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Search for Light-Mass Fractionally Charged Particles in Space with DAMPE Experiment

Using ten years of data from the DAMPE satellite, this study searched for light-mass fractionally charged particles with a charge of $\frac{2}{3}~e$ in primary cosmic rays, found no candidates, and established a 90% confidence level upper flux limit of $5.0 \times 10^{-11}\,cm^{-2}sr^{-1}s^{-1}$ for particles with a mass of 0.511 MeV $/c^{2}$ .

Original authors: F. Alemanno, Q. An, P. Azzarello, F. C. T. Barbato, P. Bernardini, X. J. Bi, H. V. Boutin, I. Cagnoli, M. S. Cai, E. Casilli, J. Chang, D. Y. Chen, J. L. Chen, Z. F. Chen, Z. X. Chen, P. Coppin, M. Y.

Published 2026-02-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: F. Alemanno, Q. An, P. Azzarello, F. C. T. Barbato, P. Bernardini, X. J. Bi, H. V. Boutin, I. Cagnoli, M. S. Cai, E. Casilli, J. Chang, D. Y. Chen, J. L. Chen, Z. F. Chen, Z. X. Chen, P. Coppin, M. Y. Cui, T. S. Cui, I. De Mitri, F. de Palma, A. Di Giovanni, T. K. Dong, Z. X. Dong, G. Donvito, J. L. Duan, K. K. Duan, R. R. Fan, Y. Z. Fan, F. Fang, K. Fang, C. Q. Feng, L. Feng, S. Fogliacco, J. M. Frieden, P. Fusco, M. Gao, F. Gargano, E. Ghose, K. Gong, Y. Z. Gong, D. Y. Guo, J. H. Guo, S. X. Han, Y. M. Hu, G. S. Huang, X. Y. Huang, Y. Y. Huang, M. Ionica, L. Y. Jiang, W. Jiang, Y. Z. Jiang, J. Kong, A. Kotenko, D. Kyratzis, S. J. Lei, B. Li, M. B. Li, W. L. Li, W. H. Li, X. Li, X. Q. Li, Y. M. Liang, C. M. Liu, H. Liu, J. Liu, S. B. Liu, Y. Liu, F. Loparco, M. Ma, P. X. Ma, T. Ma, X. Y. Ma, G. Marsella, M. N. Mazziotta, D. Mo, Y. Nie, X. Y. Niu, A. Parenti, W. X. Peng, X. Y. Peng, C. Perrina, E. Putti Garcia, R. Qiao, J. N. Rao, Y. Rong, A. Serpolla, R. Sarkar, P. Savina, Z. Shangguan, W. H. Shen, Z. Q. Shen, Z. T. Shen, L. Silveri, J. X. Song, H. Su, M. Su, H. R. Sun, Z. Y. Sun, A. Surdo, X. J. Teng, A. Tykhonov, G. F. Wang, J. Z. Wang, L. G. Wang, S. Wang, X. L. Wang, Y. F. Wang, D. M. Wei, J. J. Wei, Y. F. Wei, D. Wu, J. Wu, S. S. Wu, X. Wu, Z. Q. Xia, Z. Xiong, E. H. Xu, H. T. Xu, J. Xu, Z. H. Xu, Z. Z. Xu, Z. L. Xu, G. F. Xue, M. Y. Yan, H. B. Yang, P. Yang, Y. Q. Yang, H. J. Yao, Y. H. Yu, Q. Yuan, C. Yue, J. J. Zang, S. X. Zhang, W. Z. Zhang, Y. Zhang, Y. P. Zhang, Y. Zhang, Y. J. Zhang, Y. Q. Zhang, Y. L. Zhang, Z. Zhang, Z. Y. Zhang, C. Zhao, H. Y. Zhao, X. F. Zhao, C. Y. Zhou, X. Zhu, Y. Zhu

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

The Big Idea: Hunting for "Ghost" Particles

Imagine the universe is a giant, bustling highway filled with cosmic rays—high-speed particles zipping through space. For decades, scientists have been looking for a very specific type of "ghost" on this highway: Fractionally Charged Particles (FCPs).

In our standard understanding of physics (the "Rulebook" of the universe), every free-flying particle has a charge that is a whole number: 0 (like a neutron), +1 (like a proton), or -1 (like an electron). But some theories suggest there might be particles with "broken" charges, like 2/3 or 1/3 of an electron's charge.

Think of it like this: If an electron is a standard $1 bill, a Fractionally Charged Particle is a weird, half-cut $0.66 bill. Most physicists thought these "half-bills" were heavy and slow, but this paper asks: What if they are light and fast?

The Detective: The DAMPE Satellite

To find these ghosts, the researchers used the DAMPE satellite (nicknamed "Wukong," after the Monkey King).

