Unraveling Cosmic Secrets: Investigating Dark Matter-Nucleon Interactions with Underground Detectors

The date is November 25, 2023 and we have some interesting scientific developments to report on.

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A new study has revealed the results of the PandaX-4T experiment, where scientists extensively scrutinized dark matter-nucleon interactions using low-energy data and the Migdal effect, leading to the possibility of dismissing vast evidence for a thermal relic dark-matter model.

As many know, dark matter is a burning question in the scientific community, dodging direct detection and mystifying existing models. So elusive is its nature that we remain in the dark about what kind of particles it's made up of or what their masses might be.

Dark matter particles' inability to interact with light makes it impossible to identify them. Leading contenders among hypothetical dark matter particles include axions and weakly interacting massive particles (WIMPs).

Deep within the China Jinping Underground Laboratory, the PandaX-4T experiment employs 'xenon detectors,' breaking new ground in the quest to solve the riddle of dark matter. They also help investigate new physics phenomena like neutrinoless double beta decay while studying neutrinos.

A recent advancement in the study of dark matter–nucleon interactions involves the PandaX-4T and a peer-reviewed publication in Physical Review Letters features this momentous research work.

The PandaX-4T experiment's nerve center is an advanced dual-phase xenon time projection chamber (TPC) housing an impressive 3.7 tons of liquid xenon. This complex chamber provides the main platform for particle interactions.

One of the researchers, Dr. Ran Huo from the Shandong Institute of Advanced Technology clarifies, 'The maximum energy that dark matter can transfer to xenon nuclei is proportional to the square of the dark matter mass if we're dealing with light dark matter.'

He elaborates, 'When dark matter mass falls under several GeV, the recoil energy resulting from Xenon nuclei colliding with the dark matter hardly ever exceeds the detector's energy threshold.'

The study overcomes this hurdle by utilizing the Migdal effect to augment the experiment's sensitivity, notably for dark matter particles below 3 GeV, probing dark matter-nucleon interactions in the process.

Ionization or excitation of the atoms' electrons happens because of the Migdal effect when the dark matter travels through a substance, xenon in this case. Dark matter particles interact with the atomic nuclei's nucleons (protons and neutrons).

These interactions could result in the surrounding atoms' electrons getting excited or ionized. Consequently, these electrified electrons can acquire energy exceeding keV. As these electrons navigate the liquid xenon, they create visible signals within the detector due to electron recoils.

Dr. Yong Yang, a co-author of the study from Shanghai Jiao Tong University, elucidates, 'The Migdal effect essentially aids in extending our scope for dark matter masses below 3 GeV to probe the interactions between dark matter and nucleons.'

The thermal dark matter model hypothesizes dark matter particles to have achieved thermal equilibrium with the primordial soup in the universe's early stage. Upon the universe's cooling and expansion, these particles detached from the thermal bath preserving an abundance in the process. This occurrence is similar to a freeze-out where the dark matter particles solidify into their observed abundance.

As this model effortlessly explains the noticed relic abundance of dark matter in the universe, it has garnered considerable approval in the scientific community. In the early universe, the 'annihilation' or decay of these particles could lead to the accurate dark matter density that we discern today.

The model's scrutiny often concerns the study of specific particles such as weakly interacting massive particles (WIMPs) or other comparable candidates.

Dr. Yang adds, 'Our experiment was majorly intended for WIMP-like dark matter, where interactions between dark matter and regular matter are extremely short-ranged and a heavy 'force-mediator' (the particle that transmits the force) is presumed.'

The versatility of the PandaX-4T model assists in replicating the perceived dark matter amount via the annihilation of dark matter particles to standard model particles in the universe's early stage, presenting a diversified parameter space.

PandaX-4T's targeted approach utilized optimized low-energy data to set strict constraints on dark matter-nucleon interaction strength for dark masses ranging from 0.03 to 2 GeV.

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

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

The experiment's success in scrutinizing dark matter particles within the 0.03 to 2 GeV range offers valuable insights, refining our comprehension of a thermal dark matter model.

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

'We aim to enhance exposure, through increased data or a larger xenon target, to delve into lower dark matter–nucleon interaction cross-sections.'

'This expanded exposure holds the potential to elucidate the intricacies of the background in the low-energy domain, predominantly influenced by cathode electrodes and micro-discharging noise,' said Dr. Huo.

'On the other side, our study has no sensitivity for this interaction for dark matter lighter than 30 MeV, below which the Migdal effect cannot help us anymore. This means we need new detection methods,' acknowledged Dr. Yang.

Journal information: Physical Review Letters

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