⚛️ phenomenology

Probing sub-GeV dark matter through dark matter-electron scattering in the superheated $C_2H_2F_4$ target of the InDEx experiment

This paper projects the sensitivity of the InDEx experiment's superheated $\mathrm{C_2H_2F_4}$ target to sub-GeV dark matter via electron scattering, demonstrating that by lowering operating temperatures to achieve thresholds as low as 26.8 eV, the experiment can probe dark matter masses down to approximately 12 MeV and set competitive upper limits on dark-photon-mediated scattering cross sections.

Original authors: Utkarsh Patel, Astitva Kathait, Mala Das

Published 2026-08-27

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CC BY 4.0

Original authors: Utkarsh Patel, Astitva Kathait, Mala Das

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

For decades, the hunt for dark matter has focused on the heavyweights of the invisible universe. Scientists have built massive, ultra-sensitive detectors deep underground, waiting for a rare, heavy particle to bump into an atomic nucleus, much like a billiard ball striking another. This strategy has worked well for finding heavy particles, but it leaves a vast gap in our understanding. If dark matter is light—thousands of times lighter than a proton—it would barely nudge a heavy nucleus, making it invisible to these traditional detectors. Yet, even a light particle could kick a lightweight electron out of an atom if it hit hard enough. This possibility has opened a new frontier in physics: searching for dark matter by watching for these tiny electron kicks rather than heavy nuclear collisions.

A team of researchers in India has now mapped out how to use a specific type of detector to explore this lighter territory. The Indian Dark Matter Search Experiment, known as InDEx, currently operates deep underground in a mine, using droplets of a special chemical fluid called tetrafluoroethane. These droplets are kept in a state of "superheat," meaning they are hotter than their boiling point but remain liquid because they are under pressure and undisturbed. The experiment has traditionally been tuned to ignore electrons and only react to heavy nuclear hits, effectively acting as a shield against background noise. However, the researchers realized that by simply turning up the temperature, they could change the rules. At higher temperatures, the droplets become sensitive enough to react to the tiny energy deposited by an electron, turning the detector into a tool for finding light dark matter.

In a new study, the team calculated exactly what would happen if they operated these detectors at six different temperatures, ranging from 45 to 70 degrees Celsius. As the temperature rises, the energy required to trigger a bubble in the liquid drops dramatically. At the lowest setting, a particle needs to deposit a relatively large amount of energy to create a signal. But at the highest setting of 70 degrees, the threshold drops to a mere 26.8 electron-volts, a level of sensitivity that allows the detector to "see" dark matter particles as light as 12 million electron-volts. This is a significant leap, pushing the experiment's reach from the heavy sub-GeV range down into the realm of very light particles that were previously out of reach for this type of technology.

The researchers simulated how dark matter would interact with the electrons in the carbon, hydrogen, and fluorine atoms that make up the liquid target. They found that the fluorine atoms are the most important players in this process. Because fluorine has a higher number of electrons and a tighter grip on them, it provides the best chance for a dark matter particle to transfer enough energy to knock an electron loose and create a detectable bubble. The team calculated the expected number of events for a standard exposure of 1,000 kilogram-days, assuming a perfect scenario with no background noise. Their results show that if dark matter exists and interacts with electrons through a heavy mediator, the experiment could rule out certain interaction strengths down to a cross-section of roughly 2.7 times 10 to the power of minus 41 square centimeters for particles around 100 million electron-volts. If the interaction is mediated by a very light particle, the sensitivity reaches about 3.0 times 10 to the power of minus 37 square centimeters for particles around 83 million electron-volts.

This work does not claim to have found dark matter; rather, it provides a detailed roadmap for how the InDEx experiment can be upgraded to hunt for it. The study confirms that by adjusting the operating temperature, the same detectors currently used to look for heavy dark matter can be repurposed to search for light dark matter. This adds a new, chemically distinct tool to the global arsenal of dark matter searches, complementing the large liquid xenon tanks and silicon chips used by other groups. The researchers emphasize that these projections are based on ideal conditions. In reality, the detectors will need to be calibrated to ensure they can distinguish between a genuine dark matter signal and the background noise of natural radiation, which becomes more active as the temperature rises. Nevertheless, the study demonstrates that the path to finding light dark matter is not blocked by the limitations of current technology, but simply by the need to tune the instrument to the right frequency. By turning up the heat, the InDEx team has shown that their underground laboratory could soon be listening for the faintest whispers of the universe's lightest dark matter.

Technical Summary: Probing Sub-GeV Dark Matter through Dark Matter–Electron Scattering in the Superheated C2H2F4 Target of the InDEx Experiment

Problem and Motivation
The canonical direct-detection strategy for dark matter (DM) relies on observing nuclear recoils from elastic scattering of Weakly Interacting Massive Particles (WIMPs). However, for DM masses below a few GeV/c², the recoil energy transferred to nuclei becomes too small for conventional detectors to observe. In contrast, scattering off bound electrons remains effective, as even sub-GeV particles can transfer sufficient energy to ionize atoms or excite electrons across band gaps. While noble-liquid and solid-state targets have established the field of DM–electron scattering, there is a need to explore alternative target materials with distinct chemical and operational properties. The Indian Dark Matter Search Experiment (InDEx) currently utilizes superheated droplet detectors with tetrafluoroethane (C2H2F4) for nuclear-recoil searches. This work investigates the potential of the same InDEx detectors to probe the DM–electron channel by operating at higher temperatures where electron recoils can nucleate bubbles.

