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Experimental determination of the Dalitz plot for positronium decay using the J-PET detection system

Using the J-PET detection system, researchers performed the first measurements of the Dalitz plot for ortho-positronium decay into three photons, achieving high precision across nearly the entire phase space and confirming consistency with leading-order and next-to-leading-order QED predictions.

Original authors: Magdalena Skurzok, Steven D. Bass, Kamila Kasperska, Ermias Beyene, Neha Chug, Catalina Curceanu, Eryk Czerwinski, Manish Das, Marek Gorgol, Sharareh Jalali, Bozena Jasinska, Krzysztof Kacprzak, Tevfi

Published 2026-07-23

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Magdalena Skurzok, Steven D. Bass, Kamila Kasperska, Ermias Beyene, Neha Chug, Catalina Curceanu, Eryk Czerwinski, Manish Das, Marek Gorgol, Sharareh Jalali, Bozena Jasinska, Krzysztof Kacprzak, Tevfik Kaplanoglu, Łukasz Kapłon, Aleksander Khreptak, Tomasz Kozik, Deepak Kumar, Karol Kubat, Sumit Kumar Kundu, Edward Lisowski, Filip Lisowski, Bartłomiej Łach, Justyna Mędrala-Sowa, Wiktor Mryka, Simbarashe Moyo, Szymon Niedźwiecki, Anand Pandey, Piyush Pandey, Alessio Porcelli, Bartłomiej Rachwał, Martin Rädlera, Sushil Sharma, Ewa Stępień, Tomasz Szumlak, Pooja Tanty, Keyvan Tayefi Ardebili, Satyam Tiwari, Kavya Valsan Eliyan, Anoop K. Venadan, Paweł Moskal

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

Imagine the universe as a giant, invisible dance floor where tiny particles waltz, spin, and sometimes crash into each other. In this microscopic ballroom, there is a very special couple called "positronium." It's not a normal atom; it's a temporary partnership between an electron (a tiny particle with a negative charge) and its evil twin, the positron (the same size but with a positive charge). Because they are opposites, they are drawn together, but they can't stay together forever. When they finally meet, they annihilate each other in a flash of pure energy, turning into light.

Sometimes, this couple splits into two beams of light, like a perfect mirror reflection. But sometimes, the dance gets more complicated, and they burst into three beams of light at once. This three-beam explosion is the "ortho-positronium" decay. Physicists have long wanted to map out exactly how these three beams of light fly apart. They call this map a "Dalitz plot." Think of it like a weather map for the explosion: instead of showing rain and wind, it shows the speed and direction of every single photon (particle of light) created. By drawing this map, scientists can check if the rules of the universe—specifically the rules of Quantum Electrodynamics (QED), which is the math that describes how light and matter interact—are working exactly as predicted. If the map looks different than the math says it should, it could mean there's a new, hidden rule of physics waiting to be discovered.

The Great Three-Photon Map

In this new study, a team of scientists from Poland and Italy used a giant, high-tech detector called J-PET to finally draw this map for the first time. Imagine J-PET as a massive, hollow cylinder made of 192 long, plastic "light sticks" arranged in three layers, like the rings of a tree. Inside the very center of this cylinder sits a tiny chamber holding a radioactive source that creates our positronium couples. When the couples annihilate and burst into three photons, those light particles zoom out and hit the plastic sticks, creating tiny flashes. The detector is so fast it can catch these flashes in a fraction of a billionth of a second.

The researchers spent about 218 days collecting data, which resulted in a massive pile of information—about 950 terabytes! From this mountain of data, they managed to identify 33.42 million specific events where a positronium atom exploded into exactly three photons. They then used this data to plot the Dalitz map, showing the angles and energies of the three photons for almost the entire range of possibilities.

What They Found

The big news is that this is the first time anyone has successfully mapped the Dalitz plot for this three-photon decay across almost the whole "dance floor" of possibilities. Before this, scientists had only been able to peek at three tiny, specific corners of the map. Now, they have a full picture.

When they compared their new map to the theoretical predictions made by the math of QED, the results were a perfect match. The experimental map lined up beautifully with the "Leading Order" predictions (the basic math) and also agreed with the more complex "Next-to-Leading Order" calculations (which include tiny corrections). The team measured the angles with about 3% statistical uncertainty (a measure of how much the numbers might wiggle due to random chance) and 2-3% systematic uncertainty (a measure of how precise their tools are).

However, the map isn't perfect everywhere. In the very "corners" of the plot—where the photons have very low energy and fly off at extreme angles—the uncertainty gets much higher, reaching up to 25%. This is because the detector has a hard time catching the faintest, slowest photons. Also, the team had to be very careful to filter out "fake" events, like when a photon bounces off something inside the detector before being caught, which could trick the computer into thinking it saw a three-photon explosion when it didn't. They used clever computer simulations to subtract these background noises and isolate the real signal.

What's Next?

The paper concludes that while their current map confirms the existing rules of physics, it's not quite precise enough to spot the tiniest, most subtle effects that might exist. To see those, they would need to measure the map with even greater precision. The authors suggest that by upgrading their detector with new, more sensitive light sensors and a better data system, they could improve their precision by a factor of ten. This future upgrade would allow them to see the "corners" of the map clearly and perhaps even test the very highest levels of the QED math, looking for any tiny cracks in the theory that could lead to a revolution in our understanding of the universe. For now, though, they have successfully drawn the first complete sketch of this three-photon dance, and it looks exactly as the universe's rulebook predicted.

