Astroparticle physics

Dark matter remains elusive

WIMPs, or weakly interacting massive particles, are the leading candidates for dark matter, the 'missing' mass in the Universe. An experiment has obtained no evidence for such particles, despite an impressive increase in sensitivity.

For almost a century, scientists have claimed that the Universe is filled with dark matter, a mysterious substance that is different from everything we see around us. Since the mid-1980s, detectors have been built to look for dark matter, under the assumption that it consists of entities known as weakly interacting massive particles (WIMPs). Although these experiments are now about 100,000 times more sensitive than originally, WIMPs have not been detected. Writing in Physical Review D, the XENON Collaboration (Aprile et al.)1 again reports a null result, using data obtained over four years from a liquid-xenon detector. The lack of a signal in such experiments has sparked much speculation about the nature of dark matter, and challenges the WIMP hypothesis.

WIMPs are particles that are theorized to have properties similar to those of neutrinos. They are electrically neutral and, apart from gravitational interactions, interact only through the weak force — a feeble nuclear force that can convert neutrons to protons, and vice versa. The most striking example of the weak force is the first step of nuclear fusion in the Sun, in which two protons fuse to form deuterium. Many particle-physics models that attempt to explain dark matter predict that it should have weak interactions. The strongest argument for the existence of WIMPs is that they offer a natural explanation for why about 27% of the present-day Universe consists of dark matter2, rather than a figure close to 100%.

The most direct approach to detecting WIMPs is through their collisions with ordinary matter on Earth — WIMPs would stream through everything on the planet with an average speed of about 220 kilometres per second, owing to the rotation of the Solar System around the Milky Way's centre3. Such direct-detection experiments must be performed deep underground because cosmic rays at ground level generate a huge amount of background noise. Even then, the detectors must be carefully shielded, and both the target material and the detector itself must have minimal radioactivity, again to reduce background noise.

In the first decade of the millennium, direct-detection experiments using semiconductor technologies — mostly pioneered by the CDMS Collaboration4 — led the way in sensitivity. Such semiconductor materials have extremely low levels of radioactivity, and the detectors can identify very weak signals. These experiments are thus ideal for detecting light WIMPs that have masses between 1 and a few gigaelectronvolts (particle masses are conventionally expressed in energy units based on the electronvolt; eV).

In 2004, the XENON Collaboration began to develop an alternative 'dual-phase' liquid-xenon detection technology (Fig. 1). When a particle scatters off an atom of liquid xenon in such an experiment, it produces light that is detected by two arrays of photomultiplier tubes; it also produces electrons. The presence of electric fields causes the electrons to drift through the liquid xenon and emit light as they pass into a layer of gaseous xenon. By studying the timing and relative intensity of these two light signals, the authors could determine the 3D location of a scattering event and distinguish WIMP scattering from background noise. Xenon can be made extremely pure because it lacks long-lived radioactive isotopes (apart from 136Xe, which has such a long half-life that it is a problem only for large detectors). Liquid-xenon detectors are most sensitive to WIMPs that have masses between about 10 and 100 GeV.

Figure 1: Dual-phase xenon detection technology.

The XENON Collaboration1 has looked for evidence of weakly interacting massive particles (WIMPs) that could account for the 'dark matter' in the Universe. The authors' detector consists of a chamber filled with liquid and gaseous xenon, two arrays of photomultiplier tubes (PMTs), an electric field generated by a positively charged electrode (anode) and a negatively charged electrode (cathode), and a stronger electric field (not shown) around the liquid surface. If a WIMP were to scatter off a liquid-xenon atom in the detector, it would produce light, which would be detected by the PMTs, and electrons (e). Owing to the presence of the electric fields, the electrons would drift to the top of the liquid-xenon layer, emitting light as they pass through the surface. The XENON Collaboration used these light signals to pinpoint the exact 3D location of a scattering event and the relative intensity of the signals to distinguish WIMP scattering from ordinary radioactivity.

The authors released their first data5 in 2008, from an experiment called XENON10 at the Gran Sasso National Laboratory in Italy. The experiment used a 5.4-kilogram liquid-xenon target and produced results on a par with those achieved by the leading semiconductor-based experiments. The authors' dual-phase technology has since led to a rapid increase in WIMP-detection sensitivity, ushering in the 2010s as the 'xenon decade' in the history of direct-detection experiments. In the current paper, the XENON Collaboration present results from a follow-up experiment, XENON100.

XENON100 consists of a 62-kg liquid-xenon target and a detector that is much larger than that of XENON10. Four years of its experimental programme were marked by three periods of data taking — the latest publication combines previously released results6,7 with data from the third run. Despite the experiment's substantial improvement in sensitivity with respect to XENON10, the authors find no evidence for WIMPs. Their results suggest that, if these particles exist, they must either be much lighter or much heavier than a few GeV, or have only extremely rare interactions with ordinary matter.

The XENON100 experiment led in search sensitivity until 2014, when the first results from the Large Underground Xenon (LUX) experiment were published8. This experiment is based at the Sanford Underground Research Facility in South Dakota and uses a 250-kg liquid-xenon target. The LUX Collaboration released its final results last month9, and these are about ten times more sensitive than those of XENON100. In 2016, a newcomer called PandaX-II began running in the China Jinping Underground Laboratory (using a 580-kg target10). The first 100 days of data from PandaX-II have already reached a sensitivity similar to that of LUX11,12. However, none of these experiments has detected a signal for WIMPs.

The results from dark-matter direct-detection experiments seriously challenge, and even rule out, some simple WIMP models. However, some types of WIMP require an even greater detection sensitivity than is currently available. The hunt for WIMPs is therefore far from over, and now is a critical time for these experiments.Footnote 1


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Correspondence to Xiangdong Ji.

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Ji, X. Dark matter remains elusive. Nature 542, 172–173 (2017).

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