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January 27, 2022

Experiment fails to find dark matter

By Mo-Yu Zhou | April 28, 2011

After searching for dark matter for 100 days, the XENON100 tank failed to find any evidence of the material. While the negative result does not mean that dark matter does not exist, it does indicate that it is more elusive than scientists had previously imagined.

XENON100, which is filled with 161 kilograms of chilled liquid xenon and which is buried under 1,400 meters of rock in the Gran Sasso Underground Laboratory in Italy, was designed to detect dark matter particles. The depth at which the tank is located cannot be easily penetrated by cosmic rays, which mimic the behavior of dark matter particles. However, when a dark matter particle strikes a xenon nucleus, it recoils, resulting in light emission and ionization.

Whether or not a dark matter particle has been found is indicated by the ratio of the amount of light emitted to the amount of ionization. However, the experiment has not yielded any results that would indicate the existence of such particles.

The experiment’s results were revealed on April 4, when the research team crowded around a computer screen to view the results of their analysis while some of their collaborators watched in Zurich. The analysis covered data obtained between January and June of 2010.

In a few minutes, six dots appeared on the screen, one after the other, indicating six possible weakly interacting massive particles (WIMPs), or particles that had the desired light emission to ionization ratio. However, a closer look at three of the particles revealed that they were nothing but electronic noise. The remaining three potential WIMPs were also called into question. Background radiation was expected to create two events that looked like WIMPs, and the last extra WIMP could not be claimed as a legitimate detection of dark matter.

This experiment conflicts with other experiments, which have found WIMPs, which are low-mass versions of dark matter particles. This new research is especially significant because it calls into question how we can interpret the results of other experiments.

In addition, the XENON100 experiment hints at limits that exist on the degree to which dark matter interacts with ordinary matter — the upper limit of the interaction strength, this experiment suggests, is a 10th of the best previous estimate.

If, however, such interactions are controlled by, say, the sought-after Higgs boson, the XENON100 is capable of investigating such a relationship and the existence of the Higgs boson.

The results of this experiment also call into question some versions of the supersymmetry theory of particle physics. Supersymmetry suggests that every known particle has a corresponding particle that is unseen and also heavier. Using XENON100, physicists can test such theories.

It is also possible that there is a link between the strength of the interaction between dark and ordinary matter and the recent results of Fermilab’s Tevatron, which have yielded two hints at a new elementary particle and along with it, a new type of force.

Elena Aprile, the lead researcher, is also hopeful that once a full year’s worth of data is obtained from the XENON100, her team can claim to have truly detected a WIMP. Her team also plans to build an even larger underground tank containing a ton of xenon for future experiments.

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