Teams of astrophysicists around the world are trying to observe different possible types of dark matter (DM), hypothetical matter in the universe that does not emit, absorb or reflect light and would therefore be very difficult to detect. However, the fermionic DM, which would consist of fermions, has so far been mainly explored theoretically.
The PandaX Collaboration, a large consortium of researchers in China involved in the PandaX-4T experiment, recently conducted a study to extend the sensitive mass window for experiments aimed at directly detecting fermionic DM in the ranges above GeV to MeV or even keV.
The team recently published two articles in Physical examination letters describing the results of two investigations into the uptake of fermionic DM using data collected in the Panda X-4T experiment, a large-scale research effort to detect DM using a dual-phase time projection chamber (TPC) in China.
“With a massive DM converted to a massless neutrino, the DM mass is absorbed and converted into the kinetic energy of the neutrino and, more importantly, the recoiled electron or nuclear targets,” said Professor Shao-Feng Ge. , one of the researchers who carried out the study, told Phys.org.
“With an efficient conversion of mass into energy, according to Einstein’s relation E = mc2even keV (MeV) DM can deposit large enough recoil energy in the recoil electron (nuclei).”
The idea of observing the light fermionic DM by detecting the recoil energy resulting from the absorption of its mass appeared a few years ago and has since been explored by different groups of theoretical physicists. While these studies offered valuable theoretical predictions, these predictions had until now never been tested experimentally.
“Previous phenomenological papers have established the basic characteristics of this unique channel for DM r fermionic searches,” Prof. Ge explained. “The PandaX collaboration has worked hard to first find the predicted signals using real data.”
Theoretical studies predict that in nuclear absorption reactions, DM mass is converted into kinetic energy which charges the outgoing neutrino and nucleus. This energy, called “nuclear recoil energy”, should be approximately proportional to the square of the mass of DM, resulting in a unique mono-energetic spectrum. In their first study, the PandaX-4T collaboration attempted to detect the energy resulting from the uptake of fermionic DM by nuclei.
“This mono-energetic spectrum is drastically different from the traditional elastic scattering spectrum and has not been studied extensively before in the DM direct detection experiment,” said Dr. Yi Tao, another researcher involved in the study, at Phys.org. “As part of this PandaX-4T research, we performed dedicated studies on nuclear recoil energy reconstruction and then compared simulation and neutron calibration data.”
The researchers found that there was good consistency between the data collected by their dual-phase time projection (TPC) chamber and their theoretical model of detector response. Specifically, the signal region they scanned corresponded to the nuclear recoil energy up to 100 keV, which spans the DM mass parameter from 30 MeV/c2 at 125 MeV/c2.
Similar to nuclear absorption processes, electron absorption processes are also expected to be sensitive to DM light, but in a different mass range. In fact, electron absorption processes involve the conversion of the static mass of a hypothetical fermionic DM particle into electron kinetic energy, creating a free electron.
Theoretically, therefore, fermionic DM should induce recoil electronic signals in liquid xenon detectors that could be detected experimentally. In their second study, the PandaX-4T collaboration looked for this other potential trace of fermionic DM.
Electrons are much lighter than nuclei and therefore easier to eject during absorption processes. Therefore, electron absorption searches may be sensitive to the sub-MeV mass range.
“Also, unlike nuclear recoil signals where much of the energy is transformed into heat and cannot be detected in a liquid xenon detector, most of the electronic recoil energy is detectable,” said said Dr. Dan Zhang, another researcher who conducted the study. , told Phys.org.
“For more detailed theoretical models, different hypothetical six-dimensional operators in the four-fermion process (DM fermionic + electron -> electron + neutrino) have been studied with an effective field theory approach. It turns out that the Electronic absorption signals will be similar regardless of operators in direct detection experiments, but the interpretations on the couplings are quite different, and the comparison with other cosmological and astrophysical observations are also different.”
The electron-absorbed sub-MeV fermionic DM search performed by Dr. Zhang and the rest of the PandaX-4T collaboration did not lead to the detection of any significant signal compared to the expected background noise. Nevertheless, the team was able to set the strongest limits on the axial vector and vector interactions for DMs with a mass of several tens of keV/c2which exceed the existing astronomy and cosmology constraints for such light fermion DMs.
“About two years ago, XENON1T reported a low-energy excess, which could be interpreted as an electron absorption of 60 keV/c2 fermionic DM according to phenomenological studies,” said Dr. Zhang. “This possibility is now challenged by our data.
Recent research by the PandaX-4T collaboration highlights the potential of nuclear absorption and electron absorption processes as light-mass DM search channels. In the future, they could inspire other astrophysical collaborations around the world to do similar research.
“Once an excess is observed, the energy of the excess would indicate the mass of DM,” Professor Ning Zhou, another researcher involved in the study, told Phys.org. “For this channel, we obtained model-independent constraints on the sub-GeV DM-nucleon scattering cross section and probed down to 10^-50 cm2 region for 35 MeV/c2 Mass DM, for the first time. In addition, we study a UV-complete model with mediator Z’, which brings together the cosmology constraint, the collider constraint and our direct detection limit.”
So far, the Panda X-4T collaboration has successfully established new boundaries for experiments aimed at directly detecting fermionic DM. As their experiment is ongoing and therefore continues to collect data, the team will soon conduct additional research into the light and elusive DM.
“The data we reported is equivalent to exposing a 600 kg liquid xenon target for one year to the illumination of this hypothetical MD,” said Professor Jianglai Liu, spokesperson for the PandaX collaboration, at Phys.org. “When PandaX-4T completes in 2025, we expect 10 times greater cumulative exposure. We also hope to gain a more accurate understanding of our detector to nuclear recoil and electronic recoil signals through extensive calibrations and are excited to see how The story takes place in the future.”
Linhui Gu et al, First Search for Fermionic Dark Matter Absorption with the PandaX-4T Experiment, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.129.161803
Dan Zhang et al, Search for light fermionic dark matter absorption on electrons in PandaX-4T, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.129.161804
Jeff A. Dror et al, Direct Detection of Fermionic Dark Matter Absorption Signals, Physical examination letters (2020). DOI: 10.1103/PhysRevLett.124.181301
Jeff A. Dror et al, Sub-MeV Fermionic Dark Matter Absorption by Electron Targets, Physical examination D (2021). DOI: 10.1103/PhysRevD.103.035001
Jeff A. Dror et al, Erratum: Sub-MeV Fermionic Dark Matter Absorption by Electron Targets [Phys. Rev. D 103 , 035001 (2021)], Physical examination D (2022). DOI: 10.1103/PhysRevD.105.119903
Jeff A. Dror et al, Absorption of Fermionic Dark Matter by Nuclear Targets, Journal of High Energy Physics (2020). DOI: 10.1007/JHEP02(2020)134
Shao-Feng Ge et al, Revisiting the fermionic dark matter absorption on electron target, Journal of High Energy Physics (2022). DOI: 10.1007/JHEP05(2022)191
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