Probing pre-BBN era with scale invariant FIMP

Abstract Detecting dark matter (DM) relic via freeze-in is difficult in laboratories due to smallness of the couplings involved. However, a non-standard cosmological history Universe, prior Big Bang Nucleosynthesis (BBN), can dramatically change this scenario. In context, we study production classically scale invariant U(1) X gauge extension Standard Model (SM), recently dubbed as Scale Invariant FIMP Miracle . We assume an additional species dominates energy density Universe at early times, causing expansion rate given temperature be larger than that standard radiation-dominated case. find, out-of-equilibrium scattering processes involving particles thermal bath lead significantly suppressed DM era, thereby enhancing between visible and sector (by several orders magnitude) satisfy observed abundance, improving detection prospects for turn. invariance underlying theory leaves only four free parameters model: mass m , coupling g transition T R from scalar-dominated era power-law dependence n temperature. show, within minimal set-up, experiments like FASER, MATHUSLA, DUNE, SHiP will probing various models depending on choice { } also PLANCK bound. Moreover, presence naturally light scalar mediator, direct XENON1T, PandaX-4T or XENONnT becomes relevant Higgs-scalar mixing sin ? ? {10 -5 –10 -3 }, thus providing complementary probes freeze-in, well pre-BBN era.