# h → 2 Lepton-Jets + X

## Contact Person(s)

Stefania Gori, David McKeen, Tao Liu and Jessie Shelton

More details on this mode may be found in Section 17 of Survey of Exotic Higgs Decays (arXiv:1312.4992).

## Theoretical Motivation

Here we consider Higgs decays to two lepton-jets + X; see also one lepton-jet + X for related signatures and additional definitions and theoretical discussion. Again, for simplicity we concentrate on simple lepton-jets, consisting of a single collimated electron or muon pair. One well-studied model for a Higgs decaying to pairs of collimated muons is the NMSSM. Here the Higgs decays via h→ a a, with a subsequently decaying to muons.   Also h→ 2(μμ)+X arises in the PQ-symmetric limit of the NMSSM [1,2], where the Higgs decays to neutralinos which cascade to produce light (pseudo)scalars, h→ χ2χ2, χ2→ (a)s χ1. In the NMSSM or any singlet-augmented 2HDM model, once ma > 2mτ, the branching fraction for a→ μμ will always be suppressed by the small ratio mμ2/mτ2  ∼ 3.5×10−3. As discussed in h→ 4τ, 2μ2τ, the tiny branching fraction into h→ 4μ is not competitive. Thus if a couples proportional to mass, only the range 2mμ < ma < 2mτ is of interest for the decay h→ 4μ (+ X), in which case the muon pairs will be collimated.

Higgs decays to collimated lepton pairs may also arise in models with light vector bosons ZD that mix with the SM hypercharge gauge boson (see SM+Vector). The motivation to consider mZD << mh has been driven by dark matter models that require mZD ∼ GeV or below [3,4]. In these models, the branching fractions of ZD depend on the SM fermion gauge couplings, so electron and muon pairs are produced with comparable branching fraction unless mZD < 2mμ. Importantly, the branching fraction for h→ 2(ll) remains large even when mZD > 2mb, motivating searches for both electrons and muons in this mass range.

Dark photon models can give h→ 2 lepton-jets directly [5,6], via an initial decay h→ ZD ZD, as well as h→ 2  leptonjets + MET. Among possibilities for obtaining MET are cascade decays involving fermions, e.g.  χ2→ χ1 ZD, hidden sector showering or hadronization where ZD  is produced along with some invisible hidden sector particles, see Hidden Valleys.

## Existing Collider Studies

A collider search for h→ 2a→ 4μ was first proposed in [7], which took ma ≈ 215 MeV, as motivated by an excess in HyperCP measurements of Σ+→ pμ+μ decay [8]. This study pointed out that modifications of the (then-)standard muon isolation algorithms would be required to preserve the signal. A more careful treatment of the dominant QCD backgrounds was carried out in [9], which concluded that there were excellent prospects for discovery in early 14 TeV LHC running (considering exotic branching fractions of tens of percent).

Reference [10] performed a collider study of the Higgs decaying to multiple electron-jets plus MET through a 100 MeV ZD. Production in association with a leptonic W or Z was identified as the most promising channel, in which the dominant background is W or Z plus QCD jets. Reference [10] found that an analysis distinguishing electron-jets from QCD jets using the electromagnetic fraction and charge ratio of the jet candidates could discover the Higgs with 1 fb−1 of 7 TeV LHC data at 95% CL assuming unit branching fraction to electron-jets plus MET.

### Existing Experimental Searches and Limits

The h→ 2(μμ) signature has become established in experimental programs, beginning with the D0 search [11]. The most stringent constraints are set by the LHC, looking for Higgs decays to both prompt [12,13,14] and displaced [15] dimuon jets. As this final state is extremely clean, these searches are inclusive, and in particular do not require m=mh. Thus they are sensitive to both the h→ aa →2(μμ) decay topology and the topology h→χ2χ2→ 2(μμ) +MET.

