Dark matter from electroweak single top production

Dark matter scenarios with spin-0 mediators in the $s$ channel have be tested in well-established processes with missing energy, such as top-pair- and mono-jet-associated production. We suggest electroweak single top production in association with a dark matter pair as an alternative channel. Based on a realistic analysis for the LHC at 13 TeV, we demonstrate how to efficiently discriminate between the signal and standard model background using event kinematics. With $300{\mathrm{fb}}^{-1}$ $(3{\mathrm{ab}}^{-1})$ of data, on-shell scalar mediators with a coupling strength $g_S^t=1$ to top quarks can be probed up to masses of 180(360) GeV. Single-top-associated dark matter production should thus be included as an independent search channel in the LHC dark matter program.

Figure 1

Feynman diagrams describing dark matter production in association with a single top quark through the $t$-channel process. We show the contributions in the 4-flavor scheme (left two diagrams) and in the 5-flavor scheme (right diagram), which we use for our simulation.

Figure 2

Left: production rate for single-top-associated (red) and top-pair-associated (blue) dark matter production with an on-shell scalar mediator. Right: width-to-mass ratio for the scalar mediator. We assume $g_S^t=g_S^{\chi }=1$ and a default mass value of $m_{\chi }=1\mathrm{GeV}$.

Figure 3

Kinematic distributions for the signal process $pp\to tjS\to tj\chi \overline{\chi }$ with a scalar mediator and the relevant backgrounds: the missing transverse energy ${\cancel{E}}_T$ (left) and the transverse mass $m_T$ of the lepton and missing momenta (right). Distributions are shown for the benchmark parameters in Eq. (9) and after applying the cuts from Eq. (11).

Figure 4

Signal and background distributions of the hardest light-flavor jet rapidity (left) and the invariant mass $m_{bj}$ (right) in the benchmark scenario from Eq. (9) with a scalar mediator and after applying the acceptance cuts from Eq. (11).

Figure 5

Sensitivity to single-top-associated dark matter production with a scalar (left) and a pseudoscalar (right) mediator at the 13 TeV LHC with $300{\mathrm{fb}}^{-1}$ (red) and $3{\mathrm{ab}}^{-1}$ (black), assuming a systematic background uncertainty of 3% (plain) and 10% (dashed). Shown is the signal strength $\mu$ that can be excluded at 95% CL, as a function of the mediator mass.

Figure 6

Sensitivity to single-top-associated dark matter production with a scalar (left) and a pseudoscalar (right) mediator at the 13 TeV LHC with $300{\mathrm{fb}}^{-1}$ (red) and $3{\mathrm{ab}}^{-1}$ (black), assuming a systematic background uncertainty of 3% (plain) and 10% (dashed). Shown is the total rate $\sigma (pp\to tjS/P)\times \mathcal{B}(S/P\to \mathrm{inv})$ that can be excluded at 95% CL, as a function of the mediator mass.

Figure 7

Left: production rate for single-top-associated (red) and top-pair-associated (blue) dark matter production with an on-shell pseudoscalar (dashed) and scalar (solid) mediator. Right: width-to-mass ratios. We assume $g_{S,P}^t=g_{S,P}^{\chi }=1$ and a default mass value of $m_{\chi }=1\mathrm{GeV}$.

Figure 8

Scalar (solid) and pseudoscalar (dashed) signal distributions of the missing transverse energy ${\cancel{E}}_T$ (left) and the azimuthal separation ${\varphi }_{\ell ,b}$ between the lepton and the $b$-jet (right). We assume mediator masses of $m_{S,P}=300\mathrm{GeV}$ (red) and $m_{S,P}=50\mathrm{GeV}$ (black), setting the other parameters to $m_{\chi }=1\mathrm{GeV}$ and $g_{P,S}^t=g_{P,S}^{\chi }=1$.