The Mission: Launched in 2015, this satellite has been orbiting Earth for over 10 years, acting like a giant, high-tech cosmic rain gauge.
The Tool: It's not just a camera; it's a multi-layered sandwich of detectors.
- The Plastic Scintillator (PSD): Like a motion sensor that feels the "kick" of a passing particle.
- The Silicon Tracker (STK): Like a high-speed camera that takes a 3D photo of the particle's path.
- The BGO Calorimeter: Like a heavy sponge that soaks up the particle's energy to measure how hard it hit.

The Challenge: Why Look for "Light" Particles?

Previous searches assumed these ghost particles were heavy (like a bowling ball). Heavy things punch through the atmosphere easily.

But the scientists in this paper wondered: What if the ghosts are light (like a feather)?

The Problem: If a particle is light and has a fractional charge, it interacts with matter very differently. When it hits the atmosphere or the satellite's metal, it doesn't just punch through; it creates a "cascade" or a shower of other particles (like a snowball hitting a wall and exploding into a million tiny snowflakes).
The Result: These light particles get absorbed or scattered before they can be easily seen by ground-based detectors. They are like a whisper in a hurricane—hard to hear unless you are right there in the eye of the storm (which is why we need a satellite in space).

The Investigation: How They Searched

The team looked at 10 years of data (about 20 billion cosmic ray events). They were looking for a very specific signature:

The Charge: They filtered for particles with a charge of exactly 2/3 of an electron's charge.
The Mass: They focused on particles that were very light (between 0.2 and 1.0 times the mass of an electron).
The Filter: They used the satellite's detectors to measure the "kick" (energy loss) and the path. Since a 2/3 charge is weaker than a full 1 charge, it leaves a fainter "footprint" in the detectors.

The Analogy: Imagine you are trying to find a specific type of bird in a forest.

Most birds (electrons) are loud and leave big footprints.
You are looking for a tiny, silent bird (the 2/3 charge particle) that leaves a very faint, specific track.
You have to ignore all the loud birds and the wind noise to see if that tiny bird is there.

The Result: The Great Silence

After sifting through 10 years of data, the result was a bit anticlimactic but scientifically crucial: They found nothing.

No Ghosts: They did not see a single candidate for a light, fractionally charged particle.
The Limit: Because they didn't find any, they set a "speed limit" for how common these particles could be. They calculated that if these particles do exist, they are incredibly rare—fewer than 50 particles per square kilometer per second (a very tiny number).

Why Does This Matter?

You might ask, "If they didn't find anything, why write a paper?"

Ruling Out Theories: Science isn't just about discovery; it's also about elimination. By proving these particles aren't hiding in this specific mass range, they have narrowed down the search for new physics. It's like checking a room and confirming the keys aren't there, so you know to look in the kitchen instead.
Dark Matter Clues: Some theories suggest these particles could be a form of Dark Matter (the invisible stuff holding galaxies together). By ruling them out in this mass range, scientists are one step closer to figuring out what Dark Matter actually is.
First of Its Kind: This is the first time anyone has looked for light fractionally charged particles in primary cosmic rays using a space satellite. It opens a new door for future experiments.

The Bottom Line

The DAMPE team acted like cosmic detectives, scanning the universe for a decade to find a "half-dollar" particle. They didn't find the thief, but by proving the thief isn't hiding in the attic, they have helped the rest of the scientific community know exactly where not to look, bringing us closer to solving the mystery of the universe's hidden ingredients.

1. Problem Statement and Motivation

Theoretical Context: While the Standard Model (SM) dictates that free fundamental particles possess integer charges (0 or $\pm 1e$ ), various extensions (e.g., Grand Unification, Supersymmetry) and astrophysical mechanisms predict the existence of Fractionally Charged Particles (FCPs) with charges such as $\pm \frac{1}{3}e$ or $\pm \frac{2}{3}e$ .
The Gap: Previous searches for FCPs have predominantly focused on heavy, lepton-like particles (mass $> \text{several MeV}/c^2$ ). These heavy particles behave like Minimum Ionizing Particles (MIPs) and can penetrate the atmosphere or crust, making them detectable by ground-based or underground experiments.
The Challenge: Light-Mass FCPs (LFCPs) in the sub-MeV range (specifically $0.2 - 1 \text{ MeV}/c^2$ ) are difficult to detect from the ground. Due to their low mass, they experience significant bremsstrahlung effects when interacting with matter, leading to rapid energy loss and absorption in the atmosphere or Earth's crust.
Objective: To conduct the first dedicated search for LFCPs in primary cosmic rays using space-based data, specifically targeting particles with a charge of $\frac{2}{3}e$ and masses between $0.2 \text{ MeV}/c^2$ and $1 \text{ MeV}/c^2$ .

2. Methodology

The study utilizes data from the Dark Matter Particle Explorer (DAMPE), also known as "Wukong," a satellite-borne calorimetric detector launched in 2015.

