⚛️ phenomenology

Precision Tests of SM and new physics with the COHERENT Ge-mini and TEXONO data

This paper presents a comprehensive analysis of COHERENT Ge-mini and TEXONO germanium-based data to validate the Standard Model's weak mixing angle and establish stringent, complementary constraints on neutrino electromagnetic properties, light mediators, and sterile neutral leptons.

Original authors: Tousib Ahmed, Ayan Chattaraj, Anirban Majumdar, Newton Nath, Rahul Srivastava

Published 2026-08-12

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CC BY 4.0

Original authors: Tousib Ahmed, Ayan Chattaraj, Anirban Majumdar, Newton Nath, Rahul Srivastava

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, bustling party where tiny particles are the guests. For decades, physicists have had a very strict guest list called the "Standard Model." It's like a rulebook that perfectly predicts how most particles behave, dance, and interact. But there's a problem: the rulebook has some holes. We know neutrinos—ghostly, invisible particles that zip through everything—have mass, which the original rulebook said they shouldn't. This suggests there are secret VIPs or hidden rules we haven't discovered yet. To find them, scientists look for "Coherent Elastic Neutrino-Nucleus Scattering" (CEνNS). Think of this like a neutrino gently bumping into a whole bowling pin (the atomic nucleus) all at once, rather than just hitting a single pinball. Because the neutrino hits the whole pin together, the "bump" is much stronger and easier to feel, but it's still incredibly faint, like trying to hear a whisper in a hurricane.

Now, imagine two very different detectives trying to catch these whispers. One detective, COHERENT, uses a powerful machine that shoots neutrinos from a stopped pion source (like a high-energy particle cannon) at a bank of super-sensitive Germanium detectors. The other detective, TEXONO, sits quietly next to a nuclear reactor, waiting for the steady stream of anti-neutrinos that leak out like heat. Both are using Germanium crystals, which are so pure and cold they can feel the tiniest shiver of a particle hitting them. The big question is: Do these neutrinos behave exactly as the Standard Model predicts, or are they doing something sneaky, like having a tiny electric charge, a magnetic personality, or even turning into invisible "sterile" cousins that we can't see?

In this paper, the authors act as the ultimate referees, taking the latest, most precise data from both the COHERENT Ge-mini experiment and the TEXONO experiment to see if the neutrinos are playing by the rules. They didn't just look at the "bowling pin" hits (CEνNS); they also looked at the "pinball" hits, where neutrinos bounce off electrons (EνES), combining both clues to get a sharper picture.

Here is what they found:

First, they checked the "Weak Mixing Angle," a fundamental setting in the universe's rulebook that determines how particles interact. Using the COHERENT Ge-mini data, they measured this angle to be 0.233 with a small margin of error (+0.025, -0.024). This result is a perfect match for the Standard Model's prediction, confirming that at low energies, the universe is behaving exactly as expected. The TEXONO data wasn't precise enough to give a two-sided measurement, but it did set an upper limit of 0.285, which is still consistent with the rules.

Next, they hunted for "new physics" in the form of weird neutrino properties. They looked for things like a "millicharge" (a tiny electric charge), a "magnetic moment" (acting like a tiny magnet), and a "charge radius" (how big the neutrino's electric cloud looks).

The Magnetic Moment: The TEXONO experiment was the star here, setting a very tight limit on the electron neutrino's magnetic moment at 1.18 × 10⁻¹⁰ µB (Bohr magnetons). This is much stricter than previous tests. The COHERENT Ge-mini data also improved limits for both electron and muon neutrinos, but the TEXONO result is currently the champion for electron neutrinos.
The Millicharge: This is where combining the two types of signals (hitting nuclei and hitting electrons) made a huge difference. By adding the electron-scattering data, the sensitivity to the millicharge improved by up to three orders of magnitude (a thousand times better!). TEXONO found the electron neutrino's charge is between -1.94 and 2.04 × 10⁻¹² e (where e is the charge of an electron). This is a very strict "no" to the idea that neutrinos have a significant electric charge.
The Charge Radius: The limits they set on the neutrino's charge radius are competitive with the best other experiments, including those using dark matter detectors, but they didn't find any strange deviations.

