# h → 4b

## Contact Person(s)

David Curtin, Rouven Essig, Prerit Jaiswal, Yi-Ming Zhong
More details on this mode may be found in Section 3 of Survey of Exotic Higgs Decays (arXiv:1312.4992).

## Theoretical Motivation

Exotic Higgs decays to four b quarks can occur via a light resonance X: h→ XX → bbbb Several classes of models can lead to 4b final state :
• 2HDM+S: In two-Higgs-doublet models with an additional light singlet, the decay h → ss or h → aa, where s (a) is the mostly-singlet (pseudo)scalar is generic. Depending on tanβ, the decays s → bb or a → bb are also generic in all four 2HDM Types as long as ma, ms > 2mb.
• R-symmetry limit in the NMSSM : The additional two degrees of freedom in the NMSSM Higgs sector make a pseudo-scalar a with sizable coupling to the SM-like Higgs and SM fermions possible. Furthermore, light pseudo-scalars naturally arise in the R-symmetry limit of the NMSSM.
• Little Higgs models : Another class of models with additional potentially light pseudo-scalar is Little Higgs. The couplings of a to SM fermions are again proportional to the SM Yukawas if one imposes Minimal Flavor Violation (MFV) [1,2,3,4] in order to get rid of large flavor violation. Therefore the coupling to the b quark is typically enhanced.

## Existing Experimental Constraints

Due to large QCD backgrounds to the 4b final state, the only realistic discovery mode for h→ 4b at the LHC with √s = 7 or 8 TeV is Wh associated production. To the best of our knowledge, no such search has been performed. V (h → 2b) searches [6,7] have not yet reached SM sensitivity and are even less likely to find the softer signal from 4 b's. Searches for b (h → b b) production [8,9] do not look for an isolated lepton or large amounts of MET, which results in large backgrounds, and SUSY searches for final states containing several b-jets like [10] also typically do not require a lepton while requiring an amount of missing energy that is much too high for Vh production.
The h → aa → 4b signal could, in principle, show up in the SM h → 2b searches [11] if the pseudoscalar is light enough and therefore boosted, so that its decay to two b-jets is reconstructed as a single b-jet (see Section 7.3 of arXiv:1312.4992  for details). Recast of this SM h → b b search sets a 95  % C.L. limit of Br(h4b) <~0.7 for ma = 15  GeV (assuming no contamination from the SM h → 2b decay) while the limit gets worse with increasing ma. No limit could be set for ma > 30  GeV. In the absence of any other dedicated searches, the only existing limit is the general bound on non-standard Higgs decays of Br(h4b) <~0.5.

## Estimates of Sensitivity

The literature contains several collider studies examining h → 4b decay at the 14 TeV LHC. Refs. [12,13] considered the 4b final state in the context of VBF Higgs production, but this signature is very difficult to distinguish from QCD background. More recently the focus has been on the Wh production mode [14,15,16], where the tagged lepton greatly reduces backgrounds and enhances the discovery potential.
Ref. [14] demonstrates how a very simple 4b search could constrain h→ a a → b b b b at LHC14 (similar conclusions are reached by [15]). They define the parameter

where κhVV is the WWh coupling strength relative to the SM. Given the SM associated Higgs production cross section σWh, the cross section for the 4b signal is simply given as

With parameter choices inspired by the NMSSM, the authors of [14] consider C24b ∈ (0.1, 0.9), with 0.5 being the benchmark, which happens to define the upper range of our regime of interest Br(h→ 4b) <~0.5. Their benchmark is chosen to be mh = 120  GeV and ma = 30  GeV. For each event, two a-candidates are constructed by grouping the four b-candidates into pairs of minimal mass difference. Masses of a and h are reconstructed by looking at the invariant mass distribution of 2 or 4 b-jets (details of the backgrounds and cuts can be found in [14]). For C4b2 = 0.5, 3σ discovery is possible with 10 − 30  fb−1 of LHC14 luminosity, depending on the b-tagging efficiency at low pT, which is the main challenge of this search.
A more recent study [16] uses modern jet substructure methods to tackle a much harder signature, h → a a → 4g (also considered in [17,18,19,20]), by considering boosted Higgs production in association with a W or Z (with C24b = 1 in the language of [14]) for a 120 GeV Higgs. The analysis is split into a light pseudo-scalar (ma <~30  GeV) and heavy pseudo-scalar (ma ∈ (30  GeV, mh/2) regime). In the light scalar regime, a boosted Higgs decaying as h → a a → 4j can produce a 2, 3 or 4-pronged fat jet. The authors find that the h→ 4j signature can be observed at 3σ with 100  fb−1 of LHC14 luminosity for ma < 30  GeV, while adding 1 (2) b-tags improves the h→ 4b discovery signal to  ∼ 6 (10) σ. In the case of a heavy intermediate scalar flavor tagging is required for discovery, with 2 b-tags allowing for  ∼ 6-sigma discovery with 100  fb−1. Note that all these limits were set assuming C4b2 = 1 .

## Related Decay Modes

Jet-substructure techniques described above can also be applied to h → 4j exotic decay channel.

## References

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