The X-files: uncovering secrets of dark matter

14 April 2017

95% of the universe is unknown, and is simply described as dark matter and dark energy. Dark matter behaves like us in gravity, but does not emit light. While astrophysicists have evidence that dark matter exists using gravitational effects, the particle physics properties are still unknown. “We hope to use astrophysical observations to find out the particle nature of dark matter, more than just treating it as an X-factor,” Dr Kenny CY Ng (Croucher Fellowship, 2016) says.

There are a multitude of long standing problems in particle physics. For instance, dark matter makes up 25% of the universe, but its particle nature is still unknown. Physicists know that neutrinos have mass, but what produces it is still a mystery, and dark matter may play a role in the process. It is also not clear how many types of particles make up the whole of dark matter.

Infamously difficult to detect, many dark matter candidates exist, showing up as byproducts in different attempts to solve various particle physics problems, like neutrino mass, “so even if you don’t find dark matter in one particular search, you can still say something interesting,” Ng cheerfully notes.

Particle astrophysics deals mainly with dark matter, subatomic particles (such as neutrinos) and their relation to astrophysics and cosmology. Neutrinos, the lesser known cousin of electrons, with no charge and only weak interactions, play an important role in modern particle astrophysics. Particle physics canon previously held that neutrinos lacked mass, but recent discoveries have found that neutrinos act completely different than expected. It was discovered that neutrinos (come with three flavors) can change among themselves when they travel from point A to B! This also implies that they have mass, but are inexplicably light compared to everything we know.

Astrophysics is anything but linear; you start with a question on dark matter, and you find various tangents and byproducts, like understanding the sun or creating new tools

One of the projects Ng is most excited about involves the study of both dark matter and neutrinos. One theory about dark matter is that it interacts with other matter very weakly, making it possible to be trapped in heavy astrobodies, like the sun, and prone to self-annihilation, releasing energy. Because the center of the sun is so dense, this process is hard to study, except for neutrino production as a byproduct of the annihilation process. Neutrinos escape rather than interact the body of the Sun, and observation of these neutrinos may give evidence of the particle makeup of dark matter.

With so many unknowns, researchers have to ask what else can produce high-energy neutrinos in the sun in order to single out dark matter annihilation. For instance, cosmic rays are highly energetic, omnipresent particles, that are produced in the death of stars. When they collide with the sun, they also produce neutrinos, which are practically indistinguishable from those produced by dark matter annihilation. But these cosmic-ray interactions are affected by the magnetic fields in the Sun, which makes the problem really difficult. One way to tackle this is to look for the gamma rays that are produced together with the neutrinos. Interestingly, the observed gamma rays from the Sun is completely different and more complicated than what was thought before, meaning that there is still much to learn about cosmic rays and the Sun. “Astrophysics is anything but linear; you start with a question on dark matter, and you find various tangents and byproducts, like understanding the sun or creating new tools,” Ng says.

Some types of dark matter are very light and do not interact with anything, such as a new kind of neutrinos called sterile neutrinos, which is important in theories of neutrino mass. Fortunately, sterile neutrino dark matter can spontaneously decay into X-rays. These X-rays are monoenergetic, giving this “lighter” dark matter a unique signature among astrophysical processes. Ng is confident that the NuSTAR satellite will help to detect these dark matter. While the satellite has not yet detected the tentative signal (claimed by some other experiments) from dark matter decay, it is far more sensitive than previous probes. More data from the satellite in the future can perhaps tell us whether “sterile neutrinos” can be dark matter or not.

Ng’s interest in physics started in high school with the realization that infinitesimal particles could significantly impact the structure of the universe, and was further drawn in by the dark matter question. Later, Hong Kong’s participation in an international particle physics experiment (the Daya Bay experiment) exposed him to the interesting world of ghostly neutrinos.

Yearning for research experience, Ng joined an experimental group at UC Berkeley as a summer research assistant. “This experience taught me the thrill of doing research,” Ng says, “Physics research involves a big puzzle with many moving pieces, each small but with a lot of intricacy.” After his doctoral work at Ohio State University’s Center for Astroparticle Physics and Cosmology, Ng joined the Weizmann Institute of Science in Israel as a postdoctoral fellow.

As a theorist, Ng takes satellite data and finds new ways to analyze it to understand how astrophysical systems work on a theoretical level, including improving current ideas on how to test dark matter. Broadly, this involves calculations and simulations, “many, many particle interaction simulations” to test new research and different circumstances. “Calculations can often become too much to do by hand when we’re discussing half-known things in the universe, so simulations provide more controlled and reliable results.”

If high-energy neutrinos are detected from the sun, the next step will be to determine whether they can be distinguished from dark matter. Ng hopes to find a new way to test and affirmatively differentiate cosmic-ray and dark matter neutrinos. Like many things in physics, the research involved in creating a separation method, which may impact both neutrino and dark matter research. Research byproducts such as greater understanding of gamma-ray emission and the magnetic field of the sun might also yield better prediction of solar weather and protection for satellite communications systems.

“The astroparticle physics field is old in the sense that it was the avenue where lots of new particles were first discovered, but is still young in that there are so many new ideas and experiments, which are constantly pushing the frontier physics. The most interesting thing is that there’s always a problem, and always something to study,” Ng explains. This reliance on the latest developments makes astrophysics surprisingly collaborative as physicists rely heavily on new data and theoretical developments, giving them ample chance to get to know each other’s work. “Everything is interconnected and our limits are infinite, but chasing uncertainties is pretty fun.” 

Dr Ng received his BSc in Physics from the Chinese University of Hong Kong in 2010. Dr Ng then went to Ohio State University for PhD under the supervision of Prof. John Beacom. Starting from 2016, Dr Ng will be a postdoc fellow at the Weizmann Institute of Science in Israel. Dr Ng is particularly interested in how can one test fundamental physics in astrophysical and cosmological settings. During graduate school, he worked on various topics in astroparticle physics involving high-energy gamma rays, neutrinos, and dark matter. One of his current research interests is on realizing the Sun as a high-energy astrophysical source. Much about the high-energy Sun is unknown, in particular, its gamma-ray emission. New observational and theoretical studies will tell us more about the Sun, local cosmic rays, high energy neutrinos from the Sun, as well as the Sun as a dark matter detector. 

To view Dr Ng’s Croucher profile, please click here.