Curious physics results could shed light on dark matter


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Scientists love a mystery. It’s satisfying when a prediction turns out to be correct, but it’s intriguing when an experiment gives a result that deviates from expectations.

Several such anomalies have appeared in recent years in particle physics and astrophysics.

Sometimes such results can be explained by faulty equipment. Sometimes they go away with more rigorous measure. But sometimes they stay put and ask to be understood.

What’s interesting about some recent anomalies is that they each have the potential to be explained by the influence of an undiscovered particle or force. Any of these could be a sign of the existence of dark matter, a mysterious substance that makes up about 85% of the matter in our universe.

We call it dark matter, not only because it is impossible to see, but because it is figuratively opaque. We know very little about it even though it is ubiquitous. We know that something is responsible for the agglutination of galaxies and the movement of stars, but we have never observed it directly.

Experiments around the world have been designed exclusively to research this mysterious substance. But it’s not just direct research that could point us in the right direction. For example, the following anomalies, found in experiments on non-dark matter, may help shed light on this otherwise dark part of the universe.

Muon g-2

The muon (the heavier cousin of the electron) acts strangely in a magnetic field. Earlier this year, Fermilab’s Muon g-2 collaboration announced a measurement of how much of a muon “wobbles” in a magnetic field, confirming a result from 2001. The problem is, the two results look very different from this. predicted by the standard model.

Scientists know that the behavior of fundamental particles such as the muon is influenced by other subatomic characters. A muon can momentarily emit and reabsorb virtual versions of other fundamental particles, an action called quantum fluctuation.

To understand muon oscillation, theorists have made complicated calculations recording the effects of any possible fluctuations that might occur. If the experimental measurements and the theorists’ predictions hold up, the predictions might be missing a particle.

“It turns out that there are some great ideas on how to rectify this problem that also very naturally solve the dark matter problem,” says Jonathan Feng, a professor at the University of California at Irvine, who established this link for the first time in an article with Konstantin. Matchev a few days after the original 2001 announcement.

Specifically, Feng mentions the neutralino, a dark matter candidate predicted by supersymmetry. If theorists add the influence of virtual neutralinos to the mix, it could alter their prediction in a way that matches the experimental results.

Of course, there are also explanations which not have direct links to dark matter, and the anomaly might even disappear as the calculations become more precise.

As the Muon g-2 collaboration continues to take action, and theorists pursue their own calculations, it is possible that the theoretical and experimental values ​​of g-2 will converge. However, many physicists suspect that this anomaly will remain, and if so, it could have a natural solution thanks to dark matter.

Hubble’s tension

Our universe is expanding, but the rate of expansion of the universe, called the Hubble constant (H0), is the subject of one of the greatest disputes in modern cosmology. This is because the Hubble constant was calculated using two different methods which give irreconcilable results.

This tension has existed for decades and persists even as the measurements become more precise. Although one method uses measurements from the “first” universe (shortly after the Big Bang) and the other uses measurements from the “last” universe (closer to today), they should always arrive at the same value for H0. But with a 9% difference between the results, there is either a major experimental error or something is wrong with our current understanding of the universe.

Could dark matter explain the gap? Sophia Gad-Nasr, a doctoral student in cosmology at UC Irvine, says it’s possible, but only if dark matter decays. “The way expansion works is that the amount of matter we have is going to … kind of pull against the expanse of dark energy. [the mysterious force driving the expansion of the universe], “she said.” So if you decrease this [amount of matter] by breaking down dark matter, there will ultimately be less matter to fight against the attraction of dark energy. This would therefore give a greater number [for H0], what we are currently seeing.

But, Gad-Nasr points out, if dark matter decays, it has a lot more implications in other parts of the universe, and things start to get complicated. “We want the simplest explanation we can have, and I don’t think we’ve found one yet,” she says.

Gad-Nasr says she suspects the divergence may instead lie in our misunderstanding of dark energy.

Excess KOTO

One of the most recent curious findings that may be related to dark matter came in 2019 from J-PARC’s KOTO experiment, which studies a very rare decay of a subatomic particle called a kaon. Degradation is so rare in Standard Model predictions that the collaboration never expected to see it when they started taking data. Surprisingly, they observed four potential occurrences of rot.

In March 2020, researchers came up with three explanations to the apparent excess, including one that could be due to a new metastable particle. To not violate any further measurements, this new particle would have to decay after about 1 meter, which is characteristic of a particle that frequently appears in dark matter models.

However, the excess may not hold. The result was presented in a preliminary fashion at a workshop in 2019, and further analysis showed that it was not statistically significant.

“We are not yet ready to put all our hopes in this anomaly,” says Felix Kling, a postdoctoral fellow at SLAC National Accelerator Laboratory, who is working on a dark matter experiment called FASER that will test the KOTO anomaly.

Still, he says, researchers will need to collect more data before they can make a decision one way or the other.

“If the anomaly is really there, then what we expect to see is the longer we run, the more signal we see,” he says. “In my opinion, we should just keep an eye out for future updates to the KOTO collaboration and see what happens. “

The proton ray puzzle

In 2010, the Charge Ray Experiment with Muon Atoms (CREMA) made an incredibly accurate measurement of the proton radius by shooting a laser beam at (as the name suggests) hydrogen atoms made up of muons at the place of electrons.

Strangely, the measurement was almost 4% lower than the then official value set in 2006 by the Data Committee of the International Science Council (CODATA), derived from several spectroscopy experiments using ordinary hydrogen. The gap persisted even though CODATA updated its value every four years. In 2017, further experimental measurements also showed support for CREMA’s smaller proton radius.

The unexpected result aroused the enthusiasm of theorists. Why did the different experimental methods produce such disparate values? Some have wondered if the “new physics”, like dark matter, causes a difference between the behavior of electrons and muons, causing a discrepancy between ray measurements with ordinary and muonic hydrogen. Perhaps, as some theorists have suggested, there is a new dark matter particle that interacts with muons and not electrons, which could solve both the proton ray puzzle and the anomaly of the proton. muon g-2.

This is a possibility, although it may no longer be necessary: ​​in 2019, a spectroscopy experiment using ordinary hydrogen measured a smaller proton radius that matched the muon measurement, suggesting that the riddle is solved. Yet other spectroscopy experiments continue to produce the larger radius, leading some physicists to believe it’s not over yet.

More precise experiments are probably needed to definitively solve the proton ray puzzle.

How will we know?

Ultimately, only time – and more accurate measurements and forecasts – will tell if these anomalies persist.

“We need more development both theoretically and experimentally,” says JiJi Fan, theoretical physicist and professor at Brown University. “We need more clues beyond these to determine if these findings have a connection to dark matter.”

If there’s one thing physicists can agree on, it’s that the Standard Model can’t explain the long-standing evidence that there is something about the way our universe is made up that we don’t. do not understand.

“We have to keep looking,” Kling says. “If we don’t look everywhere we can, then we won’t find anything.

“Sometimes these abnormal results can give us the right clue. Many discoveries in physics or particle physics over the past 100 years have been due to an abnormal result that suddenly appeared that no one expected, but it was right there. Then we started to learn something new.

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