h → 2b 2τ | Exotic Higgs Decays

Contact Person(s)

Andrey Katz, Brock Tweedie, Rouven Essig and Stefania Gori

More details on this mode may be found in Section 4 of Survey of Exotic Higgs Decays (arXiv:1312.4992).

Theoretical Motivation

This channel can become very important in the case of a Higgs decays into a pair of light (pseudo)-scalars, h → a a , that further decay mainly into the third generation fermions bb or τ⁺ τ⁻. In the mass range 2m_b < m_a < m_h/2 the Higgs can have a relatively large branching ratio into aa, while both decays into bb and τ⁺ τ⁻ are allowed by phase space. In many models, e.g. the NMSSM and Little Higgs models, the couplings of a to the fermions will be roughly proportional to the SM Yukawa couplings (with some corrections that depend on tanβ), leading to Br(a → bb) ≈ 94% and Br(a → τ⁺ τ⁻) ≈ 6%. In this case ∼ 90% of all the aa decays will end up in bbbb, ∼ 10% in bb τ⁺ τ⁻ and less than 1% in τ⁺ τ⁻τ⁺ τ⁻. bb τ⁺ τ⁻ can be a reasonable compromise between branching fraction and visibility of the signal. In particular, more than 50% of the ditau decays include at least one isolated lepton.

Existing Collider Studies

This channel has attracted the attention of several research groups both in the context of the Tevatron and of the LHC. Most of the studies assumed a O(1) branching fraction for the decay h → aa.

Refs. [1,2] performed a feasibility study for this mode at the Tevatron. This study used associated production of the Higgs with a leptonic W. The study found very few sources of irreducible backgrounds, but also very small σ(Wh)×Br(h → bb τ⁺ τ⁻). This can be probably improved by ∼ 40% if this channel is combined with Z(→ l⁺ l⁻)h associated production. But probably little more can be gained at the Tevatron, and one cannot hope for more than just a few signal events in the realistic case.
This study was performed in Ref [3] for the LHC at 14 TeV , concentrating on VBF Higgs production mode. This study largely relies on a precise reconstruction of m_bb for rejection of the dominant tt background, while m_ττ and m_bbττ are not considered. The study is rather preliminary and it claims that, with 100 fb⁻¹ data, a significance of S/√B ∼2 can be reached in some parts of parameter space. It is also worth noticing that this study only considered channels with both τs decaying leptonically, and the situation can probably be significantly improved by including τ_l τ_h and maybe even τ_h τ_h final states.
Ref. [2] also very briefly discussed this search for 14 TeV LHC, considering only associated production with a W or Z, decaying leptonically. This study found this mode largely unfeasible at 100 fb⁻¹ due to very small S/B ratio.

Discussion of Future Searches at the LHC

We are not aware of any current experimental searches in this channel. Obviously, searches for a Higgs boson produced in association with b-quarks and decaying to taus [4,5] are not sensitive to our 2b2τ decay mode. Nonetheless, this CHANNEL might be a very important direction for studies of the LHC at 14 TeV. Probably, in order to have optimal reach, all three major productions modes (gluon fusion, VBF, and W/Z associated production) should be combined together. Different production modes may be dominated by different backgrounds. While tt looks indeed like a formidable background for VBF, it is possible that γ^*/Z^* + jets dominates the two other channels.

It is also worth noticing additional complications for very small values of m_a. First, as the mass of a is getting close to the Υ mass, the branching ratio a→ bb can be significantly reduced in favor of a → η+ X, leading effectively to a τ⁺ τ⁻ j event topology and opening up additional possible backgrounds from bottomonium decays (see [6] for detailed discussion and calculation of branching ratios). In addition, the τs tend to merge in this region of parameter space, failing isolated reconstruction criteria and yielding effectively a single τ-like jet instead of two. Finally, triggering on these events may be an issue. In particular, one can only be confident that associated production events are triggered with a reasonable efficiency. At LHC8, one can also probably use the low-efficiency VBF trigger. It is not clear, though, whether a search in this channel is feasible. At the 14 TeV LHC, the trigger thresholds may be too high for this type of decay, and therefore one probably has to focus on associated production.

We conclude that more dedicated feasibility studies for the LHC are needed in this particular channel.

References

: [1] U. Aglietti, A. Belyaev, S. Berge, A. Blum, R. Bonciani, et. al., Tevatron for LHC report: Higgs, [hep-ph/0612172].
: [2] M. Carena, T. Han, G.-Y. Huang, and C. E. Wagner, Higgs Signal for h → aa at Hadron Colliders, JHEP 0804 (2008) 092, [arXiv:0712.2466].
: [3] N. Adam, T. Aziz, J. Andersen, A. Belyaev, T. Binoth, et. al., Higgs Working Group Summary Report, [arXiv:0803.1154].
: [4] CMS Collaboration, Search for MSSM Neutral Higgs Bosons Decaying to Tau Pairs in pp Collisions.
: [5] ATLAS Collaboration, G. Aad et. al., Search for the neutral Higgs bosons of the Minimal Supersymmetric Standard Model in pp collisions at √s=7 TeV with the ATLAS detector, JHEP 1302 (2013) 095, [arXiv:1211.6956].
: [6] M. Baumgart and A. Katz, Implications of a New Light Scalar Near the Bottomonium Regime, JHEP 1208 (2012) 133, [arXiv:1204.6032].

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