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Demonstrating topological identification capabilities of the NEXT experiment at low pressure

This paper reports the first validation of the NEXT-100 detector's topological discrimination capabilities at low pressure (~4 bar), demonstrating a signal efficiency of 75.6% and background acceptance of 14.7% for distinguishing neutrinoless double beta decay-like events from background, which aligns with the performance of its predecessor, NEXT-White.

Original authors: J. Waiton, H. Almazán, B. Palmeiro, G. Martínez-Lema, R. Guenette, V. Álvarez, L. Arazi, I. J. Arnquist, F. Auria-Luna, S. Ayet, Y. Ayyad, C. D. R. Azevedo, F. Ballester, J. E. Barcelon, M. del Barrio

Published 2026-09-14

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: J. Waiton, H. Almazán, B. Palmeiro, G. Martínez-Lema, R. Guenette, V. Álvarez, L. Arazi, I. J. Arnquist, F. Auria-Luna, S. Ayet, Y. Ayyad, C. D. R. Azevedo, F. Ballester, J. E. Barcelon, M. del Barrio-Torregrosa, J. M. Benlloch-Rodríguez, F. I. G. M. Borges, A. Brodoline, C. Cabo, A. Castillo, E. Church, M. Cid, X. Cid, C. A. N. Conde, C. Cortes-Parra, F. P. Cossío, R. Coupe, E. Dey, P. Dietz, C. Echeverria, M. Elorza, R. Esteve, R. Felkai, L. M. P. Fernandes, P. Ferrario, P. Ferrero Mancheño, F. W. Foss, Z. Freixa, J. García-Barrena, J. J. Gómez-Cadenas, J. W. R. Grocott, J. Hauptman, C. A. O. Henriques, J. A. Hernando Morata, P. Herrero-Gómez, V. Herrero, C. Hervés Carrete, Y. Ifergan, A. F. B. Isabel, B. J. P. Jones, F. Kellerer, L. Larizgoitia, A. Larumbe, P. Lebrun, F. Lopez, N. López-March, R. Madigan, R. D. P. Mano, A. Marauri, A. P. Marques, J. Martín-Albo, A. Martínez, M. Martínez-Vara, R. L. Miller, K. Mistry, J. Molina-Canteras, F. Monrabal, C. M. B. Monteiro, F. J. Mora, K. E. Navarro, P. Novella, D. R. Nygren, E. Oblak, I. Osborne, J. Palacio, A. Para, A. Pazos, J. Pelegrin, M. Pérez Maneiro, M. Querol, J. Renner, I. Rivilla, C. Rogero, L. Rogers, B. Romeo, C. Romo-Luque, E. Ruiz-Chóliz, P. Saharia, F. P. Santos, J. M. F. dos Santos, M. Seemann, I. Shomroni, A. L. M. Silva, P. A. O. C. Silva, A. Simón, S. R. Soleti, M. Sorel, J. Soto-Oton, J. M. R. Teixeira, S. Teruel-Pardo, J. F. Toledo, C. Tonnelé, S. Torelli, J. Torrent, A. Trettin, P. R. G. Valle, M. Vanga, P. Vázquez Cabaleiro, J. F. C. A. Veloso, J. D. Villamil, L. M. Villar Padruno, A. Yubero-Navarro

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 within the subatomic world, there is a rare and mysterious event that physicists have been hunting for decades: the moment when two neutrons inside an atomic nucleus transform into two protons and two electrons, but without releasing the usual pair of ghostly particles known as neutrinos. If this happens, it would prove that the neutrino is its own antiparticle, a discovery that would rewrite the fundamental laws of physics and explain why the universe is made of matter rather than being annihilated by antimatter. To catch this fleeting event, scientists build massive, ultra-sensitive detectors filled with a heavy gas, waiting for the specific signature of two electrons emerging from a single point. The challenge is not just seeing them, but distinguishing them from the constant rain of background radiation that mimics their appearance.

A team of researchers working on the NEXT-100 experiment has taken a significant step forward in this search by proving that their detector can successfully tell the difference between a signal and background noise, even when the gas inside is kept at a lower pressure than originally planned. The experiment is housed in an underground laboratory in Spain, shielded from cosmic rays, where a large cylinder filled with xenon gas acts as a three-dimensional camera. When a particle passes through, it leaves a trail of light that the detector records, allowing scientists to reconstruct the path of the particle in three dimensions. The key to identifying the rare decay is the shape of that path: the signal they want looks like two distinct tracks starting from the same spot and ending with two bright, heavy "blobs" of energy, while most background noise looks like a single track with only one heavy end.

In this recent study, the team focused on a specific phase of their operation where the xenon gas was held at a pressure of about 4 bar, roughly four times the pressure of the atmosphere at sea level. This was a lower pressure than the 10 bar they intend to use for their main data collection, but it offered a unique opportunity to test how the detector performs under different conditions. The researchers wanted to know if the lower pressure, which allows the electron tracks to stretch out longer and become more diffuse, would ruin their ability to identify the correct shapes. To find out, they used a powerful computer simulation to predict how the tracks would behave at both low and high pressures, and then compared those predictions to real data collected from the detector. The simulation showed that at the lower pressure, the tracks do indeed become longer and the energy deposits at the ends become larger and fuzzier, but the fundamental difference between a two-track signal and a single-track background remains visible.

To test this in the real world, the scientists introduced a specific type of radioactive source into the detector that produces pairs of electrons and positrons, creating events that look very similar to the signal they are searching for. They then applied a series of careful filters to the data, removing events that were too close to the edges of the detector or that had overlapping tracks, leaving them with a clean sample to analyze. They focused on the energy deposits at the ends of the tracks, looking for the tell-tale sign of two heavy blobs versus one. By adjusting the size of the area they examined around the ends of the tracks, they found a sweet spot that maximized their ability to keep the good events while rejecting the bad ones.

