h → 2b 2μ


Contact Person(s)

David Curtin, Rouven Essig, Ze'ev Surujon, and Yi-Ming Zhong
More details on this mode may be found in Section 5 of Survey of Exotic Higgs Decays (arXiv:1312.4992).

Theoretical Motivation

The possibility of the Higgs boson decaying to (bb)(μ+μ) is intriguing. In the context of NMSSM and 2HDM+S models it represents a compromise between the very difficult but often dominant 4b mode (h→ 4b) and the spectacular but rare 4μ signature. Below we present the theoretical motivation to consider this decay mode and demonstrate the reach of a dedicated search at both Run 1 and 2 of the LHC. A detailed study will appear in [1].
The h→(bb)(μ+μ) decay mode occurs when the Higgs field couples to one or more bosons a that couple to b quarks and muons, with at least one a heavy enough to decay to bb. As discussed in SM+Scalar, the simplest realization of such a scenario is given by extending the SM to include an additional real singlet scalar. However, searching for this mode is motivated in any model with additional singlets that couple to quarks in proportion to their masses.1 This includes the 2HDM+Scalar and the well-known NMSSM.
The small coupling to muons leads to very hierarchical branching ratios,                                                                      with ε ≡ Br(a→μ+μ)/Br(abb) ∼ mμ2/3mb2 ≈ 2×10−4 in the SM+S. (Non-minimal scalar models can modify this ratio, but the ratio is in general very small.) Assuming SM Higgs production and Br(haa) = 10% leads to zero h→ 2a → 4μ events from gluon fusion at LHC Run 1, while about twenty h → 2a → 2b2μ events are expected to occur. Even though this is much less than the few hundred h→ 2a → 4b events expected from associated production, the backgrounds for the 4b search are so challenging (see h→ 4b) that the 2b2μ channel may provide much better sensitivity. This is even more attractive in non-minimal models, where e.g. tanβ can enhance the leptonic pseudoscalar branching fraction significantly (such as Type-III 2HDM). It is also possible that the Higgs decays to two pseudo scalars, ha1 a2, which have large branching fractions to 2b and 2μ, respectively. The presence of a clean dimuon resonance makes the 2b2μ decay mode very attractive for discovering SM extensions with extra singlets.

Existing Collider Studies and Experimental Searches

To the best our knowledge there have been no theoretical collider studies of this final state, and there are no limits on this decay channel from existing searches. A similar topology is searched for in hbb from associated production with a Z boson, where the Z decays to μ+μ. However, this search is not relevant for (2b)(2μ), since the required bb invariant mass was O(125 GeV), and the two muons were required to reconstruct the Z-boson. A dedicated search is therefore needed for this channel.

Proposals for New Searchs at the LHC

We estimate the discovery potential of a very simple search for h→2a → 2b 2μ with Run 1 LHC data as well as 100  fb−1 at 14 TeV. This preliminary study is simulated at parton-level for signal and backgrounds (see [1] for a more complete study).
LHC 7 and 8 TeV
We assume the Higgs is produced through gluon fusion and has a non-zero branching ratio as haa → (bb)(μ+ μ). We do not include Higgs bosons produced through VBF in our analysis, although this would slightly increase the sensitivity to this channel. The final state consists of two opposite-sign muons and two b-tagged jets and is simulated for mh = 125  GeV and ma ∈ (15, 60)  GeV. The main background is Drell-Yan (DY) production with associated jets, Z* + 2j/2c/2b, where the Z-decay/γ* produces two muons. In this preliminary estimate, we neglect backgrounds arising from lepton-misidentification of jets, diboson production VV, and tt production, which are expected to be subdominant to DY. (The tt background has a total cross section comparable to DY + jets but does not contribute significantly in the low dimuon invariant mass region [2,3], and also typically produces a sizable amount of MET that is not present for the signal.)
Both signal and background are simulated to lowest order at parton-level in MadGraph 5. The signal is renormalized by the NLO gluon-fusion cross section σggF=19.3 pb [4].
Figure 1: Expected 95% C.L. sensitivity to Br(h→ aa → bb μ+ μ) for 25 fb−1 data at 8 TeV LHC. The solid, dashed, and dotted lines show the limits for at least one b-tag, two b-tags, and no b-tag, respectively.
We demonstrate 95% C.L. sensitivity of Br(haabb μ+ μ) with respect to ma in Fig. 1. For ma ≤ 25  GeV, the bb from a-decay are collimated enough to fail our simple reconstruction cuts. A more sophisticated substructure analysis is required in this regime [1]. For details of the analysis and upper limits on the Br(haa) in the NMSSM see arXiv:1312.4992.
LHC 14 TeV
We repeat the study with identical cuts for 100 fb−1 of data at the 14 TeV LHC. The gluon fusion NLO Higgs production cross section is σggF=49.85  pb [4]. 
Figure 2: Expected 95% C.L. sensitivity to Br(h→ aa → bb μ+ μ) for 100 fb−1 of data at 14 TeV LHC. The solid, dashed, and dotted lines show the limits for at least one b-tag, two b-tags, and no b-tag respectively.

The expected 95% C.L. sensitivity of the 14 TeV LHC is shown in Fig. 2. For details of the analysis and upper limits on the Br(haa) in the NMSSM see arXiv:1312.4992.
Our simple parton-level study demonstrates that  ∼ 10−4 − 10−3 sensitivity to Br(h→ 2a → 2b2μ) is possible at the LHC. We will investigate this channel more closely in [1], but these preliminary results already strongly suggest conducting a corresponding search with available Run 1 data.


[1]D. Curtin, R. Essig, Z. Surujon, and Y.-M. Zhong, to appear.
[2]CMS Collaboration, S. Chatrchyan et. al., Measurement of the Drell-Yan Cross Section in pp Collisions at √s=7 TeV, JHEP 1110 (2011) 007, [arXiv:1108.0566].
[3]S. Gonzalez, E. Ros, and M. Vos, Analysis of the process pp → b b h/Ab b μμ in the MSSM with mA < 125 GeV.
[4]LHC Higgs Cross Section Working Group, S. Dittmaier et. al., Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables, [arXiv:1101.0593].


1If the coupling is through gauge interactions, fully leptonic final states are generally the preferred discovery channel, see hZa, ZZD and hZDZD4l.
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