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Measurement of solar $pp$ neutrino flux with the new PandaX-4T data

Using upgraded PandaX-4T Run 2 data combined with previous results, researchers report the first positive indication of solar $pp$ neutrino--electron scattering below 165 keV, yielding a measured flux of $(8.5 \pm 3.5)\times 10^{10}$ $\mathrm{cm^{-2}s^{-1}}$ that is consistent with Standard Solar Model predictions.

Original authors: PandaX Collaboration, Peiyuan Chen, Wei Chen, Xiaohua Chen, Xun Chen, Yunhua Chen, Chen Cheng, Xiangyi Cui, Yuxin Cui, Manna Deng, Roni Dey, Yingjie Fan, Deqing Fang, Xuanye Fu, Zhixing Gao, Yujie Ge

Published 2026-07-03

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: PandaX Collaboration, Peiyuan Chen, Wei Chen, Xiaohua Chen, Xun Chen, Yunhua Chen, Chen Cheng, Xiangyi Cui, Yuxin Cui, Manna Deng, Roni Dey, Yingjie Fan, Deqing Fang, Xuanye Fu, Zhixing Gao, Yujie Ge, Lisheng Geng, Xunan Guo, Xuyuan Guo, Zichao Guo, Chencheng Han, Ke Han, Changda He, Jinrong He, Ruquan Hou, Houqi Huang, Junting Huang, Yule Huang, Xiangdong Ji, Yonglin Ju, Xiaorun Lan, Chenxiang Li, Mingchuan Li, Peiyuan Li, Shuaijie Li, Tao Li, Yangdong Li, Yuan Li, Zhiyuan Li, Qing Lin, Jianglai Liu, Yuanchun Liu, Yunyang Luo, Yugang Ma, Yajun Mao, Yue Meng, Binyu Pang, Ningchun Qi, Xiangxiang Ren, Dong Shan, Xiyuan Shao, Manbin Shen, Wenliang Sun, Xuyan Sun, Yi Tao, Yueqiang Tian, Yuxin Tian, Anqing Wang, Guanbo Wang, Hao Wang, Haoyu Wang, Jiamin Wang, Lei Wang, Meng Wang, Qiuhong Wang, Shaobo Wang, Shibo Wang, Siguang Wang, Wei Wang, Xu Wang, Zhou Wang, Yuehuan Wei, Weihao Wu, Yuan Wu, Mengjiao Xiao, Xiang Xiao, Yuhan Xie, Kaizhi Xiong, Jianqin Xu, Yifan Xu, Binbin Yan, Xiyu Yan, Yong Yang, Shunyu Yao, Peihua Ye, Chunxu Yu, Zhe Yuan, Youhui Yun, Minzhen Zhang, Peng Zhang, Shibo Zhang, Shu Zhang, Siyuan Zhang, Tao Zhang, Wei Zhang, Yang Zhang, Yingxin Zhang, Yuanyuan Zhang, Kangkang Zhao, Li Zhao, Jiaxu Zhou, Jiayi Zhou, Jifang Zhou, Ning Zhou, Xiaopeng Zhou, Zhizhen Zhou, Chenhui Zhu

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

The Big Picture: Catching Ghosts from the Sun

Imagine the Sun is a giant, glowing furnace in the sky. Deep inside, it is constantly cooking hydrogen into helium, a process that releases tiny, invisible particles called neutrinos. These particles are like cosmic ghosts; they zip through the entire Earth without hitting anything, making them incredibly hard to catch.

The most common type of these solar ghosts comes from the very first step of the Sun's cooking process (called the "pp" branch). Scientists have known about them for decades, but catching them at very low energies is like trying to hear a whisper in a hurricane.

This paper reports on a new attempt to catch these "whispers" using a giant tank of liquid xenon called PandaX-4T, located deep underground in China.

The Detector: A Giant, Invisible Net

Think of the PandaX-4T detector as a massive, ultra-pure swimming pool filled with liquid xenon, sitting 2,400 meters underground (under a mountain) to block out cosmic rays from space.

How it works: When a solar neutrino (the ghost) finally bumps into a xenon atom in the pool, it creates a tiny flash of light and a small electric charge.
The Upgrade: Before this new run (called "Run 2"), the scientists upgraded the "net." They fixed the lights (photomultiplier tubes) to see dimmer flashes, improved the water filtration to remove radioactive "dirt," and added a new system to catch radon gas.
The Goal: They wanted to see if they could detect the faint signal of solar neutrinos bouncing off electrons in the xenon, specifically at very low energy levels (below 165 keV), which is a region that had never been clearly seen before.

