Constraints and Projections for Millicharged Dark Matter in the Sun with Water Cherenkov Neutrino Detectors

This paper demonstrates that the lower energy thresholds of Super-Kamiokande and the future Hyper-Kamiokande water Cherenkov detectors enable them to constrain previously unexplored parameter space for lighter millicharged dark matter in the Sun, offering sensitivity to fractional abundances nearly an order of magnitude below current IceCube limits.

The Gist

Imagine the universe is filled with invisible "ghosts" called Dark Matter. We know they exist because of their gravity, but we don't know what they are made of. One popular theory suggests these ghosts might have a tiny, tiny electric charge—so small it's like a speck of dust compared to a lightning bolt. Scientists call these "millicharged particles."

This paper is a detective story about how we can catch these ghosts using the Sun as a giant trap and underwater telescopes as our eyes.

The Setup: The Sun as a Cosmic Vacuum Cleaner

The Sun is huge and has a massive gravitational pull. Think of it as a giant vacuum cleaner floating in space. As millicharged particles drift through the galaxy, some get sucked into the Sun's gravity.

Once inside, they crash into the Sun's atoms. Because these particles have a tiny electric charge, they interact with the Sun's matter more strongly than normal dark matter would. They lose energy, slow down, and get stuck. Over billions of years, the Sun acts like a bucket, filling up with these captured particles.

The Problem: The "Too Heavy" Trap

There's a catch. If these particles get too heavy, they might bounce off the Sun's hot core and escape back into space. This is called evaporation.

• Previous studies (using the IceCube detector in Antarctica) said, "We can only see these particles if they are heavier than 5 GeV (a specific unit of mass)."
• The author of this paper says, "Wait a minute! If these particles interact strongly enough, they get stuck even if they are lighter. We can look for particles as light as 2 GeV."

The Solution: The Water Detectors

To find these particles, we need to see what happens when they meet. When a positive millicharged particle meets a negative one inside the Sun, they annihilate (destroy each other) and create a burst of neutrinos (ghostly particles that travel through space).

We need to catch these neutrinos.

• IceCube is a detector buried in the ice. It's great at seeing heavy particles and high-energy signals, but it has a "blind spot" for lighter, lower-energy signals.
• Super-Kamiokande (Super-K) and the future Hyper-Kamiokande (Hyper-K) are massive tanks of ultra-pure water in Japan. They use special lights to detect the faint blue flashes (Cherenkov radiation) left behind by neutrinos.

The Analogy: Imagine trying to hear a whisper in a noisy room.

• IceCube is like a microphone tuned to hear loud shouts. It misses the whispers.
• Super-K and Hyper-K are like high-quality microphones that can hear the whispers (lower energy neutrinos) that IceCube misses.

The New Findings

The author ran the numbers to see what these water detectors could find:

1. Filling the Gap: Super-Kamiokande can now look for millicharged particles with masses between 2 and 28 GeV. This is a range of masses that IceCube couldn't see before. It's like finding a missing piece of a puzzle that everyone else ignored.
2. The "Tiny Fraction" Discovery: Most of the universe's dark matter is likely not millicharged; it's probably just a tiny, tiny fraction of the total.
   • IceCube could only see these particles if they made up about 1 in 20,000 of all dark matter.
   • Super-K can see them if they make up 1 in 50,000.
   • Hyper-K (the future detector) will be so sensitive it can find them if they are as rare as 1 in 200,000.
3. The "Bound State" Wall: There is a limit to how strong the charge can be. If the charge is too strong, the particles get stuck in "cages" (bound states) with heavy atoms in the Sun and can't annihilate to make neutrinos. The paper calculates exactly where this "ceiling" is, ensuring we don't look in places where the signal would be zero.

The Bottom Line

This paper argues that we don't need to wait for new, expensive technology to find these specific types of dark matter. By using existing (Super-K) and upcoming (Hyper-K) water tanks in Japan, we can hunt for lighter and rarer millicharged particles than ever before.

It's like realizing that while your big, powerful telescope can see distant galaxies, your smaller, more sensitive microscope can actually see the tiny bacteria hiding right under your nose. The author shows that by looking at the Sun through these water "microscopes," we can finally test a whole new range of possibilities for what dark matter might be.

Technical Summary

Technical Summary: Constraints and Projections for Millicharged Dark Matter in the Sun with Water Cherenkov Neutrino Detectors

Problem Statement
Millicharged Dark Matter (DM), a candidate arising from extensions of the Standard Model with an additional $U(1)'$ gauge symmetry, interacts with Standard Model particles via an ultralight dark photon. While extensively studied, constraints on millicharged DM weaken significantly when the particles constitute only a small fraction ($f_\chi$) of the total DM relic abundance. Existing constraints from the Cosmic Microwave Background (CMB), Big Bang Nucleosynthesis (BBN), and direct detection experiments leave an open window in the "strongly-coupled" regime (where the DM-baryon cross-section exceeds the Sun's transition cross-section, $\sigma \gtrsim 10^{-35} \text{ cm}^2$). Recent work by Berlin and Hooper [47] proposed using the IceCube Neutrino Observatory to constrain this regime via high-energy solar neutrinos from DM annihilation, deriving limits down to $f_\chi \simeq 5 \times 10^{-5}$ for masses $30\text{--}100$ GeV. However, IceCube's high energy threshold limits its sensitivity to lighter DM masses, leaving a gap in the parameter space for $m_\chi < 30$ GeV.

