Direct Detection of Millicharged Particles from Evaporating Primordial Black Holes
This paper investigates the direct detection of millicharged particles emitted by evaporating primordial black holes via Hawking radiation, utilizing electron recoil signals from XENONnT, JUNO, and Super-Kamiokande to establish stringent constraints on the particles' mass and charge parameters.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Deep in the quiet corners of the universe, where the laws of physics are pushed to their absolute limits, there exists a theoretical possibility that could rewrite our understanding of matter. For decades, scientists have searched for particles that are not part of the standard family of atoms and light we see every day. Among the most intriguing candidates are "millicharged particles," hypothetical objects that carry a tiny fraction of the electric charge of an electron. If they exist, they would be ghostly, slipping through ordinary matter almost unnoticed, yet they might hold the key to mysteries like dark matter. Another cosmic enigma involves primordial black holes, which are not the massive, star-swallowing giants formed by collapsing stars, but rather tiny, ancient remnants created in the first fraction of a second after the Big Bang. While these black holes are too small to see directly, theory suggests they are not truly black; they slowly leak energy and particles into space, a process known as Hawking evaporation. The question that drives this research is simple yet profound: could these ancient, evaporating black holes be pumping a steady stream of millicharged particles into our galaxy, waiting to be caught by the most sensitive detectors on Earth?
A researcher set out to answer this by calculating whether such particles could survive the journey from the depths of space to the underground laboratories where physicists hunt for them. They focused on two specific populations of these primordial black holes, one group that began with a mass of one quadrillion grams and another that started at eight times that weight. As these black holes evaporate over billions of years, they emit a spectrum of particles. The researcher modeled how these particles, carrying a minuscule electric charge, would travel through the magnetic fields of our galaxy, the Milky Way. This journey is not a straight line; the particles are constantly buffeted by magnetic winds and lose energy as they interact with the sparse matter of interstellar space. The researcher had to account for how the solar wind, the stream of charged particles blowing from our Sun, would further filter the flow before these particles even reached Earth.
Once the particles arrive at our planet, they face their final and most difficult obstacle: the Earth itself. To reach the detectors, which are buried deep underground to shield them from cosmic noise, the particles must pass through thousands of meters of rock. This "overburden" acts as a thick shield, stripping away energy from any passing particle. The researcher calculated exactly how much energy a millicharged particle would lose as it drilled through the rock, depending on its mass and speed. They found that only those particles with enough initial energy and the right amount of charge could survive the trip and reach the sensitive instruments without being stopped or slowed too much to be detected.
The researcher then simulated what would happen if these surviving particles struck the detectors. They looked at three major experiments: XENONnT, a tank of liquid xenon deep in Italy; JUNO, a massive liquid scintillator detector in China; and Super-K, a giant water tank in Japan. Each of these facilities is designed to see the faint flash of light produced when a particle bumps into an electron. The researcher calculated the expected number of these "electron recoil" events for different combinations of particle mass and charge. They compared these predictions against the actual data already collected by these experiments. If the detectors had seen more flashes than expected from known sources like neutrinos, it would be a sign that millicharged particles were present. However, the data showed no such excess.
The result of this careful analysis is a set of strict new boundaries on where these particles can exist. The researcher found that for the heavier group of primordial black holes, the emission of heavier millicharged particles is strongly suppressed, meaning the detectors would not see them. For the lighter, hotter black holes, the emission is more efficient, but the detectors still saw nothing. By combining the null results from XENONnT, JUNO, and Super-K, the researcher was able to rule out a vast range of possible masses and charges for these particles. Specifically, they found that if these particles exist, they cannot have the specific combinations of mass and charge that would have produced a detectable signal in these experiments. The study demonstrates that the low-energy sensitivity of XENONnT is particularly powerful, allowing it to probe the highest mass regions for these particles, while the other detectors provide complementary constraints.
This work does not prove that millicharged particles do not exist, but it does significantly narrow the search space. It tells us that if these ghostly particles are being produced by evaporating primordial black holes, they are either much lighter or much more weakly charged than the models predicted, or perhaps they do not exist in the quantities required to be seen by current technology. The researcher also noted that while other cosmic events, like supernova explosions, might disturb the flow of these particles, the continuous injection from black holes throughout the galaxy ensures that the population remains steady enough to be studied. Ultimately, this study turns the silence of the detectors into a powerful statement, defining the limits of what is possible in the hidden sector of particle physics and guiding future searches toward the remaining, unexplored territories.
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