The Challenge: The "Static" Problem

The main problem is that the detector is surrounded by natural radioactivity. Imagine trying to listen to a single violin (the solar neutrino) in a room where a thousand people are coughing and shuffling their feet (radioactive backgrounds like Argon-39 and Krypton-85).

To solve this, the team had to:

Clean the room: They used a special distillation tower to scrub out the "coughing" gases (Argon and Krypton) from the xenon.
Filter the noise: They separated the data into two groups: one where the "noise" was high (High-Ar) and one where it was low (Low-Ar), allowing them to compare and subtract the background.
Blind Analysis: To avoid cheating (even accidentally), the scientists kept the most important part of the data hidden (blinded) until they were sure their math and models were perfect. Only then did they "unblind" the results.

The Results: Hearing the Whisper

After analyzing 1.9 years' worth of data (1.9 tonne-years of exposure), the team found something exciting:

The Signal: They detected a signal that matches the prediction for solar pp neutrinos.
The Confidence: They are 2.2 sigma confident. In the world of particle physics, this is like being 97% sure you heard the violin, even though you aren't 100% certain yet (scientists usually want 5 sigma, or 99.9999%, to claim a "discovery").
The Measurement: They measured the flow of these neutrinos to be roughly 8.5 (with an uncertainty of 3.5) per square centimeter per second. This number matches the "Standard Solar Model," which is the textbook theory of how the Sun works.

The "First" Claim: This is the first time anyone has seen a positive sign of these specific low-energy solar neutrinos bouncing off electrons below 165 keV. Previous experiments (like Borexino) saw them, but at higher energies or with different methods. This proves that liquid xenon detectors are a viable new tool for this job.

Why This Matters

Think of this as finding a new pair of glasses.

Old Glasses (Liquid Scintillators): Good at seeing solar neutrinos, but they have their own limitations and background noises.
New Glasses (Liquid Xenon): This experiment proves that liquid xenon detectors can also see these solar ghosts, and they see them with a completely different set of "background noises."

This gives scientists a second, independent way to check if our understanding of the Sun is correct. If the numbers from the xenon tank match the numbers from the liquid scintillator tanks, it confirms our solar models are solid.

What's Next?

The paper concludes that while this is a success, the "static" (background noise) is still too loud to hear the neutrinos perfectly. To get a clearer picture in the future, they need:

Bigger tanks: More xenon means more chances to catch a neutrino.
Cleaner tanks: Even better ways to remove the Argon and Krypton "coughs."
Better shielding: To block out the material-induced noise.

The team suggests that future, even larger detectors (like PandaX-xT) will use these lessons to become precision instruments for both studying the Sun and hunting for Dark Matter.

In short: The PandaX-4T team successfully upgraded their underground detector, cleaned out the radioactive noise, and caught a faint, positive signal of the Sun's most common neutrinos at low energies, confirming that the Sun is burning exactly as our theories predict.

Technical Summary: Measurement of Solar pp Neutrino Flux with PandaX-4T Run 2 Data

Problem and Motivation
Solar proton-proton (pp) neutrinos constitute the primary source of solar neutrinos, originating from the first step of hydrogen burning. Precise measurements of the pp flux are essential for testing the Standard Solar Model (SSM) and the solar luminosity constraint. Furthermore, because the pp neutrino endpoint is approximately 420 keV, their oscillation probability is governed predominantly by vacuum mixing, making them a critical probe for the transition region of the Mikheyev-Smirnov-Wolfenstein (MSW) effect and potential physics beyond the Standard Model.

Direct measurement of pp neutrinos is experimentally challenging because the resulting electronic-recoil (ER) spectrum is restricted to low energies (below a few hundred keV) and lacks sharp spectral features, making it difficult to distinguish from intrinsic radioactive backgrounds. While liquid scintillator detectors like Borexino have achieved high-significance measurements, liquid xenon (LXe) time projection chambers (TPCs) offer a complementary approach due to their low energy thresholds, large homogeneous targets, three-dimensional position reconstruction, and strong self-shielding capabilities.

