New angular and other cuts to improve the Higgsino signal at the LHC

Motivated by the fact that naturalness arguments strongly suggest that the supersymmetry (SUSY)-preserving Higgsino mass parameter $\mu$ cannot be too far above the weak scale, we reexamine Higgsino pair production in association with a hard QCD jet at the High Luminosity LHC. We focus on ${\ell }^{+}{\ell }^{-}+{\cancel{E}}_T+j$ events from the production and subsequent decay, ${\tilde{\chi }}_2^0\to {\tilde{\chi }}_1^0{\ell }^{+}{\ell }^{-}$, of the heavier neutral Higgsino. The novel feature of our analysis is that we suggest angular cuts to reduce the important background from $Z(\to \tau \tau )+j$ events more efficiently than the $m_{\tau \tau }^2<0$ cut that has been used by the ATLAS and CMS Collaborations. Other cuts, needed to reduce backgrounds from $t\overline{t}$, $WWj$, and $W/Z+\ell \overline{\ell }$ production, are also delineated. We plot out the reach of LHC14 for 300 and $3000{\mathrm{fb}}^{-1}$ and also show distributions that serve to characterize the Higgsino signal, noting that Higgsinos may well be the only superpartners accessible at LHC14 in a well-motivated class of natural SUSY models.

Figure 1

Distribution in $m_{\tau \tau }^2$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV introduced in the text, along with SM backgrounds after $\mathbf{C}1$ cuts augmented by $n_j=1$.

Figure 2

Sketch of a ditau background event to the dilepton plus jet plus ${\cancel{E}}_T$ signature in the transverse plane of the event. Here ${\ell }_1$ and ${\cancel{E}}_{T1}$ denote the transverse momentum of the lepton and of the vector sum of the neutrinos from the decay of the first tau, and likewise ${\ell }_2$ and ${\cancel{E}}_{T2}$. ${\cancel{E}}_T(\mathrm{tot})$ is the resultant ${\cancel{E}}_T$ in the event. Notice that because the taus are expected to be relativistic, ${\ell }_i$ and ${\cancel{E}}_{Ti}$ vectors are nearly collimated along the direction of the $i$th tau ($i=1$, 2).

Figure 3

Distribution in ${\varphi }_1$ vs ${\varphi }_2$ plane for $\tau \overline{\tau }j$ background after $\mathbf{C}1$ cuts, requiring also that $n_j=1$.

Figure 4

Distribution in ${\varphi }_1$ vs ${\varphi }_2$ plane for $t\overline{t}$ background after $\mathbf{C}1$ cuts, requiring also that $n_j=1$.

Figure 5

Distribution in ${\varphi }_1$ vs ${\varphi }_2$ plane for signal point BM1 after $\mathbf{C}1$ cuts, requiring also that $n_j=1$.

Figure 6

Distribution in $n(\text{jet})$ for three SUSY BM models with $\mu =150$, 200, and 300 GeV, along with SM backgrounds after $\mathbf{C}1$ and the angular cuts described in the text.

Figure 7

Distribution of the hardest jet $p_T(j_1)$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV and for SM backgrounds after $\mathbf{C}1$ and angular cuts.

Figure 8

Distribution of ${\cancel{E}}_T$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV and for SM backgrounds after $\mathbf{C}1$ and angular cuts.

Figure 9

Distribution of the transverse momentum of the hard lepton $p_T({\ell }_1)$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV and for SM backgrounds after $\mathbf{C}1$ and the angular cuts.

Figure 10

Distribution of the softer lepton $p_T({\ell }_2)$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV SUSY BM models and for SM backgrounds after $\mathbf{C}1$ and angular cuts.

Figure 11

Distribution in $H_T(\ell \overline{\ell })$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV and for SM backgrounds after $\mathbf{C}1$ and angular cuts.

Figure 12

Distribution of ${\cancel{E}}_T/H_T(\ell )$ for three SUSY BM models with $\mu =150$, 200, and 300 GeV and for SM backgrounds after $\mathbf{C}1$ cuts and angular cuts.

Figure 13

Distribution in $m(\ell \overline{\ell })$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV and for SM backgrounds after $\mathbf{C}2$ cuts.

Figure 14

Distribution in $\mathrm{\Delta }\varphi (\text{jet},{\cancel{E}}_T)$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV and for SM backgrounds after $\mathbf{C}2$ cuts.

Figure 15

Distribution in $m_{cT}({\ell }^{+}{\ell }^{-},{\cancel{E}}_T)$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV and for SM backgrounds after $\mathbf{C}2$ cuts.

Figure 16

Distribution in $E_T(\text{jet})/{\cancel{E}}_T$ for three SUSY BM models with $\mu =150$, 200, and 300 GeV, along with SM backgrounds after $\mathbf{C}2$ cuts.

Figure 17

Distribution in $E_T(jet)-{\cancel{E}}_T$ for the three SUSY BM models with $\mu =150$, 200, and 300 GeV and for SM backgrounds after $\mathbf{C}2$ cuts.

Figure 18

Distribution in $\mathrm{\Delta }\varphi (\mu \overline{\mu })$ for three SUSY BM models with $\mu =150$, 200, and 300 GeV, along with SM backgrounds after $\mathbf{C}3$ cuts.

Figure 19

Distribution of $m({\ell }^{+}{\ell }^{-})$ for the three SUSY BM models with $\mu =150$, 300, and 200 GeV and for the SM backgrounds after $\mathbf{C}3$ cuts.

Figure 20

The projected $5\sigma$ reach and 95% C.L. exclusion of the HL-LHC with $3000{\mathrm{fb}}^{-1}$ in $\mu$ for four different NUHM2 model lines with (a) $\mathrm{\Delta }m=4$, (b) $\mathrm{\Delta }m=8$, (c) $\mathrm{\Delta }m=12$, and (d) $\mathrm{\Delta }m=16\mathrm{GeV}$ after $\mathbf{C}3+m(\ell \overline{\ell })<m_{{\tilde{\chi }}_2^0}-m_{{\tilde{\chi }}_1^0}$ cuts. We also list the total background in each frame in case the reader wishes to estimate the statistical significance of the signal for different choices of integrated luminosity.

Figure 21

The projected $5\sigma$ reach and 95% C.L. exclusion contours for LHC14 with 300 and $3000{\mathrm{fb}}^{-1}$ in the $m_{{\tilde{\chi }}_2^0}$ vs $\mathrm{\Delta }m$ plane after $\mathbf{C}4$ cuts. Also shown is the current 95% C.L. exclusion (ATLAS) and the projected 95% C.L. exclusions from two different analyses for the HL-LHC [43].