Prof. Tao Liu and his collaborators, Mr. Xuzixiang LOU (HKUST) and Prof. Jing Ren (Institute of High Energy Physics, Chinese Academy of Sciences, Beijing), recently proposed a pioneering experimental methodology for application in astroparticle physics with potential for far-reaching impact. Based on the observational data for pulsars accumulated over the last few decades, they proposed to construct Pulsar Polarization Arrays to explore new phenomena in astrophysics and fundamental physics. As one scientific case for the use of this new tool, Prof. Liu and his collaboration demonstrated its strong capability to detect ultralight axion-like wave dark matter. The groundbreaking concept of Pulsed Polarization Arrays and its demonstration results were published in the prestigious journal Physical Review Letters [Phys. Rev. Lett. 130, 12401 (2023)] on March 23, 2023.

There are a large number of stably rotating millisecond pulsars in the Milky Way galaxy. These pulsars are known to be extraordinary astronomical clocks and linear polarizers. As astronomical clocks, millisecond pulsars can be used to form a galaxy-scale interferometer, which is called a Pulsar Timing Array (PTA), to detect nano-Hertz gravitational waves (GWs). In pulsar timing observations for detecting the GWs, the polarization of pulses is usually also recorded for calibration purposes. Based on the readily available polarization data accumulated over decades for the PTA programs, Prof. Tao Liu and his collaboration proposed to develop Pulsar Polarization Arrays to give full play to the relevant telescope resources. By cross-correlating the polarization data from the arrayed pulsars, astronomers will be able to explore physics with a common correlated polarization signal at galaxy scale. Using this capability to study the ultralight axion-like wave dark matter as one of the mainstream dark matter candidates is such a scientific target. The ultralight axion-like wave dark matter is characterized by a strong wave nature, due to its de Broglie wavelength of astronomical scale, and meanwhile, can modulate the position angle of linearly polarized pulsar light when it travels across the galactic dark matter halo. The pulsar polarization array is thus especially suited for the detection of ultralight axion-like wave dark matter. Currently, the real data analyses are being either performed or planned in collaboration with several world-leading PTA collaborations, which include the Parkes PTA collaboration (based on the Parkes Radio Telescope) in Australia, the Chinese PTA collaboration (based on the Five Hundred Meter Aperture Spherical Telescope) in China, and the MeerTime collaboration (based on the MeerKAT Radio Telescope) in South Africa.

This research also highlights research opportunities for outstanding undergraduate students. Mr. Xuzixiang LOU, who was a key contributor and co-author of this work, was an student in the elite International Research Enrichment (IRE) undergraduate program at HKUST between 2017 – 2021. Mr. Lou was involved in research projects under Prof. Tao Liu’s close supervision during his second through fourth years of undergraduate study, working on topics ranging from collider physics to dark matter physics. He joined the project on Pulsed Pulsar Arrays in his final undergraduate year and played a key role in advancing this research work. Xuzixiang Lou was awarded the 2021 Academic Achievement Medal at HKUST in recognition of his outstanding academic performance. He will join the high energy physics research group at the State University of New York at Stony Brook as a PhD student in Fall 2023 to continue his study of fundamental physics.