Evading Dark Matter Bounds through NLSP-Assisted Freeze-Out with Long-Lived Signatures

In this work, we explore a conversion-driven freeze-out scenario, where the next-to-lightest stable particle (NLSP) sets the dark matter (DM) abundance through the process ``NLSP SM $\leftrightarrow$ DM SM". Although DM is produced via a freeze-out mechanism, its interaction strength with the visible sector can range from weak to feeble couplings. This results in a vast, largely unexplored parameter space that evades current direct, indirect, and collider bounds, while remaining testable in the near future. We study this mechanism in the context of an alternative $U(1)_{B-L}$ model, where four chiral fermions are required to cancel gauge anomalies, unlike the usual case with three right-handed neutrinos. The observed relic abundance is successfully reproduced within this framework. The viable parameter space can be probed by future direct detection experiments, while remaining inaccessible to indirect searches. Our results show that the DM relic density is highly sensitive to the NLSP-SM interaction strength and the mass difference between the NLSP and DM, but not to the DM-SM direct interaction. For certain parameter choices, the NLSP decays to DM via two or three body processes involving an extra gauge boson and SM particles, leading to long-lived decays outside the CMS or ATLAS detectors at the LHC. In contrast, if the decay proceeds via a CP-odd Higgs, it occurs promptly within the detector. We investigate prospects for detecting such long-lived NLSPs at the proposed MATHUSLA detector, with similar expectations for the ongoing FASER experiment. Finally, we find that choosing arbitrarily small values of the gauge coupling or BSM fermionic mixing angle can violate successful BBN predictions.