Missing particle associated with two bottom quarks at the LHC: Mono-$b$ versus $2b$ with razor variables

The extended Higgs sector, such as by a second Higgs doublet of type II, provides portals to the dark sector which contain missing particles at the LHC, e.g., dark matter particles. In this paper, working in the simplified model and taking into consideration the wide decay-width effect of the mediator, we analyze the characteristic signatures of mono-$b+\mathrm{MET}$ and $2b+\mathrm{MET}$ at the LHC. The latter signature was believed to be ineffective, while we found that, with the aid of razor shape analysis, it should be as important as the mono-$b$ signature. In the region of relatively low mediator mass (below a few hundred GeV), by requiring a signal-to-background ratio greater than a few percent, the $2b$-tagged razor analysis has comparable sensitivity to the mono-$b$ search; it is even better for a mediator lighter than $\sim 200\mathrm{GeV}$.

Figure 1

The distributions of $N_j$, $p_T(b_1)$, MET and $\mathrm{\Delta }{\varphi }_{\min }(j_i,{\cancel{E}}_T)$ for the NW and WW scenarios with mediator masses of 100 GeV and 1 TeV at 8 TeV LHC.

Figure 2

The distributions of $N_b$, $M_R$, $R^2$ and $\mathrm{\Delta }\varphi (J_1,J_2)$ for NW and WW scenarios with mediator mass 100 and 500 GeV at 14 TeV LHC. More explanations for the relative size of distributions in backgrounds can be found in the text.

Figure 3

The projected two-dimensional fit function on top of Monte Carlo data for all backgrounds. Upper panels: project to the $M_R$ integrating $R^2$; lower panels: project to $R^2$ integrating $M_R$.

Figure 4

The 95% CL exclusion limits on $Y_b$ in terms of the mono-$b$ analysis and shape analysis at 14 TeV $3000{\mathrm{fb}}^{-1}$ HL-LHC; the latter analysis is demonstrated for two cases, respectively, having signal-to-background ratios greater than 1% and 5%. The left panel ($Y_{\chi }=1$) compares the NW and WW scenarios, while the right shows the influence of varying $Y_{\chi }$ in the WW scenario. The region of ${\mathrm{\Gamma }}_A>m_A$ is marked with a grid in both panels. In the left panel, we also show the ${\mathrm{\Gamma }}_A/m_A$ as a second Y axis because it is monotonically determined by $Y_b$ in the region $m_A\gg m_b$, $m_{\chi }$. In the right panel, different lower bounds of the grid correspond to the different $Y_{\chi }$ with the same line type.