Recent searches for fermion-light dark matter by the PandaX-4T collaboration

PandaX-4T liquid xenon time projection chamber. Credit: PandaX Collaboration.

Teams of astrophysicists around the world are trying to observe the different possible types of dark matter (DM), a hypothetical substance in the universe that does not emit, absorb or reflect light and is therefore extremely difficult to detect. However, so far Fermionic DM has been explored mainly theoretically, which would be made of fermions.

The PandaX Collaboration, a large consortium of researchers in China participating in the PandaX-4T experiment, recently conducted a study aimed at expanding the sensitive cluster window for experiments aimed at direct detection of fermionic DM from the higher GeV to MeV or even keV ranges.

The team recently published two research papers Physical review letters Determines the results of two searches for DM fermion uptake using data collected as part of the Panda X-4T experiment, a large-scale research effort aimed at detecting DM using a biphasic time-projection chamber (TPC) in China.

“With the conversion of the massive DM into a massless neutrino, the mass of the DM is absorbed and converted into the kinetic energy of the neutrino and more importantly the bouncing electron or nuclear targets,” Professor Shao Feng Jie, one of the researchers who carried out the study, told Phys.org.

With the effective mass-to-energy conversion, according to Einstein’s relation E=mc2, even keV (MeV) DM can deposit a sufficiently large recoil energy in the recoil electron (nucleus). “

The idea of ​​observing DM fermion light by detecting the recoil energy generated by the absorption of its mass first came about a few years ago, and has since been explored by various groups of theoretical physicists. While these studies have provided valuable theoretical predictions, these predictions have not yet been experimentally tested.

“Previous phenomenological papers established the essential features of this unique channel for DM r fermion searches,” explained Professor Ji. “The PandaX collaboration worked hard to first look for the predicted signals using real data.”

Theoretical studies predict that in nuclear absorption reactions, the mass of DM is converted into kinetic energy that charges the neutrinos and nuclei out. This energy, known as the “nuclear recoil energy”, should be roughly proportional to the square of the DM’s mass, resulting in a unique mono-energy spectrum. In their first study, the PandaX-4T collaboration attempted to detect the energy released by nuclei uptake of fermionic DM.

“This single-energy spectrum is very different from conventional elastic scattering spectrum and has not been ad hoc searched before in a direct detection experiment of DM,” Dr. Yi Tao, another study co-author, told Phys.org. “As part of the PandaX-4T research, we conducted studies dedicated to reconstructing the energy of the nuclear recoil and then compared simulation data and neutron calibrations.”

The researchers found that there was good consistency between the data collected by the biphasic time projection chamber (TPC) and the theoretical model of the detector response. More specifically, the signal region they scanned corresponds to a nuclear recoil energy of 100 keV, which covers a DM mass parameter of 30 MeV/c.2 to 125 meV/c2.

In a similar fashion to nuclear absorption processes, electronic absorption processes are also expected to be sensitive to DM photosensitivity, but in a different mass range. In fact, electronic absorption processes refer to the conversion of the hypothetical static mass of a DM fermion particle into the kinetic energy of electrons, resulting in a free electron.

Recent searches for fermion-light dark matter by the PandaX-4T collaboration

A figure representing the absorption of xenon by dark matter. Credit: PandaX Collaboration.

Theoretically, DM fermion should induce electronic backscattering signals in experimentally detectable liquid xenon detectors. In their second study, the PandaX-4T collaboration looked for this other potential effect of fermentic DM.

Electrons are much lighter than nuclei and therefore easier to eject during absorption processes. Therefore, electron absorption searches can be sensitive to the sub-MeV mass range.

“Additionally, unlike nuclear recoil signals where much of the energy is quenched in heat and undetectable in a liquid xenon detector, most of the electronic recoil energy is detectable,” according to Dr. Dan Zhang, another researcher who conducted the study, Phys said. org.

To obtain more detailed theoretical models, different 6D virtual operators were studied in a four-fermion process (DM fermion + electron -> electron + neutrino) using an effective field theory approach. It turned out that the electronic absorption signals will be similar regardless of which operators are in Direct detection experiments, but the interpretations on the couplings are very different, and the comparison with other cosmological and astrophysical observations is also different.”

The search for the electron-absorbing fermion sub-DM conducted by Dr Zhang and the rest of the PandaX-4T collaboration did not detect any significant signals over the expected background. Nevertheless, the team was able to place the strongest limits on the axial and vector interactions of DMs with a mass of several tens of keV/c.2which exceeds current astronomy and cosmology limitations for such light fermions.

About two years ago, XENON1T reported a low energy excess, which can be explained as an electron absorption of 60 keV/c.2 “This possibility is now being challenged by our data,” said Dr. Zhang, according to phenomenological studies.

Recent searches by the PandaX-4T collaboration highlight the potential of nuclear adsorption and electron adsorption processes as channels to search for DM light mass. In the future, it could inspire other astrophysics partnerships around the world to conduct similar searches.

“As soon as any excess energy is observed, the excess energy indicates DM mass,” Prof. Ning Zhu, another co-author of the study, told Phys.org. For this channel, we obtained model-independent constraints on the cross-section of the sub-GeV DM-nucleon scattering and probe down to 10^-50 cm.2 area for 35 meV/c2 DM block, first time. In addition, we study a complete UV model with a Z medium, which combines the cosmological limitations, the collider limitation, and our direct detection limitation. “

To date, the Panda X-4T collaboration has succeeded in setting new frontiers for experiments aimed at the direct detection of fermionic DM. Since their experiment is ongoing and therefore still collecting data, the team will soon be conducting additional searches for the light and elusive DM.

“The data we reported is equivalent to exposing a 600-kg liquid xenon target for one year to light this hypothetical DM,” Professor Jianglai Liu, spokesperson for the PandaX Collaboration, told Phys.org. “When PandaX-4T ends in 2025, we expect a 10-fold greater cumulative exposure. We also expect to have a more accurate understanding of our detector’s nuclear recoil and electronic backscatter signals via comprehensive calibrations and are excited to see how the story unfolds in the future.”

more information:
Linhui Gu et al, first research on fermion dark matter absorption using the PandaX-4T experiment, Physical review letters (2022). DOI: 10.1103/PhysRevLett.129.161803

Dan Zhang et al, Research on the absorption of fermion-light dark matter on electrons in PandaX-4T, Physical review letters (2022). DOI: 10.1103/PhysRevLett.129.161804

Jeff A. Dror et al., Direct Detection of Signals from Fermion Dark Matter Absorption, Physical review letters (2020). DOI: 10.1103/PhysRevLett.124.181301

Jeff A. Dror et al., Sub-MeV fermion dark matter absorption by electronic targets, physical review d (2021). DOI: 10.1103/PhysRevD.103.035001

Jeff A. Dror et al., Erratum: Sub-MeV fermion dark matter absorption by electron targets [Phys. Rev. D 103 , 035001 (2021)]And the physical review d (2022). DOI: 10.1103/PhysRevD.105.119903

Jeff A. Dror et al., Fermion Dark Matter Absorption by Nuclear Targets, Journal of High Energy Physics (2020). DOI: 10.1007 / JHEP02 (2020) 134

Shao-Feng Ge et al, Revisiting fermion dark matter absorption on an electron target, Journal of High Energy Physics (2022). DOI: 10.1007 / JHEP05 (2022) 191

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