First dark matter search using WINERED spectrograph sets new lifetime constraints

Dark matter is an elusive type of matter that does not emit, absorb or reflect light and is thus impossible to detect using conventional techniques employed in particle physics. In recent years, groups of physicists worldwide have been trying to observe this matter indirectly using advanced detectors and equipment, by detecting signals other than electromagnetic radiation that could be linked to its activity or interactions with other matter.

Researchers at Tokyo Metropolitan University, PhotoCross Co. Ltd, Kyoto Sangyo University and other collaborating institutions recently released the findings of the first search for dark matter that relied on data collected by WINERED, a near-infrared and high-dispersion spectrograph mounted on a large telescope in Chile.

Their paper, published in Physical Review Letters, sets the most stringent constraints to date on the lifetime of dark matter particles with masses between 1.8 and 2.7 eV.

“Our recent paper builds upon our earlier work published in Physical Review D, where we first highlighted the pivotal role of state-of-the-art infrared spectrographs—such as NIRSpec on the James Webb Space Telescope and WINERED on the Magellan Telescope—in the search for dark matter,” Wen Yin, first author of the paper, told Phys.org.

“The idea took shape during a visit with my old friend Taiki who left academia. During that visit, Taiki Bessho introduced me to the WINERED instrument and connected me with Yuji Ikeda, an engineer and astronomer whose expertise proved essential.”

Yin had been working on dark matter theories and trying to identify effective ways to detect it for a while. The visit from his colleague Bessho and his subsequent connection with Ikeda thus sparked important discussions that led to their collaboration on various research projects, eventually leading to this study.

The recent paper by Yin and his colleagues was the result of several years of research that involved both particle physics theorists and experts in advanced instruments. Its primary objective was to search for dark matter using the WINERED high-dispersion spectrograph mounted on one of the Magellan telescopes in Chile.

The Magellan telescopes are two optical telescopes with mirrors that are 6.5 meters in size, which is situated at the Las Campanas Observatory in the Chilean Atacama Desert. The WINERED spectrograph, mounted on one of these telescopes, allowed the researchers to search for the extremely narrow spectral lines predicted to arise when dark matter particles decay into photons.

“Our strategy focused on dwarf spheroidal galaxies, which are thought to be dark matter–rich and are expected to exhibit very narrow spectral features,” explained Yin. “For an analogy, consider how a prism disperses white light into its component colors: as the continuous background light spreads out over many wavelengths, its intensity at any single wavelength diminishes, whereas a narrow emission line remains concentrated.

“Thus, by achieving very high spectral resolution, we can effectively suppress the dispersed background and isolate any narrow dark matter signal.”

To further enhance the accuracy of their search, the researchers also employed a technique known as nodding, which allowed them to subtract the bright sky background from the data collected by the telescope with the WINERED spectrograph mounted on it. In addition, they corrected for Doppler shifts in the data by combining data from multiple targets, as this would allow them to isolate any possible dark matter signal detected from other signals originating from the Earth.

“Our study established stricter lower limits on the lifetime of dark matter particles than those set by previous experiments in the relevant mass range,” said Yin. “Remarkably, with just about four hours of observation, we demonstrated that high-resolution infrared spectroscopy can reach the sensitivity necessary to probe the eV mass range, thereby validating our earlier proposal published in Physical Review D.

“This not only challenges and refines existing theoretical models of dark matter decay but also opens up a new observational avenue that can be applied to other telescopes—such as the Subaru Telescope—and additional targets in the future.”

While Yin and his colleagues did not observe any definitive signal that could be associated with the decay of dark matter particles into photons, they were able to set new constraints on the lifetimes of dark matter in the 1.8–2.7 eV mass range. As they observed some excesses in the data collected by WINERED, they are now planning to carry out additional analyses to determine whether they could be linked to a possible dark matter signal.

In addition, the researchers plan to continue exploring possible technological advancements that could contribute to the detection of signals linked to dark matter. Specifically, they believe that the development of new spectrographs especially tailored for dark matter searches could play a key role in the future observation of these signals.

“In fact, in another paper published last year, Taiki, Yuji, and I proposed a new spectrograph design tailored for dark matter detection,” added Yin. “The key idea is to relax the spatial resolution requirements—which are not as critical for dark matter searches—thus allowing the instrument to be installed on a less competitive small-aperture telescope, while still achieving sensitivity comparable to that of WINERED@Magellan for dark matter detection under identical observational conditions.

“If realized, this approach could secure considerably more observation time and provide access to a wider range of targets, thereby enhancing our dark matter search capabilities.”

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