New constraints on physics within and beyond the standard model from the latest CONUS datasets

The CONUS collaboration reports a $3.7\sigma$ observation of coherent elastic neutrino-nucleus scattering at the Leibstadt reactor and utilizes combined datasets from Brokdorf and Leibstadt to establish new, improved constraints on neutrino magnetic moments, millicharges, non-standard interactions, light new mediators, and the Weinberg angle, thereby advancing the search for physics within and beyond the Standard Model.

The Gist

Imagine the universe is a giant, noisy party. For decades, physicists have been trying to hear a specific, very quiet whisper: the sound of a neutrino (a tiny, ghost-like particle) bumping into an entire atom all at once. This phenomenon is called Coherent Elastic Neutrino-Nucleus Scattering (CEνNS). It's like a mosquito hitting a bowling ball; the mosquito barely makes a dent, but if you have enough of them, you might feel a tiny vibration.

The CONUS Collaboration is a team of scientists who built a super-sensitive "ear" (a detector) to listen for these vibrations near nuclear power plants. This paper is their latest report card, summarizing what they heard at two different locations: a power plant in Brokdorf, Germany, and a newer one in Leibstadt, Switzerland.

Here is the breakdown of their findings in plain English:

1. The Setup: Two Different Listening Posts

Think of the experiment as a high-stakes game of "Whisper in a Storm."

• The Storm: Nuclear reactors are incredibly loud sources of neutrinos, but they also create a lot of background noise (heat, radiation, cosmic rays).
• The Ears: The scientists used Germanium detectors (special crystals) buried deep underground to block out the noise.
• The Move: They started in Brokdorf (Germany) and later moved to Leibstadt (Switzerland). The new spot in Switzerland had less rock overhead (less "shielding" from cosmic rays), which usually makes things noisier. However, they upgraded their equipment, making the "ears" much more sensitive. They could now hear vibrations as small as a single atom's worth of energy (about 160 electron-volts).

2. The Big Breakthrough: Finally Hearing the Whisper

For years, they were looking for this signal but only saw hints of it.

• The Result: At the new Swiss site, they finally caught the signal with 3.7 sigma significance. In the world of physics, this is like being 99.9% sure you heard the whisper and not just the wind.
• The Match: The sound they heard matched the "Standard Model" (the rulebook of physics we already know) perfectly. It's like tuning a radio and finally finding the station exactly where the map said it would be.

3. The Real Goal: Hunting for "New Physics"

Just because they heard the standard whisper doesn't mean the job is done. The real excitement is finding out if there are other sounds hiding in the noise—signs of New Physics (particles or forces we haven't discovered yet). They used their data to check for four specific "ghosts":

A. The Magnetic Ghost (Neutrino Magnetic Moment)

• The Idea: Do neutrinos have a tiny magnetic pull, like a microscopic magnet?
• The Finding: They didn't find a magnet. However, they tightened the rules. They can now say with high confidence that if neutrinos are magnetic, they are weaker than a specific limit. They improved their previous "no magnet" limit, getting closer to the best measurements in the world.

B. The Tiny Charge Ghost (Neutrino Millicharge)

• The Idea: Do neutrinos have a tiny electric charge, even though we think they are neutral?
• The Finding: Again, no charge was found. But they improved the limit, saying, "If they have a charge, it's smaller than 1.76 out of 10 trillionths of an electron's charge."

C. The Invisible Handshake (Non-Standard Interactions)

• The Idea: Maybe neutrinos have a secret way of talking to matter that isn't in the standard rulebook. Imagine if neutrinos could shake hands with atoms in a way we didn't know about.
• The Finding: They didn't find a new handshake. However, they managed to solve a puzzle that confused other experiments. Other detectors saw a "double band" of possibilities (like two different answers to a math problem). Because CONUS finally detected the signal clearly, they could narrow it down and say, "The new physics scale must be at least 145 GeV." This pushes the search for new particles to higher energies.

D. The Invisible Messenger (Light Mediators)

• The Idea: Maybe there are new, super-light particles acting as messengers between neutrinos and atoms, changing how they interact.
• The Finding: They didn't find these messengers. But they set new, stricter limits on how strong these messengers could be. They lowered the "coupling" (how strongly they interact) to levels as low as 4 in 10 million.

4. Measuring the "Weinberg Angle"

• The Concept: In physics, there's a number called the Weinberg angle that describes how the weak nuclear force and electromagnetism are related. It's like a dial that sets the rules of the universe.
• The Finding: Using their new data, the team measured this dial. They found a value of 0.28. This is very close to what the Standard Model predicts, but slightly different (about 1 standard deviation away). It's a precise measurement that helps physicists check if the universe's rulebook is written correctly at low energies.

Summary

The CONUS team successfully upgraded their experiment, moved to a new location, and for the first time, clearly detected neutrinos bouncing off atomic nuclei. While they didn't find any "new" particles or forces (which would have been a Nobel Prize-level discovery), they did something equally important: they tightened the net.

They proved that if new physics exists, it is hiding even deeper than we thought. They have set the strictest limits yet on several theories, effectively telling other scientists, "If you are looking for new particles, don't look here; they aren't this strong." This clears the path for future experiments to hunt for even more elusive secrets of the universe.

