Mono-Higgs + MET in BSM Physics

• Mono-Higgs plus MET channel is a collider signature marked by a directly produced Higgs recoiling against significant missing energy from invisible particles.
• It probes BSM scenarios including dark matter, sterile neutrinos, SUSY, extra dimensions, and hidden sectors through distinctive operator interactions.
• Experimental strategies leverage varied decay channels and advanced multivariate techniques to optimize signal extraction and constrain coupling parameters.

The Mono-Higgs Plus Missing Energy channel serves as a distinctive collider signature in probing physics beyond the Standard Model (BSM), particularly through the production of a Standard Model-like Higgs boson in association with large missing transverse energy (MET) carried by invisible particles. The channel is relevant for dark matter searches, exotic Higgs decays, hidden sectors, neutrino mass models, and extra-dimensional scenarios. Mono-Higgs signals are fundamentally distinguished from mono-jet, mono-photon, and mono-vector boson channels by the unique way in which the Higgs boson is sourced—not via initial state radiation (ISR), but as a direct probe of BSM interaction structures involving the Higgs (Carpenter et al., 2013).

1. Key Signatures and Theoretical Motivation

Mono-Higgs plus MET events are characterized by the presence of a reconstructed Higgs boson (via, e.g., $h\to b\bar{b}$, $h\to\gamma\gamma$) recoiling against substantial missing transverse energy, attributed to new invisible particles such as dark matter candidates, sterile neutrinos, pseudo-goldstinos, axion-like particles, or Kaluza-Klein (KK) excitations. These events arise in various frameworks:

• Dark Matter Searches: Higgs-portal models produce $pp \to h + \chi\chi$ with MET from escaping dark matter particles. The process may proceed via scalar, vector, or fermion mediators, often detailed by effective field theory (EFT) or simplified models (Carpenter et al., 2013, Berlin et al., 2014, Bhowmik et al., 2020, Bahl et al., 2021, Cheung et al., 11 Dec 2024).
• Sterile Neutrino Models: Heavy neutrinos produced via an off-shell $W/Z$ or directly in Higgs decays $pp\to hN N$ generate MET when the sterile neutrinos escape detection (Basso, 2015, Cheung et al., 11 Dec 2024).
• Supersymmetry (SUSY): Neutralinos or other weakly interacting states from the decay of extended Higgs sectors (e.g., NMSSM) or from processes involving pseudo-goldstinos (multi-sector SUSY breaking) produce $h +$MET or $hh +$MET signatures (Petersson et al., 2012, Baum et al., 2017, Dai et al., 2022).
• Extra Dimensions: Higgs-bulk mixing gives rise to invisible decays when the Higgs is coupled to bulk scalars, or produces MET when KK modes are kinematically accessible (Diener et al., 2013, Ghosh et al., 2014).
• Hidden Sector Portals: Axion-like particles (ALPs), singlet scalars, and other weakly coupled light states can yield mono-Higgs signals, often studied with higher-dimensional effective operators (Cheung et al., 11 Dec 2024, Berlin et al., 2014).

Mono-Higgs channels access couplings and operator structures less explored by conventional mono-jet/photon signatures; the Higgs itself directly probes electroweak symmetry-breaking and scalar sector dynamics.

2. Model Implementations and Operator Structures

Mono-Higgs production is modeled within several theoretical frameworks:

• EFT Operators:
  • Higgs Portal (scalar DM): $\lambda |H|^2\chi^2$ (Carpenter et al., 2013).
  • Higher-dimensional operators: e.g., $$(1/\Lambda^4)(\bar{X}\gamma^\mu X)(W^a_{\nu\mu}H^\dagger t^a D^\nu H)$$ for fermion DM (Berlin et al., 2014).
  • ALP interactions: $$(C_{aH}/\Lambda^2)(\partial_\mu a)(\partial^\mu a)(\phi^\dagger\phi)$$ (Cheung et al., 11 Dec 2024).
  • Sterile neutrino-Higgs: $(\lambda_3/M_*)(H^\dagger H)(N N)$ (Cheung et al., 11 Dec 2024).
• Simplified Models:
  • Heavy vector mediators ($Z'$): $pp \to Z' \to h A^0$ with $A^0 \to$ invisible (Berlin et al., 2014).
  • Two-Higgs-doublet and singlet extensions: Additional CP-even/odd scalars decaying to Higgs plus invisible states (NMSSM topologies) (Baum et al., 2017).
  • Higgs-bulk mixing in extra dimensions: Trilinear coupling $gH^\dagger H\Phi$ yielding invisible Higgs decays (Diener et al., 2013).
• Supersymmetric Scenarios:
  • Neutralino-pseudosgoldstino mixing with prompt decays to Higgs and missing energy (Dai et al., 2022).
  • Gravitino-goldstino chains: $h \to \chi_1^0 G$, followed by $\chi_1^0 \to G \gamma$ (Petersson et al., 2012).

