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The Relentless Hunt for Dark Matter

Started Nov-21 by BlearyIrish; 29 views.

From: BlearyIrish


Dark matter does not emit, radiate, or absorb light, but most models predict that dark matter particles should—on rare occasions—interact with ordinary matter. Since the late 1980s, physicists have been deploying experiments deep underground in an effort to detect the gentle impacts of individual particles of dark matter. Over the past fifteen years or so, the collective sensitivity of these experiments has been increasing at an exponential rate, doubling each year or so on average. This staggering rate makes Moore’s Law seem stagnant in comparison.

Two independent experimental collaborations—XENON [1] and PandaX-II [2]—have recently taken the next steps in this relentless march of progress. The former group has built the world’s largest dark matter detector, called XENON1T. It utilizes a 2000-kg target of liquid xenon, housed in a 10-m-tall water tank located 1.4 km underground in the low-background environment of central Italy’s Gran Sasso National Laboratory (see Fig. 1). PandaX-II, by contrast, is located 2.4 km below ground in the China Jinping Underground Laboratory in Sichuan, China, and consists of 584 kg of liquid xenon. The reason both experiments use xenon is twofold. First, it is highly unreactive, helping to maintain the required low rate of background events. Second, its nucleus is relatively high in mass (containing 131 nucleons on average), providing a big target for incoming dark matter particles. If one of these particles were to pass through the Earth and then collide with a xenon nucleus in XENON1T or PandaX-II, that interaction could produce a faint but detectable signal of light (scintillation) and electric charge (ionization). Observing even a handful of such events would put us well on the way to identifying the nature of the mysterious substance that makes up our Universe’s dark matter. But even nondetections constitute progress, in that they reveal to us what the dark matter is not.

The recent publications from the XENON and PandaX-II collaborations do not claim to have detected any particles of dark matter, but the lack of such events can be used to place upper limits on the likelihood—or cross section—with which dark matter particles interact with ordinary matter. Because of the unprecedented size of these experiments, the reported limits are the most stringent to date on the dark matter’s proclivity to interact with nuclei. For a dark matter particle with a mass of 100 GeV, for example, each of these collaborations rule out cross sections for such interactions that are larger than about 10
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