Data Source:
- Timeframe: January 1, 2016, to December 31, 2025 (10 years of on-orbit data).
- Exclusions: Data from the South Atlantic Anomaly (SAA), instrumental dead time, calibration periods, and a giant solar flare (Sept 9–13, 2017) were removed.
- Effective Exposure: Approximately $2.4 \times 10^8$ seconds (76% of total operational time).
Simulation and Modeling:
- LFCP Model: Created using the Geant4 toolkit, simulating ionization, bremsstrahlung, and multiple scattering.
- Parameters: Charge fixed at $\frac{2}{3}e$ ; Mass varied from $0.2$ to $1 \text{ MeV}/c^2$ .
- Backgrounds: Monte Carlo (MC) simulations of electrons and protons generated from a hemispherical isotropic source with power-law energy spectra.
Selection Criteria (Event Reconstruction):
1. Preselection:
  - Fiducial Cut: Events must traverse the full detector without hitting edges to ensure shower containment.
  - Hadron Rejection: 3D topological shower profiles in the BGO calorimeter are used to exclude hadrons.
  - Energy Cut: Deposited energy $> 10 \text{ GeV}$ to overcome geomagnetic cutoff.
2. Trigger Selection: Events must satisfy the High Energy Trigger (HET) pattern, requiring significant energy deposition in the first three layers of the BGO calorimeter.
3. Track Selection: High-quality trajectories reconstructed by the Silicon-Tungsten tracker (STK) using the Kalman filter algorithm, matching the BGO track within 15 mm.
4. Charge Selection:
  - Detectors: Plastic Scintillator Detector (PSD) and STK.
  - PSD: Requires penetration of both layers; charge corrected for path length and quenching.
  - STK: Uses the first four sub-layers; requires at least three effective signals to reduce charge fluctuation.
  - Resolution: PSD resolution $\approx 0.06e$ ; STK resolution $\approx 0.04e$ .
Signal Definition:
- The signal region is defined based on the charge distribution (PSD and STK).
- The region is set to the Most Probable Value (MPV) + $2.5\sigma$ (where $\sigma$ is the combined Landau width and Gaussian sigma).
- Borders: PSD $> 0.73e$ , STK $> 0.78e$ .
- Efficiency: The signal region retains $\approx 67.4\%$ of LFCP signals while effectively rejecting electron backgrounds.
Systematic Uncertainties:
- Estimated at 2.37%, derived from the differences in selection efficiencies between MC electrons and on-orbit data (trigger, track, and charge selection).

3. Key Results

Observation: No LFCP candidates were observed within the defined signal region in the 10-year dataset. The charge distributions of on-orbit data align perfectly with MC electron simulations, showing no excess in the fractional charge region.
Flux Upper Limits:
- For LFCPs with a mass of $0.511 \text{ MeV}/c^2$ and charge $\frac{2}{3}e$ , the flux upper limit at the 90% confidence level (CL) is determined to be:
  $\Phi_{upper} = 5.0 \times 10^{-11} \text{ cm}^{-2}\text{sr}^{-1}\text{s}^{-1}$
- This limit was calculated using the formula $\Phi = \frac{N_{obs}}{T_{exp} A_{eff} \epsilon_{region} (1-\delta)}$ , where $N_{obs}$ is set to 2.44 (Feldman-Cousins limit for zero observed events).
Mass Dependence: The study provides flux upper limits for the entire mass range of $0.2 \text{ MeV}/c^2$ to $1 \text{ MeV}/c^2$ . The limits generally remain on the order of $10^{-11} \text{ cm}^{-2}\text{sr}^{-1}\text{s}^{-1}$ .

4. Key Contributions

First Space-Based Search for Light-Mass FCPs: This is the first study to specifically target sub-MeV FCPs in primary cosmic rays using space-borne data, overcoming the atmospheric absorption limitations that hinder ground-based searches.
Utilization of DAMPE Capabilities: Demonstrated the high-precision charge resolution of the DAMPE PSD and STK detectors ( $\sim 0.04-0.06e$ ) and the effectiveness of the BGO calorimeter in distinguishing electromagnetic showers for low-mass particle identification.
Comprehensive Constraints: Established stringent upper limits on the flux of $\frac{2}{3}e$ particles, significantly narrowing the parameter space for theories predicting light-mass fractionally charged dark matter candidates.

5. Significance

Theoretical Impact: The results place strong constraints on Beyond Standard Model (BSM) theories and astrophysical production mechanisms that predict light-mass FCPs. The non-detection suggests that if such particles exist, their flux in the local universe is extremely low.
Dark Matter Implications: Since FCPs are a candidate for dark matter, these limits help refine the search space for dark matter models involving millicharged or fractionally charged particles.
Experimental Benchmark: The study sets a new benchmark for future space-based cosmic ray experiments aiming to detect exotic low-mass particles, validating the methodology of using high-precision calorimetric detectors in orbit for this specific physics goal.