Over the past half century, astronomers have faced an embarrassing problem: galaxies spin too fast. When astronomers measure the speed of stars on the outskirts of galaxies, it is much faster than expected. It is as if a cloud of invisible matter surrounds almost all the galaxies in the universe. This substance interacts gravitationally and neither absorbs nor emits light. Astronomers even have a name for this ghostly matter: dark matter.
The problem is, despite decades of efforts, no direct evidence of dark matter has been observed. Scientists working at the CERN laboratory in Europe have developed facilities that offer new capabilities in the search for this elusive substance. They recently published their first results.
NA64
Using CERN’s NA64 facility, scientists used an energetic beam of muons to search for a type of dark matter that had been missed by previous searches. The effort follows a long history of experiments in the search for dark matter with specific properties.
While the evidence of rapidly rotating galaxies is very strong evidence for the existence of dark matter, its properties are unknown, with the possible mass of individual dark matter particles spanning a wide range. In the case of light, one theory suggests that individual particles have much less mass than electrons. On the heavy side, individual dark matter particles can be 30 times the mass of the Sun.
Since the 1990s, various experiments have ruled out some possibilities. For example, most scientists now reject superheavy dark matter, preferring models in which individual dark matter particles are atomic or smaller. In the early 2000s, the scientific community favored models in which dark matter particles ranged from the mass of a proton to a few thousand times heavier. However, with the launch of the Large Hadron Collider in 2010, the world’s most powerful particle accelerator, dark matter in this form is increasingly frowned upon.
The NA64 facility is designed to search for possible lighter forms of dark matter. Instead of trying to detect it directly, the NA64 experiment relies on the fact that dark matter does not interact with normal matter as a way to detect it.
Conservation of energy is a fundamental principle of physics. It says that energy can neither be created nor destroyed. If you measure the energy of a system at one point in time, no matter what happens, it stays the same. It’s like a bank account that doesn’t pay interest. Whatever you deposit, you can withdraw. If these two numbers are not balanced, someone has stolen some of your money.
The basic principle of the NA64 test is similar. Energetic muons hit the target and interact with atomic nuclei. After the collision, the energy of the debris is measured. If the energy after the collision is less than the energy before the collision, then the energy somehow escaped and went undetected. One possibility is that a dark matter particle was created. Since dark matter does not interact, it would pass through the detector without interacting. Basically, you know it’s there because you haven’t seen it.
The NA64 experiment looked for dark matter ranging from 0.5% to 50% of the mass of a proton. In addition to the range of masses not fully explored using the muon beam, this range was also fortuitous for other reasons.
Muons are essentially heavy electrons. They have electric charge and spin properties similar to electrons, but muons are heavier. Having an electric charge and spin means that muons act like tiny magnets, and the magnetic properties of muons have been mysterious for the past few decades. The name given to this magnetic property is “g-2 muon” and scientists have predicted and measured the g-2 muon very accurately. They agree, digit by digit, for seven digits, and then disagree on the eighth. The measurement was made by the Muon g-2 Collaboration and the prediction was made by the Muon g-2 Theory Initiative.
The existence of a discrepancy between data and predictions is not inherently surprising. After all, measurement and theoretical prediction rarely match exactly. However, if the theory is correct and the measurement accurate, the two should be close and should agree within the stated uncertainties.
Future experiments
The most recent measurements and predictions do not match the stated uncertainties, and this has sparked a firestorm of debate in the scientific community. When predictions and highly accurate measurements disagree, it’s often a sign of an impending discovery. Therefore, any muon measurement can help resolve this situation.
The NA64 collaboration examined 20 billion muon collisions, looking for collisions with the right amount of energy lost. None found. This result rules out both a range of dark matter scenarios and specific explanations for the g-2 muon puzzle.
The NA64 experiment is still under development, and future developments are expected to increase the number of muons studied by a thousandfold. Along with increasing the beam, the improved equipment results in a tenfold reduction in measurement uncertainties associated with mismeasurement of muon energy. Once these two developments are achieved, the resulting device will significantly improve the experimental capabilities, and it is possible that future measurements will be able to find the elusive dark matter.
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