“The Last Holdout” –LHC Scientists See a Gateway to the ‘Dark-World’


Dark Matter Map



 “It’s entirely plausible that there could exist multiple forms of matter and energy that we have not yet discovered,” cosmologist and particle physicist, Dan Hooper told The Daily Galaxy, referring to the possibility that dark particles could couple with the Higgs boson creating a portal to the dark sector of the Universe. “If there are even very feeble interactions between the known particles and those we have not yet discovered we could hope to see the known particles occasionally disappear and be replaced by invisible hidden particles.”

“We know for sure there’s a dark world, and there’s more energy in it than there is in ours. It’s possible that the Higgs could actually decay into these long-lived dark particles,” said LianTao Wang, a University of Chicago (UChicago) physicist

The Higgs Gateway –Portal to the Dark Universe

“One particularly interesting possibility is that these long-lived dark particles are coupled to the Higgs boson in some fashion—that the Higgs is actually a portal to the dark world,” said Wang, referring to the last holdout particle in physicists’ grand theory of how the universe works, discovered at the Large Hadron Collider (LHC) particle accelerator at CERN in 2012. 

Now that they’ve identified the Higgs boson, scientists at the Large Hadron Collider have set their sights on an even more elusive target. A new paper outlines a method to directly detect particles from the ‘dark world’ using the LHC. Until now we’ve only been able to make indirect measurements and simulations, such as the visualization of dark matter.

Wang, who studies how to find signals in large particle accelerators, along with scientists from UChicago and UChicago-affiliated Fermilab, think they may be able to lead us to its tracks; in a paper published in Physical Review Letters, they laid out an innovative method for stalking dark matter in the LHC (image below) by exploiting a potential particle’s slightly slower speed.

“The Next Big Thing” –Hunt for Double Higgs Boson Opens a New Frontier

Dark World –95 percent of the Universe

While the dark world makes up more than 95 percent of the universe, scientists only know it exists from its effects—like a poltergeist you can only see when it pushes something off a shelf. For example, we know there’s dark matter because we can see gravity acting on it—it helps keep our galaxies from flying apart.

Theorists think there’s one particular kind of dark particle that only occasionally interacts with normal matter. It would be heavier and longer-lived than other known particles, with a lifetime up to one tenth of a second. A few times in a decade, researchers believe, this particle can get caught up in the collisions of protons that the LHC is constantly creating and measuring.

The only problem is sorting out these events from the rest; there are more than a billion collisions per second in the 27-kilometer LHC, and each one of these sends subatomic chaff spraying in all directions.

Wang, UChicago postdoctoral fellow Jia Liu and Fermilab scientist Zhen Liu (now at the University of Maryland) proposed a new way to search by exploiting one particular aspect of such a dark particle. “If it’s that heavy, it costs energy to produce, so its momentum would not be large—it would move more slowly than the speed of light,” said Liu, the first author on the study.

Dark Particle –Moves More Slowly Than Speed of Light

That time delay would set it apart from all the rest of the normal particles. Scientists would only need to tweak the system to look for particles that are produced and then decay a bit more slowly than everything else.

The difference is on the order of a nanosecond—a billionth of a second—or smaller. But the LHC already has detectors sophisticated enough to catch this difference; a recent study using data collected from the last run found the method should work, plus the detectors will get even more sensitive as part of the upgrade that is currently underway.

“We anticipate this method will increase our sensitivity to long-lived dark particles by more than an order of magnitude—while using capabilities we already have at the LHC,” Liu said.

Experimentalists are already working to build the trap with the LHC turned back on at the end of September 2021, after boosting its luminosity by tenfold, all three of the major detectors will be implementing the new system, the scientists said. “We think it has great potential for discovery,” Liu said. 

“Dark Energy’s Known Unknown” — Could It Be the Symmetron Field That Pervades Space Much Like the Higgs Field

“Through new ideas using precision timing information  and fine-imprints in the calorimetry at CERN LHC” Zhen Liu told The Daily Galaxy, “we can significantly expand our knowledge of Higgs decays into long-lived particles. Global efforts in the next generation of Higgs factories, such will be fully geared toward exploring these intriguing possibilities centered around the Higgs gateway. These Higgs factories will include the International Linear Collider (ILC) under consideration in Japan and the Compact Linear Collider (CLIC) at CERN, while the other two are circular: the Future Circular Collider (FCC-ee) at CERN and the Circular Electron Positron Collider (CEPC) in China.

“If the particle is there, we just have to find a way to dig it out,” Wang said. “Usually, the key is finding the question to ask.”

The Daily Galaxy, Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via University of Chicago. Avi was formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL) 

Image credits: Dark Matter Kids Survey top of page. LHC image with thanks to Laurent Egi

[Editors Note: Zhen Liu sources —Enhancing Long-Lived Particles Searches at the LHC with Precision Timing Information and Enhancing sensitivities to long-lived particles with high granularity calorimeters at the LHC]


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