Beyond-the-Standard Model Higgs Physics using the ATLAS Experiment

. Recent searches for the Higgs boson in the context of beyond the Standard Model performed by the ATLAS experiment are presented: high mass Higgs boson searches, lepton ﬂavour violating Higgs decay, NMSSM, constraint from the search for three photons. The interpretation based on the measurements of Higgs couplings is shown, along with the constraint on the Higgs boson invisible decays. The search for invisible decays of a Higgs boson produced in association with a Z boson was performed with both √ s = 7 and 8 TeV while the rest were performed using the √ s = 8 TeV data of proton-proton collisions collected by the ATLAS experiment. No signiﬁcant excess of data over the predicted background is observed and limits are placed in certain quantities depending on the searches. − 12 GeV), Minimal Composite Higgs models ( f > 710(780) GeV MCHM4(5)), Two Higgs Doublet Models (Alignment limit within 1 σ ), Higgs to invisible decays ( BR inv < 0 . 23) and subsequent constraint on Higgs-portal dark matter. Much better results with the new data from the √ s = 13 TeV run at the LHC are expected.


Introduction
The ATLAS experiment at the LHC discovered the Higgs boson in July 2012. So far, there have been many measurements on its couplings and mass [1][2][3][4][5], the results are consistent with the Standard Model (SM) predictions. Nevertheless, these measurements have not yet excluded a large range of other extensions of the SM.
Exploring whether there are additional Higgs bosons or exotic decay of the SM Higgs boson can give us direct evidences about physics beyond the Standard Model (BSM). Those include the two Higgs doublet models (2HDMs) [6-9], next-to-minimal superymmetric SM (NMSSM) [10,11], composite Higgs models [12,13], which favour the existence of other Higgs bosons in the high-mass regime, as well as lepton flavour violating (LFV) Higgs decay, three photons Higgs decay. There are strong evidences of dark matter from astrophysical observations which could be explained by the existence of weakly interacting massive particles (WIMPs, see Ref. [14] and the references therein). The observed Higgs boson might decay to dark matter or other stable or long-lived particles which do not interact significantly with a detector [15][16][17][18][19] leading to Higgs boson invisible decay.
The report is organized as follows: Sec. 2 describes the high mass-Higgs boson searches, Sec. 3 describes the LFV Higgs decay search, Sec. 4 describes the search for Higgs bosons in the context of NMSSM, Sec. 5 describes the search for a Higgs boson decaying to three photons or more, Sec. 6 summarizes some constraints on BSM models using Higgs boson couplings and mass measurements, Sec. 7 describes the combination of the searches for invisible decays of the Higgs boson. Finally concluding remarks are in Sec. 8. Details about the ATLAS detector can be found in Ref. [20].

NMSSM
The NMSSM contains an additional pseudoscalar Higgs boson (a), generally assumed to have a mass lower than the observed Higgs boson (h) since its mass is protected by a Peccei-Quinn symmetry. A search for the decay to a pair of the lightest neutral pseudoscalar Higgs a of either the 125 GeV Higgs (h) or a second CP-even Higgs (H) was performed [25], where one a boson decays to 2 μ and the other decays to 2 τ . As a result, the most stringent upper limit on the branching ratio of the h boson decaying to the non-SM particles was set at 3.5% for m a = 3.75 GeV (see Fig. 3).

Search for 3 photons
A search for events with at least 3 γ was done [26]. The model-independent interpretations are the first of their kind. One of the interpretations was performed for a SM Higgs boson (h) decaying to four photons via a pair of intermediate pseudoscalar particles (a). Limits on the cross section times BR(h → aa) × BR(a → γγ) 2 was set to be < 10 −3 σ SM for 10 GeV < m a < 62 GeV (see Fig. 4).

Beyond the Standard Model constraints via Higgs boson couplings
BSM constraints [27] were performed using the measured production and decay rates of the Higgs boson (γγ, ZZ, W W , Zγ, bb, τ τ , & μμ; tth with h → γγ, bb & multileptons). The constraints include the probe on the scaling of the couplings with mass, setting limits on parameters in extensions of the SM such as composite Higgs boson, 2HDMs. They are described in details below.