Methodology and Experimental Setup
This study reports a measurement using the PandaX-4T LXe TPC located at the China Jinping Underground Laboratory (CJPL-II), utilizing data from Run 2 (July 5, 2024 – January 23, 2026). The analysis corresponds to an exposure of 1.9 tonne·yr, representing a threefold increase over the previously published Run 0 results.

Detector Upgrades: Prior to Run 2, the detector underwent significant upgrades, including the replacement of the cathode, redesigned voltage-dividing bases for photomultiplier tubes (PMTs) to extend linearity up to 40,000 photoelectrons, and new PTFE field-cage panels with enhanced reflectivity. The external ultrapure-water shield was upgraded to an active water Cherenkov veto system.
Purification and Background Control: The liquid xenon was continuously circulated through hot zirconium getters. A dedicated "reverse mode" distillation system was employed to reduce $^{222}$ Rn activity to a minimum of $2.9 \pm 0.1$ $\mu$ Bq/kg. To address elevated $^{39}$ Ar levels, two online argon-distillation campaigns were conducted, splitting the data into "High-Ar" (233.8 live days) and "Low-Ar" (204.2 live days) periods.
Data Processing: A unified reconstruction chain was implemented, merging previously separate low-energy (dark matter) and high-energy (neutrino) pipelines. This chain identifies S1 signals down to a single PMT hit and improves the identification of S2 signal pileup.
Analysis Strategy: A blind spectral analysis was performed on ER data across a 20–1000 keV energy range. The analysis utilized a log-likelihood fit to simultaneously model the High-Ar and Low-Ar datasets. The signal model included solar pp and $^7$ Be neutrino-electron scattering spectra, convolved with a five-parameter detector energy response model. Backgrounds were modeled using components from detector materials (DetM, SSP), noble-gas impurities ( $^{39}$ Ar, $^{85}$ Kr, $^{222}$ Rn chains), and radioactive xenon isotopes.

Key Contributions and Improvements

Increased Exposure: The analysis leverages a 1.9 tonne·yr exposure, significantly larger than previous PandaX-4T neutrino results.
Enhanced Detector Response: Upgrades to PMT bases and the adoption of a unified reconstruction chain improved energy linearity and reduced systematic uncertainties, particularly at low energies.
Background Suppression: The implementation of reverse-mode distillation significantly reduced $^{222}$ Rn activity, while targeted argon distillation campaigns allowed for the separation and modeling of $^{39}$ Ar backgrounds.
Blind Analysis: A multi-stage unblinding procedure was employed to optimize the fit range (20–1000 keV) and fiducial volume (1.60 $\pm$ 0.03 tonnes) without biasing the signal extraction.

Results
The spectral fit yielded a solar pp+ $^7$ Be neutrino normalization factor of $\mu = 1.42 \pm 0.63$ , consistent with the SSM prediction. By isolating the statistical uncertainty (0.22) and calculating the systematic uncertainty (0.59), the authors derived the pp flux.

Run 2 Result: The fitted pp flux is $(8.5 \pm 3.7) \times 10^{10}$ cm $^{-2}$ s $^{-1}$ .
Combined Result: When combined with the published Run 0 data, the uncertainty is reduced to $(8.5 \pm 3.5) \times 10^{10}$ cm $^{-2}$ s $^{-1}$ .
Significance: The combined data disfavor the zero-pp hypothesis with a statistical significance of 2.2 $\sigma$ . This constitutes the first positive indication of solar pp neutrino–electron scattering below an electronic-recoil energy of 165 keV.

The dominant systematic uncertainties arise from $^{85}$ Kr and $^{39}$ Ar, which exhibit spectral degeneracy with the solar pp neutrino signal.

Significance and Claims
The paper claims that this measurement establishes liquid xenon detectors as a complementary platform for low-energy solar neutrino physics. The result is consistent with the Standard Solar Model prediction within 1 $\sigma$ . The authors emphasize that the systematic uncertainties in this measurement are entirely distinct from those of gallium and liquid scintillator experiments, providing an independent cross-check.

The authors note that current sensitivity is limited by statistics and, more critically, by the spectral degeneracy between the smooth pp recoil spectrum and low-energy radioactive backgrounds. They conclude that future multi-tonne liquid xenon TPCs (e.g., PandaX-xT), with controlled intrinsic backgrounds and external veto systems, will be capable of extending this capability to serve as precision probes of low-energy solar neutrinos while simultaneously advancing dark matter searches.