The team also looked for "mediators"—hypothetical light particles that could carry new forces between neutrinos and matter. They tested two types: "vector" mediators (like a new kind of photon) and "scalar" mediators (like a new kind of Higgs boson). They found a beautiful complementarity: TEXONO, with its low-energy reactor neutrinos, is the best at spotting very light mediators (below a few MeV), while COHERENT Ge-mini, with its higher-energy beam, is better at spotting heavier mediators (up to 200 MeV). Together, they cover a wide range of possibilities that neither could cover alone.

Finally, they investigated "Sterile Neutral Leptons" (SNLs)—heavy, invisible cousins of neutrinos that might be produced if a regular neutrino gets a "boost" from a new force. They found that TEXONO is excellent at spotting these heavy cousins up to about 10 MeV, while COHERENT Ge-mini can push that limit up to 50 MeV. In the range of 20 to 40 MeV, the COHERENT Ge-mini data provides some of the strictest limits in the world, effectively ruling out certain scenarios where these heavy neutrinos could be hiding.

In short, the paper concludes that while the Standard Model is holding up incredibly well under this new, high-precision scrutiny, these Germanium detectors have become powerful tools for searching for the "ghosts" in the machine. They haven't found the new physics yet, but they have drawn a much tighter map of where it could be hiding, proving that the combination of stopped-pion and reactor experiments is a winning strategy for the future of particle physics.

Technical Summary: Precision Tests of SM and New Physics with the COHERENT Ge-mini and TEXONO Data

Problem Statement
The Standard Model (SM) of particle physics, while successful, faces challenges regarding neutrino masses and potential deviations in neutrino properties. Coherent Elastic Neutrino-Nucleus Scattering (CE $\nu$ NS) has emerged as a powerful probe for both SM precision tests and Beyond the Standard Model (BSM) physics due to its large cross-section scaling with the square of the neutron number. Recent experimental advancements have provided high-statistics CE $\nu$ NS datasets using germanium detectors: the COHERENT Ge-mini experiment at the Spallation Neutron Source (SNS) and the TEXONO experiment at the Kuo-Sheng Reactor Neutrino Laboratory (KSNL). While previous analyses have utilized these datasets individually or focused on specific parameters, a comprehensive numerical analysis combining both CE $\nu$ NS and Elastic Neutrino-Electron Scattering (E $\nu$ ES) signals to simultaneously constrain SM parameters and a broad class of BSM scenarios using these specific germanium-based datasets was lacking.

Methodology
The authors performed an extensive numerical analysis of the latest datasets from COHERENT Ge-mini (February–May 2025 campaign) and TEXONO (D70 and D50 detectors). The methodology involved:

Theoretical Framework:
- SM Calculations: Derived differential cross-sections for CE $\nu$ NS and E $\nu$ ES within the SM, incorporating nuclear form factors (Klein-Nystrand parametrization) and quenching effects (Lindhard model).
- BSM Scenarios: Extended the framework to include:
  - Neutrino Electromagnetic (EM) Properties: Magnetic moment, millicharge, charge radius, and anapole moment.
  - Neutrino Generalized Interactions (NGIs): Light vector ( $U(1)_{B-L}$ ) and scalar mediators.
  - Sterile Neutrino Upscattering: Production of sterile neutral leptons (SNLs) via transition dipole portals and generalized scalar/vector interactions.
- Signal Combination: Unified the analysis by combining nuclear recoil (CE $\nu$ NS) and electron recoil (E $\nu$ ES) channels, treating detector effects and systematic uncertainties consistently.
Experimental Simulation:
- COHERENT Ge-mini: Modeled four Inverted Coaxial Point Contact High Purity Germanium (ICPC HPGe) detectors (Ge-21, 25, 26, 28) with sub-keV thresholds. Simulated fluxes from pion-decay-at-rest ( $\pi$ -DAR) and muon-decay-at-rest ( $\mu$ -DAR).
- TEXONO: Modeled two electro-cooled Point-Contact Germanium (ECPCGe) detectors (D70, D50) using reactor antineutrino fluxes based on the Huber-Mueller model.
- Statistical Analysis: Employed a Poissonian $\chi^2$ test statistic for COHERENT Ge-mini and a Gaussian $\chi^2$ for TEXONO. Nuisance parameters for signal normalization (flux, mass, form factors) and background normalization were profiled out to derive constraints.

