Dark Matter Hunt

Today, we can estimate the age of the cosmos, the number of galaxies, and even the number of atoms in the universe (1078to 1082), but among the myriad things we don’t know—not counting the things we don’t know that we don’t know—one of the most elusive is dark matter. Nearly two and a half kilometers underground in Sichuan Province, in the deepest laboratory of its type in the world, scientist are hunting for the most mysterious particles in the universe with the Particle and Astrophysical Xenon Detector. It’s called PandaX for short. It is the fi rst large scale experiment in China in the hunt for dark matter, and it is a promising experiment, indeed. But, it’s not the technology or the theories or even the device itself that gives the experiment such potential, but rather the choice of laboratory, Jinping Mountain. In a northern bend in the Yalong River, an 18-kilometer tunnel leads to the China Jinping Deep Underground Laboratory, built under a mountain of marble. The low-radiation in the rock above prevents cosmic rays and particles called muons from causing trouble with the search for dark matter, which, even under optimal circumstances, is unbelievably diffi cult to detect. There are labs attempting to detect dark matter in Korea, Japan, Canada, England, France, Germany, and the United States, but none have succeeded in producing stable, satisfactory results. Currently, PandaX is just starting out but aims to be the most sensitive dark matter detector in the world in the next four years. The “detector”is what is termed a Time Projection Chamber, or TPC, using liquid xenon, currently running at 125 kilograms of xenon and later planning to move on to over a tonne. To little fanfare and to keep funds fl owing in, the PandaX experiment released their fi rst results late in 2014. Their results? Nothing.

The team of approximately 40 scientists—including four Chinese universities and two US universities—published their fi ndings in Science China: Physics, Mechanics and Astronomy, having recorded four million events. Only about ten thousand of those left energy signatures having to do with dark matter, with 46 hitting the isolated, inner xenon target. Those proved to be background radiation.

Frustratingly, even though it cannot be reliably detected, dark matter, put simply, exists. It makes up a great deal of our universe and can be seen in the velocity of spiral galaxies like our own Milky Way and the disparities in known luminous mass (stars) and gravitational mass. The discovery of dark matter would change the Standard Model of physics as we know it, and, despite being one of China’s greatest discoveries in physics, it would have wide-ranging effects on how we understand things like extra dimensions and supersymmetry.

The lack of results means that China has simply joined the ranks of nations looking for dark matter, and it’s important to remember that no results are still results. Indeed, China’s PandaX project may end in abject, expensive failure, but, as China barrels into a new world of modern scientifi c endeavor, fi nding these bashful particles is only a matter of time.