The CERN experiment helps narrow the hunt for dark matter

Over the past half century, astronomers have faced an embarrassing problem: galaxies rotate too fast. When astronomers measure the velocities of stars on the outskirts of galaxies, they are much faster than expected. It is as if a cloud of invisible matter surrounds almost every galaxy in the Universe. This matter interacts gravitationally and neither absorbs nor emits light. Astronomers even have a name for this ghostly substance: dark matter.

The problem is that, despite decades of effort, no direct evidence of dark matter has been observed. Scientists working at the CERN laboratory in Europe have created a facility that will provide new capabilities in the search for this elusive substance. They recently published their first results.

NA64

Using the CERN NA64 facility, scientists used a high-energy muon beam to search for a form of dark matter that has been overlooked by previous searches. This effort follows in the footsteps of a long history of experiments in search of dark matter with specific properties.

While the evidence of rapidly rotating galaxies is very strong evidence that dark matter exists, its properties are unknown, with the possible mass of individual dark matter particles spanning a large range. On the light side, one theory suggests that individual particles have a much lower mass than an electron. 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 several possibilities; for example, most scientists now rule out very heavy dark matter, preferring models in which individual dark matter particles are atomic or smaller in size. In the early 2000s, the scientific community favored models in which dark matter particles ranged from the mass of a proton to several thousand times heavier than that. However, with the start of operations in 2010 of the Large Hadron Collider, the most powerful particle accelerator in the world, this form of dark matter is becoming increasingly disadvantaged.

The NA64 facility was designed to search for possible lighter forms of dark matter. Rather than trying to detect it directly, the NA64 experiment relies on the fact that dark matter does not interact with ordinary matter as a way to detect it.

Conservation of energy is a central principle of physics. He says that energy can neither be created nor destroyed. If you measure the energy of a system at one time, it will stay the same no matter what happens. It’s like a bank account that pays no interest. Whatever you deposit, you can withdraw. If the two numbers don’t balance, someone stole some of your money.

The basic principle of the NA64 experiment is similar. High-energy muons crash into a target, interacting 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 has somehow escaped, undetected. One possibility is that a particle of dark matter was created. Because dark matter does not interact, it would have traveled through the detector without interacting. Basically, you know it’s there because you haven’t seen it.

The NA64 experiment searched for dark matter in the range of about 0.5% to 50% of the mass of a proton. In addition to being a range of masses that had not been fully explored using a muon beam, this range was fortuitous for other reasons as well.

Muons are essentially heavy electrons. They have the same electric charge and spin characteristics as 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 two decades. The name given to this magnetic property is the “g-2 muon”, and scientists have predicted and measured the g-2 muon with great precision. They agree, digit by digit, on 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.

Having data and predictions that disagree is not inherently surprising. After all, measurement and theoretical prediction rarely match exactly. However, if the theory is correct and the measurement is correct, the two should be close and should agree within the stated uncertainties.

Future experiments

The most recent measurements and predictions disagree with the stated uncertainties and this has caused a firestorm of discussion in the scientific community. When predictions and highly accurate measurements disagree, it’s often a sign of an impending breakthrough. Thus, any measurement of muons can help resolve the situation.

The NA64 collaboration studied 20 billion muon collisions, looking for collisions with just the right amount of missing energy. None were observed. This result has ruled out a number of dark matter scenarios, as well as ruling out several explanations for the g-2 muon mystery.

The NA64 experiment is still being developed, and future improvements are expected to create a thousandfold increase in the number of muons to be studied. Along with the increased radius, the improved equipment will result in a tenfold reduction in the measurement uncertainties associated with mismeasurement of muon energy. When these two improvements are achieved, the resulting apparatus will significantly improve the experiment’s capabilities, and it is possible that future measurements could find the elusive dark matter.