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Theoretical Physics Letters
2023 ° 03(10) ° 0631-4506
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We would like to thank Sam Harper for his invaluable input and discussions around the trigger analysis. SM is supported in part through the NExT Institute and the STFC Consolidated Grant ST/L000296/1. CHS-T(SS) is supported in part(full) through the NExT Institute. SS acknowledges the use of the IRIDIS High Performance Computing Facility, and associated support services at the University of Southampton, in the completion of this work. YW’s work is supported by the Natural Science Foundation of China Grant No. 12275143, the Inner Mongolia Science Foundation Grant No. 2020BS01013 and the Fundamental Research Funds for the Inner Mongolia Normal University Grant No. 2022JBQN080. QSY is supported by the Natural Science Foundation of China under the Grants No. 11875260 and No. 12275143.
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Unlike other realisations of the 2-Higgs Doublet Model (2HDM), the so-called Type-I allows for a very light Higgs boson spectrum. Specifically, herein, the heaviest of the two CP-even neutral Higgs states, H, can be the one discovered at the Large Hadron Collider (LHC) in 2012, with a mass of ≈ 125 GeV and couplings consistent with those predicted by the Standard Model (SM). In such a condition of the model, referred to as ‘inverted mass hierarchy’, the decay of the SM-like Higgs state into pairs of the lightest CP-even neutral Higgs boson, h, is possible, for masses of the latter ranging from MH/2 ≈ 65 GeV down to 15 GeV or so, all compatible with experimental constraints. In this paper, we investigate the scope of the LHC in accessing the process gg → H → hh → b ¯bτ τ by performing a Monte Carlo (MC) analysis aimed at extracting this signal from the SM backgrounds, in presence of a dedicated trigger choice and kinematic selection. We prove that some sensitivity to such a channel exists already at Run 3 of the LHC while the High-Luminosity LHC (HL-LHC) will be able to either confirm or disprove this theoretical scenario over sizable regions of its parameter space.
In the SM of particle physics, it is well known that the Higgs boson [1,2] is responsible for the generation of fermion and gauge boson masses through what is called Spontaneous Symmetry Breaking (SSB) [3, 4]. Such a mechanism also predicts a self-interaction for the Higgs state. The measurement of such a self-coupling is the only experimental way to understand the SSB mechanism and to reconstruct the Higgs potential responsible for it. This is an important (and challenging) task also because it can shed some light on possible Beyond the SM (BSM) effects that may affect Higgs self-couplings in general. The LHC has started a new campaign of measurements after the recent upgrade, the socalled Run 3. This will involve, among other things, measuring ever more precisely the coupling of the SM-like Higgs boson to other SM particles or even progressing towards the measurement of its self-coupling. The LHC is also capable of measuring new decays of the SM-like Higgs boson into non-SM particles. Current results from the ATLAS and CMS experiments indicate that the measured SM-like Higgs signal rates in all channels agree well with the SM theoretical predictions at the ∼ 2σ level [5, 6]. However, there are several pieces of evidences, both theoretical (the hierarchy problem, the absence of gauge coupling unification, etc.) and experimental (neutrino masses, the matter-antimatter asymmetry, etc.), which indicate that the SM could not be the ultimate description of Nature but should be viewed as a low-energy effective theory of some more fundamental one yet to be discovered. There exist several BSM theories that address these weaknesses of the SM while identifying the 125 GeV scalar particle as a part of an extended scalar sector. One of the simplest extensions of the SM is the 2HDM, which contains two Higgs doublets, Φ1 and Φ2, which give masses to all fermions and gauge bosons. The particle spectrum of the 2HDM is as follows: two CP-even (h and H, with mh < mH, one of them being identified with the SM-like Higgs boson with mass 125 GeV: H in our case), one CP-odd (A) and a pair of charged (H±) Higgs bosons. According to the latest experimental results from both ATLAS and CMS, the presence of non-SM decay modes of the SM-like Higgs boson is not completely ruled out. Both experiments have set upper limits on the Branching Ratio (BR) of such non-SM decays which are 12% for ATLAS [5] and 16% for CMS [6]. The LHC experiments are expected to soon constrain the BRs of such non-SM decays beyond the 5-10% level using indirect measurements [7,8]. There exist several BSM models that possess such non-SM decays of the SM-like Higgs boson: non-minimal scenarios of Supersymmetry [9] such as the Next-to-Minimal Supersymmetric Standard Model and new Monimal Supersymmetric Standard Model (NMSSM/mMSSM) [10–13], models for Dark Matter (DM) [14–17], scenarios with first order Electro-Weak (EW) phase transitions [18,19] and an extended Higgs sector [20,21]. It is then crucial to use LHC Higgs measurements to test BSM models that predict such exotic SM-like Higgs decays (i.e., into non-SM particles). In the 2HDM, if the heavy CP-even H is the observed SM-like Higgs boson, then H can decay into a pair of light CP-even Higgs states, H → hh, or CP-odd ones, H → AA. The phenomenology of such decays of the observed SM-like Higgs boson is studied in Refs. [22–25] for the case of the 2HDM, with an emphasis on the so-called Type-I (see below).
The Type-I is an intriguing realisation of the 2HDM as it allows for the so-called inverted mass hierarchy scenario, wherein the Higgs boson discovered at the LHC on 4 July 2012 can be identified as the heaviest CP-even Higgs state of this construct, H, with a mass of 125 GeV or so and couplings to fermions and gauge bosons similar to those predicted in the SM. Such a configuration specifically implies that there is then a lighter CP-even Higgs state, h, into pairs of which the heavy one can decay: i.e., via H → hh. Needless to say, this can be realised without contradicting any of the theoretical requirements of self-consistency of the 2HDM or current experimental results, whether coming for measurements of the discovered Higgs boson or null searches for companions to it. In fact, the latter have primarily been concentrating on other realisations of the 2HDM, where only the standard mass hierarchy is actually possible (i.e., mh ≈ 125 GeV < mH), thereby altogether missing out on the possibility of optimising searches for very light neutral Higgs states in general. Specifically, here, by looking for H → hh signals in the 2HDM Type-I, we have concentrated on the following mass range: 15 GeV < mh < mH/2 The production of the heavy CP-even Higgs state (the SM-like Higgs boson) at the LHC was pursued via gluon-gluon fusion, gg → H, indeed, the dominant channel, while we have focused on the hh → b ¯bτ τ decay pattern, where the two heavy leptons where tagged through their (different flavour) electron and muon decays. By performing a sophisticated MC analysis of signal versus background, we have shown that both Run 3 of the CERN machine and its HLLHC phase can offer sensitivity to this 2HDM Type-I signal, in the presence of very low mass trigger thresholds (on the electrons and muons) already implemented for Run 3 and also possible at the HL-LHC. We have done so by adopting several BPs capturing representative mh values over the aforementioned interval after a fine scanning of the whole 2HDM Type-I parameter space, of which they are therefore representative examples amenable to further scrutiny by the LHC collaborations. Finally notice that, if the collision energy of the LHC increases from 13 TeV to 14 TeV, the production rate of the signal process gg → H can increase by 10%, as 16 shown in [65] (with the dominant backgrounds, tt¯ and Zb¯b, scaling similarly or less), lending further scope to our analysis in the near future.
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