Probing compressed Higgsinos at the FASER experiment

Abstract

In the Minimal Supersymmetric Standard Model (MSSM), compressed Higgsinos spectrum ($Δm^0 \lesssim 1$ GeV) occurs when $|μ| \ll |M_1|, |M_2|$ and ${\rm sign}(M_1\cdot M_2)<0$, which leads to a long-lived next-to-lightest neutralino. Such a long-lived neutralino could be copiously produced at the LHC, however escape the detection at the LHC main detectors. We examine the discovery potential at the FASER experiment and find that the FASER 2 could cover the neutral Higgsino mass up to about 130 GeV with mass splitting between 4 to 30 MeV. It is complementary to both the LHC Higgsino search in the $Δm^{0,\pm} \gtrsim 1$ GeV region, and displaced vertex and disappearing track searches of charginos with $Δm^\pm \lesssim 1$ GeV.

Figures (4)

• Figure 1: The samples in the $(\left| M_{1}\right|, \Delta m^{0})$ plane (left) and $(\Delta m^{\pm}, \Delta m^{0})$ plane (right), color-coded by $\Gamma_{\tilde{\chi}^0_2}$ (left) and $\tan\beta$ (right), respectively, for $M_2=\pm 2$ TeV and $\mu=100$ GeV. Gray points denote charged Higgsino LSPs.
• Figure 2: Experimental constraints on the Higgsino-like electroweakinos in the plane of mass splittings versus the mass of the neutralino (left) and chargino (right). Projected sensitivities from FASER 2 are shown for $\tan\beta = 2$ (solid) and $\tan\beta = 50$ (dashed) in the left panel.
• Figure 3: Density map of $p_{\tilde{\chi}_2^0}$ versus $\theta_{\tilde{\chi}_2^0}$ for $\tilde{\chi}_2^0$ production at the LHC, shown for $\mu = 110~\mathrm{GeV}$. The color scale represents the differential cross section per bin in unit of fb. Vertical dashed lines indicate the angular acceptances of FASER (orange) and FASER 2 (blue), and the black dashed curve shows $p^{\tilde{\chi}_2^0}_{\rm T} = m_{\tilde{\chi}_2^0}$.
• Figure 4: Left: Coverage of LHC searches and FASER 2 in the $(|M_1|,|M_2|)$ plane, with heat map indicating the proper lifetime of $\tilde{\chi}_2^0$. Right: Projected sensitivity of the LHC and FASER 2 in the $(\mu,\tan\beta)$ plane, with the color indicating the value of the $M_2/M_1$ ratio that yields the maximal sensitivity of FASER 2.