Technical Summary

Technical Summary: New constraints on physics within and beyond the standard model from the latest CONUS datasets

Problem and Motivation
Coherent elastic neutrino-nucleus scattering (CEνNS) was predicted in the 1970s but remained undetected for decades due to the tiny nuclear recoil energies involved. Its detection has recently opened a new window for investigating physics within and beyond the Standard Model (SM). While previous experiments established CEνNS using pion-decay-at-rest (πDAR) sources and solar neutrinos, the CONUS collaboration previously reported the first detection using reactor antineutrinos. This paper addresses the need to refine these measurements and utilize the full experimental systematics to place stringent constraints on various Beyond the Standard Model (BSM) scenarios, including neutrino electromagnetic properties, non-standard interactions (NSIs), light mediators, and the low-energy running of the Weinberg angle.

Methodology and Experimental Setup
The study analyzes data from two distinct experimental sites: the Brokdorf nuclear power plant (KBR) in Germany and the Leibstadt nuclear power plant (KKL) in Switzerland.

• Experimental Configuration: The experiment utilizes high-purity germanium (HPGe) detectors housed in a compact shield with passive (lead, polyethylene) and active (muon veto) background reduction components.
  • KBR Site: Operated from 2018–2022 with a 3.9 GW thermal reactor at a distance of 17.1 m. Detectors were deployed with a threshold of ~210 eV using a CAEN DAQ system in the final run (Run-5), and a Lynx DAQ system in earlier runs.
  • KKL Site: Operations began in 2023 at a 3.6 GW boiling water reactor, 20.7 m from the core. The setup was refined to lower the detection threshold to 160–180 eV, achieving near-100% trigger efficiency at the threshold.
• Datasets: Three primary datasets are analyzed:
  1. KBR Lynx: An extended dataset (Runs 1, 2, 4, 5) used for constraining neutrino electromagnetic properties.
  2. KBR CAEN: The final dataset from Brokdorf (Run-5) with upgraded DAQ and pulse shape discrimination (PSD), used for vector NSI and light mediator constraints.
  3. KKL CAEN: The first dataset from Leibstadt, used for the first CEνNS detection at a reactor site and subsequent BSM analyses.
• Analysis Framework: The analysis employs a maximum likelihood approach fitting reactor ON and OFF data simultaneously. The framework incorporates full systematics, including background modeling (Monte Carlo simulations and empirical components), neutrino flux uncertainties, energy calibration, and signal quenching (modeled via the Lindhard parameter $k$). The statistical treatment uses a profile likelihood ratio test to determine 90% confidence level (C.L.) limits.

Key Contributions and Results

1. CEνNS Detection and SM Validation:
   The KKL CAEN dataset yielded a 3.7σ observation of CEνNS, showing good agreement with SM predictions. This confirms the viability of reactor-based CEνNS detection with high-purity germanium detectors.

2. Neutrino Electromagnetic Properties:
   Using the extended KBR Lynx dataset, the paper improves limits on the neutrino magnetic moment ($\mu_\nu$) and neutrino millicharge ($q_\nu$):

   • $\mu_\nu < 5.18 \times 10^{-11} \mu_B$
   • $|q_\nu| < 1.76 \times 10^{-12} e_0$
     These represent improvements over previous CONUS results and approach the sensitivity of the GEMMA experiment.

3. Non-Standard Interactions (NSIs):
   The analysis of vector-type NSIs using the KBR CAEN and KKL CAEN datasets allows for the resolution of the characteristic "double-band" structure in the $\{\epsilon^u_{ee}, \epsilon^d_{ee}\}$ parameter space, a feature arising from the degeneracy of the weak charge squared.

   • The new physics scale is constrained to $\Lambda_{NSI} = 145$ GeV (improved from 135 GeV in previous runs).
   • The KKL data, with the actual signal observed, provides tighter constraints than the KBR data alone.

4. Light Mediator Models:
   Constraints were placed on light scalar and vector mediators (coupled universally or via $B-L$ charge) by analyzing both CEνNS and elastic neutrino-electron scattering ($E\nu eS$) channels.

   • Scalar Mediators: Couplings down to $2 \times 10^{-6}$ (universal) and $2.7 \times 10^{-6}$ (KKL) were probed.
   • Vector Mediators: Couplings down to $4 \times 10^{-7}$ were constrained in the $E\nu eS$ channel. The results are competitive with other experiments like Coherent and PandaX-XenonT, particularly at lower mediator masses.

5. Weinberg Angle Determination:
   For the first time, the Weinberg angle ($\sin^2 \theta_W$) was determined using CEνNS and reactor antineutrinos at a momentum transfer of $\sim 10$ MeV.

   • Result: $\sin^2 \theta_W = 0.28^{+0.03}_{-0.04}$ (68% C.L.).
   • This value is approximately $1\sigma$ away from the SM prediction but is compatible with it and consistent with the Coherent experiment's results.

6. Neutrino Charge Radius:
   A two-dimensional analysis of the neutrino magnetic moment and charge radius ($\langle r^2_{\nu e} \rangle$) was performed, yielding constraints on the charge radius in the range of $[-31.3, -24.1] \cup [-1.6, 5.5] \times 10^{-32} \text{ cm}^2$.

Significance
The paper claims that these results demonstrate the maturity of the CONUS experiment as a precision tool for testing the Standard Model and searching for new physics. By successfully detecting CEνNS at a reactor site and incorporating full experimental systematics, the collaboration has established competitive limits on a broad class of BSM models. The resolution of the NSI double-band structure and the first determination of the Weinberg angle at the MeV scale via reactor antineutrinos are highlighted as significant milestones. The work positions CONUS as a competitive player in the global effort to probe neutrino properties and new physics, with future improvements expected as the CONUS+ experiment continues data taking with increased active mass and improved detector capabilities.