Contact Person(s)

Stefania Gori, David McKeen, Tao Liu and Jessie Shelton     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 m_a > 2m_τ, the branching fraction for a→ μμ will always be suppressed by the small ratio m_μ²/m_τ²  ∼ 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 2m_μ < m_a < 2m_τ is of interest for the decay h→ 4μ (+ X), in which case the muon pairs will be collimated.      

Existing Collider Studies

A collider search for h→ 2a→ 4μ was first proposed in [7], which took m_a ≈ 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 Z_D. 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_4μ=m_h. 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 2m_μ < m_a < 2m_τ.¹ This search limits σ(p p → 2a+X)Br(a→μμ)² α_gen < 0.24 fb at 95% CL over almost all of the mass range in consideration, where α_gen is a (model-dependent) fiducial acceptance. This translates to a limit Br(h→ aa)Br(a→ μμ)² α_gen < 1.2×10⁻⁵ for m_h=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 (Z_D)χ₁, a (Z_D)→ μμ in Fig. 1, for masses m_a (m_ZD) = 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 α_gen and the full experimental efficiency may no longer hold.   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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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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