From underground detectors to cosmic secrets: exploring dark matter-nucleon interactions

In a new study, scientists report results from the PandaX-4T experiment, setting strict limits on dark matter-nucleon interactions using low-energy data and the Migdal effect, thus excluding a significant parameter space for a thermal relic dark matter model.

Dark matter is one of the great mysteries of science, evading direct detection and defying traditional models. It’s so shrouded in mystery that we don’t even know what dark matter particles are or what their mass is.

This is because dark matter particles do not interact with light, making them impossible to detect. The main candidates for dark matter particles are axions and weakly interacting massive particles (WIMPs).

Deep in China Jinping’s underground laboratory, the PandaX-4T experiment stands as a beacon in the quest to unlock the mysteries of dark matter. The experimental program uses “xenon detectors” to explore dark matter, study neutrinos and investigate new physics, such as neutrinoless double beta decay.

Today, scientists reported progress in the search for dark matter-nucleon interactions using the PandaX-4T. The results are published in Physical Examination Letters.

The PandaX-4T experiment and the Migdal effect

At the heart of the PandaX-4T experiment is a state-of-the-art two-phase xenon temporal projection chamber (TPC) housing 3.7 tonnes of liquid xenon in a sensitive volume. This sophisticated chamber serves as the main arena for interactions between particles.

Co-author Dr Ran Huo from Shandong Advanced Technology Institute explained: “For light dark matter, the maximum energy that dark matter can transfer to xenon nuclei is proportional to the mass of matter black squared. »

“When the mass of dark matter is less than several GeV, the recoil energy due to the collision of dark matter with xenon nuclei has almost no chance of exceeding the energy threshold of the detector.”

The PandaX-4T experiment exploits the Migdal effect to overcome this challenge by improving the sensitivity of the experiment, particularly to low-mass dark matter particles below 3 GeV, with the aim of probing dark matter-nucleon interactions .

The Migdal effect involves the ionization or potential excitation of electrons in atoms, making up the material (in this case, xenon) through which dark matter passes. Nucleons (protons and neutrons) present in atomic nuclei interact with dark matter particles.

These interactions can lead to the excitation or ionization of electrons from surrounding atoms. As a result, these electrons can acquire energies higher than keV. When these excited electrons pass through the liquid xenon, they generate detectable signals indicating the recoil of the electrons in the detector.

“Simply put, the Migdal effect helps us extend our reach to dark matter masses below 3 GeV to probe dark matter-nucleon interactions,” said Dr. Yong Yang, co-author of the study. Shanghai Jiao Tong University.

A thermal model of dark matter

In a thermal model of dark matter, it is assumed that dark matter particles were in thermal equilibrium with the primordial particle soup of the early universe. As the universe expanded and cooled, these particles decoupled from the thermal bath while preserving a certain abundance.

This process is similar to a freeze, where dark matter particles freeze until they reach their observed abundance.

The thermal model of dark matter is particularly attractive because it provides a natural mechanism to explain the relic abundance of dark matter observed in the universe. The “annihilation” or decay of these particles in the early universe would have produced the correct density of dark matter that we observe today.

This model often involves consideration of specific types of particles, such as weakly interacting massive particles (WIMPs) or other candidates with similar properties.

“Our experiment was primarily designed for WIMP dark matter, in which case the “force mediator” (particle responsible for transmitting force between dark matter and ordinary matter) is assumed to be very heavy, so the interaction is extremely short. at a distance,” Dr. Yang noted.

The flexibility of the PandaX-4T model helps reproduce the observed amount of dark matter through the annihilation of dark matter particles into standard model particles in the early universe, exhibiting a diverse parameter space.

PandaX-4T’s targeted approach used optimized low-energy data to define tight constraints on the dark matter-nucleon interaction strength for dark masses ranging from 0.03 to 2 GeV.

“The new analysis directly tests a kind of thermal model of dark matter – pairs of dark matter annihilating into ordinary matter via the dark photon in the early universe – and eliminates a substantial parameter space that was previously considered plausible” , explained Dr. Huo.

Essentially, the study refines our understanding by narrowing down potential scenarios for interactions with dark matter via the dark photon, which is the mediator.

Building on findings

The experiment’s success in examining dark matter particles in the 0.03 to 2 GeV range provides valuable information, refining our understanding of a thermal model of dark matter.

The researchers highlight two possible avenues for future studies with the PandaX-4T.

“We aim to improve the exposure, through increased data or a larger xenon target, to delve deeper into weaker dark matter-nucleon interaction cross sections.”

“This expanded exposure has the potential to elucidate the subtleties of the background in the low-energy domain, mainly influenced by cathode electrodes and micro-discharge noise,” said Dr. Huo.

“On the other hand, our study has no sensitivity to this interaction for dark matter below 30 MeV, below which the Migdal effect can no longer help us. This means that we need new methods of detection,” acknowledged Dr. Yang.

© 2023 Science X Network