# h → 1 Lepton-Jet + X

## Contact Person(s)

Stefania Gori, Tao Liu and Jessie Shelton
More details on this mode may be found in Section 16 of Survey of Exotic Higgs Decays (arXiv:1312.4992).

## Simple Lepton-Jets

Higgs decays to collimated pairs of leptons (here l = e,μ but not τ) - often called "lepton jets" - have been a focus of much experimental and theoretical work. For simplicity we focus on simple lepton-jets, consisting of a collimated pair of either electrons or muons, in contrast to complex lepton-jets, which have a larger and more variable particle content that can involve hadrons. Searches for collimated pairs of leptons are typically carried out inclusively, that is, no attempt to reconstruct the Higgs mass (and the lepton-jet resonances) is made, so the same searches constrain decays both with and without MET, although events with MET (or other Higgs daughter products, such as soft jets) will typically have reduced acceptance. Here we consider Higgs decays to one lepton-jet+X; we consider Higgs decays to two lepton-jets+X elsewhere. There are well-motivated signals that produce a single lepton-jet only or dominantly, and exclusive analyses targeting these states, e.g., with a MET requirement or a lepton-jet resonance reconstruction, can yield meaningful sensitivity to these decays.
The opening angle of two partons coming from a parent particle X is  ∼ ∆R ≅ 2 mX/pT,X. For hXX, h at rest, pT,X  ∼ 50 GeV, so partons from the X decay are then typically separated by ∆R < 0.2 when mX < 5 GeV. Thus collimated leptons may arise if the parent particle X has a mass of the order of 10 GeV or less. Of course the transition between isolated leptons and collimated leptons happens smoothly as a function of the parent particle mass mX.

## Theoretical Motivation

One theory that realizes the decay h → (μμ) + MET is [1,2] the PQ-symmetric limit of the NMSSM. Decays of the Higgs to h→χ1χ2 or h→χ2χ2, with subsequent decays χ2 → χ1 s,  χ2 → χ1 a, give Higgs decay signatures with missing energy in the final state. If s (or a) has mass ~ O(1) GeV or below, its branching fraction into light leptons becomes appreciable (O(10%)); see discussion of 2HDM+Scalar. The resulting signatures are dileptons + MET for h→ χ1χ2 and four leptons + MET for h→ χ2χ2. If decays to muons dominate, typically we will have pT(a,s) >> ma,s, and the daughter muons will be collimated: ∆Rll <~0.1.
Dark photons ZD also realize this signature. Possible connections with cosmic ray anomalies [3,4] and (g−2)μ [5] have stimulated interest in mZD ∼ 1 GeV, but dark photons decay often to e+e, μ+μ even for large mZD. (For discussion of uncollimated leptons from dark vectors, see here and here.) In supersymmetry, h→ χ2χ1 decay (with χ2ZDχ1) can occur but is only important if the decay h→ χ2χ2 is kinematically forbidden. In a more general Hidden Valley, the role of the neutralinos χi may be played instead by hidden sector mesons Kv, πv or similar states, and the ZD may be composite. Also possible is hZD ZD, followed by ZD→ lepton jet on one side and ZD→ invisible states on the other. Similar signatures can be obtained in haa if a is coupled to a hidden sector.
Relevant decays that may lack MET include h→ (μμ) + (jj), e.g. in ha a where one a decays to muons and the other to hadrons. The same can occur for hZD ZD but is less likely to be experimentally important, since four-lepton final states are more common than for haa.

## Existing Collider Studies

A dedicated analysis for

(1)

is presented in [2], which focuses on a benchmark with a light scalar(pseudoscalar) resonance s(a) with a mass of 1 GeV (see Table below.)

Table 1: Mass parameters of the h → collimated leptons + MET benchmark model [2].
The analysis focuses on W± h production where the W decays leptonically, so the signal contains one hard lepton from the W decay, two collimated muons, and MET. With 20 fb−1 data, and

(2)

assumed, a sensitivity S/√B > 6σ can be achieved at the 8 TeV LHC.
This analysis is sensitive to other decay modes. If the light resonance is a vector, then a wider range of masses should be considered. Another possibility arises in the NMSSM from the decay chain h→ χ2χ2 → (μμ)(ττ)+MET or  (μμ)(bb) +MET. It is also of interest, for future work, to consider gluon fusion and VBF production, where lepton-jet or even dilepton triggers may yield a reasonable acceptance for this decay mode.

## Existing Experimental Searches and Limits

Existing experimental bounds from LHC searches do not place any limits on the branching fraction Br(h→ μ+μ− +MET) in the collimated/low-mass regime. Leptons arising from very light parents will typically fail standard isolation requirements. Potentially significant bounds come from dedicated searches for lepton-jets, where modified lepton isolation criteria are applied, and low mass ranges are considered. Searches for lepton-jets have been pursued by both CMS [6] and ATLAS [7,8,9].  None of the ATLAS analyses is sensitive to a final state with a single, prompt, simple lepton-jet. The CMS search [6] looks for p p(ϕ→ μ+μ)+X, for new bosons ϕ with masses below 5 GeV, using only 35 pb−1 of 7 TeV data. But selection cuts of |ημμ| < 0.9 and pT,μμ > 80 GeV are applied for the muon pair, eliminating most of the signal for the decay mode Eq. (1).

## Proposals for New Searches at the LHC

A search for h→ one lepton-jet (or one light resonance)+ MET is highly motivated on both theoretical and experimental sides. For h→ (μμ) + MET and ceff = 0.1, and mμμ=1 GeV, a sensitivity S/√B > 6 σ can be achieved, using the 8 TeV LHC data [2]. Good sensitivity for probing a wider range of masses should be expected.

## References

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[2] J. Huang, T. Liu, L.-T. Wang, and F. Yu, Supersymmetric Exotic Decays of the 125 GeV Higgs BosonPhys. Rev. Lett. 112 (2014) 221803, [arXiv:1309.6633].
[3] N. Arkani-Hamed, D. P. Finkbeiner, T. R. Slatyer, and N. Weiner, A Theory of Dark Matter, Phys.Rev. D79 (2009) 015014, [arXiv:0810.0713].
[4] M. Pospelov and A. Ritz, Astrophysical Signatures of Secluded Dark Matter, Phys.Lett. B671 (2009) 391-397, [arXiv:0810.1502].
[5] M. Pospelov, Secluded U(1) below the weak scale, Phys. Rev. D80 (2009) 095002.
[6] 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].
[7] 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].
[8] 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].
[9] 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].

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