Monojet Signatures from Heavy Colored Particles: Future Collider Sensitivities and Theoretical Uncertainties.

In models with some colored particle $\mathcal{Q}$ that can decay into a dark matter candidate $X$, the hadron collider process $pp\rightarrow\mathcal{Q}\bar{\mathcal{Q}}\rightarrow X\bar{X}+$jets gives rise to events with significant transverse momentum imbalance. In the limit that the colored particles and dark matter candidates masses are very close, the signature becomes monojet-like, and experimental limits from searches at Large Hadron Collider (LHC) become much less constraining. In this paper, we study the current and anticipated experimental sensitivity to such particles at the High-Luminosity LHC at $\sqrt{s}$ = 14 TeV with $\mathcal{L}$ = 3 ab$^{-1}$ of data and the proposed High-Energy LHC at $\sqrt{s}$ =27 TeV with $\mathcal{L}$ = 15 ab$^{-1}$ of data. We estimate the reach for various scenarios of heavy colored particles $\mathcal{Q}$, including spin-0 and 1/2 particles in the triplet and octet representations of QCD. The identification of the nature of $\mathcal{Q}$ is very important to identify the physics scenario behind the monojet signature. Therefore we also study the dependence of the cross section and of observables built from the leading jet in the $pp\rightarrow\mathcal{Q}\bar{\mathcal{Q}} + j $ process on the quantum numbers of $\mathcal{Q}$. Using the state-of-the-art Monte Carlo suites MadGraph5_aMC@NLO+Pythia8 and Sherpa, we find that when these observables are calculated at next-to-leading order (NLO) in QCD with parton shower matching and multijet merging, the residual theoretical uncertainties associated with the renormalization, factorization, and parton shower scales, as well as those associated with parton distribution functions, are comparable to differences observed when varying $\mathcal{Q}$ itself. We find, however, that the precision achievable with next-to-next-to-leading order (NNLO) calculations, where available, could resolve this dilemma.