Key Contributions and Results

Weak Mixing Angle ( $\sin^2 \theta_W$ ):
- Derived a low-energy determination of $\sin^2 \theta_W$ from COHERENT Ge-mini: $0.233^{+0.025}_{-0.024}$ ( $1\sigma$ ), consistent with SM predictions.
- Provided the first constraint using TEXONO 2026 data, yielding an upper limit of $\sin^2 \theta_W \leq 0.285$ ( $1\sigma$ ).
- The COHERENT Ge-mini result offers precision comparable to combined COHERENT CsI+LAr analyses and superior to previous reactor-based CE $\nu$ NS determinations.
Neutrino Electromagnetic Properties:
- Magnetic Moment: Combined CE $\nu$ NS and E $\nu$ ES improved sensitivity. TEXONO set a competitive bound of $\mu_{\nu_e}^{eff} \leq 1.18 \times 10^{-10} \mu_B$ ( $90\%$ CL), surpassing COHERENT (CsI+LAr) and Dresden-II limits.
- Millicharge: The inclusion of E $\nu$ ES significantly enhanced sensitivity, improving bounds by up to three orders of magnitude. TEXONO provided a tight constraint of $q_{\nu_e} \in [-1.94, 2.04] \times 10^{-12} e$ .
- Charge Radius & Anapole Moment: Constraints were comparable to existing CE $\nu$ NS results (e.g., CONUS+, Dresden-II) and more stringent than those from solar neutrino E $\nu$ ES analyses by dark matter detectors (LZ, XENONnT). The anapole moment limits were derived directly from charge radius constraints.
Neutrino Generalized Interactions (Light Mediators):
- Demonstrated striking complementarity between the two experiments based on mediator mass.
- TEXONO: Dominated constraints in the light mediator regime ( $M_X \lesssim 100$ keV) due to the low-energy reactor spectrum.
- COHERENT Ge-mini: Provided leading constraints for heavier mediators ( $10 \text{ MeV} \lesssim M_{Z'} \lesssim 200 \text{ MeV}$ ) due to the broader SNS energy spectrum.
- In the heavy mediator regime ( $M_X \gtrsim 100$ keV), scalar mediators were constrained more strongly than vector $B-L$ mediators.
Sterile Neutral Leptons (SNL) Upscattering:
- Transition Dipole Portal: TEXONO reached $\mu_{\nu_e N}^{eff} \sim 1.2 \times 10^{-10} \mu_B$ for $m_N \lesssim 1$ MeV. COHERENT Ge-mini extended the kinematic reach to $\sim 50$ MeV, providing stringent constraints in the $20-40$ MeV range.
- Generalized Interactions: Both experiments surpassed projected DUNE Near Detector sensitivities within their respective kinematic mass reaches ( $m_N \lesssim 10$ MeV for TEXONO, $m_N \lesssim 50$ MeV for Ge-mini), probing parameter space beyond anticipated accelerator-based coverage in the low-mass regime.

Significance and Claims
The paper claims that current germanium-based CE $\nu$ NS experiments constitute a "powerful low energy laboratory" for precision electroweak tests and complementary probes of BSM physics. The authors emphasize that:

Complementarity: The combination of stopped-pion (SNS) and reactor sources with germanium detectors allows for a broad exploration of parameter space, covering neutrino electromagnetic properties, light mediators, and sterile neutrino portals across different mass scales.
Synergy of Channels: The inclusion of the E $\nu$ ES channel is critical, particularly for constraining neutrino millicharge and magnetic moments, offering sensitivity improvements of several orders of magnitude over CE $\nu$ NS-only analyses.
Competitive Limits: The derived constraints on neutrino magnetic moments and millicharges from TEXONO are competitive with or superior to existing limits from other terrestrial, astrophysical, and cosmological probes.
Future Potential: The results suggest that with increased statistics, larger detector arrays, and improved thresholds, these experiments will further consolidate CE $\nu$ NS as a primary tool for discovering physics beyond the SM.

The authors maintain a modest tone regarding the TEXONO weak mixing angle determination (an upper limit only) and acknowledge that direct comparisons of sterile neutrino limits across different experiments require careful consideration of source-dependent effective couplings.

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