When backgrounds become signals: neutrino interactions in xenon-based dark matter detectors

This paper analyzes recent electron and nuclear recoil data from XENONnT, PandaX-4T, and LUX-ZEPLIN to probe both Standard Model and Beyond the Standard Model physics, highlighting how xenon-based dark matter detectors can uniquely complement dedicated neutrino experiments by providing sensitivity to solar neutrino interactions and flavor-dependent new physics effects.

The Gist

Imagine the universe is filled with a ghostly substance called "dark matter." Scientists have built massive, ultra-sensitive underwater cameras (filled with liquid xenon) deep underground to catch these ghosts. These cameras are designed to spot a tiny flash of light when a dark matter particle bumps into a xenon atom.

However, there's a problem: the universe is also filled with a different kind of ghostly particle called a "neutrino." These are tiny, almost massless particles streaming from the Sun. They are so sneaky that they can bump into the same xenon atoms and create a flash of light that looks almost identical to a dark matter bump.

For a long time, scientists treated these neutrino bumps as "noise" or "background static" that ruined their search for dark matter. This paper is about a clever twist: What if we stop trying to ignore the noise and start listening to it instead?

Here is what the authors did, explained simply:

1. The Two Types of "Bumps"

When a neutrino hits the xenon, it can do two things, like a billiard ball hitting another ball:

• The Heavy Hit (Nuclear Recoil): The neutrino hits the heavy nucleus (the core) of the xenon atom. This is like a cue ball hitting a heavy bowling ball. It's hard to see, but it happens. This is called Coherent Elastic Neutrino-Nucleus Scattering (CEνNS).
• The Light Tap (Electron Recoil): The neutrino hits the tiny electrons orbiting the atom. This is like a ping-pong ball hitting a feather. It's easier to see, but it's usually a very faint signal. This is called Neutrino-Electron Scattering (νES).

2. Turning "Background" into "Signal"

The researchers took data from three giant experiments (XENONnT, PandaX-4T, and LUX-ZEPLIN). Instead of throwing away the data that looked like neutrinos, they treated it as a treasure trove of information.

They asked: "Can we use these dark matter detectors to learn about the Sun and the laws of physics?"

The answer is yes. Even though these detectors aren't as precise as dedicated neutrino labs, they have a superpower: they can detect a specific type of neutrino (the "tau" neutrino) that other experiments struggle to see. It's like having a microphone that picks up a specific musical note that other microphones miss.

3. What They Learned (The "Detective Work")

By analyzing the "noise," the team tested several theories about how the universe works:

• Checking the Sun's Recipe: They measured how many neutrinos are coming from the Sun. They found the numbers match the "recipe" scientists have been using for decades (the GS98 solar model). It's like tasting a soup and confirming the chef used exactly the right amount of salt.
• Testing the Rules of Physics: They checked if the "Weak Mixing Angle" (a fundamental rule of how particles interact) changes at low energies. Their results say: "The rules are working exactly as the Standard Model predicts." No cheating found yet!
• Hunting for "Ghostly" Properties: They looked for signs that neutrinos might have secret properties, like a tiny magnetic charge or a tiny electric charge (millicharge).
  • The Analogy: Imagine looking for a ghost that might have a faint glow. They didn't find the glow, but they proved that if the ghost does have a glow, it must be incredibly dim. They set the strictest limits yet on how "bright" these neutrino ghosts can be.
• New Particles? They looked for evidence of a new, invisible force carrier (a "light mediator") that might connect particles in a way we don't understand. Again, they didn't find it, but they narrowed down the search area significantly.

4. The Big Picture

The paper concludes that while these dark matter detectors were built to find dark matter, they are accidentally becoming excellent tools for studying neutrinos.

• The "Tau" Advantage: They are the first to use this data to get a good look at the "tau" flavor of neutrinos, filling in a missing piece of the puzzle that other experiments can't see.
• The "Noise" is Useful: What was once considered a nuisance (neutrino background) is now a valuable signal. It helps scientists understand the Sun and test the fundamental laws of physics.

In short: The authors took the "static" on their radio (neutrino bumps) and tuned it in to listen to the music of the universe. They confirmed the music is playing the right notes, and they proved that even the quietest instruments (dark matter detectors) can hear the faintest instruments (tau neutrinos) in the orchestra.

Technical Summary

Technical Summary: "When backgrounds become signals: neutrino interactions in xenon-based dark matter detectors"

Problem Statement
Direct detection dark matter experiments utilizing dual-phase time projection chambers (TPCs) filled with noble liquids (specifically xenon) are primarily designed to search for Weakly Interacting Massive Particles (WIMPs). However, these detectors face a fundamental limitation known as the "neutrino fog," where solar neutrinos scattering elastically off nuclei (Coherent Elastic Neutrino-Nucleus Scattering, CE$\nu$NS) and atomic electrons (Neutrino-Electron Scattering, $\nu$ES) produce signals indistinguishable from potential dark matter interactions at low energies. While traditionally viewed as an irreducible background, the recent observation of CE$\nu$NS from $^8$B solar neutrinos by XENONnT and PandaX-4T has transformed these events into a new probe for low-energy neutrino physics. The challenge lies in leveraging these detectors' large target masses and background rejection capabilities to study Standard Model (SM) parameters and Beyond the Standard Model (BSM) scenarios, particularly those involving the $\tau$ neutrino flavor, which is difficult to access with other low-energy probes.

