Monojet and direct detection constraints on real scalar dark matter: EFT and a simple UV completion

TL;DR

This work confronts real scalar dark matter φ with LHC monojet and direct-detection constraints, contrasting an EFT description up to dimension six (φ-SMEFT) with a simple UV completion featuring vector-like quarks (VLQs). It shows that EFT bounds can overstate or misrepresent UV constraints when high-$E_T^{miss}$ data are included, due to breakdown of the EFT, possible on-shell mediator production, and scale choices in simulations; restricting to low-$E_T^{miss}$ bins improves EFT–UV agreement. The VLQ UV model provides distinctive collider signatures and a clearer interpretation of EFT limits, with matching relations linking EFT Wilson coefficients to VLQ Yukawas. Direct-detection and relic-density analyses reveal priority regions where collider searches offer complementary coverage, especially for flavor-off-diagonal couplings and for DM masses where DD is less sensitive. Looking ahead, HL-LHC significantly enhances sensitivity, while the combination of collider and DD constraints yields a comprehensive probe of real scalar dark matter in both EFT and UV frameworks.

Abstract

We consider constraints that can be placed on certain invisible scalar particles through monojet studies at the LHC and compare them with those from direct detection experiments when interpreted as dark matter. Whereas direct detection constraints are typically more restrictive, we identify regions of parameter space where monojet studies provide important complementary bounds. We carry out our analysis using both a $φ$SMEFT for real scalar particle pairs coupled to standard-model fields through operators of up to dimension six, and a simple UV completion with vector-like quarks, with both the scalars and the vector-like quarks being odd under a $\mathbb{Z}_2$ symmetry, while the SM particles are even. The vector-like quarks can only decay into a jet and an invisible scalar, and we recast the current ATLAS monojet data to constrain their parameter space. Comparison of the two descriptions yields some insight into interpreting dark matter constraints obtained with EFTs.

Figures (13)

• Figure 1: A few representative Feynman diagrams of the process $p p \to \phi \phi j$ (monojet) for VLQS (upper pannel) and $\rm \phi SMEFT$ (bottom pannel).
• Figure 2: Allowed (solid) and projected (hatched) 95$\%$ CL regions at luminosities $\mathcal{L}=140$ and $3000~\rm fb^{-1}$ respectively for VLQS parameters ($y_{q/d}^d, y_{q/d}^s$) at the chosen benchmark, $y_1=1, \kappa=\lambda=0, M_{Q/D}=3~\rm TeV$ (top left), and selected $\rm \phi SMEFT$ WCs (remaining panels) shown in yellow. The DM mass is chosen to be $m_\phi=1$ GeV. The dashed lines in the top-left panel indicate perturbativity limits.
• Figure 3: Comparison of the allowed parameter region for a UV complete model obtained in two different manners: directly constraining the UV completion (VLQS) (purple), translating constraints placed on the EFT ($\rm \phi SMEFT$) (blue) via the matching equations \ref{['eq:matching1']} for $M_{Q}=3$ TeV and $m_{\phi}=1$ GeV. The left plot shows the result when using all the exclusive $\rm E{\!\!\!/}_T$ bins, and the centre panel excludes the EM10 $\rm E{\!\!\!/}_T$ bin. The right panel uses only bins with $\rm E{\!\!\!/}_T<350$ GeV. The gray dashed lines indicate the perturbativity limit for $y_{q/d}^{d/s}$.
• Figure 4: Distribution of the truth-level invariant mass of two $\phi s$ and the jet ($M(\phi,\phi,j)$, top panel) and Missing pT ($\rm E{\!\!\!/}_T$, bottom panel) for the monojet process $pp\to \phi\phi j$ at $\sqrt{s}=13$ TeV LHC. The left figures consist of the full set of events, whereas in the right figures, events with at least one $\rm M(\phi,j)\in [M_Q-2\Gamma_{Q}, M_Q+2\Gamma_{Q}]$ are vetoed. The NP-scale ($M_{Q/D}$ and $\Lambda$) is set to 3 TeV and $\rm m_{\phi}=1$ GeV. The parameters for the VLQS $y_{q/d}^d = y_{q/d}^s = y_1 = 1$ and all the relevant $\rm \phi SMEFT$$\mathcal{C} = 1$. The subplots in the bottom panel present the deviation of the observed ATLAS events from the standard model prediction, quantified by pull.
• Figure 5: Feynman diagrams of $d\bar{s}\to \phi\phi$ in VLQS (left) and $\rm \phi SMEFT$(right) .