Seeking leptoquarks in the $t\overline{t}$ plus missing energy channel at the high-luminosity LHC

The $t\overline{t}$ plus missing energy channel is one of the most efficient to detect third-generation leptoquarks (LQs). It offers an important test to models which explain flavor anomalies in $B$ meson decays. We outline a search strategy in the channel, relying on tagging the tops and on observables constructed out of the tops, and we assess the reach on LQs of the future high-luminosity LHC program. We find that with $3{\mathrm{ab}}^{-1}$ a vector (scalar) LQ decaying 50% (100%) to top and neutrino can be excluded up to masses of 1.96 TeV (1.54 TeV). We also indicate several observables that, in case of a future discovery in the channel, can be used to distinguish a scalar LQ from a vector LQ. The implications of our findings to models addressing the recent flavor anomalies are finally discussed.

Figure 1

Leading diagrams for the pair production of LQs at the LHC.

Figure 2

Cross section values at the 14 TeV LHC for the QCD pair production of vector (blue line) and scalar (red line) LQs. For the vector $U_1$, we indicate the cross sections in both the minimal coupling $k=0$ (dashed line) and the Yang-Mills $k=1$ (continuous line) scenarios. The cross sections are calculated at (next-to-)leading order in QCD for the (scalar) vector LQ.

Figure 3

Normalized distributions for background and a signal from a LQ $U_1$ of 1.7 TeV (in the YM case). (Left) Invariant mass of the $p_T$-leading jet. The red dotted lines mark the window around the top mass considered in this analysis. (Right) Invariant mass of the second-leading jet.

Figure 4

HL-LHC reach. Integrated luminosity required to exclude at 95% C.L. (black line) or to observe at $3\sigma$ (blue dashed line) a scalar LQ $S_3$ (upper plot) and a vector LQ $U_1$ (lower plots) as a function of their mass. For $U_1$, the plot on the left (right) refers to the YM (MC) scenario with $k=1(0)$.

Figure 5

Normalized distributions of top observables for the background (black dotted curve) and the signals from a scalar LQ $S_3$ of 1.3 TeV (red curve) and from a vector LQ $U_1$ of 1.7 TeV with $k=1$ (blue curve). The tagging of the tops and the following cuts are applied: ${\cancel{E}}_T>500\mathrm{GeV}$, $M_{tt}>800\mathrm{GeV}$. (Left, upper) Distributions of the $M_{T2}$ variable as defined in Eq. (9). (Right, upper) Invariant mass distributions of the reconstructed top pair. [Left (right), lower] $p_T$ distribution of the reconstructed leading (second leading) top.

Figure 6

Normalized distributions of the azimuthal angular separation between the two reconstructed tops for the background (black dotted curve) and the signals from a scalar LQ $S_3$ of 1.3 TeV (red curve) and from a vector LQ $U_1$ of 1.7 TeV with $k=1$ (blue curve). The top tagging procedure and the following cuts are applied: ${\cancel{E}}_T>500\mathrm{GeV}$, $M_{tt}>800\mathrm{GeV}$.

Figure 7

Coupling vs mass parameter space for the vector LQ $U_1$ model preferred by $B$-physics anomalies compared to 95% C.L. limits obtained in our analysis in the $t\overline{t}$ plus missing energy channel. The green band is extracted from [40] and represents the $1\sigma$ region preferred by low-energy flavor observables, and the gray band indicates the current limit on $m_{U_1}$, from the CMS analysis [29] at $\sqrt{s}=13\mathrm{TeV}$ and $35.9{\mathrm{fb}}^{-1}$. The blue lines are our lower bounds on the $U_1$ mass at $\sqrt{s}=14\mathrm{TeV}$ with a luminosity of 300 (dashed line) and 3000 (dotted line) ${\mathrm{fb}}^{-1}$. The plot on the left (right) refers to the YM (MC) scenario.

Figure 8

Coupling vs mass parameter space for the scalar LQ $S_3$ model preferred by $B$-physics anomalies compared to 95% C.L. limits obtained in our analysis in the $t\overline{t}$ plus missing energy channel. The $1\sigma$ region preferred by low-energy flavor observables (green band) is extracted from [39], and the gray band indicates the current limit on $m_{S_3}$, from the CMS analysis [29] at $\sqrt{s}=13\mathrm{TeV}$ and $35.9{\mathrm{fb}}^{-1}$. The blue lines are our lower bounds on the $S_3$ mass at $\sqrt{s}=14\mathrm{TeV}$ with a luminosity of 300 (dashed line) and 3000 (dotted line) ${\mathrm{fb}}^{-1}$.