Contact Person(s)

David Curtin, Rouven Essig, Zhen Liu, and Ze'ev Surujon More details on this mode may be found in Section 10 of Survey of Exotic Higgs Decays (arXiv:1312.4992).

Theoretical Motivation

h→ ZZ_D As discussed in SM+V, many theories feature a hidden U(1) sector with small kinetic or mass mixing the the SM photon and Z-boson. This possibility often arises in connection to dark matter, but similar phenomenology can also arise in hidden valley models, see Hidden Valley. The minimal setup to generate h → Z Z_D decay involves a kinetic mixing term between the hypercharge gauge boson and the dark U(1) gauge boson

(1)

where hatted quantities are fields before their kinetic terms are canonically renormalized by a shift of B_μ. In the canonical basis, SM matter has dark milli-charge and there is mass mixing between the SM Z-boson and Z_D. The dominantly dark vector mass eigenstate has photon-like couplings to SM fermions (proportional to the small mixing ϵ) up to O(m_ZD²/m_Z²) corrections. It is also possible to have pure mass mixing after EWSB via operators of the form h Z^μ Z′_μ, but in this case additional constraints from parity violating interactions and rare meson decays apply, see [1,2,3]. Generically, new physics similar to that which generates kinetic mixing may also generate dimension-6 terms of the form H^† HB^μνZ_Dμν/Λ². Once the Higgs acquires a VEV, this term yields the coupling in Eq. (1). h→ Za Next we consider the decay h→ Z a. This is motivated by, for example, by the 2HDM+S or the NMSSM, where one of the CP-odd Higgs masses can be small. The relevant interaction Lagrangian in terms of mass eigenstates h and a is with an Yukawa term:

(2)

with g = √{(g²+g′²)/2} sin(α− β) sinθ_a. α is the mixing angle between the doublet scalars, tanβ = v_u/v_d and θ_a is the mixing angle between the uneaten doublet pseudoscalar A and the singlet pseudoscalar. Since the higgs coupling to ZZ and W⁺ W⁻ is also proportional to sin(α− β), the SM-like rates in those channels (as well as the diphoton mode) favor the decoupling limit α = π/2 − β. θ_a can be constrained by direct LEP and Tevatron searches for the CP-odd Higgs, but the SM-like higgs could still have large branching fractions to Za [4].

(3)

and the overall size of θ_a does not affect its branching ratios. For the length of the LHC program it will likely be safe to take Br(h→ Za)=10% as a benchmark point. In the next section, we discuss the experimental constraints on this mode. Depending on the mass of this pseudoscalar, the dominant decay mode could be bb, τ⁺τ⁻ or μ⁺μ⁻ (ss). We consider all of these cases when proposing searching strategies.

Existing Collider Studies

Up to different branching ratios and some angular correlations the final states for h → Z Z_D and h → Z a are identical. As such, collider studies and experimental searches for one channel generally apply to both. The two relevant parameters to define a simplified model for this channel are

(4)

for X = a, Z_D and y = some SM particle, where the different a, Z_D branching ratios lend different importance to different choices of y. There have not been many collider studies specifically performed for the h → Za mode. Ref. [4] pointed out that this channel may be very large in the context of the NMSSM. Ref [5,6,7] discussed heavy non-SM-like Higgs decaying into Za. More searches have been inspired by looking for a Z_D. The phenomenology of a Z_D with mass mixing to the Z has recently been discussed in [8,1,2,3] (see also, e.g., [9,10,11,12] for earlier work), including collider phenomenology of h→ ZZ_D, h→ γZ_D and h→ Z_DZ_D decays, as well as low energy constraints from dark photon experiments, g−2 of the muon, meson physics and electroweak precision observables. In [3], the authors designed a search for pp→ h→ ZZ_D→ e⁺e⁻μ⁺μ⁻. The backgrounds considered are Z(→l⁺l⁻)jj, j faking l (probability ∼ 0.1%) and leptonic tt (reducible), as well as h→ ZZ^*, Zγ^*, ZZ and Z→ 4l (irreducible). The authors of [3] assumed only mass mixing of the form ε_Z m_Z² Z^μ Z_Dμ. For m_ZD=10 GeV and ε = 10⁻⁶, they find that the luminosity at 14 TeV LHC needed to exclude (2σ), observe (3σ) and discover (5σ) the Z_D is 42, 95 and 260 fb⁻¹, respectively. For half of those values of m_ZD and mixing ε_Z the numbers are slightly smaller: (33,75,210) fb⁻¹.

