Now that scientists have found the Higgs boson, the Large Hadron Collider will be looking for an even more elusive target: dark matter. We are surrounded by dark matter and dark energy - invisible substances that bind galaxies, but do not give themselves away. The new work outlines an innovative method for searching for dark matter by the Large Hadron Collider by exploiting the relatively slow speed of a potential particle.
The dark world of dark matter
“We know for sure that there is a dark world and there is more energy in it than ours,” says Lien-Tao Wong, a physics professor at the University of Chicago who studies signal search in large particle accelerators like the LHC. The work was published in Physical Review Letters.
Although the dark world makes up more than 95% of the universe, scientists know about its existence only from the effects it has - like a poltergeist, which is visible only if something moves on a shelf or when a light blinks. For example, we know about the existence of dark matter, because we see its gravitational effect - it helps our galaxies not to fly apart.
Theorists believe that there is one special kind of dark particle that can periodically interact with ordinary matter. It will be heavier and will last longer than other known particles - up to one tenth of a second. According to scientists, several times a decade such a particle can be caught in collisions of protons at the LHC, which occur and are measured constantly.
“One particularly interesting possibility is that these long-lived dark particles are somehow related to the Higgs boson - that the Higgs is actually a portal to the dark world,” Wong says, referring to the last particle physicists discovered in the grand theory of how the universe works. … The Higgs boson was discovered at the LHC in 2012. "Perhaps the Higgs can actually decay into these long-lived particles."
The only problem is to separate these events from the rest; at the 27-kilometer LHC, there are more than a billion collisions per second, each of which sends out subatomic spray in all directions.
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Scientists have proposed a new search method using one particular aspect of such a dark particle. "If it is so heavy, it requires energy to produce, so the momentum will not be large - it will travel slower than the speed of light," says Jia Liu, the study's first author.
This time delay will make it stand out from the rest of the particles. Scientists only need to tweak the system to find the particles it produces. The difference lies in the range of a nanosecond - one billionth of a second - or less. But the LHC already has smart enough detectors to pick up on this difference; a recent study using data collected from the last launch of the collider showed that this method should work, plus the detectors will become even more sensitive with the upcoming upgrade.
“We expect this method to increase our sensitivity to long-lived dark particles by several orders of magnitude - while leveraging the capabilities that the LHC already has,” says Liu. Experimenters are already working on creating a trap. When the LHC turns on again in 2021, after a 10-fold increase in luminosity, all three main detectors will be equipped with a new system.
“We think there is great potential for discovery. If there is a particle, we will have a way to extract it."
Ilya Khel
