Experiment fails to answer science mystery
AKen Hicks
s scientists push the limits of knowledge, known physical laws are becoming ever more precise, whereas the mystery deepens for a few unexplained phenomena.
For example, the known physical forces that explain the basis of all chemistry, biology and other observed phenomena here on Earth are gravity, electric/magnetic, and nuclear strong and weak forces. Each of these forces has a wellknown mathematical formula that is fully tested by experiments. Together, they make up what physicists call the Standard Model.
As technology improves, more precise experiments become possible, leading to more stringent testing of those physical laws. In the past few years, no new evidence has been found in laboratory experiments for any physics beyond the Standard Model.
Yet astronomy provides clear evidence that there must be something beyond the Standard Model. For example, we cannot explain the dense clustering of galaxies, nor other observed phenomena, without dark matter.
Assuming that dark matter exists, we also surmise that it interacts weakly with regular matter. Imagine a subatomic collision between a dark-matter particle and a proton. Based on what we know from astronomy, most of the time these particles just pass by one another. This is, in part, because both the proton and the dark-matter particle are extremely small. Very rarely are they expected to have a head-on collision and exchange energy.
In late May, scientists conducting an experiment called XENON1T, which can detect collisions between dark matter and regular matter, announced their results after a year of data-taking. This result was much anticipated in the scientific community because it is the most sensitive experiment to date for dark-matter collisions.
The reason that XENON1T was so sensitive is that it used more than a ton of ultra-pure liquefied xenon gas, making it one of the biggest detectors yet built. A collision between dark matter and the nucleus of xenon is expected to give off a tiny flash of light. This detector was equipped with light-sensing electronics capable of seeing even the smallest light flash.
The result was a big fat zero. No dark-matter collisions were detected within the expected range of mass for dark-matter particles. It’s possible that the dark-matter particle could be less massive than expected, but in that case, the astronomy observations would not agree with mathematical models of dark matter.
The bottom line is that, even with improvements in technology and ever bigger detectors being built, we still can’t find evidence of dark matter interacting with regular matter except via the gravitational force (the latter is not a process that we can detect in laboratory experiments).
It appears as if the proton and the dark-matter particles pass through each other like two ghost ships in the night, never colliding. So, the mystery of “what is the particle nature of dark matter?” deepens with the null result from XENON1T.
Will it help to build even more sensitive instruments, with the hope of one day detecting dark-matter collisions? There are two problems with this. First, there are natural phenomena such as radioactive decay and cosmic rays that interfere with the detectors. For example, XENON1T was built in a tunnel that goes below the Gran Sasso mountain in Italy, to shield it from cosmic rays.
To continue building bigger and better detectors is expensive, and, at some point, the cost becomes almost prohibitive. Even when many countries cooperate with combined funding, as was the case for XENON1T, still there is a limit. For now, nature is winning, and we are left to wonder about the nature of dark matter.