Searching for dark matter: Are scientists looking in the wrong direction?

Dark matter is believed to be the most abundant form of matter in the Universe, consisting of about five times more mass than the ordinary matter found in stars and galaxies. There’s just one problem: Despite half a century of effort, scientists have found only indirect evidence of its existence. Now, a measurement from an ultra-precise new facility suggests that scientists may be looking in the wrong direction.

Something is not right in the Universe. Galaxies move too fast and rotate too fast to be explained by the observed matter in the cosmos and the accepted laws of physics. The most popular explanation for these strange observations is that the Universe is covered with a large amount of invisible — and invisible — matter. This matter, tentatively called dark matter, can interact gravitationally with ordinary forms of matter, but dark matter neither emits nor absorbs light.

While several candidate forms of dark matter have been proposed and rejected, since the late 1990s the scientific community has settled on a favored dark matter candidate called the WIMP (Weakly Interacting Massive Particle). If WIMPs are real, the consensus is that they are stable subatomic particles, electrically neutral, with masses in the range of perhaps 100 to 100,000 GeV. (GeV is a unit of energy connected to mass by Einstein’s equation E = mc². For context, a proton has a mass of about 1 GeV.)

According to popular theory, WIMPs cannot be seen, although they use gravity. Unlike ordinary matter, which aggregates into stars and planets, WIMP dark matter is organized as vast clouds of “gas” that surround and permeate galaxies, including our own Milky Way. If such clouds exist, a gas of dark matter is found in the Solar System and forms a “WIMP wind” that constantly passes through the Earth. Depending on the mass of individual WIMP particles, if you raise your fist, there is probably one WIMP particle in that volume.

Scientists searched for this WIMP wind without success, despite a series of increasingly sensitive detectors. Since the mid-1980s, detectors have increased their sensitivity a million times. One of the current, world-class, detectors is called LUX-Zeplin (LZ). It uses 10 tons of liquid xenon, cooled to about -150 °F (-100 °C), and is located about 4850 feet underground (1478 meters) at the Sanford Underground Research Facility (SURF) in an abandoned mine gold.

An array of photomultiplier tubes designed to detect signals from particle interactions occurring inside LZ’s liquid xenon detector. (Credit: Matthew Kapust/Sanford Underground Research Facility)

This depth shields the detector from constant barrage of cosmic rays from space, which can mimic dark matter signals. Only seven of the 10 tons of xenon in the detector’s central core are used in subsequent analyses; Events where signals are seen in the three tons of liquid xenon that form a shell around the inner core are rejected. This ensures that any possible WIMP detection is well quantified.

The LZ experiment waits for a WIMP particle to pass through the detector and hit a xenon atom. The guess is that debris from that collision will pass through the apparatus and leave a visible signal. After running for 280 days (out of an expected 1,000), the LZ scientists did not see WIMPs pass through their detector. They recently reported their results at the TeVPA and LIDINE conferences. A scientific paper is under preparation, although some caution is necessary because the published results have not yet been peer-reviewed. (Early release of measurements is common for large experiments like LZ, and it is very rare that peer review significantly changes the measurement as it was first reported.)

The measurement announced by LZ shows a five-fold improvement in the range of WIMP masses and interaction probabilities excluded. While the existence of WIMPs remains a possibility, the current measurement makes their existence more likely. There remains one set of masses that LZ has not been able to exclude. The analysis they report does not consider WIMP masses below 9 GeV.

Given the mass range favored by most WIMP theories (100 – 100,000 GeV), if dark matter turns out to be very bright, it is possible that dark matter is not a WIMP. LZ is constantly analyzing its data. The fact that they avoided masses below 9 GeV is not because they have no data there; instead, low mass analysis is very difficult and will require more care before any results can be obtained.

Future, larger, liquid xenon experiments will be able to make more sensitive measurements, however, there is a limit to how much WIMP experiments can improve. You see, the Earth is constantly being showered with particles called neutrinos. These neutrinos come from both nuclear reactions in the sun and from cosmic rays from space. Effectively, the Earth exists in a neutrino fog.

Unfortunately, neutrino interactions in WIMP detectors look like dark matter interactions, so when dark matter experiments are sensitive enough to detect this neutrino fog, it will be impossible to detect the interactions of dark matter. It’s like trying to hear a whisper at a rock concert.
A detector with a sensitivity about 10 times better than LZ can detect neutrino fog. It will probably take 10 to 15 years to develop detectors with that level of sensitivity. Once reached, traditional WIMP searches are no longer possible.

Does this mean we are nearing the end of the road for finding dark matter? Not really. After all, the unexpected movement of galaxies requires an explanation. Dark matter may be a form of matter that is not a WIMP. In fact, some scientists have turned their attention to a lighter dark matter candidate, called axions. Axions, like WIMPs, have not been detected, but axion detection requires different technologies than WIMP particles. The search for dark matter will continue.