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The ATAR, a dense and compact vertex detector for the PIONEER experiment

This paper presents the design and 2026 development status of the ATAR, a high-granularity active target composed of 48 low-gain avalanche strip detectors with precise timing and spatial resolution, which serves as a critical component of the PIONEER experiment at PSI to achieve a 0.01% measurement accuracy of the charged-pion branching ratio.

Original author: Tilman Rohe (Paul Scherrer Institut, Villigen, Switzerland), for the PIONEER collaboration

Published 2026-11-09

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CC BY 4.0

Original author: Tilman Rohe (Paul Scherrer Institut, Villigen, Switzerland), for the PIONEER collaboration

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 author. For technical accuracy, refer to the original paper. Read full disclaimer

In the subatomic world, particles called pions are fleeting messengers that decay almost instantly after being created. When a positively charged pion breaks apart, it usually transforms into a muon and a ghostly particle called a neutrino. Very rarely, however, it chooses a different path, decaying directly into a positron and a neutrino. This rare event is a fundamental test of how the universe works, specifically a principle called lepton universality, which suggests that nature treats different types of light particles with equal weight, regardless of their mass. Theoretical calculations predict exactly how often this rare decay should happen, but the most precise measurements we have so far fall slightly short of that prediction. Scientists need to know if this gap is just a measurement error or a sign of new physics hiding in the shadows. To solve this, they must count these rare events with a precision that has never been achieved before, requiring a detector capable of seeing the tiniest details of a particle's final moments.

A team of researchers known as the PIONEER collaboration is building a machine to make this measurement possible at the Paul Scherrer Institut in Switzerland. Their goal is to measure the ratio of pion decays into electrons versus muons with an accuracy of one hundredth of a percent, a level of precision that matches the best theoretical predictions. The heart of this new machine is a device called the ATAR, a dense and compact stack of silicon sensors designed to act as a fully instrumented target. Unlike traditional experiments where particles pass through empty space before hitting a detector, the ATAR stops the incoming pion beam directly inside the sensor layers. This allows the machine to capture the exact moment a pion stops, how it decays, and the energy of the resulting particles with unprecedented detail. The device is composed of a stack of 48 low gain avalanche strip detectors (LGADs), where each sensor plane contains 100 micro strips, stacked to create a total target thickness of about six millimeters.

The challenge for the PIONEER team is that the rare decay they are hunting looks very similar to a much more common background event. When a pion decays into a muon, that muon can subsequently decay into an electron, creating a signal that mimics the rare direct decay. To separate the two, the ATAR relies on extreme speed and spatial precision. The silicon sensors are capable of measuring time with a resolution of two hundred picoseconds, which is fast enough to distinguish between the initial pion stop and the subsequent decay of its daughter particle. By recording the full electrical waveform of the signal, the system can even tell the difference between two pulses arriving just two nanoseconds apart in the same channel. This timing capability allows the researchers to filter out the common decay chains, keeping the rare events they need while rejecting millions of background signals.

Beyond timing, the ATAR measures the energy deposited by particles as they pass through or stop in the silicon. A pion that stops in the target leaves a distinct energy signature of about fourteen million electron volts, while a muon that stops leaves a much smaller deposit of four point one million electron volts. This difference in energy, combined with the precise timing, helps the team identify the specific type of particle involved in each event. The sensors are arranged in a stack where each layer is rotated ninety degrees relative to the one before it, creating a three-dimensional map of the particle's path. This spatial information is crucial for reconstructing the shape of the event and ensuring that the positron from the rare decay is correctly identified and tracked all the way to the surrounding calorimeter, a large detector that measures the total energy of the particles.

To handle the massive amount of data generated by this high-speed recording, the electronics are placed outside the immediate path of the particles to avoid interfering with the energy measurements. The system uses fast amplifiers and specialized chips that digitize the waveforms in real time, capturing the shape of the signal to analyze the pulse height and timing later. The team is currently developing prototypes of this system, with a test version called ATAR 8, consisting of a stack with 8 to 10 sensor layers, scheduled for testing in late 2026. Early simulations and tests suggest that this approach could reject background noise by a factor of five hundred while still keeping half of the rare events they are looking for. If successful, the PIONEER experiment will provide a definitive measurement of the pion decay ratio, either confirming our current understanding of the universe or revealing a crack in the standard model that points toward new, undiscovered physics.

Technical Summary: The ATAR for the PIONEER Experiment

Problem Statement
The PIONEER experiment at the Paul Scherrer Institut (PSI) aims to measure the charged-pion branching ratio to electrons versus muons ( $R_{\pi}^{e/\mu}$ ) with a precision of 0.01%. This target represents an order-of-magnitude improvement over the current world average, which is dominated by the PiENu collaboration's results from over a decade ago. The primary challenge in achieving this precision is the "tail fraction" of events: approximately 1% of direct $\pi^+ \to e^+ \nu$ decays (which produce a mono-energetic 69 MeV positron) are reconstructed with energy below the analysis threshold due to incomplete shower collection in the calorimeter (e.g., lateral leakage or interactions with material). To reach the 0.01% goal, this tail fraction must be determined at the 1% level. Furthermore, the experiment requires a high pion stop rate of 300 kHz, leading to significant pile-up where multiple decay chains are active simultaneously, necessitating a detector capable of resolving overlapping signals in time and space.

