String theory’s nightmare: Ghostly five-particle family promises to uncover dark matter
For decades, physicists have chased the dream of unifying all of nature’s forces under a single theory. String theory has long been a top candidate, offering a single mathematical framework that could combine quantum physics and general relativity.
However, the problem with string theory is that it is so flexible that it can explain almost anything, making it nearly impossible to test or disprove.
Now, a new study flips the usual approach. Instead of asking what string theory can explain, a team of physicists searched for what it absolutely can’t. This led them to 5-plet, a strange five-member particle group that, if detected through the Large Hadron Collider, could seriously shake string theory, and even offer a clue to the mystery of dark matter.
“If we detect a 5-plet, it’s a double win. We’d have disproven key predictions of string theory and simultaneously uncovered new clues about dark matter,” Rebecca Hicks, one of the study authors and a PhD student at the University of Pennsylvania (UPenn), said.
The quest for string theory’s limitations
The current foundations of physics rest on two major theories. First is the Standard Model, which explains all known particles and three of the four forces (electromagnetism, the strong, and the weak nuclear forces). The second is Einstein’s general relativity, which describes gravity and the shape of spacetime.
However, these two theories don’t fit together as they speak different mathematical languages. String theory tries to fix that. It imagines that all particles are tiny vibrating strings and suggests that space has extra hidden dimensions, up to 10 or 11 in total. While the math works out beautifully, the theory is hard to test.
Its predictions often involve energies much higher than what we can achieve in current experiments, like those at the Large Hadron Collider (LHC). That’s why the study authors used a different approach. They searched for particle patterns that string theory can’t produce under any known version.
During their search, they found something missing—a specific group of five related particles, known as a 5-plet. These groups are like a bigger cousin of common particle families, such as doublets (pairs like the electron and its neutrino).
The 5-plet includes a type of particle called a Majorana fermion, a particle that is its own antiparticle. The researchers searched through all known constructions in string theory but couldn’t find a single one that produces this five-member structure. “We scoured every toolbox we have, and this five-member package just never shows up,” Jonathan Heckman, one of the study authors and a theoretical physicist at UPenn, said.
If a 5-plet is discovered, it could have two major impacts. First, it would mean current string theory models are incomplete or wrong, because they can’t explain the existence of such a particle. That would force physicists to rethink one of the most ambitious theories in science.
Second, one of the particles in the 5-plet (Majorana fermion) is a promising candidate for dark matter, the invisible substance that makes up about 85% of the universe’s matter. So far, dark matter has never been directly detected, but if the 5-plet shows up at the right energy and with the right behavior, that could change.
So what’s stopping us from detecting the 5-plet?
The challenge now is to find out if the mysterious five-particle family exists in reality. The LHC might be able to create them, but it won’t be easy. First, these particles are extremely heavy, possibly weighing up to 10,000 times more than a proton. This means they would only appear in the most energetic proton collisions.
“As the masses of these particles climb toward a trillion electron volts, the chance of creating them drops dramatically. The LHC has to slam protons together hard enough to conjure these hefty particles out of pure energy,” Hicks added.
Second, even if created, these particles would be hard to detect. Most of them decay almost immediately into faint signals, leaving behind disappearing tracks in detectors. These tracks start strong but vanish partway through, like footprints in snow that suddenly stop.
The study authors already looked for such clues. They used data from ATLAS, one of the LHC’s massive detectors, and studied past searches designed for other hypothetical particles. They reanalyzed them for signs of the 5-plet.
No signals turned up, but this helped them set a lower limit: if these particles exist, they must be heavier than 650–700 GeV, which is about five times heavier than the Higgs boson. “This early result is already a strong statement; it means lighter 5-plets don’t exist. But heavier ones are still very much on the table,” Heckman explained.
Now, the researchers plan to continue searching as the LHC gets upgrades and becomes more sensitive in future runs.
Source: Interesting Engineering
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String theory’s nightmare: Ghostly five-particle family promises to uncover dark matter/String theory’s nightmare: Ghostly five-particle family promises to uncover dark matter