Monojet Studies at the LHC

• Monojet studies at the LHC are experimental searches characterized by a high-pT jet recoiling against large missing transverse energy, indicating the production of invisible or weakly interacting particles.
• Experimental methodologies employ stringent event selection, advanced ISR jet analysis, and machine learning techniques to suppress Standard Model backgrounds and enhance signal sensitivity.
• Interpretations range from effective field theory approaches to UV-complete models, providing constraints on dark matter candidates, compressed supersymmetry, and invisible Higgs decays while guiding future collider upgrades.

Monojet studies at the Large Hadron Collider (LHC) constitute a pivotal experimental and theoretical strategy to search for new physics scenarios involving non-interacting or weakly interacting invisible particles such as dark matter candidates. The defining signature is a high-transverse-momentum (pₜ) jet recoiling against large missing transverse energy (MET), resulting from particles that escape the detector without interacting. These studies bridge collider physics, dark matter phenomenology, effective field theory modeling, and searches for a broad spectrum of new-physics signatures, including compressed supersymmetric spectra, inelastic dark matter, feebly interacting particles, and invisible Higgs decays.

1. Theoretical Foundations and Signal Topologies

Monojet signatures arise when new, stable (or long-lived), neutral particles are produced in association with a high-pₜ jet, the latter typically originating from initial-state radiation (ISR) or from the decay of a heavy colored particle. The generic process is: $pp \rightarrow \text{Invisible States} + j$ where "Invisible States" may be a dark sector neutralino pair (Arbey et al., 2013), a pair of inelastic dark matter states (χ, χ*) (Bai et al., 2011), vector, scalar, or fermionic dark matter (Roy et al., 18 Sep 2025, Kumar et al., 2015), or other new particles such as feebly interacting massive particles (FIMPs) (Claude et al., 2022).

The simplified model and effective field theory (EFT) paradigms play central roles in modeling these interactions. The EFT approach supplements the Standard Model (SM) by higher-dimension operators: $$\mathcal{L}_{\text{EFT}} = \frac{1}{\Lambda^2} (\bar{\chi} \Gamma \chi)(\bar{q} \Gamma q)$$ with Λ the new physics scale. In contrast, simplified models introduce explicit mediators, such as a heavy s-channel axial vector or pseudoscalar, whose mass and couplings control the signal kinematics (Belwal et al., 2017).

If kinematic conditions allow, more complex signal topologies can supplement or dominate the canonical ISR monojet signature. For instance, in R-parity–conserving supersymmetry, monojet-like events may also arise from the decay of a squark or gluino close in mass to the lightest neutralino, with the high-pₜ jet coming from the colored particle's decay, as in: $$pp \rightarrow \tilde{q} \tilde{\chi}^{0},\ \tilde{q} \rightarrow q + \tilde{\chi}^{0}$$ (Lara et al., 2022).

2. Experimental Methodologies and Analysis Techniques

Monojet analyses at the LHC involve stringent event selection criteria designed to maximize sensitivity to new physics while suppressing SM backgrounds, particularly Z(→νν̄)+jets, W(→ℓν)+jets, and top quark pair production. Key components include:

• Requirement of a leading jet with high pₜ (e.g., > 500 GeV (Drees et al., 2012), > 1200 GeV (Claude et al., 2022))
• Large missing transverse energy (e.g., MET > 150–450 GeV (Bai et al., 2011, Drees et al., 2012))
• Vetoes on isolated leptons and additional jets (e.g., no second jet with pₜ > 100 GeV)
• Suppression of events with b-tagged jets to reduce top background (Drees et al., 2012)

SM backgrounds are simulated and calibrated using a combination of Monte Carlo generators (MadGraph5, Pythia8, Sherpa), fast detector simulation (Delphes), and data-driven techniques (e.g., using Z(→ℓ⁺ℓ⁻)+jets for background estimation (Drees et al., 2012)). Systematic uncertainties, including those on background normalization, lepton/veto efficiency, and jet energy scale, are incorporated into statistical procedures such as a χ² fitting of NP yields in exclusive MET bins (Roy et al., 18 Sep 2025).

Recent developments in analysis methodologies include the use of machine learning classifiers, notably deep neural networks trained on 2D histograms of jet kinematics (pₜ, η), allowing statistical sensitivity to be expressed as a function of S/√B (signal-to-background significance) (Arganda et al., 2021). Additional techniques for operator discrimination, such as jet energy profiles (JEPs), have been shown to improve the separation between quark- and gluon-initiated jets and to enable the differentiation of underlying dark matter operators (Agrawal et al., 2013).

3. Principal Scenarios and Phenomenological Applications

Monojet studies have been extensively applied to a wide range of NP scenarios:

a) Inelastic and Non-Minimal Dark Matter

Inelastic dark sector models (χ, χ) produce monojet events accompanied by potentially displaced hadronic activity (e.g., displaced pions) (Bai et al., 2011). The mass splitting Δ between χ and χ controls both the kinematics and the lifetime of χ*, affecting where (and whether) displaced tracks are observed in the detector. For Δ ≳ 1 GeV, energetic, displaced pions allow for further background rejection, providing a unique handle over background-only monojet searches.

b) Compressed Supersymmetric Spectra

Monojet searches are particularly sensitive to scenarios where SUSY particles (e.g., stops, squarks, gluinos) are nearly degenerate with the LSP, making their decay products too soft to be detected in standard channels (Drees et al., 2012, Schwaller et al., 2013, Arbey et al., 2015, Han et al., 2013, Lara et al., 2022). The sensitivity is enhanced in the so-called "co-annihilation corridor", which is phenomenologically motivated by thermal relic density calculations (Arbey et al., 2013).