Methodology
The study evaluates the sensitivity of the InDEx C2H2F4 target to DM–electron scattering across six operating temperatures: $T = 45, 50, 55, 60, 65,$ and $70^\circ\text{C}$ .

Detector Physics: The detectors operate as threshold devices based on the Seitz "heat-spike" model. The critical energy ( $E_c$ ) required to nucleate a bubble decreases with increasing temperature. The authors adopt the Seitz critical energy as the effective deposited-energy threshold ( $E_{th}$ ) for electron recoils, ranging from $0.61 \text{ keV}$ at $45^\circ\text{C}$ down to $26.8 \text{ eV}$ at $70^\circ\text{C}$ .
Ionization Response: The ionization response of the C2H2F4 target is computed in the isolated-atom approximation. The target is treated as an incoherent superposition of Carbon, Hydrogen, and Fluorine atoms. Bound-state wavefunctions are derived from Roothaan–Hartree–Fock calculations, while outgoing electrons are modeled as positive-energy continuum solutions in a hydrogenic potential. The calculations utilize the DarkART code.
Signal Modeling: The analysis assumes the Standard Halo Model for the local DM velocity distribution. It considers a dark-photon-type mediator in two limiting cases:
1. Heavy mediator (Contact interaction): Form factor $F_{DM}(q) = 1$ .
2. Light mediator (Long-range interaction): Form factor $F_{DM}(q) = (\alpha m_e / q)^2$ .
Limit Setting: Projected 90% confidence level (C.L.) exclusion limits are derived for a benchmark exposure of $10^3 \text{ kg days}$ under a zero-background assumption. The limits are calculated using the standard Poisson condition for zero observed events ( $N_{exp} \leq 2.3$ ).

Key Contributions

First Characterization of Superheated Liquids in the Electron Channel: This work provides the first theoretical characterization of a superheated-liquid (bubble-nucleation) target specifically for DM–electron scattering, adding a refrigerant-based target to the landscape of sub-GeV DM searches.
Temperature-Dependent Sensitivity: The study quantifies how varying the operating temperature tunes the energy threshold, directly extending the kinematic reach to lower DM masses.
Target Composition Analysis: The analysis decomposes the ionization signal by atomic species, identifying Fluorine as the dominant contributor due to its four-fold multiplicity in C2H2F4 and its larger effective nuclear charge, which provides a harder momentum-transfer tail compared to Carbon.

Results

Threshold and Mass Reach: Lowering the threshold from $0.61 \text{ keV}$ ( $45^\circ\text{C}$ ) to $26.8 \text{ eV}$ ( $70^\circ\text{C}$ ) shifts the kinematic low-mass edge from $m_\chi \approx 181 \text{ MeV}$ down to $m_\chi \approx 12 \text{ MeV}$ .
Exclusion Limits:
- Contact Interaction: At the lowest threshold ( $70^\circ\text{C}$ ), the projected 90% C.L. limit reaches $\sigma_e \approx 2.73 \times 10^{-41} \text{ cm}^2$ near $m_\chi \approx 101 \text{ MeV}$ .
- Long-Range Interaction: At the same threshold, the limit reaches $\sigma_e \approx 3.03 \times 10^{-37} \text{ cm}^2$ near $m_\chi \approx 83 \text{ MeV}$ .
Comparison with Existing Experiments: The projected sensitivity is competitive with current constraints from XENONnT, PandaX, and DAMIC-M, particularly in the mass range between $20 \text{ MeV}$ and $80 \text{ MeV}$ for the contact interaction. For the long-range interaction, the projection is comparable to leading limits from DAMIC-M over much of the sub-GeV window. The InDEx projection extends the reach to masses as low as $\sim 12 \text{ MeV}$ , a region where superheated liquids retain a distinctive reach due to their low thresholds.

Significance and Claims
The authors claim that the electron channel extends the physics reach of the InDEx experiment from nuclear recoils to sub-GeV DM masses, down to $\mathcal{O}(10 \text{ MeV})$ . This provides a complementary probe to the existing nuclear-recoil program, specifically enhancing sensitivity to DM candidates that couple predominantly to leptons (leptophilic DM), which would be invisible to nuclear-recoil searches.

The paper emphasizes that these results are "best-case" projections based on zero-background and unit-efficiency assumptions. The authors note that a realistic assessment requires incorporating measured electron-recoil nucleation responses and background rates in the gamma-sensitive regime. Furthermore, the theoretical treatment relies on the isolated-atom approximation, neglecting $\mathcal{O}(10 \text{ eV})$ bonding effects on valence orbitals, which the authors identify as a refinement for future work. Despite these caveats, the study demonstrates that a superheated refrigerant target, already deployed underground, can achieve cross-section sensitivities relevant for sub-GeV DM with moderate exposure.

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