Rare quadruplet 'top quarks' created at world's largest atom smasher

Close-up illustration of subatomic particle. — (Image credit: Shutterstock)

The world's largest atom smasher has "given birth" to a set of four ultraheavy particles — called top quarks.

The formation of these chubby-but-tiny quadruplets, at the Large Hadron Collider in Geneva, Switzerland, has long been predicted by the Standard Model, the prevailing physics theory that governs subatomic interactions. But new physics theories suggest they could be created much more often than the Standard Model predicts. Finding more of such foursomes is the first step in testing those theories. The new findings were announced at the LHCP 2020 Conference.

In 2011, the LHC assumed the mantle as the world's most powerful particle accelerator. The LHC — a ring of nearly 10,000 powerful magnets arrayed around a circle with a circumference of about 16.5 miles (27 km) — accelerates two beams of protons in opposite directions and crashes them into one another at an energy of 13 trillion electron volts, or 6.5 times more energetic than the Fermilab Tevatron. They also collide about 100 times more frequently than was possible at the Tevatron.

In the recent experiments, the researchers were looking for the simultaneous production of two sets of top quark/antiquark pairs. The Standard Model predicts these more complex collisions should occur about 70,000 times less frequently than collisions that create a single pair, according to the ATLAS team. When hunting for new particles, it's important to know how likely it is that the observed number of collisions occurred by chance, which can be quantified by a result's "sigma."

In particle physics, the gold standard for declaring a discovery is a sigma of 5 or greater, which means there's about 1 in 3.5 million odds that the current observation occurred due to random fluctuations. A sigma of 3 means that the observed signal is expected to happen by accident in 1 in 740 experiments, and is considered "evidence" of an observation, according to Fermilab. The evidence for top-quark quadruplet production isn't strong enough to claim a new discovery just yet.

Physicists searched for top-quark quartets in data collected by ATLAS and CMS between 2015 and 2018. The ATLAS experiment team announced that they have seen the production of four top quarks with a sigma of 4.3. Meanwhile, in a paper published to the European Physical Journal C, researchers from the CMS experiment reported an observed sigma of just 2.6 for their quadruplet top quarks. Prior to conducting the experiment, both ATLAS and CMS expected a significance of about 2.6 sigma.

The high significance seen by ATLAS could be mere chance. Or, it could be an indication that four top quark production is more common than the Standard Model predicts, which could mean that this measurement is the first hint of some new and unexpected physics. "Additional data from the next LHC run — along with further developments of the analysis techniques employed — will improve the precision of this challenging measurement," the researchers said in a statement.

Diane Lincoln is the author of "Science Discovery Files," a non-fiction science book published by World Scientific. She is a former science teacher, with two decades of

experience bringing fascinating science stories to life for her students. She holds a Doctorate in Education and has taught graduate-level courses in science education to

teachers wanting to hone their skills. She has written for online media outlets such as Live Science and Science News for Students. Diane has won awards for her fictional

stories and poetry in print media, and she has published a book of poetry.