The results were encouraging. The team demonstrated that even at this lower pressure, the detector could identify the double-electron tracks with an efficiency of about 76 percent, meaning it successfully kept three-quarters of the true signals. At the same time, it rejected about 85 percent of the background noise, accepting only about 15 percent of the unwanted single-electron tracks. These numbers are comparable to the performance of their previous, smaller detector, which operated at a higher pressure, proving that the technology is robust and scalable. The study confirms that the lower pressure does not prevent the detector from seeing the crucial topological features needed for the search. While the researchers note that their current computer models do not yet capture every detail of the low-pressure environment, the agreement between their simulations and the real data gives them confidence that their methods are sound. This work establishes a solid baseline for the next phase of the experiment, where the detector will operate at full pressure, bringing the hunt for neutrinoless double beta decay closer to a definitive answer.

Technical Summary: Demonstrating topological identification capabilities of the NEXT experiment at low pressure

Problem and Context
The primary objective of the NEXT (Neutrino Experiment with a Xenon TPC) programme is the detection of neutrinoless double beta decay ( $0\nu\beta\beta$ ) in $^{136}$ Xe. Achieving this requires not only high energy resolution (sub-1% FWHM) but also effective background suppression through topological discrimination. While the previous demonstrator, NEXT-White, established the technology's capabilities at 10 bar, the current NEXT-100 detector is undergoing commissioning and initial physics runs at a lower pressure of $\sim$ 4 bar (Low Pressure Run, LPR).

Operating at low pressure presents specific challenges for topological reconstruction. Lower pressure results in longer particle tracks, reduced specific energy loss, and increased diffusion, which alters the morphology of the tracks compared to high-pressure (HPR) conditions. The paper addresses the need to validate the detector's topological performance in this LPR regime and to establish a baseline discrimination method that can be improved upon in future high-pressure analyses.

Methodology
The analysis utilizes data from dedicated high-energy calibration runs of the NEXT-100 detector, employing an external $^{228}$ Th source. The decay chain of $^{228}$ Th produces 2615 keV gamma rays, which induce pair production ( $e^+e^-$ ). These events are selected as a proxy for $0\nu\beta\beta$ signals because they produce two electron-like tracks originating from a shared vertex, mimicking the topology of the signal of interest. The analysis focuses specifically on the Double Escape Peak (DEP) events ( $\sim$ 1593 keV), where the positron annihilates and the resulting 511 keV photons escape the detector, leaving a distinct two-blob topology.

The methodology involves the following steps:

Data Selection: A series of quality cuts are applied to isolate well-reconstructed DEP events, including fiducial volume cuts, a requirement for a single reconstructed track, and a cut to remove tracks with overlapping blobs.
Topological Reconstruction: Charged particle trajectories are reconstructed as 3D "hits" from SiPM waveforms. These hits are grouped into voxels and connected via a Breadth First Search algorithm to form tracks.
Blob Identification: To distinguish signal (two Bragg peaks) from background (single Bragg peak), the algorithm identifies track extrema as "blobs." The paper introduces an improved algorithm for determining blob centers at low pressure. Unlike previous fixed-radius methods, this new approach uses a "scan radius" to identify the voxel that encapsulates the maximum energy within a fixed "blob radius," thereby optimizing the center relocation for the larger, more diffuse blobs characteristic of LPR.
Optimization: A parameter scan is performed on the blob radius, scan radius, and voxel size. The optimal parameters are selected by maximizing a Figure of Merit (FOM), defined as $FOM = e_i / \sqrt{b_i}$ , where $e_i$ is the signal efficiency and $b_i$ is the background acceptance after the $i$ -th cut.
Monte Carlo (MC) Validation: A GEANT4-based MC study is conducted to characterize the theoretical effects of pressure on track topology (length, blob size, specific energy loss) to provide context for the data-driven observations.

Key Results
The study successfully demonstrates the topological discrimination capabilities of NEXT-100 at $\sim$ 4 bar:

Pressure Effects: MC studies confirm that at 4 bar, true electron tracks are approximately 2.6 times longer and have a blob size roughly 2.3 times larger than at 10 bar, with specific energy loss reduced by a factor of $\sim$ 3. Data observations align qualitatively with these MC predictions.
Discrimination Performance: Using the optimized cut-based method (specifically a cut on the energy of the second blob, $E_{blob2}$ $E_{b l o b 2}$ ), the analysis achieves:
- Signal Efficiency: $75.6 \pm 1.9 \text{ (stat.)} ^{+3.4}_{-4.1} \text{ (syst.)} \%$ for double-electron tracks.
- Background Acceptance: $14.7 \pm 0.4 \text{ (stat.)} ^{+0.7}_{-0.8} \text{ (syst.)} \%$ for single-electron tracks.
Comparison: These results are consistent with the performance of the NEXT-White detector at 10 bar, which achieved a signal efficiency of $71.6 \pm 1.5 \%$ and background acceptance of $20.6 \pm 0.4 \%$ .

Significance and Claims
The paper claims to present the first validation of NEXT-100's topological performance and the first topological analysis conducted at low pressure within the programme. The primary significance lies in establishing a "baseline" cut-based topological discrimination method for the NEXT-100 detector. This baseline serves as the reference point for all future analyses, against which more sophisticated techniques (such as track deconvolution or convolutional neural networks) will be compared and implemented in future high-pressure runs. The results confirm that despite the unique challenges of low pressure, the detector maintains excellent topological discrimination capabilities, validating the scalability of the technology for future tonne-scale detectors.

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