The best existing limits on prompt h→ 2 (μμ)+X come from the recent CMS analysis [14], which was performed with the full 8 TeV data set. This search, like the previous CMS search [13], only covers the range 2mμ < ma < 2mτ.1 This search limits σ(p p → 2a+X)Br(a→μμ)2 αgen < 0.24 fb at 95% CL over almost all of the mass range in consideration, where αgen is a (model-dependent) fiducial acceptance. This translates to a limit Br(haa)Br(a→ μμ)2 αgen < 1.2×105 for mh=125 GeV. Outside this mass range, the 35 pb−1 search of Ref. [12] extends to 5 GeV, placing limits of σ(p p → 2a+X)Br(a→ μμ)2 ϵ < 125 fb, where ϵ is again an acceptance.
We have reinterpreted the results of Ref. [14]  for the cascade decay h→ χ2χ2, χ2→ a (ZD1, a (ZD)→ μμ in Fig. 1, for masses ma (mZD) = 0.4 GeV (blue), 1 GeV (green), and 3 GeV (red). Dark vector branching fractions to muons are taken according to the tree-level computation of SM+Vector, while a reference branching fraction Br(a→ μμ)=0.1 is assumed. Caution should be used in interpreting the recast limits for the smallest values of m2−m1, which is furthest from the spectra considered in Ref. [14], as in this region the linear relation between αgen and the full experimental efficiency may no longer hold.
Figure 1: Approximate bounds on the branching fraction for h→ χ2χ2, assuming (left) Br(χ2a χ1) = 1, and (right) Br(χ2ZD χ1) = 1, as a function of mχ1, from [14]. Here solid lines indicate mχ2 = 50 GeV and dotted lines mχ2=60 GeV, while red, green, and blue correspond to ma, ZD = 3 GeV, 1 GeV, and 0.4 GeV respectively. For a we use a reference Br(a→ μμ) = 0.1.

Searches in electron-jets are more challenging. Nonetheless, searches for h→ 2 electron-jets have been carried out, targeting Wh associated production first at CDF with 5.1 fb−1 data [16], and later at ATLAS with 2.04 fb−1 of 7 TeV data [17] and inclusively for pairs of hard electron-jets (e.g. from squark production) with 5 fb−1 of 7 TeV data [18]. It is challenging to reinterpret the first two searches as a limit on Higgs decays to simple electron-jets, as both require > 2 tracks per electron-jet, to better reject photon conversions.

## References

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[11] D0 Collaboration, V. Abazov et. al., Search for NMSSM Higgs Bosons in the h → aa → μμ μμ,  μμ ττ Channels Using pp Collisions at √s = 1.96 TeV, Phys.Rev.Lett. 103 (2009) 061801, [arXiv:0905.3381].
[12] CMS Collaboration, S. Chatrchyan et. al., Search for Light Resonances Decaying into Pairs of Muons as a Signal of New Physics, JHEP 1107 (2011) 098, [arXiv:1106.2375].
[13] CMS Collaboration, S. Chatrchyan et. al., Search for a Non-Standard-Model Higgs Boson Decaying to a Pair of New Light Bosons in Four-Muon Final States, [arXiv:1210.7619].
[14] Search for a non-standard-model higgs boson decaying to a pair of new light bosons in four-muon final states, Tech. Rep. CMS-PAS-HIG-13-010, CERN, Geneva, 2013.
[15] ATLAS Collaboration, G. Aad et. al., Search for displaced muonic lepton jets from light Higgs boson decay in proton-proton collisions at √s=7 TeV with the ATLAS detector, Phys.Lett. B721 (2013) 32-50, [arXiv:1210.0435].
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[17] ATLAS Collaboration, G. Aad et. al., Search for WH production with a light Higgs boson decaying to prompt electron-jets in proton-proton collisions at √s=7 TeV with the ATLAS detector, New J.Phys. 15 (2013) 043009, [arXiv:1302.4403].
[18] ATLAS Collaboration, G. Aad et. al., A Search for Prompt Lepton-Jets in pp Collisions at √s=7 TeV with the ATLAS Detector, Phys.Lett. B719 (2013) 299-317, [arXiv:1212.5409].

### Footnotes:

1It also requires the two lepton jet masses to be within 0.1 GeV of each other, meaning it is insensitive to decays h → a1 a2 with a1 ≠ a2.
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