Existing Searches & Limits

Our focus is the hZX vertex (X = a, Z_D). No direct search for h → Za or ZZ_D has been performed to be best of our knowledge, but there are several channels and other searches at LEP, Tevatron and LHC that are sensitive to this interaction term. * LEP The hZX vertex not only give rise to the h→ ZX decay, but also opens the channel e⁺e⁻→ Z^* → h X at LEP. Related searches include e⁺ e⁻ → h a, ZZ′→ 4b [13], 4τ [13] and 2b 2τ [13]. For Br(h→ Za)=10%, these searches are not constraining because the cross section for e⁺e⁻→ Z^* → h a is at the sub-fb level. * Tevatron and LHC   The most relevant existing search sensitive to h→ ZZ_D and h→ Za is h→ ZZ^* → 4 l by CMS [14] and ATLAS [15], where 4 l stands for electrons and muons. The cleanliness of the 4l decay makes these existing searches very sensitive to ZZ_D or Za decaying into leptons. The leptonic h → ZZ^* searches divide the four leptons of each event into two pairs, the "leading" pair (likely to have come from an on-shell Z) and the "subleading" pair (from the off-shell Z^*). With The subleading dilepton mass distributions from ATLAS and CMS using 20 + 5 fb⁻¹ data set it is easy to estimate limits on h → Z X decay. We show our translated 95% CL bounds on this exotic branching fraction for different m_X > 12 GeV in Fig. 1, see arXiv:1312.4992 for more details. The bound on Br(h → ZX) ×Br(X → ll) is <~10⁻⁴ − 10⁻³ for 12 GeV <~m_X <~34 GeV and l = e, μ. The situation is more ambiguous for pseudoscalars. Their branching ratios are more model-dependent in general, and their Yukawa couplings usually imply that a → ττ is enormously preferred over e, μ. Bounds for X → ττ could also be derived from the leptonic h → Z Z^* searches but would be much weaker. Nevertheless this may be the preferred discovery channel for 2HDM+S and NMSSM type models, where Br(h → Z a) could easily be 10% and Br(a → ττ) is generally O(0.05-1), see 2HDM+S.

Proposals for New Searches at the LHC

For m_{a, ZD} > 12 GeV it seems likely that LHC14 searches inspired by h→ ZZ^* will constrain h → Za in the a→ 2τ modes, while LHC7+8 already gives significant direct bounds to h → Z Z_D → 4 l. A Z + lepton jet search would be able to set strong limits in particular for very light Z_D. Care must be taken to correctly account for challenging quarkonium backgrounds. Identifying promising search strategies will be the subject of future work. Here we list new experimental and theory results pertinent to this exotic higgs decay channel.

• Adam Falkowski, Roberto Vega-Morales, Exotic Higgs decays in the golden channel, arxiv:1405.1095

 

References

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[2]H. Davoudiasl, H.-S. Lee, and W. J. Marciano, Dark Side of Higgs Diphoton Decays and Muon g-2, Phys.Rev. D86 (2012) 095009, [arXiv:1208.2973].

[3]H. Davoudiasl, H.-S. Lee, I. Lewis, and W. J. Marciano, Higgs Decays as a Window into the Dark Sector, [arXiv:1304.4935].

[4]N. D. Christensen, T. Han, Z. Liu, and S. Su, Low-Mass Higgs Bosons in the NMSSM and Their LHC Implications, [arXiv:1303.2113].

[5]G. Mahlon and S. J. Parke, Using Spin Correlations to Distinguish Zh from ZA at the International Linear Collider, Phys.Rev. D74 (2006) 073001, [hep-ph/0606052].

[6]S. Chang and A. Menon, Discovering Nonstandard Higgs bosons in the H→ZA Channel Decay to Multileptons, JHEP 1302 (2013) 152, [arXiv:1211.4869].

[7]M. M. Almarashi and S. Moretti, LHC Signals of a Heavy CP-even Higgs Boson in the NMSSM via Decays into a Z and a Light CP-odd Higgs State, Phys.Rev. D85 (2012) 017701, [arXiv:1109.1735].

[8]H. Davoudiasl, H.-S. Lee, and W. J. Marciano, Muon Anomaly and Dark Parity Violation, Phys.Rev.Lett. 109 (2012) 031802, [arXiv:1205.2709].

[9]A. Hook, E. Izaguirre, and J. G. Wacker, Model Independent Bounds on Kinetic Mixing, Adv.High Energy Phys. 2011 (2011) 859762, [arXiv:1006.0973].

[10]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].

[11]R. Schabinger and J. D. Wells, A Minimal spontaneously broken hidden sector and its impact on Higgs boson physics at the large hadron collider, Phys.Rev. D72 (2005) 093007, [hep-ph/0509209].

[12]K. Babu, C. F. Kolda, and J. March-Russell, Implications of generalized Z - Z-prime mixing, Phys.Rev. D57 (1998) 6788-6792, [hep-ph/9710441].

[13]ALEPH, DELPHI, L3, OPAL, LEP Working Group for Higgs Boson Searches Collaboration, S. Schael et. al., Search for neutral MSSM Higgs bosons at LEP, Eur.Phys.J. C47 (2006) 547-587, [hep-ex/0602042].

[14]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.

[15]ATLAS Collaboration, 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, ATLAS-CONF-2013-013

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