h → 2b + MET

Contact Person(s)

David Curtin, Tao Liu, and Brock Tweedie
More details on this mode may be found in Section 18 of Survey of Exotic Higgs Decays (arXiv:1312.4992).

Theoretical Motivation

Decays of the form h→ bb+MET can be classified into two main topologies, assuming a primary two-body decay stage h→ X1 X2:
1. X1 → MET,  X2bb+MET,
2. X1 → MET,  X2bb.
Here, X1,2 are intermediate on-shell particles (possibly the same particle undergoing different decays), and X1 is either stable and invisible, or decays invisibly.1 The bb pair may either be resonant or nonresonant in general for first class of decays, though we will mainly assume that it is resonant. The second class is resonant by definition. Below, theoretical motivations and experimental search strategies will be discussed.
• NMSSM in PQ-symmetry limit: h→ χ1 χ2 (topology 1, resonant)
Here, X1 and X2 represent the lightest and the next-to-lightest neutralinos χ1 and χ2, respectively, with χ2 decaying to χ1 plus a scalar or pseudoscalar of the extended Higgs sector. For details on the decay h→ χ1χ2 (and h→ χ2χ2), see NMSSM+F or [2,3]. If the scalar is heavier than 2 mb, its decays are typically dominated by bb. The signatures at colliders will then be one or two b-jets + MET, depending on how collimated the two b quarks are.
If the mass difference mχ2mχ1 is larger than the Z mass, the decay χ2 → χ1 Z is opened and the Z-boson can further decay into a bb pair. However, this decay tends to be kinematically disfavored.
• νSM: h→ νN (topology 1, resonant or non-resonant)
In the νSM, the Higgs can decay into an active neutrino and a sterile neutrino via the neutrino portal Yukawa interaction. In this case, we identify X1 = ν and X2 = N, and the topology is the same as in the PQ-symmetric NMSSM. The mass mixing between RH sterile neutrinos and LH active neutrinos allow the RH neutrinos to decay via N → νZ(*)→ νbb. For more details, refer to SM+F.
• Other models: ha a, ZD ZD, η1 η2 (topology 2)
In the R limit of the NMSSM (NMSSM+S) it is possible for a to decay competitively into singlinos as well as bottom quarks. In that case, the decay h→ 2a → 2b + MET may be realized. Dark vector extensions (SM+V) will usually have an invisible decay mode ZDνν, so the 2b + MET final state can occur (even if it may not be the first discovery channel for such a model). Finally, it is of course possible to imagine a more complicated hidden sector (see e.g. Hidden Valley) where h→ η1 η2 and η1bb but η2 is invisible or decays invisibly.

Existing Collider Studies

As the kinematics of h→ bb+MET can be significantly different from the standard h→ bb decay, dedicated analyses are required to search for it. Inspired by the PQ-limit of the NMSSM, a dedicated study of this process has recently been performed [3,4]. The signals from gluon fusion and vector boson fusion production would be overwhelmed by QCD backgrounds (similar to SM h→bb), even if they could be triggered on, so the analysis focuses on vector boson associated production, triggering on leptonic boson decays. As an illustration, Zh with Z→ e+e+μ is considered. In addition to two neutralinos χ12, this includes a spin-0 state s (either scalar or pseudoscalar) that decays to bb. The study is based on two benchmark models in the PQ-limit of the NMSSM, with their parameters presented in Table I. The main backgrounds include Zbb, Zcc, Zc+ Zc and tt+jets.
 mh mχ2 mχ1 ms 125 GeV 80 GeV 10 GeV 45 GeV 125 GeV 80 GeV 30 GeV 20 GeV
Table 1: Benchmark masses used for the hbb + MET collider analyses of [3,4].
The analysis includes basic detector effects but no pile-up simulation. Jet substructure tools [5] are also applied to investigate b-tagged fat-jets. These analyses indicate that  ∼ 2 σ exclusion sensitivity to Br(h→ χ1 χ2 → 2b + MET) = 0.2 is possible at the 14 TeV LHC with 300  fb−1, though it is very challenging, and more realistic studies are needed.

Existing Experimental Searches and Limits

Although the signature h → bb + MET is well-motivated, dedicated experimental searches have not yet been performed. There are similarities to the SM higgs decay h→bb, but the generally softer bottom quarks and lower rate make this a more challenging signal to detect. The h→ bb searches from (W→ lν)h, (Z→ ll)h and (Z→ νν)h production by both the CMS and the ATLAS collaborations [6,7] have only recently achieved SM sensitivity, yielding no constraints on the rarer 2b + MET final state. The (Z→ νν)h search could in principle be sensitive to the exotic Higgs decay from ggF and VBF production channels, with the orders-of-magnitude larger production rate offsetting the subdominant exotic Br. However, the jet pT and MET cuts are quite high and would likely eliminate almost all of the signal. This underlines the need for dedicated searches.

References

[1]B. Batell, C. E. M. Wagner, and L.-T. Wang, Constraints on a Very Light Sbottom, [arXiv:1312.2590].
[2]P. Draper, T. Liu, C. E. Wagner, L.-T. Wang, and H. Zhang, Dark Light-Higgs BosonsPhys. Rev. Lett. 106 (2011) 121805, [arXiv:1009.3963].
[3]J. Huang, T. Liu, L.-T. Wang, and F. Yu, Supersymmetric Exotic Decays of the 125 GeV Higgs BosonPhys. Rev. Lett. 112 (2014) 221803, [arXiv:1309.6633].
[4]J. Huang, T. Liu, L.-T. Wang, and F. Yu, Supersymmetric Sub-Electroweak Scale Dark Matter,  the Galactic Center Gamma-ray Excess, and Exotic Decays of the 125 GeV Higgs Boson, [arXiv:1407.0038].
[5]J. M. Butterworth, A. R. Davison, M. Rubin, and G. P. Salam, Jet substructure as a new Higgs search channel at the LHCPhys.Rev.Lett. 100 (2008) 242001, [arXiv:0802.2470].
[6]CMS Collaboration, Search for the standard model Higgs boson produced in association with W or Z bosons, and decaying to bottom quarks, 2013. CMS PAS HIG-13-012.
[7]ATLAS Collaboration, Search for the bb decay of the Standard Model Higgs boson in associated (W/Z)H production with the ATLAS detector, 2013. ATLAS-CONF-2013-079.

Footnotes:

1
A logical third option that leads to this final state would be a decay into a pair of bottom-partners, that each subsequently decay to b+MET. However, this option is now almost entirely ruled out [1].

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