinos [11, 17] or in the framework of Little Higgs / T production at the LHC have scenarios [11, 18]. Experimental searches for di-Higgs plus E been performed for a signal of four bottom quarks and missing energy in the context of SUSY [19, 20]. This search targets a signature of Higgsino pair production, followed by a decay chain with Higgs bosons and Goldstinos in the final state [10]. Since the analysis is optimized for very light Goldstinos produced via this specific decay chain, its reinter/T pretation in other scenarios is limited. A systematic exploration of the di-Higgs plus E channel at the LHC is still lacking. Our goal is to provide a minimal, simple framework to exploit the full potential of the / T signature. As a matter of LHC to search for new hidden sectors with a di-Higgs plus E fact, the search strategy for this signature strongly depends on the masses and decays of the relevant part (...truncated)