Technical Summary: Experimental Determination of the Dalitz Plot for Positronium Decay using the J-PET Detection System

Problem Statement
The decay of ortho-positronium (o-Ps) into three photons ( $o\text{-Ps} \to 3\gamma$ ) is a fundamental process in quantum electrodynamics (QED). While the total decay rate and energy spectrum have been studied, the complete kinematic distribution of the final state photons—represented by the Dalitz plot—has remained largely unexplored. Previous experiments were limited by restricted kinematic coverage, measuring only three specific angular configurations with limited statistics. Consequently, the full phase space of the three-body decay had not been mapped, preventing sensitive tests of differential QED amplitudes and higher-order corrections (specifically next-to-leading order, NLO, at $O(\alpha)$ ) in the angular distribution.

Methodology
The study utilized the Jagiellonian Positron Emission Tomograph (J-PET), a detector system based on 192 plastic scintillator strips arranged in three concentric cylindrical layers. The experimental setup included a central annihilation chamber containing a porous XAD-4 polymer matrix and a $^{22}\text{Na}$ positron source (0.7 MBq). The XAD-4 material was selected for its high probability of o-Ps formation (29% of 3 $\gamma$ annihilations).

The experimental procedure involved:

Data Acquisition: The J-PET operated in a trigger-less mode using Field Programmable Gate Arrays (FPGAs), recording time-over-threshold (TOT) and timing information for photon interactions.
Event Selection: Candidates for $o\text{-Ps} \to 3\gamma$ decays were identified by requiring three photon hits in different scintillator strips within a 2.5 ns coincidence window.
Background Suppression:
- TOT Cuts: A TOT range of 1–17 ns was selected to isolate annihilation photons from de-excitation photons.
- Geometric and Temporal Cuts: Events were required to have a decay plane distance $d < 5$ cm from the annihilation origin and an emission time difference $\delta t_{13} < 1.5$ ns between the first and third hits.
- Angular Correlations: To suppress background from two-photon annihilation with scattering, the sum of the two smallest relative angles ( $\theta_2 + \theta_1$ ) was required to be greater than 190 $^\circ$ .
Signal Extraction: A one-dimensional histogram was constructed from the experimental Dalitz distribution. This was fitted using a $\chi^2$ minimization with three components: reconstructed signal, misreconstructed signal (simulated via GEANT4), and background.
Efficiency Correction: The final distribution was corrected for the detector's overall reconstruction efficiency, determined via Monte Carlo simulations.

Key Contributions

First Full-Phase-Space Measurement: This work presents the first experimental measurement of the Dalitz plot for $o\text{-Ps} \to 3\gamma$ decay covering almost the entire available kinematic phase space.
Angular Representation: The study provides the Dalitz plot in an angular representation, mapping the distribution of relative angles between the three emitted photons.
Quantitative Precision: The measurements achieve approximately 3% statistical uncertainty and 2–3% systematic uncertainty across most of the phase space (excluding "corner" regions where systematic errors reach up to 25%).
Validation of QED: The results provide a direct experimental comparison with theoretical predictions, confirming consistency with both leading-order (LO) and next-to-leading-order (NLO) QED calculations within current experimental uncertainties.

Results
The analysis identified 33.42 million $o\text{-Ps} \to 3\gamma$ signal events from approximately 218 effective days of data collection (950 TB of processed data). The resulting Dalitz plot shows a distribution consistent with QED predictions.

Agreement with Theory: The experimental data aligns with the leading-order QED predictions.
Sensitivity to Higher Orders: While the current precision (2–3% systematic) is sufficient to confirm LO predictions, the paper notes that the relative difference between LO and LO + $O(\alpha)$ corrections in the angular distribution varies between 1.5% and 2.5%. The current uncertainties are on the order of these differences, meaning the experiment has not yet resolved the NLO corrections but establishes the baseline for doing so.
Limitations: The measurement excludes the region corresponding to low-energy photons correlated with relative angles $\theta_{ij} > 170^\circ$ due to detection threshold limitations (photons below ~30 keV).

Significance and Claims
The paper claims that this work demonstrates the feasibility of using the J-PET system to investigate the $o\text{-Ps} \to 3\gamma$ decay process with high precision. The primary significance lies in establishing a foundation for future, more comprehensive measurements.

Future Potential: The authors state that with an upgraded modular J-PET detector (equipped with silicon photomultipliers and improved electronics) and a higher activity source, statistical uncertainties could be reduced by a factor of 10.
Scientific Goal: Such improvements would allow for the reconstruction of the Dalitz plot over a wider kinematic range (including low-energy regions) and enable the experimental testing of higher-order QED effects ( $O(\alpha)$ and beyond) and searches for charge-conjugation symmetry breaking.
Current Status: The paper modestly concludes that while the current results agree with LO QED, the precision is not yet sufficient to definitively test the NLO corrections, which remain a target for future iterations of the experiment.

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