Contact Person(s) Stefania Gori, Jessie Shelton, David Curtin, Rouven Essig More details on this mode may be found in Sec. 11 of the Survey of Exotic Higgs Decays, arXiv:1312.4992.

Theoretical Motivation

Similarly to the discussion in the previous section, two classes of models can give a Higgs to four-lepton signature, with two pairs of electrons and/or muons reconstructing the same resonance:

• As discussed in § SM+V, models with an additional U(1)_D gauge group may lead to the h→ Z_D Z_D decay, followed by Z_D→l⁺l⁻. Branching ratios for h→ Z_D Z_D as large as  ∼ 10% are allowed in certain regions of parameter space. In particular, in non-minimal models this vector boson may be non-Abelian or composite; see Hidden Valley [1].
• Models predicting a sizable branching ratio for h→ aa, where a is CP-odd scalar, can also lead to the 4l signature. Such pseudoscalars can arise in 2HDM+S models. However, for m_a > 2m_τ, Br(a→l⁺l⁻)/Br(a→ττ) ∼ m_l²/m_τ² ∼  3×10⁻³ (8×10⁻⁸) for l = μ (e). For this reason, we will focus on models with dark gauge bosons. Searches that exploit the more dominant 4τ and 2τ2μ decay modes of the pseudoscalar pair are discussed on the h→ 4τ page.

Existing Collider Studies

The authors of [2] investigate the feasibility of probing h→ Z_D Z_D→ 4l at Tevatron and at the LHC. In particular, they perform an estimation of the reach at the 14 TeV LHC for several benchmark scenarios. They show that there are very good prospects for detecting this Higgs decay mode, even for small Higgs branching ratios. Furthermore, Ref. [3] shows that a light Higgs boson could have been discovered sooner in h→ Z_D Z_D→ 4l than in the traditional decay modes, γγ, ττ, with the 7 TeV LHC data. In particular, the authors claim that, even for Br(h→ Z_D Z_D) ∼ O(1%), one could have expected 5 events with the first  fb⁻¹ of 7 TeV LHC data.

Existing Experimental Searches and Limits

Searches for h→ aa→ 4μ were performed by the CMS collaboration with 5 fb⁻¹ of data at √s=7 TeV [4] and 20 fb⁻¹ at √s=8 TeV [5]. For these searches, a refers to a spin-0 boson with a mass between 250 MeV and 2m_τ. Differences in the acceptance between this signal and h→ Z_D Z_D→4μ should be modest for this range of boson masses, and the limits from these searches at CMS are directly applicable. The 8 TeV search [5] is more sensitive and results in a limit Br(h→ Z_D Z_D→4μ) < 4.7×10⁻⁵ for m_h=125  GeV and 250  MeV < m_ZD < 2m_τ. Limits can be obtained from SM Higgs searches as well as from a plot reported as part of a ZZ cross section measurement. To estimate limits on exotic Higgs decays to four leptons, we simulate Higgs (produced in gluon fusion) decays to dark photons, h→ Z_D Z_D, followed by Z_D→l⁺l⁻, in the dark photon model of § SM+V. We begin by considering the SM h→ Z Z^* analyses, which are conducted with the full 7+8 TeV datasets in both experiments. The CMS search [6] requires four isolated leptons within kinematic acceptance, forming two OSSF pairs. The invariant mass of the OSSF pair that minimizes |m_ll − m_Z| is denoted m₁, while the remaining OSSF pair invariant mass is denoted m₂. The pair invariant masses must satisfy

(1)

Events in which any OSSF pair has invariant mass m_ll < 4 GeV are rejected, to suppress backgrounds from quarkonia. To compare to public data, we study the set of four-lepton events with four-lepton invariant mass in the range m_4l ∈ (121.5, 130.5) GeV. The requirement that one OSSF pair of leptons lies within a Z window means that frequently h→ Z_D Z_D events are not reconstructed as a pair of resonances: if m_ZD = 20 GeV, for instance, a lepton pair with invariant mass near m_Z can only be obtained by taking one lepton from each Z_D decay. Since events with two electrons and two muons cannot be mispaired in this way, for m_ZD < 40 GeV, eeμμ events cannot contribute to the reach at all. As m_ZD increases, the fraction of events which are reconstructed as a pair of resonances increases, so that when m_ZD = 60 GeV, nearly all leptons are correctly paired. Details on the procedure to extract limits on h→ Z_D Z_D from this search are presented in the Survey of Exotic Higgs Decays, arXiv:1312.4992. The resulting 95% CL limits are shown in the red line in Fig. 1. To translate between limits on h→ Z_DZ_D and h→ Z_D Z_D→ 4l we point out that, as seen in § SM+V, for 10  GeV <~m_ZD <~60  GeV, Br(Z_D→l⁺l⁻) ≅ 0.3. This implies that Br(h→ Z_DZ_D→4l) ≅ 0.09×Br(h→ Z_DZ_D). The ATLAS SM h→ Z Z^*→ 4l search [7] is similar in spirit to the CMS search. The major difference for our purposes is that the acceptance is tighter for the OSSF lepton pair minimizing |m_ll−m_Z|,

(2)

This reduces the overall acceptance for the BSM signal, leading to weaker limits than those from CMS. At low masses, the best limits are found from control regions in the ATLAS ZZ cross section measurement with 20 fb⁻¹ of 8 TeV data [8]. Here, events are again required to have exactly four leptons, which can be paired into two OSSF pairs. Now when there is a choice of possible OSSF pairings, the assignment which minimizes |m₁ − m_Z | + |m₂−m_Z| is chosen. This still has some probability of mis-pairing h→ Z_D Z_D events (e.g., for m_ZD=25 GeV, only 55% of signal events passing the selection criteria are correctly paired). Note that, unlike the SM h→ Z Z^* analyses, there is no restriction on the invariant mass of the four leptons.     Fig. 1 shows the resulting limits in blue (along with those from CMS's h→ ZZ^∗ search), of order 10⁻³, on Higgs branching fractions to dark vector bosons that further decay to lepton pairs. These limits, while impressive, are easy to improve at low masses by simply looking for OSSF pairs which minimize |m₁−m₂|, instead of a distance from the Z peak. As backgrounds are already zero for most bins, improving signal acceptance is the most likely to improve reach.

References

[1] M. J. Strassler and K. M. Zurek, Echoes of a hidden valley at hadron colliders, Phys.Lett. B651 (2007) 374-379, [hep-ph/0604261].

[2] S. Gopalakrishna, S. Jung, and J. D. Wells, Higgs boson decays to four fermions through an abelian hidden sector, Phys.Rev. D78 (2008) 055002, [arXiv:0801.3456].

[3] A. Martin and T. S. Roy, The Gold-Plated Channel for Supersymmetric Higgs via Higgsphilic Z', [arXiv:1103.3504].

[4] 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].

[5] 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.

[6] Properties of the Higgs-like boson in the decay H to ZZ to 4l in pp collisions at sqrt s =7 and 8 TeV, Tech. Rep. CMS-PAS-HIG-13-002, CERN, Geneva, 2013.

[7] Measurements of the properties of the Higgs-like boson in the four lepton decay channel with the ATLAS detector using 25 fb?1 of proton-proton collision data, Tech. Rep. ATLAS-CONF-2013-013, CERN, Geneva, Mar, 2013.

[8] ATLAS Collaboration, Measurement of the total ZZ production cross section in proton-proton collisions at √s = 8 TeV in 20 fb^-1 with the ATLAS detector, Tech. Rep. ATLAS-CONF-2013-020, CERN, Geneva, Mar, 2013.

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