Methodology
This work investigates the sensitivity of water Cherenkov detectors—specifically Super-Kamiokande (Super-K) and the future Hyper-Kamiokande (Hyper-K)—to solar millicharged DM. The analysis follows the framework established in Ref. [47] but adapts it to the lower energy thresholds of water-based detectors.

1. Accumulation and Distribution: The study assumes the Sun captures millicharged DM via gravitational scattering. In the strongly-coupled regime, the Sun is optically thick to DM, leading to efficient capture. Unlike previous studies that imposed a lower mass cutoff of $\sim 5$ GeV due to evaporation concerns, this work extends the analysis down to $m_\chi = 2$ GeV. It argues that in the strongly-coupled regime, evaporation is suppressed because up-scattered particles undergo additional scatterings before escaping, retaining them within the Sun.
2. Bound-State Formation: The analysis accounts for the formation of Coulomb bound states between negatively charged DM ($\chi^-$) and solar nuclei (specifically Thorium, $Z=90$, to ensure conservative limits). If the binding energy exceeds the solar core temperature, the annihilation rate between $\chi^+$ and bound $\chi^-$ is exponentially suppressed due to the repulsive Coulomb barrier.
3. Annihilation and Signal: The DM annihilates primarily into Standard Model fermion pairs ($\chi^+\chi^- \to f\bar{f}$), with a focus on the $\tau^+\tau^-$ channel. The resulting neutrino flux is calculated using the PPPC4DM$\nu$ package, accounting for secondary cascades and neutrino oscillations.
4. Detection Strategy: The study utilizes a 10-year observation timescale. It calculates the expected muon neutrino signal from the Sun and compares it against the atmospheric muon neutrino background. A fiducial angular cut is applied based on the angular resolution of water Cherenkov detectors to isolate solar events.
5. Statistical Analysis: Constraints are set using a Poisson-based 95% confidence level (CL) exclusion criterion, requiring a minimum total event count to avoid spurious limits in low-statistics regions.

Key Contributions

• Extension to Lower Masses: By leveraging the lower energy threshold of water Cherenkov detectors ($\sim 100$ MeV), this work extends the search for solar millicharged DM down to $m_\chi = 2$ GeV, addressing the "evaporation suppression" regime where IceCube is insensitive.
• Complementary Parameter Space: The analysis demonstrates that Super-K and Hyper-K probe a mass range ($2\text{--}30$ GeV) that is largely inaccessible to IceCube, particularly for smaller DM fractions ($f_\chi$).
• Improved Sensitivity to Fractional Abundance: The study projects that Hyper-K can probe DM fractions as small as $f_\chi \simeq 5 \times 10^{-6}$, which is nearly an order of magnitude below current IceCube limits.

Results

• Super-Kamiokande: For a DM fraction of $f_\chi = 10^{-4.5}$, Super-K can constrain previously unexplored parameter space in the mass range $m_\chi = 5\text{--}28$ GeV. At lower fractions, Super-K remains robust at low masses ($m_\chi \sim 2$ GeV) where IceCube's exclusion region retreats to higher masses.
• Hyper-Kamiokande: The projected 10-year reach of Hyper-K bridges the gap between Super-K and IceCube, providing sensitivity across the entire $m_\chi < 100$ GeV range. Its peak sensitivity shifts to $m_\chi \sim 20$ GeV due to its larger fiducial volume and enhanced sensitivity at higher neutrino energies.
• Comparison with Existing Limits: While direct detection, accelerator searches, and balloon/rocket experiments (RRS, XQC) provide constraints, they either lose sensitivity at moderate-to-large couplings or weaken rapidly as $f_\chi$ decreases. The water Cherenkov constraints presented here access a region of parameter space invisible to these other methods, particularly for small $f_\chi$.

Significance
The paper claims that water Cherenkov detectors provide a "powerful and complementary probe" to IceCube for millicharged DM in the strongly-coupled regime. The primary significance lies in filling the low-mass gap left by IceCube and significantly improving the sensitivity to the fractional abundance of millicharged DM. The results suggest that a combined analysis of IceCube (high mass) and water Cherenkov detectors (low mass) forms a comprehensive strategy for indirect detection of strongly-coupled millicharged DM. The work reinforces the utility of neutrino astronomy not only for astrophysical sources but also for probing the nature of Dark Matter, motivating dedicated searches at upcoming detectors like Hyper-K, KM3NeT, and IceCube-Gen2.