Methodology and Detector Design
The PIONEER apparatus centers on two main components: a high-resolution, highly segmented LYSO crystal calorimeter and a fully instrumented, dense, and compact active target known as ATAR (Active Target).

The ATAR Structure: The ATAR is a 2 cm $\times$ 2 cm $\times$ 6.6 mm stack composed of 48 Low Gain Avalanche Detector (LGAD) strip sensors. Each sensor is a trench-isolated silicon strip detector (2 $\times$ 2 cm $^2$ , 120 $\mu$ m thick) containing 100 micro-strips with a 200 $\mu$ m pitch. To achieve 3D positional tracking, adjacent sensor planes are rotated by 90 $^\circ$ . Inactive silicon is minimized, leaving only ~5 $\mu$ m of support wafer.
Signal Processing and Timing: The design leverages the fast rise time of LGADs to achieve ~200 ps timing precision. By recording full waveforms, the system aims to resolve pulse pairs separated by as little as 2 ns. This capability is critical for distinguishing a pion stop from its subsequent decay products within the same channel.
Readout Electronics: To avoid interfering with the calorimeter's energy measurement, the readout electronics are placed upstream, outside the calorimeter's acceptance angle. The system utilizes:
- Amplification: Analog FAST3 chips (16 channels each) with adjustable gain to distinguish Minimum Ionizing Particles (MIPs), moving pions/muons, and particle stops.
- Digitization: Two strategies are being evaluated. The first uses the HD-SoC chip (Nalu Scientific) sampling at 1–2 Gs/s with a long buffer (2048 samples). The second employs the Sampic system (IJCLab Orsay) with ultra-high sampling rates (1.6–10 Gs/s) but shorter buffers, utilizing daisy-chaining to re-trigger and capture multiple pulses within a short window.
Event Reconstruction: The system utilizes pulse height measurements to identify specific energy deposits: ~14 MeV for a stopping pion, ~4.1 MeV for a stopping muon, and a distinct distribution for a traversing positron. Topology reconstruction, including per-hit energy deposits, is being investigated using both rule-based approaches and transformer techniques to reject background.

Key Contributions and Development Status
The paper details the design and development status of the ATAR as of late 2026. Key contributions include:

Prototype Validation: The paper reports on the "ATAR 8" prototype, a stack of 8 to 10 sensor layers scheduled for testing in the PSI piM1 beamline in December 2026. This prototype uses 10 $\times$ 10 mm $^2$ sensors with 48 strips, mounted back-to-back on FR-4 frames with 15 cm flex cables.
Sensor Production: The first PIONEER-specific production of 2 $\times$ 2 cm $^2$ strip sensors is ongoing at FBK, with delivery expected in early 2027.
Characterization of Non-Linearity: The paper notes that LGADs exhibit gain suppression (non-linearity) dependent on charge concentration and impact angle. While a power-law model fits tracks traversing the sensor, the behavior regarding large, local charge deposits (such as muon stops) is scheduled for investigation in a 2026 test beam.

Results and Claims
The paper presents design projections and early simulation results rather than final experimental data.

Background Rejection: Simulations and early results suggest that selecting events with a characteristic pion stop deposit followed by a matching positron deposit can preserve 80% of $\pi^+ \to e^+ \nu$ events while reducing the muon decay-at-rest background by three orders of magnitude.
Pile-up Handling: The combination of fast timing and spatial segmentation is projected to provide a rejection factor of 160 for the $\pi \to \mu \to e$ decay chain while retaining ~60% of direct $\pi \to e$ events. Full topology reconstruction is estimated to offer an additional factor of 500 in rejection power.
Digitization Performance: Tests of the HD-SoC chip at 1 Gs/s showed good linearity, though there is an estimated 8% probability of failing to resolve two consecutive pulses within a 2 ns window.

Significance
The paper positions the ATAR as the critical innovation enabling the PIONEER experiment to meet its precision goals. By providing precise timing, spatial information, and pulse height data, the ATAR allows for the rigorous identification and subtraction of the "tail fraction" events that previously limited measurement accuracy. The successful operation of the ATAR 8 prototype in late 2026 is presented as a necessary step toward the staged startup of the full experiment following the PSI HIPA upgrade, ultimately aiming to provide a probe of lepton universality and a more accurate determination of CKM matrix elements via pion beta decay in later phases.

Technical Summary: The ATAR for the PIONEER Experiment

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