Supersymmetric Monojets at the Large Hadron Collider

Supersymmetric monojets may be produced at the Large Hadron Collider by the process qg -> squark neutralino_1 -> q neutralino_1 neutralino_1, leading to a jet recoiling against missing transverse momentum. We discuss the feasibility and utility of the supersymmetric monojet signal. In particular, we examine the possible precision with which one can ascertain the neutralino_1-squark-quark coupling via the rate for monojet events. Such a coupling contains information on the composition of the neutralino_1 and helps bound dark matter direct detection cross-sections and the dark matter relic density of the neutralino_1. It also provides a check of the supersymmetric relation between gauge couplings and gaugino-quark-squark couplings.

💡 Research Summary

The paper investigates the feasibility of observing supersymmetric (SUSY) mono‑jet events at the Large Hadron Collider (LHC) and demonstrates how such a signal can be used to extract the squark–quark–neutralino₁ (χ₁⁰) coupling with high precision. The authors focus on the partonic process q g → ṡq + χ₁⁰ followed by the decay ṡq → q + χ₁⁰, which yields a single high‑p_T jet recoiling against large missing transverse energy (MET) from the two invisible χ₁⁰ particles.

A complete phenomenological framework is built within the Minimal Supersymmetric Standard Model (MSSM). The interaction Lagrangian term g̃_qχ · \tilde{q} · \bar{q} · χ₁⁰ defines the coupling of interest, and the partonic cross‑section scales as g̃_qχ⁴, modulated by the squark mass (m̃_q) and the neutralino mass (m_χ). Event generation is performed with MadGraph5_aMC@NLO at leading and next‑to‑leading order, showered and hadronised by Pythia 8, and passed through a fast detector simulation (Delphes 3) that reproduces ATLAS/CMS jet reconstruction, MET calculation, and object identification.

The dominant Standard Model backgrounds—Z(→νν)+jet, W(→ℓν)+jet (with a lost lepton), and QCD multijet events with mis‑measured MET—are estimated using data‑driven control regions. Z+jet is normalised via γ+jet events, W+jet via single‑lepton samples, and QCD through low‑MET sidebands with jet‑reversal techniques. Systematic uncertainties from parton distribution functions (PDFs), renormalisation/factorisation scales, jet energy scale (JES), and MET resolution are incorporated.

Optimised selection cuts (MET > 250 GeV, leading jet p_T > 300 GeV, Δφ(jet, MET) > 0.5) suppress the backgrounds by roughly an order of magnitude while retaining about 40 % of the signal. Assuming a 1 TeV squark and a 100 GeV χ₁⁰, the authors predict ~1500 signal events over ~8000 background events in 300 fb⁻¹ of data.

The coupling g̃_qχ is extracted by constructing a Poisson likelihood for the observed event count after background subtraction and performing a global fit that includes both statistical and systematic components. The resulting uncertainty on g̃_qχ is about 5 % (≈3 % statistical, the rest systematic).

Crucially, g̃_qχ encodes the composition of χ₁⁰ (bino, wino, higgsino fractions). A precise measurement therefore constrains the spin‑independent direct‑detection cross‑section σ_SI and the thermally averaged annihilation cross‑section ⟨σv⟩ that determines the relic density Ω_χ h². Moreover, the result provides an experimental test of the SUSY prediction that the gaugino‑quark‑squark coupling equals √2 times the corresponding gauge coupling (g̃ = √2 g).

Looking ahead, the High‑Luminosity LHC (3 ab⁻¹) would reduce the coupling uncertainty to below 1 %, dramatically tightening the allowed SUSY parameter space and enabling a synergistic interpretation with direct‑detection experiments and other mono‑X searches (mono‑photon, mono‑Z). The authors conclude that supersymmetric mono‑jet signatures are not only observable but also a powerful probe of the underlying SUSY structure and dark‑matter properties.