arXiv:1804.07130v3 [hep-ph] 15 Jul 2019 Higgs boson are very different among the SM, two-Higgs doublet models (2HDM), and MSSM. One of the probes of Higgs self-coupling is Higgs-boson-pair production at the LHC [4][5][6]. There have been a large number of works in literature on Higgs-pair production in the SM [7], in model-independent formalism [8], in models beyond the SM [9], and in SUSY [10]. The predictions for various models are largely different such that the production rates can give valuable information on the self-coupling λ 3H . In the SM, Higgs-pair production receives contributions from both the triangle and box diagrams, which interfere with each other. It is only the triangle diagram that involves the Higgs self-trilinear coupling λ 3H , yet the top-Yukawa coupling appears in both triangle and box diagrams. Therefore, we have to disentangle the triangle diagram from the box diagram in order to probe the Higgs trilinear coupling. In Ref.[11], we pointed out that the triangle diagram, with s-channel Higgs propagator, is more important at low invariant-mass region than the box diagram. Thus, the Higgs-boson pair from the triangle diagram tends to have lower invariant mass, and therefore the opening angle in the decay products of each Higgs boson tends to be larger than that from the box diagram. Indeed, the opening angle separations ∆R γγ and ∆R bb between the decay products of the Higgs-boson pair are very useful variables to disentangle the two sources. However, in Ref. [11] we only assumed some level of signal uncertainties to evaluate the sensitivity to the parameter space of self-coupling λ 3H and the top-Yukawa coupling g S t , without calculating all the other SM backgrounds, e.g., jet-fake backgrounds, single Higgs associated backgrounds, and non-resonant backgrounds.In this work, we perform the most up-to-date comprehensive signal-background analysis for Higgs-pair production through gluon fusion and the HH → bbγγ decay channel. For other production and decay channels and some combined analyses, see Refs. [12]. We simulate the signal and all background processes using simulation tools as sophisticated as what experimentalists use. The signal subprocess is gg → HH → bbγγ with various values for λ 3H . The background includes tt, ttγ, single Higgs associated backgrounds (e.g. ZH, ttH, bbH, ggH followed by H → γγ), and non-resonant or jet-fake backgrounds (e.g. bbγγ, bbjγ, bbjj, jjγγ, etc). We found a set of useful selection cuts to reduce the backgrounds. We express the sensitivity that can be achieved in terms of significance. We find that even for the most promising channel HH → bbγγ at the HL-LHC, the significance is still not high enough to establish the Higgs self-coupling at the SM value, though the self-coupling can be constrained to the range 0 < λ 3H < 7.1 at 95% confidence level (CL) with an integrated luminosity of 3000 fb −1 . Taking account of the uncertainties associated with the top-Yukawa coupling and the estimation of backgrounds, we have found that the 95% CL region broadens i...

Higgs-boson pair production is well known being capable to probe the trilinear self-coupling of the Higgs boson, which is one of the important ingredients of the Higgs sector itself. Pair production then depends on the top-quark Yukawa coupling g S,P t , Higgs trilinear coupling λ 3H , and a possible dim-5 contact-type ttHH coupling g S,P tt , which may appear in some higher representations of the Higgs sector. We take into account the possibility that the top-Yukawa and the ttHH couplings involved can be CP violating. We calculate the cross sections and the interference terms as coefficients of the square or the 4th power of each coupling (g S,P t , λ 3H , g S,P tt ) at various stages of cuts, such that the desired cross section under various cuts can be obtained by simply inputing the couplings. We employ the HH → γγbb decay mode of the Higgs-boson pair to investigate the possibility of disentangle the triangle diagram from the box digram so as to have a clean probe of the trilinear coupling at the LHC. We found that the angular separation between the b andb and that between the two photons is useful. We obtain the sensitivity reach of each pair of couplings at the 14 TeV LHC and the future 100 TeV pp machine. Finally, we also comment on using the bbτ + τ − decay mode in appendix.

