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).
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 lepton−jets + 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 StudiesA collider search for h→ 2a→ 4μ was first proposed in , which took ma ≈ 215 MeV, as motivated by an excess in HyperCP measurements of Σ+→ pμ+μ− decay . 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 , which concluded that there were excellent prospects for discovery in early 14 TeV LHC running (considering exotic branching fractions of tens of percent). Reference  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  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 LimitsThe h→ 2(μμ) signature has become established in experimental programs, beginning with the D0 search . The most stringent constraints are set by the LHC, looking for Higgs decays to both prompt [12,13,14] and displaced  dimuon jets. As this final state is extremely clean, these searches are inclusive, and in particular do not require m4μ=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 , which was performed with the full 8 TeV data set. This search, like the previous CMS search , 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(h→ aa)Br(a→ μμ)2 αgen < 1.2×10−5 for mh=125 GeV. Outside this mass range, the 35 pb−1 search of Ref.  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.  for the cascade decay h→ χ2χ2, χ2→ a (ZD)χ1, 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. , 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(χ2 → a χ1) = 1, and (right) Br(χ2→ ZD χ1) = 1, as a function of mχ1, from . 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 , and later at ATLAS with 2.04 fb−1 of 7 TeV data  and inclusively for pairs of hard electron-jets (e.g. from squark production) with 5 fb−1 of 7 TeV data . 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.
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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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