Research Article | Open Access
Qiurong Mou, Sibo Zheng, "A Heavy Scalar at the LHC from Vector-Boson Fusion", Advances in High Energy Physics, vol. 2018, Article ID 9314613, 6 pages, 2018. https://doi.org/10.1155/2018/9314613
A Heavy Scalar at the LHC from Vector-Boson Fusion
A hypothetical scalar mixed with the standard model Higgs appears in few contexts of new physics. This study addresses the question what mass range is in the reach of TeV LHC given different magnitudes of mixing angle , where event simulations are based on production from vector-boson fusion channel and decays into SM leptons through or . It indicates that heavy scalar mass up to GeV and GeV can be excluded by integrated luminosity of 300 and 3000 , respectively, for larger than .
After the discovery of Higgs scalar at the LHC [1, 2], Standard Model (SM) as the effective field theory (EFT) of weak scale is established. While this EFT has not been violated at nowadays astrophysical and collider experiments, it must be incomplete in the light of a few indirectly experimental as well as theoretic hints. One of most robustly experimental hints arises from Plank and WIMP data , which suggests that there should be a new particle beyond SM serving as the thermal dark matter. On the other hand, one of theoretic challenges is the need of some novel mechanism to stabilize the divergence involving Higgs mass.
In a few new physics models attempted to complete the EFT of SM such as SM with doublets and supersymmetry, there usually exists a new scalar of the same spin, parity, and quantum numbers with SM Higgs but with heavier mass. Unless forbidden by some hidden symmetry, it generally mixes with the SM Higgs. If so, such scalar may leave signatures at dark matter facilities which are in the reach of TeV mass scale. See, e.g., [4–6] for very recent studies on this subject.
Alternatively, can mix with the SM Higgs and be examined at the LHC. Due to mixing effect couplings to SM particles are similar to those of SM Higgs but with an universal scaling factor related to mixing angle smaller than unity. As a result, the diboson decay channels with referring to or boson dominate others for mass above GeV. In this case, is mainly generated at the LHC through gluon-gluon fusion (GGF) and vector-boson fusion (VBF) channels similar to the SM Higgs . Early constraints [8–10] on the model parameters were obtained according to measurements on SM Higgs couplings, decay width, and direct detection at the LHC. Updated analyses based on the TeV LHC data [11–17] can be found in [18–23].
In this paper, we will employ the techniques reported in  and study the prospect for the discovery of at the TeV LHC through processes of VBF production and subsequent diboson decays to SM leptons final states (The analysis here is more general than in the earlier version, which focused on the model interpretations of diboson excess reported in .). One reason is that although the GGF channel yields larger contribution to the production cross section than the VBF channel, the contribution to SM background cross section arising from GGF process is also larger than VBF process. Moreover, the ratio between GGF and VBF contribution to the production cross section declines from about ~10 to ~2.5 when increases from GeV to TeV.
The paper is organized as follows. In Section 2, we briefly discuss general parameterization of mixing effect in a model-independent way. The key point is that only two model parameters appear in the following study of direct detection. In Section 3, we address the production cross sections from VBF channel at the LHC for mass above GeV. Our main results are presented in Section 4, where we show the luminosities required for the exclusion and discovery. Finally we conclude in Section 5.
Without mixing effect such as in the case of scalar dark matter model, the mass squared matrix for state vector reads as where and denote the mass of and , respectively. There is little chance for direct detection on this kind of scalar at the LHC . In contrast, in the case of mixing effect mass squared matrix in (1) should be replaced with where characterizes the mixing effect.
At present status only small mixing effect is allowed based on the LHC searches such as dijet, diphoton, and four-lepton signals. The mass eigenvalues can be approximated to be (Note that the analytic rather than approximations here will be utilized for the numerical calculation in the next section.) together with their couplings to SM particles relative to SM Higgs Here, refers to the SM vector bosons and fermions, and the mixing angle is given by From (3) to (5) one finds that the productions and decays of these two scalars are totally determined by heavier mass and mixing angle after identifying as the SM-like Higgs. The magnitude of has been up bounded to be less than ~0.2 at CL in the light of precise measurement [9, 10] on the SM Higgs couplings at the TeV LHC, and it will be improved to be of order ~0.04 at the future TeV LHC with designed integrated luminosity .
3. Vector-Boson Fusion
In this section, we address event simulation for the production cross section from VBF channel and branching ratios at the TeV LHC. In particular, we use package FeynRules  to generate model files prepared for MadGraph5 , which includes Pythia 6  for parton showering and hadronization and the package Delphes 3  for fast detector simulation.