The precise operator structure critically affects kinematic distributions (e.g., MET spectrum), decay branching ratios, and signal rates. Validity of the EFT may break down for large event-by-event momentum transfer, requiring unitarity cuts or UV completions.

3. Production Mechanisms and Event Topologies

Mono-Higgs signatures manifest via several key mechanisms and topologies:

• Direct Higgs Radiation at the Vertex: Unlike ISR-dominated mono-X channels, the Higgs is produced as part of the BSM interaction vertex (e.g., $gg\to Z'\to h+ A^0$). This makes mono-Higgs highly sensitive to the underlying Higgs-DM (or Higgs-hidden sector) coupling structure (Carpenter et al., 2013, Berlin et al., 2014).
• Resonant Production: Heavy mediators decaying into Higgs and invisible states (e.g., heavy neutrino $N\to \nu H$ (Basso, 2015), $B\to AA\to (h\chi)(h\chi)$, or $B\to AA\to(hh)(\chi\chi)$ in extended scalar sectors (Blanke et al., 2019)).
• Higgs Cascade Decays: In NMSSM, heavy Higgses decay to a SM Higgs plus a lighter scalar (itself decaying invisibly to neutralinos), producing $$gg\to \Phi_2\to h_{\rm SM}+\Phi_1 \to h_{\rm SM}+\chi_1\chi_1$$ (Baum et al., 2017).
• Associated Production with Invisible Sector: $pp\to h\chi\chi$, $pp\to hN N$, or $pp \to h a a$ with subsequent invisible decays (Bhowmik et al., 2020, Cheung et al., 11 Dec 2024).

Event selection is tailored to the Higgs decay mode (diphoton, $b\bar{b}$, four-lepton) and analysis strategies typically exploit high MET, invariant mass windows, and b-tagging for background suppression.

4. Experimental Strategies, Channels, and Backgrounds

Mono-Higgs analyses exploit multiple experimental strategies:

• Decay Channels:
  • $h\to\gamma\gamma$: Small branching ratio ($\sim0.2\%$) but clean background and excellent MET resolution; dominant sensitivity in many studies (Carpenter et al., 2013).
  • $h\to b\bar{b}$: Highest branching ratio ($\sim57\%$), with substantial backgrounds from $t\bar{t}$, $W/Z + b\bar{b}$, and QCD; requires aggressive b-tagging and boosted jets techniques (Bhowmik et al., 2020, Basso, 2015).
  • $h\to ZZ^*\to 4\ell$, $h\to \ell\ell jj$: Small branching fractions, lower backgrounds.
  • $hh\to bbbb$, $bbWW^*$, $bb\gamma\gamma$ for Higgs-pair plus MET signals (Dai et al., 2022, Blanke et al., 2019).
• Signal Regions and Event Selection:
  • MET thresholds (e.g., $>100$–$200$ GeV) to suppress QCD, $t\bar{t}$, and electroweak backgrounds.
  • Kinematic cuts: $p_T^{\gamma} >$ 30–50 GeV, invariant mass windows for Higgs and di-object reconstruction (e.g., $115 < m_{\gamma\gamma} < 135$ GeV).
  • Lepton vetoes, jet multiplicity cuts, and angular variables ($\Delta R$, $\Delta\phi$) for further discrimination.
• Multivariate and Machine Learning Methods: Advanced analyses employ Boosted Decision Trees (BDTs), Artificial Neural Networks (ANNs), or multi-variate approaches for signal-background separation, using a suite of kinematic observables (e.g., global event properties, jet substructure, Razor variables) (Bhowmik et al., 2020, Blanke et al., 2019).

