# h → 2τ + MET

## Contact Person(s)

David Curtin, Tao Liu and Zhen Liu
More details on this mode may be found in Section 19 of Survey of Exotic Higgs Decays (arXiv:1312.4992).

## Theoretical Motivation

hX1 X2 → τ+ τ + MET is another new class of exotic Higgs decays. As for the 2b + MET final state, the two most important non-excluded topologies are
1. X1 → MET,  X2→ τ+ τ + MET
2. X1 → MET,  X2→ τ+ τ.
Here X1,2 are intermediate particles which can be either the same or different, and the τ+ τ pair can be either resonant or non-resonant (though this resonance would be difficult to reconstruct with taus).
• PQ-limit of NMSSM: h→ χ1 χ2 (topology 1, resonant)
As discussed in detail in NMSSM+F (see also [1,2]), X1,2 represent the lightest and next-to-lightest neutralinos in this limit, and we can get decay chains similar to those that lead to hbb + MET. The second neutralino χ2, which may for example be mostly-bino, decays into χ1s and/or χ1a. If s or a has a mass 2 mτ < ms/a < 2 mb, it dominantly decays into τ+ τ via mixing with the MSSM higgs doublets. In this case the τ+ τ pair is resonant.
• νSM: h→ νN (topology 1, non-resonant)
In the νSM, the Higgs can decay into an active neutrino and a sterile neutrino via Yukawa interaction [3]. The mass mixing between RH sterile neutrinos and LH active neutrinos then make the RH neutrinos decay via N → τ+ W−(*)→ τ+ τvτ and its conjugate (given Majorana N), or/and N → νZ(*)→ ντ+ τ. Here the τ+ τ are generally non-resonant, though in some cases they could sit on the Z resonance. For more details, see SM+F.
• Other models: ha a, ZD ZD, η1 η2 (topology 2)
It is possible to realize topology 2 as a possibly subdominant mode in dark vector models (SM+V), in the R-NMSSM (NMSSM+S) via a decaying to singlinos and taus if it satisfies 2 mτ < ma < 2 mb, or in a complicated hidden sector (Hidden Valley).

## Existing Collider Studies

A preliminary analysis for topology 1 is in progress, based on a benchmark model inspired by the PQ-limit of the NMSSM which is presented in Table I [4].
 mh mχ2 mχ1 ms 125 GeV 80 GeV 10 GeV 8 GeV
Table 1: Mass parameters used for the h → τ+ τ + MET collider analysis [4].

Given the large mass hierarchy between χ2 and its decay products χ1 and s (here a scalar or pseudoscalar), as well as the fact that ms/2mτ is only O(1), the τ+τ pair produced in this decay tends to be highly collimated, forming a "ditau-jet" (much like some of the cases discussed in h→ 4τ and references therein). The study is focused on Higgs events from associated production with a leptonic Z boson (Ze+e, μ+μ), due to the very large expected QCD backgrounds for other production modes. The distinguishing features of this signal are therefore two leptons with their invariant mass falling in the Z mass window, one ditau-jet and moderate amount missing energy. The dominant backgrounds in this analysis are Z+jets, tt+jets, and diboson+jets. They can be greatly reduced by cutting on the number of tracks in the ditau-jet candidate (QCD jets have more tracks than ditaus) and requiring the reconstructed h to be back-to-back with the Z. This preliminary analysis suggests that extracting the h → χ1 χ2 → 2τ+ MET signal with sub-unity branching fractions is extremely challenging at the 14 TeV LHC [4].

## Existing Collider Searches and Limits

Though these decays are motivated in several theoretical contexts, there are no dedicated experimental searches yet, and the allowed parameter space is still mostly open. The main constraints could come from h→ τ+ τ searches [5,6], hWW* searches [7,8] and the stau pair search by ATLAS [9]. However, in all of these analyses the selection cuts are too aggressive to pick up the exotic higgs decay efficiently. Some LHC searches might partly pick up some special corners, though we will not attempt to delineate these regions here.

## References

[1]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].
[2]J. Huang, T. Liu, L.-T. Wang, and F. Yu, Supersymmetric Exotic Decays of the 125 GeV Higgs Boson, Phys. Rev. Lett. 112 (2014) 221803, [arXiv:1309.6633].
[3]A. de Gouvea, GeV seesaw, accidentally small neutrino masses, and Higgs decays to neutrinos, [arXiv:0706.1732].
[4]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]CMS collaboration, Search for the Standard-Model Higgs boson decaying to tau pairs in proton-proton collisions at sqrt(s) = 7 and 8 TeV, 2013. CMS Public Note CMS-PAS-HIG-13-004.
[6]ATLAS Collaboration, G. Aad et. al.Search for the Standard Model Higgs boson in the H to τ+ τ− decay mode in √s=7 TeV pp collisions with ATLASJHEP 1209 (2012) 070, [arXiv:1206.5971].
[7]CMS collaboration, Evidence for a particle decaying to W+W− in the fully leptonic final state in a standard model Higgs boson search in pp collisions at the LHC, 2013. CMS Public Note CMS-PAS-HIG-13-003.
[8]ATLAS collaboration, Measurements of the properties of the Higgs-like boson in the W W* → l νl ν decay channel with the ATLAS detector using 25 fb-1 of proton-proton collision data, 2013. ATLAS Public Note ATLAS-CONF-2013-030.
[9]ATLAS collaboration, Search for electroweak production of supersymmetric particles in final states with at least two hadronically decaying taus and missing transverse momentum with the ATLAS detector in proton-proton collisions at √s=8 TeV, 2013. ATLAS Public Note ATLAS-CONF-2013-028.

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