3.1. Production Cross Section
We show in Figure 1 the strengths of cross sections for two different four-lepton final states, where the dependence on the mixing angle can be understood as follows. Firstly, according to (4), the VBF induced cross section is proportional to . Secondly, with the definition on branching ratios , where ( is a SM fermion or vector boson), depends on the magnitude of relative to . Unlike which is determined by the mixing effects in quadratic term of scalar potential, is directly related to the cubic term in the scalar potential, which is model-dependent. For example, in the minimal supersymmetric standard model, the ratio is small for above GeV , which implies that mildly depends on the mixing angle. In contrast, can be important in models such as extended Higgs doublet models, where will be related to parameters such as mixing angle and quadratic and cubic terms in the scalar potential. For simplicity, we consider the case in which can be ignored.
The parameter space composed of mixing angle and heavy scalar mass is subject to both direct and indirect constraints. Current direct constraints include the TeV LHC bounds such as fb [32, 33] and fb  in the mass region below GeV, as well as  and  in the mass region above GeV. For illustration, we have shown in Figure 2 direct constraint on mixing angle in low mass region. On the other hand, indirect constraints include precision measures on the Yukawa couplings of SM Higgs to SM fermions and vector bosons. Global fits such as in  imply that above has been excluded. Other indirect constrains arising from measurements on precision electroweak observables may also be useful to constraint the mixing angle. In this sense, the constraint on mixing angle from indirect detection is much stronger than that from direct detection.
3.2. Events Selection
Let us now stress the event selections for the two different SM lepton final states from VBF channel. The primary SM backgrounds to the first process include dilepton + jets and QCD multijets. For simplicity, we consider the main contributions arising from dilepton plus jets channels and adopt the cuts used by the CMS VBF analysis  for event selection: where and are the transverse momentum of the first (second) leading lepton and jet, respectively; is pseudorapidity of (); and is the rapidity difference and invariant mass of the two leading jets (leptons), respectively. Parameter is defined as with referring to the azimuthal angle between the dilepton transverse momentum and . Any event with an additional jet with GeV is rejected. We refer the reader to  for more details.
For the second process we consider the main contributions arising from electron pair + jets + and muon pair + jets + . Cuts  for event selection in this channel are given by where denotes the of the dilepton system. Any event which includes the third lepton with GeV is rejected in order to suppress SM background. More details can be also found in .
Cuts in (6) and (8) will be applied to the 14 TeV LHC simulations for conservation, the validity of which is guaranteed by the following facts. At first, there is little difference between the 8 TeV LHC and 14 TeV LHC except the collision energy, which means the cut on the pseudorapidity of the first two leading jets should remain unchanged. Second, the kinetic distribution of the signal events and the main SM backgrounds have similar changing trends when one modifies these cuts. Take the representative mass GeV for example. The effects on the ratio of signal over background events are less than two times due to variations on the cuts in (6) and (8). See Table 1 for details.
Following the definition and about significance for exclusion and discovery, respectively, we show the values of needed for exclusion and discovery at the 14 TeV LHC in Figure 3. Systematic uncertainties are neglected in both the signal and the background simulations.
The left plot therein addresses the decay . In the left one, we observe that for mass up to GeV can be excluded via final state for , respectively; and the discovery limit approaches to GeV for . Furthermore, for HL-LHC with , mass up to GeV can be excluded via this channel for , respectively; and the discovery limits reach GeV for , respectively.
The right plot in Figure 3 addresses the decay . It shows that for mass up to GeV can be excluded through final state for . Moreover, for the HL-LHC mass up to GeV can be excluded via the same channel for , respectively; and the discovery limit reaches GeV for . The exclusion limits via the decay are relatively weaker in comparison with the decay.
Hypothetical scalar similar to the SM Higgs often appears in a complete model of quantum field theory. This work is devoted to study a heavy scalar mixed with SM Higgs at the TeV LHC through VBF channel. We have simulated events arising from diboson decays such as and , where both exclusion and discovery limits are revealed according to different magnitudes of mixing effect. Our study demonstrates that such type of heavy scalar with mass up to GeV and GeV can be excluded by the TeV LHC with and , respectively, for larger than .
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors do not have a direct financial relation with any commercial identity mentioned in the paper that might lead to conflicts of interest for any of the authors.
This work is supported in part by the National Natural Science Foundation of China under Grant no. 11775039.
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Copyright © 2018 Qiurong Mou and Sibo Zheng. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP3.