SuperCDMS collaboration tightens limits on dark matter detection

For almost a century, dark matter has remained elusive, prompting scientists to devise more innovative methods for detection. The Super Cryogenic Dark Matter Search (SuperCDMS) collaboration recently reanalyzed previous experimental data, focusing on Bremsstrahlung radiation and the Migdal effect to seek dark matter. In a unique approach, they collaborated with geologists to account for the Earth’s atmosphere and inner composition’s interaction with dark matter particles, improving energy dissipation understanding. This analysis has resulted in one of the most stringent constraints on dark matter detection, paving the way for future searches. Noah Kurinsky, a staff scientist at SLAC and the study’s corresponding author, emphasizes the importance of advancing detection sensitivities and modeling processes for the dark matter community.

Invisible scattering

In experiments like SuperCDMS, physicists search for indications that dark matter has interacted with atomic nuclei, like those found in silicon and germanium materials.

Traditionally, it was assumed that when dark matter collides with a nucleus, the collision is elastic. This means the energy lost by the dark matter particle is transferred to the nucleus, causing both particles to recoil, similar to billiard balls scattering.

However, in recent years, researchers have proposed the possibility of inelastic collisions. In such cases, the energy from the collision is transferred to other detectable particles, such as photons or electrons. This potential scenario could enhance the sensitivity of dark matter detection experiments, offering new avenues for uncovering the mysterious nature of dark matter.

Example of an energy spectrum from the maximum likelihood fit for a Migdal signal model for a WIMP with a mass of 0.5 GeV/c2 and a cross section of 3×10−37 cm2 (black dashed curve). The data (blue histogram) have been logarithmically binned and overlaid with the background models (colored solid curves). The thick black line is the sum of all the models, signal and background. Normalization of the surface background model components (TL, SG and GC) are described in Sec. 5b. The plot on the bottom shows the residual between data and the model with the 1σ statistical uncertainty indicated by the shaded region. Credit: Physical Review D (2023). DOI: 10.1103/PhysRevD.107.112013

The SuperCDMS experiment, already highly sensitive to dark matter, aimed to understand the likelihood of detecting specific signals in its data. Daniel Jardin, a co-author of the study and a postdoctoral scholar at Northwestern University, played a key role in leading the analysis.

The team focused on two potential scenarios for inelastic collisions: Bremsstrahlung radiation and the Migdal effect. Bremsstrahlung is a known phenomenon where a charged particle, like a nucleus, emits radiation upon deceleration. In the context of a dark matter detector, this could happen if a dark matter particle collides with a nucleus, transferring some energy to a photon instead of just recoiling. Detecting such a photon would suggest a high-speed unknown particle, possibly dark matter, collided with the nucleus.

The Migdal effect, although not yet experimentally proven, proposes that a dark matter particle striking a nucleus displaces it from the center of its electron cloud. Shortly afterward, the electron cloud readjusts and emits detectable electrons. Scientists have calculated how this signal would appear in dark matter detectors if it occurs.

While the reanalysis didn’t find direct evidence of dark matter, each analysis extended the experiment’s limits to detect lower-mass particles. Previously, SuperCDMS ruled out dark matter with masses as low as that of a proton. By accounting for Bremsstrahlung, the experiment can now exclude even lower masses, down to about a fifth of the proton’s mass, and potentially lower masses if considering the hypothetical Migdal effect. These advancements set the stage for further advancements in dark matter searches.

When Earth gets in the way

The researchers didn’t stop at applying existing ideas to their data; they pushed the boundaries further. Along with extending the lowest limits of dark matter detection, they also considered the upper limit. They realized that if dark matter interacts strongly enough, it might interact with Earth’s atmosphere and surface on its way to the underground detector. This interaction could lead to an upper limit where Earth itself blocks the dark matter.

To determine this upper limit, the team modeled how Earth’s atmosphere and inner layers affect dark matter particles as they travel towards the SuperCDMS detector in the Soudan Mine. Working with geologists, they accurately assessed the soil and rock composition surrounding the detector.

By calculating the energy loss of dark matter particles due to interactions along their path, the researchers established upper limits on their interaction strength, depending on where the particles originated, be it directly above the detector or on the opposite side of the Earth.

After analyzing the SuperCDMS data with the new models and upper limits, the team expanded the range of particle masses the experiment could detect but didn’t find evidence of dark matter interactions. Despite this, their analysis represents one of the most sensitive searches for ultralight dark matter, providing valuable insights from the existing data.

As Daniel Jardin explained, they put significant effort into the experiment and aim to make the most of it. The mass and interaction properties of dark matter remain mysteries, so researchers continue their determined quest to explore the unknown realms of dark matter.

Source: SLAC National Accelerator Laboratory

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