c) Higgs-Portal and Invisible Higgs Decays

Monojet signatures also arise in searches for invisible decays of the Higgs boson (e.g., H → invisible), especially in Higgs-portal dark matter scenarios (Djouadi et al., 2012). The process pp → H + j, with H decaying invisibly, can be constrained by requiring high MET. Current monojet searches place direct 95% CL constraints on the rate of invisible Higgs decays at the level of the SM Higgs production rate, with prospects for improvements at higher luminosity and energy (Djouadi et al., 2012).

d) Feebly Interacting and Freeze-In Dark Matter

Monojet searches probe FIMP scenarios and freeze-in mechanisms where couplings are too small for thermal equilibration, but where the production of FIMPs via momentum-dependent interactions (e.g., through a gluophilic Z′ portal) nevertheless leads to detectable MET signals at colliders (Claude et al., 2022).

e) Effective Field Theory and UV-Complete Models

The interpretation of monojet searches in terms of EFT operators is common, but limitations exist. Validity requires momentum transfers below the mediator mass, and higher-order terms (such as dimension-8 operators or double mediator exchange diagrams) become significant near current bounds (Belwal et al., 2017, Roy et al., 18 Sep 2025). UV completions, such as vector-like quark extensions, provide a more accurate description once on-shell mediator production is relevant (Roy et al., 18 Sep 2025).

4. SM Backgrounds, Theoretical Uncertainties, and Precision Improvements

A defining challenge in monojet searches is the accurate prediction and control of SM backgrounds. The dominant irreducible background is Z(→νν̄)+jets, followed by W(→ℓν)+jets and tt̄. Theoretical predictions have evolved to include:

• Next-to-leading-order (NLO) and next-to-next-to-leading-order (NNLO) QCD corrections
• Full NLO electroweak (EW) corrections, which are essential at high pₜ due to large negative Sudakov logarithms (e.g., a −15% to −25% correction in the TeV region (Denner et al., 2012))
• Application of jet vetoes and phase-space–dependent renormalization/factorization scales to stabilize predictions (Denner et al., 2012, Chakraborty et al., 2018)

State-of-the-art event simulations use multijet merging at NLO (e.g., Sherpa MEPS@NLO, MadGraph5_aMC@NLO+Pythia8) (Chakraborty et al., 2018). Theoretical uncertainties from scale choices, parton distribution functions, and parton shower modeling are at the 10–30% level and are comparable to differences induced by hypothetical changes in the produced particle’s quantum numbers. Achieving NNLO accuracy is required for the robust discrimination of new physics scenarios based on kinematic distributions.

5. Complementarity with Direct Detection and Broader Constraints

Collider monojet studies complement direct and indirect detection experiments for dark matter. In scenarios where operators produce suppressed signals in direct detection (e.g., momentum- or velocity-dependent interactions (Barducci et al., 2016), or flavor off-diagonal couplings (Roy et al., 18 Sep 2025)), collider bounds become the leading constraint. Conversely, for velocity-independent, spin-independent operators, direct detection is typically more sensitive, but monojet searches still constrain otherwise inaccessible regions (such as low-mass DM or combinations with DD-insensitive interference) (Roy et al., 18 Sep 2025, Kumar et al., 2015).

In Higgs-portal models, LHC constraints on the branching ratio of H to invisible final states translate into upper bounds on the DM–nucleon cross section, which can be more stringent than those of direct search experiments for m_χ < M_H/2 (Djouadi et al., 2012). Combining LHC monojet results with jets+MET, leptonic, and multilepton channels provides complete coverage of SM extensions, particularly in compressed or co-annihilation scenarios (Schwaller et al., 2013, Arbey et al., 2015, Lara et al., 2022).

6. Operator and Model Discrimination

Monojet cross sections and kinematics alone often do not reveal the spin, Lorentz structure, or coupling type of the mediator between the SM and dark sector. Advancements include:

• Jet energy profile (JEP) techniques to distinguish quark- and gluon-initiated jets, improving exclusion limits and facilitating operator discrimination at the 95% CL if a signal is discovered (Agrawal et al., 2013)
• MET spectrum shape analysis to differentiate heavy dark matter from massless invisible states (e.g., heavy dark matter exhibiting a harder MET fall-off than neutrinos) (Franzosi et al., 2015)
• Multimodal deep learning classifiers trained on kinematic histograms to identify and diagnose new physics models underlying a monojet excess (Arganda et al., 2021)

Such analyses enable statistically robust separation of signal models—such as EFTs with quark vs. gluon couplings, or identification of heavy mediator vs. effective operator dominance—even within the broad "monojet" category.

7. Future Prospects and Experimental Outlook

The sensitivity of monojet searches is poised to increase substantially with future LHC upgrades (HL-LHC at 14 TeV, HE-LHC at 27 TeV), driven by both higher luminosity (e.g., 3 ab⁻¹) and increased collision energy (Chakraborty et al., 2018). The extension of signal acceptance to higher jet pₜ and MET thresholds, reduction of systematic uncertainties (from e.g. 5% to 1%), and improvements in analysis technology (neural networks, advanced multivariate techniques) will further sharpen discovery and exclusion reach in the coming decade (Schwaller et al., 2013, Arganda et al., 2021).

At the theoretical level, moving beyond the EFT paradigm to simplified models and their proper matching to explicit UV completions is expected to play a critical role, particularly as more data are accumulated at higher energies where the limitations of EFT become acute (Belwal et al., 2017, Roy et al., 18 Sep 2025). The combination of precision Standard Model calculations, advanced signal modeling, broader experimental coverage, and multi-channel approaches ensures that monojet analyses will remain a cornerstone in the search for new physics at hadron colliders.