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

The latest research documents the outcomes from the PandaX-4T experiment, establishing rigorous constraints on the interplay between dark matter and nucleons via low-energy evidence and the Migdal effect, discounting notable ranges for a thermal relic dark matter construct.

Dark matter constitutes one of the profound enigmas within the realm of science, remaining elusive to direct observation and challenging conventional paradigms. Its secretive nature is so profound that the actual identity and mass of dark matter particles remain unknown.

Since dark matter particles are not responsive to electromagnetic radiation, they are undetectable. Axions and weakly interacting massive particles (WIMPs) are considered prime suspects for dark matter particles.

Located in the China Jinping Underground Laboratory, the PandaX-4T experiment shines as a beacon in the pursuit of understanding the arcane nature of dark matter. The scientific endeavor utilizes ‘xenon detectors’ to scrutinize dark matter, evaluate neutrinos, and research emergent physics phenomena, including neutrinoless double beta decay.

The endeavor has achieved strides in detecting interactions between dark matter and nucleons employing the PandaX-4T. These advances are documented in Physical Review Letters.

Delving into the PandaX-4T experiment and the Migdal effect

Within the PandaX-4T experiment rests a cutting-edge dual-phase xenon time projection chamber (TPC) containing a formidable 3.7 tons of liquid xenon within a supersensitive volume. This chamber is the epicenter for the detection of particle interactions.

Co-investigator Dr. Ran Huo of the Shandong Institute of Advanced Technology stated, “For light dark matter candidates, the maximal kinetic energy transferable to xenon nuclei is directly proportional to the square of the dark matter’s mass.”

“As the mass of dark matter falls below several GeV, the likelihood of the recoil energy from dark matter collisions with xenon nuclei surpassing the sensitive threshold of the detector becomes exceedingly slim.”

To surpass this obstacle, the PandaX-4T experiment capitalizes on the Migdal effect, amplifying the experiment’s acuteness towards lighter dark matter particles under 3 GeV, in pursuit of dark matter-nucleon interaction possibilities.

The Migdal effect pertains to the potential ionization or excitation of atomic electrons due to the presence of dark matter traveling through a material, xenon in this context. The atomic nuclei’s nucleons (protons and neutrons) may encounter impacts from dark matter particles.

These impacts have the potential to result in atomic electron excitation or ionization. Consequently, these electrons might attain energies surpassing keV levels. When these charged electrons traverse through liquid xenon, they generate discernible signals, indicating electron recoils inside the detector.

“To put it succinctly, the Migdal effect enables us to broaden our hunt for dark matter entities below 3 GeV in weight, scrutinizing the connections they have with nucleons,” commented Dr. Yong Yang, a contributing author from Shanghai Jiao Tong University.

Modeling thermal dark matter

In the model of thermal dark matter, these cosmic particles are postulated to have achieved thermal equilibrium with the primordial particle broth in the early universe. The ensuing universe’s expansion and temperature drop led to these particles separating from the thermal bath while maintaining a specific abundance.

This separation is comparable to a freeze-out, where the dark matter constituents establish their observed abundance by stabilizing.

The thermal dark matter model is enticing because it explains the current known quantity of dark matter in the cosmos through a spontaneous mechanism. The annihilation or disintegration of these cosmic particles in the primitive universe would have resulted in the contemporary density of dark matter.

Typically, this paradigm contemplates various specific particle types, like weakly interacting massive particles (WIMPs) or similarly behaving alternatives.

“Our probe was fundamentally intended for WIMP-style dark matter, wherein the ‘force-mediator’ (the force-conveying particle between dark matter and ordinary matter) is hypothesized to be extraordinarily massive, thus rendering the interaction incredibly localized,” observed Dr. Yang.

Through the flexibility of the PandaX-4T prototype, a replication of the observed dark matter amount is feasible, caused by the obliteration of dark matter constituents into ordinary model particles during the cosmos’ nascent era, revealing an expansive parameter domain.

The meticulous selection process employed by PandaX-4T, utilizing optimized low-energy data, set stiff limitations on the vigor of interactions between dark matter and nucleons for masses spanning from 0.03 to 2 GeV.

“Our recent analysis directly scrutinizes a form of thermal dark matter hypothesis—dark matter duos annihilating into ordinary material via a dark photon in the early universe—and discards a significant area previously deemed viable,” elucidated Dr. Huo.

In essence, the exploration narrows down the feasible hypothesis for dark matter impacts via the dark photon, representing the mediator.

Expanding upon discoveries

The triumphs of the experiment in scrutinizing dark matter particles between the ranges of 0.03 to 2 GeV yield precious data, refining our grasp of the thermal dark matter archetype.

The research team identified two prospective trajectories for upcoming endeavors utilizing the PandaX-4T.

“Our objective is to augment exposure by amassing more data or broadening the xenon target to delve deeper into lower dark matter–nucleon interaction cross-sections.”

“This increased exposure is anticipated to shed light on the subtleties of the low-energy background, predominantly affected by cathode electrodes and micro-discharging sound,” revealed Dr. Huo.

“Alternatively, we acknowledge that this study possesses no sensitivity to these types of interplay for dark matter lighter than 30 MeV, beneath which threshold the Migdal effect can no longer aid us. This indicates a necessity for innovative detection methods,” conceded Dr. Yang.

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