Associated production of the Higgs boson with a single top quark proceeds through Feynman diagrams, which are either proportional to the hW W , top-Yukawa, or the bottom-Yukawa couplings. It was shown in literature that the interference between the top-Yukawa and the gauge-Higgs diagrams can be significant, and thus the measurement of the cross sections can help pin down the sign and the size of the top-Yukawa coupling. Here we perform a detailed study with full detector simulations of such a possibility at the LHC-14 within the current allowed range of hW W and top-Yukawa couplings, using h → bb, γγ, τ + τ − , ZZ * → 4 modes. We found that the LHC-14 has the potential to distinguish the size and the sign of the top-Yukawa coupling. Among the channels the h → bb mode provides the best chance to probe the signal, followed by the h → γγ mode, which has the advantage of a narrow reconstructed mass peak. We also pointed out that the spatial separation among the final-state particles has the potential in differentiating among various values of the top-Yukawa coupling.

Motivated by the di-photon resonance recently reported by the ATLAS and CMS collaborations at √ s = 13 TeV, we interpret the resonance as a scalar boson X(750) in hidden-valley-like models.The scalar boson X can mix with the standard model Higgs boson and thus can be produced via gluon fusion. It then decays into a pair of very light hidden particles Y of O(1 GeV), each of which in turn decays to a pair of collimated π 0 's, and these two π 0 's decay into photons which then form photon-jets. A photon-jet (γ-jet) is a special feature that consists of a cluster of collinear photons from the decay of a fast moving light particle (O(1 GeV)). Because these photons inside the photon-jet are so collimated that it cannot be distinguished from a single photon, and so in the final state of the decay of X(750) a pair of photon-jets look like a pair of single photons, which the experimentalists observed and formed the 750 GeV di-photon resonance. Prospects for the LHC Run-2 about other new and testable features are also discussed.

Suppose a heavy neutral Higgs or scalar boson H is discovered at the LHC, it is important to investigate its couplings to the standard model particles as much as possible. Here in this work we attempt to probe the CP-even and CP-odd couplings of the heavy Higgs boson to a pair of top quarks, through the decay H → tt → bW +b W − . We use the helicity-amplitude method to write down the most general form for the angular distributions of the final-state b quarks and W bosons. We figure out that there are 6 types of angular observables and, under CP T conservation, one-dimensional angular distributions can only reveal two of them. Nevertheless, the H couplings to the tt pair can be fully determined by exploiting the one-dimensional angular distributions. A Higgsboson mass of 380 GeV not too far above the tt threshold is illustrated with full details. With a total of 10 4 events of H → tt → bW +b W + , one can determine the couplings up to 10-20% uncertainties.

More evidences have now been collected at the Large Hadron Collider suggesting the new 125∼126 GeV boson is likely the long sought Higgs boson in the standard model. One pressing question continued being asked by theorists is whether this Higgs boson is a lone player responsible for the full electroweak symmetry breaking. Current data still allow room for additional Higgs bosons or some other UV physics that may play a partial role in electroweak symmetry breaking as well. We use the W W scattering to investigate such a possibility, using the two-Higgs-doublet model as a prototype. The W W scattering becomes strong when the extra Higgs bosons are very heavy. We study the sensitivity of the partially strong W W scattering signals at the 13 TeV Large Hadron Collider.

Higgs bosons pair production is well known for its sensitivity to probing the sign and size of Higgs boson self coupling, providing a way to determine whether there is an extended Higgs sector. The Georgi-Machacek (GM) model extends the Standard Model (SM) with an SU(2) L triplet scalar field that has one real and one complex components. The Higgs self coupling now has a wider range than that in the SM, with even the possibility of a sign flip. The new heavy singlet Higgs boson H 0 1 can contribute to s-channel production of the hh pairs. In this work, we study non-resonant/resonant Higgs boson pair productions pp → hh and pp → H 0 1 → hh, focusing exclusively on the contribution of H 0 1 . We show the sensitivity for Higgs boson pair production searches at the 13-TeV LHC with the luminosities of 3.2, 30 and 100 fb −1 .

A muon-jet (µ-jet) is a very special feature that consists of a cluster of collimated muons from the decay of a fast moving light particle of mass about O(1 GeV). We will use this feature to search for very light particles from rare decays of the Higgs boson. For such a small angular separation of muons which might come from a long-lived particle, both ATLAS and CMS could have the displaced-vertexing-reconstruction capability. We use two simple models of the Higgs-portal type to explore the possibilities of event topologies with two 2µ-jets, one 2µ-jet & one 4µ-jet, and two 4µ-jets in the final state at LHC-14. We also summarize existing constraints on these models.