Mass scaling of couplings
In this analysis, the observed rates in different channels were used in a fit to determine how the Higgs boson couplings to other particles scale with the masses of those particles [28]. Each coupling (with fermion f and boson V) was scaled in terms of "vacuum expectation value (vev)" M and mass scaling parameter (in the SM vev v ≈ 246 GeV and → 0) as where m is the mass of the particle. As a result of the fit, the best fit value is consistent within one standard deviation with the SM (see Fig. 5

Minimal composite Higgs model
Minimal Composite Higgs Models (MCHM) [12,13] provides a possible explanation for the scalar naturalness problem. It suggests that the Higgs boson is a composite, pseudo-Nambu-Goldstone boson rather than an elementary particle. Higgs couplings are modified as functions of compositeness scale-f: ξ = v 2 /f 2 . There are different MCHM models with different modifications of the Higgs coupling such as MCHM4 [12] with The SM is recovered in the limit ξ → 0, namely f → ∞. A fit was performed in the parameter space of [κ F , κ V ] (see Fig. 6). As a result, limits at 95% CL are obtained in the order of obs (exp), for MCHM4: f > 710 (510) GeV, MCHM5: f > 780 (600) GeV.

Two Higgs Doublet Models
2HDMs models introduce two complex SU (2) cos(α)/ sin(β) − sin(α)/ cos(β) − sin(α)/ cos(β) cos(α)/ sin(β) couplings of the two neutral, CP-even Higgs bosons to vector bosons relative to their SM values to be: with the convention: sin(β − α) ≥ 0. SM-like alignment limit is retrieved at cos(β − α) = 0. Figure 7 shows the regions of the [cos(β − α), tan β] plane that are excluded at 95% CL for each of the four types of 2HDMs, overlaid with the exclusion limits expected for the SM Higgs sector. A physical boundary of κ V ≤ 1 is there in all four 2HDM types and restricts the profile likelihood ratio. The data are consistent with the alignment limit within approximately one standard deviation or better in each of the model.

Higgs invisible decays
Limits on the branching ratio of Higgs boson invisible decay [27] are set and subsequently used to constrain on a Higgs-portal dark matter model [29].

The combination of Higgs invisible decay channels
Direct searches for the SM Higgs boson h decaying into invisible particles used the following strategies: selecting events with large missing transverse momentum (E miss T ), using particles produced associated with the Higgs boson to tag the Higgs, assuming productions (and acceptance) as in the SM where BR(h → ZZ → 4ν) = 1.2 × 10 −3 . The SM branching ratio is very little so any observation would be an indication of BSM physics. They include the analyses: , where the first analysis contains the 7 TeV data. Table 4 shows upper limits on the BR(h → inv) at 95% CL and other CL for different analyses and their statistical combination. The combination of the invisible channels only resulted in BR(h → inv) < 0.25 obs (0.27 exp). Combined result was then combined statistically with the measured visible decay rates of the Higgs boson. A physical boundary of BR inv > 0 was required. The most general result uses independent coupling parameters κ W , κ Z , κ t , κ b , κ τ , κ μ , κ g , κ γ , κ Zγ and BR inv . Upper limit of 0.23(0.24) obs(exp.) were set at 95% CL. The likelihood scan for that statistical fit is shown in Fig. 8.

Higgs Portal Interpretation
The 90%CL upper limit of 0.22 (0.23) obs (exp) on BR(H → inv) was used to constrain on a Higgs-portal dark matter scenario [29], where the Higgs boson is the only mediator for interactions between WIMPs and other SM particles. It is sensitive for WIMP's mass < m h /2. Limits are set on the cross section between WIMPs and nucleons as a function of the WIMP mass, as shown in Fig. 9. The form factor (Higgs-nucleon coupling) f N is taken to be 0.33 +0. 30 −0.07 [29].

Conclusion
The ATLAS collaboration has performed many searches for BSM Higgs bosons using the 7 TeV and 8 TeV datasets: high mass Higgs search, LFV Higgs decay, Higgs decays to pseudoscalar particles in the context of NMSSM, Higgs decays to four photons via pseudoscalar particles, Higgs decay into invisible particles. No significant excess for BSM Higgs physics has been observed yet. Precise measurements of the Higgs boson couplings allow to constrain new phenomena: mass scaling ( : 0.018 ± 0.039, M : 224 + 14 − 12 GeV), Minimal Composite Higgs models (f > 710(780) GeV MCHM4(5)), Two Higgs Doublet Models (Alignment limit within 1σ), Higgs to invisible decays (BR inv < 0.23) and subsequent constraint on Higgsportal dark matter. Much better results with the new data from the √ s = 13 TeV run at the LHC are expected.