Methodology
The authors analyze the latest nuclear recoil (NR) and electron recoil (ER) datasets from three major collaborations: XENONnT (XnT), PandaX-4T (P4T), and LUX-ZEPLIN (LZ).

• Theoretical Framework: The study calculates expected event rates for CE$\nu$NS and $\nu$ES processes, accounting for solar neutrino fluxes (primarily $pp$, $^7$Be, $^8$B, and $hep$) and neutrino oscillation probabilities (MSW mechanism). The cross-sections are formulated to include SM predictions and various BSM modifications, such as neutrino charge radii, magnetic moments, millicharges, and non-standard interactions (NSI).
• Statistical Analysis:
  • For ER data (sensitive to $\nu$ES), a Poissonian least-squares function is employed to handle low event counts in energy bins, incorporating nuisance parameters for background uncertainty ($\alpha$) and solar neutrino flux normalization ($\beta$).
  • For NR data (sensitive to CE$\nu$NS), a Gaussian least-squares function is used, accounting for specific background components (accidental coincidences, neutrons, electron recoils) and flux uncertainties.
  • For P4T, the authors perform both a spectral analysis of the US2 dataset and an integrated analysis of US2 plus paired data to maximize sensitivity to flux normalization and coupling parameters.
• BSM Scenarios: The analysis tests deviations in the weak mixing angle ($\sin^2\theta_W$), neutrino charge radii (specifically the $\tau$ flavor), neutrino magnetic moments, neutrino millicharges, and light vector mediators (specifically the $L_\mu - L_\tau$ model).

Key Contributions

1. Flavor-Dependent Probes: The work highlights the unique capability of xenon-based detectors to probe the $\tau$ neutrino component of solar neutrinos. By combining data from different experiments, the authors constrain the $\tau$ flavor contribution to CE$\nu$NS, a parameter largely inaccessible to reactor and accelerator-based experiments which are dominated by $\nu_e$ and $\nu_\mu$ fluxes.
2. Global Fits and Constraints: The paper provides updated constraints on several fundamental parameters:
   • Weak Mixing Angle: Determinations of $\sin^2\theta_W$ at low energies using both ER and NR channels, representing some of the lowest energy measurements available.
   • Neutrino Charge Radius: Constraints on the diagonal $\tau$ neutrino charge radius ($\langle r^2_{\nu_\tau} \rangle$), which are improved when combined with global fits of $\nu_e$ and $\nu_\mu$ data.
   • Electromagnetic Properties: Limits on the effective solar neutrino magnetic moment and millicharge derived from ER data, which are among the most stringent laboratory constraints currently available.
   • New Physics: Constraints on flavor-preserving Non-Standard Interactions (NSI) and the $L_\mu - L_\tau$ light vector mediator model, utilizing the distinct kinematic sensitivities of NR and ER channels.
3. Flux Normalization: The analysis constrains the normalization of the $^8$B and $hep$ solar neutrino fluxes. The combined XnT and P4T data yield a measurement of the $^8$B flux consistent with the GS98 solar model and set an upper limit on the $hep$ flux that is competitive with, though slightly weaker than, the world-best constraints from SNO.

Results

• Standard Model Consistency: The data show good overall agreement with SM predictions. The ratio of data to SM predictions for the $\tau$ flavor is constrained to be $< 2.5$ at $1\sigma$ confidence level.
• Flux Measurements: The combined analysis yields $\Phi_{^8B} = 6.7^{+2.1}_{-1.7} \times 10^6 \text{ cm}^{-2}\text{s}^{-1}$ ($1\sigma$), consistent with the GS98 model prediction of $5.46 \times 10^6 \text{ cm}^{-2}\text{s}^{-1}$. The $hep$ flux is constrained to $\Phi_{hep} < 19 \times 10^4 \text{ cm}^{-2}\text{s}^{-1}$ ($90\%$ CL).
• BSM Limits:
  • Magnetic Moment: The effective solar neutrino magnetic moment is constrained to $\mu_{\nu s} < 7.8 \times 10^{-12} \mu_B$ (XnT), $14 \times 10^{-12} \mu_B$ (P4T), and $11 \times 10^{-12} \mu_B$ (LZ) at $90\%$ CL.
  • Millicharge: Constraints on the effective solar neutrino millicharge are found to be in the range of $\sim 10^{-13} e_0$.
  • Mediators: The study sets competitive limits on the $L_\mu - L_\tau$ light vector boson, particularly in the low-mass region where ER data dominate, and in the higher-mass region where NR data are more sensitive.
• Precision: While the precision of these measurements is currently lower than dedicated neutrino experiments, the results are statistically significant and provide complementary information, especially regarding the $\tau$ sector and low-energy regimes.

Significance
The paper asserts that the detection of solar neutrinos in dark matter detectors represents a paradigm shift, turning a limiting background into a valuable signal for low-energy neutrino physics. The primary significance lies in the ability of these detectors to probe the $\tau$ neutrino flavor component and test flavor-dependent new physics, which is difficult to achieve with other low-energy sources. Furthermore, the work demonstrates that xenon-based TPCs can provide competitive constraints on neutrino electromagnetic properties (magnetic moments and millicharges) and the weak mixing angle at the lowest accessible energies. The authors conclude that as background rejection improves and exposure increases, these detectors will play an increasingly crucial role in refining solar neutrino flux measurements and probing BSM physics, effectively complementing the global picture provided by reactor, accelerator, and dedicated solar neutrino experiments.