The oven slowly cools. While it's hot, the dough rises vigorously, but once the temperature drops, the process stalls. That's roughly how dark matter was born in the 'freeze-out' model: in the hot early cosmos, particles collided and abounded, but as the universe expanded and cooled, their number froze, like baked goods set in place.
However, two sensitive detectors, DAMIC-M and PandaX, peered into the realm of ultra-light particles (with masses from three to a thousand electron masses) and saw nothing. The simple 'freeze-out' with an additional ultra-light force carrier Standard Model didn't work: the calculated density of such dark matter would be higher than observations allow. The authors didn't discard the idea; instead, they asked: what if the oven wasn't that hot? Or the dough was added later?
The second scenario is hybrid: a small fraction of dark matter arose not from collisions but from the decay of the inflaton (the field that inflated the universe). It's like quietly adding ready-made buns to a cooling oven. Even a tiny admixture changes the final picture and shifts predictions into a testable region. Thus, the work turns a negative search result into a map of new possibilities: dark matter might hide not in ultra-weak but in moderately weak interactions, and to catch it, we just need to tune the instruments to a different mode.
as written by the authors arXiv:2606.12408
Recent results from the DAMIC-M and PandaX collaborations have excluded the standard freeze-in production of dark matter for masses in the range $$3~\mathrm{MeV} \lesssim m_\chi \lesssim 1~\mathrm{GeV}$$ in the context of extensions of the Standard Model featuring an additional ultra-light $$U(1)_{\rm X}$$ gauge boson. In this work, we analyze the constraints induced by DAMIC-M and PandaX results on the reheating temperature in freeze-in models at stronger coupling, or when a non-thermal source (such as inflaton decay) comes into play. We identify viable scenarios in which the DM relic abundance is correctly reproduced while evading current experimental bounds on the electron-scattering cross section, $$\overline{\sigma}_\mathrm{e}$$. In particular, we show that for reheating temperatures below the electroweak scale, Boltzmann suppressed production can be compensated by stronger couplings, bringing freeze-in scenarios within present experimental reach. Finally, we study a hybrid scenario in which a small branching ratio of inflaton decay seeds a nonzero initial dark-matter abundance. We show that such contributions can significantly modify freeze-in predictions across broad regions of parameter space, offering an additional pathway for probing extremely feeble interactions.
The temperature of the electroweak transition (about 100 GeV) is a hundred million times hotter than the Sun's core — at that temperature, the electromagnetic and weak forces were still a single force.