Long-lived $B-L$ symmetric SSM particles at the LHC

We investigate the collider signatures of neutral and charged long-lived particles (LLPs), predicted by the supersymmetric $B-L$ extension of the Standard Model (BLSSM), at the Large Hadron Collider (LHC). The BLSSM is a natural extension of the minimal supersymmetric Standard Model (MSSM) that can account for nonvanishing neutrino masses. We show that the lightest right-handed sneutrino can be the lightest supersymmetric particle (LSP), while the next-to-the LSP (NLSP) is either the lightest left-handed sneutrino or the left-handed stau, which are natural candidates for the LLPs. We analyze the displaced vertex signature of the neutral LLP (the lightest left-handed sneutrino) and the charged tracks associated with the charged LLP (the left-handed stau). We show that the production cross sections of our neutral and charged LLPs are relatively large, namely of order $\mathcal{O}(1)\mathrm{fb}$. Thus, probing these particles at the LHC is quite plausible. In addition, we find that the displaced dilepton associated with the lightest left-handed sneutrino has a large impact parameter that discriminates it from other SM leptons. We also emphasize that the charged track associated with the left-handed stau has a large momentum with slow moving charged tracks, hence it is distinguished from the SM background, and therefore it can be accessible at the LHC.

Figure 1

Total decay rates of the NLSP including the two- and three-body decays: ${\tilde{\nu }}_{L_1}$ (left) and ${\tilde{\tau }}_L$ (right) as a function of the mass difference $m_{\mathrm{NLSP}}-m_{\mathrm{LSP}}$, with the LSP is $({\tilde{\nu }}_R^{\mathrm{Im}}{)}_1$. The points are obtained by the following inputs: $0.15\le \mu \le 9\mathrm{TeV}$, $M_A\simeq 1.8\mathrm{TeV}$, $2\le \tan \beta \le 10$, ${\mu }^{\prime }\simeq 2.3\mathrm{TeV}$, $M_{A^{\prime }}\simeq 1.4\mathrm{TeV}$, $2\le \tan {\beta }^{\prime }\le 4$, $g_{BL}\simeq 0.5$, $-0.5\le \tilde{g}\le -0.3$, $M_1\simeq 1.2\mathrm{TeV}$, $0.8\le M_2\le 5.5\mathrm{TeV}$, $M_3\simeq 1.2\mathrm{TeV}$, $0.3\le m_{{\tilde{L}}_{11(22)}}^2\le {10}^3\mathrm{TeV}$, $0.1\le m_{{\tilde{L}}_{33}}^2\le 15\mathrm{TeV}$, $m_{{\tilde{N}}_{11}}^2\simeq 10\mathrm{GeV}$, and $m_{{\tilde{N}}_{22(33)}}^2\simeq 10\mathrm{TeV}$.

Figure 2

Feynman diagrams depicting the production mechanism of the LLPs (${\tilde{\nu }}_{L_1}$ and ${\tilde{\tau }}_L$) at the LHC.

Figure 3

Schematic diagram for the LLP displaced vertex.

Figure 4

Displaced distance of the LLPs: ${\tilde{\nu }}_{L_1}$ (left) and ${\tilde{\tau }}_L$ (right) as function of its mass, where the LSP is $({\tilde{\nu }}_R^{\mathrm{Im}}{)}_1$. Horizontal (blue and red) lines correspond to the upper limits on the lifetime of the LLP so that particle reaches the ECAL and the MuC. Here, we have used the same inputs of Fig. 1.

Figure 5

Probability distribution of the neutral LLP (left) and the charged LLP (right) to decay within the detector regions $a$ and $b$, as explained in the text, versus their displaced distance $c{\tau }_0$. Here, red and blue curves denote $\gamma =0.75$ and $\gamma =1$, respectively, and solid curves refer to region $a$ while dashed curves refer to region $b$. Again, we have used the same inputs as in Fig. 1.

Figure 6

$L_{xy}$ for the neutral LLP, ${\tilde{\nu }}_{L_1}$, which is calculated from Eq. (9).

Figure 7

The angular separation in $\varphi$ between dilepton and ${\cancel{E}}_T$.

Figure 8

Impact parameter distribution for the displaced electron or muon pair produced from $Z$ decay at the ECAL or the MuC, respectively.

Figure 9

$L_{xy}$ for the charged LLP, ${\tilde{\tau }}_L$, which is calculated from Eq. (9).

Figure 10

Left: The momentum $p$ of the track associated with the charged LLP, ${\tilde{\tau }}_L$; Right: $\beta$ distribution for the charged LLP, which is calculated from the track information.