Signs of heavy Higgs bosons at CLIC:
An road to the Electroweak Phase Transition
We analyse the sensitivity of the proposed Compact Linear Collider (CLIC) to the existence of beyond the Standard Model (SM) Higgs bosons through their decays into pairs of massive gauge bosons and SM-like Higgses , considering CLIC centre of mass energies TeV and TeV. We find that resonant di-Higgs searches at CLIC would allow for up to two orders of magnitude improvement w.r.t. the sensitivity achievable by HL-LHC in the mass range . Focusing then on a real singlet extension of the SM, we explore the prospects of heavy Higgs searches at CLIC for probing the regions of parameter space yielding a strongly first order electroweak phase transition that could generate the observed matter-antimatter asymmetry of the Universe. Our study illustrates the complementarity between CLIC and other possible future colliders like FCC-ee in probing singlet extensions of the SM, and shows that high-energy colliders provide a powerful means to unravel the nature of electroweak symmetry breaking in the early Universe.
A key goal of the present and future collider physics programme is to reveal the structure of the (scalar) sector responsible for electroweak symmetry breaking (EWSB) in Nature. While ongoing ATLAS and CMS analyses at the Large Hadron Collider (LHC) show that the properties of the discovered Higgs particle are close to those expected for the Standard Model (SM) Higgs boson Khachatryan:2016vau; ATLAS:2017ovn; CMS:2018lkl, it still needs to be determined whether the scalar sector is realised in its most minimal form, i.e. consisting of one doublet, or has a richer structure, containing additional states. Non-minimal scalar sectors are very well-motivated, arising naturally in the context of weakly coupled completions of the SM that address the hierarchy problem.At the same time, extensions of the SM scalar sector could provide the means to address a key open question at the interface of particle physics and cosmology, namely the generation of the cosmic matter-antimatter asymmetry, via electroweak (EW) baryogenesis Morrissey:2012db.
Among the proposed future collider experiments, the Compact Linear Collider (CLIC) would be a multi-TeV collider Aicheler:2012bya; CLIC:2016zwp, combining the high-energy reach with the clean collision environment of an electron-positron machine. CLIC would operate in three energy stages, corresponding to centre of mass (c.o.m.) energies GeV, TeV, TeV, providing an ideal setup to study the properties of the Higgs sector. In this respect, very sensitive direct probes of the existence of new, heavier Higgs bosons, possible with TeV and TeV c.o.m. energy configurations, are highly complementary to precise measurements of the properties of the 125 GeV Higgs boson, and may yield the dominant probe of a non-standard Higgs sector.
In this work we analyse the reach of CLIC in searching for heavy Higgs bosons which decay to a pair of massive gauge bosons or a pair of 125 GeV Higgs bosons. This allows to assess the direct sensitivity of CLIC to non-minimal Higgs sectors, and to compare it with that of the HL-LHC, providing at the same time a benchmark for sensitivity comparison with other possible future high-energy collider facilities like FCC(-ee and -hh). In addition, we assess the capability of CLIC heavy Higgs searches in probing the nature of the EW phase transition in the context of a general real singlet scalar extension of the SM Profumo:2007wc; Barger:2007im; Espinosa:2011ax. This scenario can capture the phenomenology of the Higgs sector in more complete theories beyond the SM such as the NMSSM (see Ellwanger:2009dp and references therein) or Twin Higgs theories Chacko:2005pe. At the same time, the singlet scalar extension of the SM constitutes a paradigm for achieving a strongly first order EW phase transition that could generate the observed matter-antimatter asymmetry of the Universe.
The paper is organised as follows: In Section 2 we discuss the main aspects of Higgs production at CLIC, as well as the various computational tools we use for our analysis. In Section 3 we assess the CLIC sensitivity in direct searches of heavy scalars decaying into EW gauge boson pairs. In Section LABEL:subsec:hh we focus instead on heavy scalar decays into a pair of 125 GeV Higgses. In Section LABEL:sec:xSM we discuss the implications of these results for a singlet scalar extension of the SM, and the possibility of exploring the nature of the EW phase transition in this scenario via direct scalar searches at CLIC. Finally we conclude in Section LABEL:conclusionsNS.
2 Heavy Higgs boson production at the Compact Linear Collider
The three dominant processes contributing to Higgs boson production at a high-energy electron-positron collider are , and (see e.g. Figure 1). Assuming a heavy scalar with SM-like properties, we compute the production cross section111For , the outgoing electrons are required to satisfy , GeV. as a function of the scalar mass for each of the three processes and for , , TeV, shown in Figure 2. We show both the case of unpolarized electron and positron beams (solid lines) and the possibility of using beam polarization, which can constitute a strong advantage in searching for new physics MoortgatPick:2005cw, assuming for definiteness an electron-positron beam polarization (dashed lines)222Here, corresponds to a fully left-handed polarized beam and to a fully right-handed polarized beam. in the ballpark of the expected CLIC operation setup.
As highlighted in Figure 2, the dominant Higgs production mechanism for both and TeV is the vector boson fusion (VBF) process . We also emphasize that the setup GeV does not allow to probe high values of , and moreover it does not yield as many kinematical handles to disentangle the heavy scalar signal from SM backgrounds. In the rest of the paper we then focus on as Higgs production mechanism in CLIC, considering and TeV as c.o.m. energies. The respective projected integrated luminosities we consider are 1500 fb and 2000 fb CLIC:2016zwp. In all our subsequent analyses, we simulate CLIC production of the new scalar via using MadgraphaMC@NLO Alwall:2014hca with a subsequent decay into the relevant final state, and assuming electron and positron polarized beams with in all our analyses. We then shower/hadronise our events with Pythia 8.2 Sjostrand:2014zea and use Delphes deFavereau:2013fsa for a simulation of the detector performance with the Delphes Tune for CLIC studies UlrikeGitHub; AlipourTehrani:2254048 (see also Potter:2016pgp).
3 Searching for heavy scalars in final states with TeV
We examine here the CLIC potential to search for new scalars via decays into EW gauge bosons (). We focus on leptonic final states in Section LABEL:Sec4l and in Section LABEL:Sec2l2nu, and leave hadronic final states (requiring a more involved analysis, but being very promising due to the large branching fraction and the clean environment of CLIC) for a future analysis. We restrict our analysis to a CLIC c.o.m. energy TeV for our studies, as our results will show that the projected sensitivity for TeV would not be competitive with that of HL-LHC. In addition, for the final state analysis of Section LABEL:Sec2l2nu, we focus on the signal decay channel: we have found that the projected sensitivity of this channel is significantly larger than the one that can be achieved for the signal channel, and thus disregard the latter in the following.