Backgrounds vary by channel and topology, but significant contributions come from $t\bar{t}+$jets, $W/Z+$ heavy flavor, diboson, and QCD multi-jet. Diphoton and four-lepton modes suffer lower backgrounds but smaller rates.

5. Constraints, Sensitivity, and Model Discrimination

Mono-Higgs plus MET analyses provide powerful probes of BSM interactions:

• Sensitivity to Coupling Constants and Operator Structures:
  • Higgs-ALP interaction: Bounds on $C_{aH}/\Lambda^2$ via $pp\to h a a$ (Cheung et al., 11 Dec 2024).
  • Sterile neutrino coupling: Limits on $\lambda_3/M_*$ via $pp\to h NN$ (Cheung et al., 11 Dec 2024).
  • DM-Higgs coupling strength: Constraints on portal and higher-dimensional operators via signal rates, branching ratios, and missing energy spectrum (Berlin et al., 2014, Bhowmik et al., 2020).
• Mass Coverage:
  • Studies set coupling limits for mass ranges $m_a, m_N$ between 1–60 GeV; improved sensitivity at the HL-LHC with $L=3$ ab$^{-1}$ (Cheung et al., 11 Dec 2024).
  • Exclusion reach for most models extends to hundreds of GeV in invisible mass, depending on coupling strength and background systematics. For Higgsino scenarios, sensitivity is model-dependent and often limited by signal-to-background at the $\sim$1% level with stringent systematic control required (Baer et al., 2014, Carpenter et al., 2021).
• Model Discrimination:
  • Kinematic distributions (e.g., $p_T$ of Higgs, MET spectrum, invariant masses) permit discrimination between different decay topologies (direct $h+$invisible, cascade decays, pair production) and allow reinterpretation in terms of simplified models (Bahl et al., 2021).
  • Decay topology classification (1vs1, 2vs1 balanced, etc.) is robust against spin assignments but sensitive to number and nature of mediators/invisible particles (Bahl et al., 2021).

Experimental constraints from ATLAS and CMS analyses are recast in terms of acceptance $\times$ efficiency maps and visible cross section upper limits, facilitating reinterpretation across broad classes of BSM scenarios.

6. Impact on BSM Physics and Future Prospects

Mono-Higgs plus MET searches yield significant impact:

• Complementarity: These channels probe couplings directly involving the Higgs sector and provide complementary reach to monojet, monophoton, and multilepton searches (Carpenter et al., 2013, Berlin et al., 2014).
• Probing Hidden and Dark Sectors: Mono-Higgs signatures are effective in constraining Higgs-portal ALPs, hidden sectors, and sterile neutrinos, often accessing interactions otherwise invisible to direct detection or low-energy experiments (Cheung et al., 11 Dec 2024).
• Neutrino Mass and Supersymmetry: Mono-Higgs signals test models of neutrino mass generation (symmetry-protected seesaw) and natural SUSY scenarios predicting light higgsinos, pseudo-goldstinos, and enhanced Higgs-pair plus MET rates (Basso, 2015, Dai et al., 2022).
• Extended Scalar Sector and Extra Dimensions: NMSSM and extra-dimensional models are probed through dominant exotic decays of heavy Higgses and invisible Higgs widths (Baum et al., 2017, Diener et al., 2013, Ghosh et al., 2014).

Future Directions:

• High-luminosity LHC runs and improved experimental techniques (e.g., boosted object tagging, refined ML) promise greater sensitivity and expanded parameter coverage (Bhowmik et al., 2020, Blanke et al., 2019, Cheung et al., 11 Dec 2024).
• The presentation of results in terms of decay topologies and acceptance cross-section maps (Bahl et al., 2021) allows for efficient reinterpretation for diverse BSM models.
• Precision measurements in mono-Higgs channels, in conjunction with invisible Higgs decay searches, could help disentangle operator structures and constrain hidden sector physics with unprecedented sensitivity.

The Mono-Higgs Plus Missing Energy channel thus constitutes a central probe for both Standard Model validity and the search for new interactions, dark matter, and hidden sectors at contemporary and future collider experiments.