How 'Dark' Dark Matter Can Be: SJTU Published Latest Research Results in Nature
Recently, the PandaX collaboration led by Shanghai Jiao Tong University published a research paper in the journal Nature, analyzing the latest research findings of the PandaX-4T liquid xenon detection experiment, aiming to understand the "darkness" of dark matter in terms of its electromagnetic properties. The paper, titled "Limits on the luminance of dark matter from xenon recoil data," presents the international community's best measurement results in this area. It was published online on May 17, 2023, in the journal Nature.
The basic particles that make up known matter in the universe all possess electromagnetic properties and interact through the exchange of photons. Therefore, these particles all exhibit "luminance," even if they are electrically neutral. For example, neutrons are uncharged, but since they are composed of charged quarks, they still have a strong residual "luminance." Even electrically neutral fundamental particles such as neutrinos can generate extremely small electromagnetic distributions, such as radial charge distribution, linear charge distribution, circular current distribution, vortex circular current distribution, and so on, due to higher-order quantum corrections. These correspond to quantities like mean-square charge radius, electric dipole moment, magnetic dipole moment, anapole moment, respectively, resulting in nonzero "luminance." So, does the universe contain matter particles without "luminance"?
A substantial amount of astronomical and cosmological observations have confirmed that there is a mysterious substance in the universe, known as dark matter, which is more than five times more abundant than known matter. This matter is electrically neutral and has not been "seen" by humans using electromagnetic methods, hence it is referred to as "dark" matter. However, whether dark matter particles possess "luminance" has been a fundamental question that scientists all over the world have been trying to answer.
Since 2009, the PandaX collaboration has been utilizing the extremely low radiation background environment of the Jinping Underground Laboratory to develop three generations of liquid xenon detectors (PandaX-I, PandaX-II, PandaX-4T) and conducting a series of experimental studies on dark matter and neutrinos. Dark matter in the universe can penetrate the Earth and reach the underground laboratory. If there is an interaction between dark matter and ordinary matter, it can result in recoiling signals through collisions with xenon atoms in the detector, which are observed as xenon scintillation (S1) and ionization (S2) signals. The working principle of the detector is shown in the diagram below.
Diagram showing photon-mediated interaction in a xenon detector.
The new generation PandaX-4T experiment is located in the B2 experimental hall of the second phase of the Jinping Underground Laboratory. It is the world's first operational multi-ton-scale liquid xenon detection experiment and started running over a year earlier than similar experiments such as the LZ experiment in the United States and the XENONnT experiment in Europe. Building upon the achievements of the previous two generations, PandaX-4T has undergone upgrades and improvements, resulting in significantly enhanced detector performance. In 2021, the experiment released its first batch of dark matter detection data with an exposure of 0.63 ton-years and provided the most sensitive search results for dark matter and xenon nucleus interactions through extremely short-range interactions.
The assembly of the PandaX-4T Time Projection Chamber (TPC) detector
Unlike the conventional assumption of extremely short-range interactions, if dark matter possesses nonzero "luminance," it can exhibit long-range interactions with xenon nuclei through the exchange of photons, resulting in distinct recoil characteristics. Building upon the effective field theory approach developed in collaboration with Professor Wick Haxton, a member of the National Academy of Sciences, the PandaX team translates different electromagnetic effects into combinations of effective operator matrix elements. This allows them to derive the corresponding dark matter collision rates and recoil features. The main background sources in the detector arise from rare radioactive processes and electron collisions, which have different distributions in S1 and S2 compared to the collision signals of dark matter with atomic nuclei. By constructing a reliable model for the dark matter signal and background response based on calibration data, the team enhances their ability to discern different electromagnetic properties of dark matter.
The distribution of scintillation signals (S1) and ionization signals (S2, ne) in the first batch of dark matter detection data from PandaX-4T.
Based on the initial dark matter detection data from the PandaX-4T experiment, the PandaX team conducted a systematic search for the possible electromagnetic properties of dark matter using a likelihood fitting method. The study revealed no dark matter signals exceeding the background: for instance, in the case of the dark matter mean-square charge radius, the expected number of background events in the signal region was approximately 10, while only 6 collision events were observed in the experimental data, consistent with statistical fluctuations of the background. Based on the observed data, the team provided the strongest constraints on various electromagnetic properties of dark matter internationally.
It is worth noting that PandaX provided the world's first experimental upper limit on the dark matter mean-square charge radius, with the strongest limit of 1.9×10^(-10) fm^2 occurring near a dark matter mass 40 times that of a proton. This upper limit is 10,000 times smaller than the upper limit on the neutrino charge mean-square radius and corresponds to a size approximately 100,000 times smaller than that of a proton! PandaX also improved the measurements of other electromagnetic properties of dark matter by a factor of 3 to 10 compared to the previous best international results. The PandaX research utilized the most sensitive data on xenon nucleus recoils and provided the current best upper limits on the "luminance" of dark matter, significantly advancing our quantitative understanding of how "dark" dark matter truly is.
Constraints on the luminance of dark matter
University is the first author of the paper. Professor Ning Zhou from the same school is the corresponding author of the paper. Professor Jianglai Liu, Vice Director of the Tsung-Dao Lee Institute and a Distinguished Professor in the School of Physics and Astronomy, serves as the Chief Scientist of the PandaX collaboration.
This work is the result of the contributions and efforts of all participating institutes of the PandaX Collaboration, under the leadership of the hosting institute, Shanghai Jiao Tong University. The collaboration has constructed and operated the PandaX-4T apparatus, and performed the data processing, calibration and data selections. J.Liu is the Collaboration Spokesperson. N.Z. initiated the effective field theory studies and the application to the dark matter electromagnetic properties with the PandaX data. X.N. and N.Z. performed the calculation of theoretical models, data analysis and hypothesis tests. The paper draft was prepared by X.N. and N.Z., extensively edited by J.Liu and reviewed by X.J., L.Geng and Y.Yang. All authors approved the final version of the manuscript.
Paper link：https://www.nature.com/articles/s41586-023-05982-0