Physicists spy on particle X in Large Hadron Collider data

As atoms and subatomic particles swirl and collide in the magnetic coil of CERN’s Large Hadron Collider (LHC), detectors monitoring their collisions and the high-energy debris they produce transform what they see into data – lots of data.

The vast majority of this data is fluff that CERN filters out automatically. But each year that the LHC operates, CERN estimates, produces 90 petabytes of recorded data, enough to fill 90,000 typical 1-terabyte hard drives. CERN, like a space opera of the 1960s, stores a large part of it on giant banks of magnetic tapes in a shiny room near the Franco-Swiss border. That’s too much data for a human to sift through easily.

It’s perhaps no surprise that hidden gems are buried deep within these storage banks, waiting to be discovered. Particle physicists have discovered such a gem: a strange particle with a strange name, X(3872). If they’re right, it could be a look back at the very first flickers of time – what the universe looked like in the first millionth of a second after the Big Bang. They published their findings in the journal Physical Review Letters on January 19.

They’ve only scratched the surface of what this particle looks like. “Theoretical predictions from different groups did not agree with each other,” says Yen Jie Lee, a particle physicist at MIT and one of the researchers.

X(3872) sounds like the name of a cryptid, and its previous sightings have indeed been fleeting. The first came in 2003, when scientists at the Belle experiment, a particle accelerator in Tsukuba, Japan, north of Tokyo, spotted X(3872) as they knocked electrons together. Unfortunately, X (3872) decayed and disappeared too quickly for scientists to learn much.

[Related: Physicists close in on the exceedingly short life of the Higgs boson]

Instead, scientists thought they could find X(3872) in what is called quark-gluon plasma. The nuclei of atoms contain clusters of protons and neutrons, but these tiny particles are actually made from even smaller particles called quarks. To make larger particles, quarks are bound together by gluons, even smaller particles, acting as agents of small nuclear force.

At extremely, extremely high temperatures – billions of degrees – protons, neutrons and other similar particles break apart and dissolve into a mush of high-energy quarks and gluons. It’s quark-gluon plasma.

It wasn’t until the 21st century that physicists were able to create it. One method that has proven effective is heavy ion collision: smashing atomic nuclei together at very high speeds. Fortunately, the LHC experiments had smashed heavy lead atoms together, leaving behind trails of data in the quark-gluon plasma for researchers to wade through.

But it is not that simple. “No one has tried to detect X(3872) in heavy ion collisions before, because it’s a very difficult task,” says Lee.

The LHC typically collides with smaller particles like protons, but larger particles like atomic nuclei leave behind a lot more debris. “In proton-proton collisions there are about a few dozen particles produced in one event, while in heavy ion collisions there are usually several thousand or even 10,000 particles per event,” explains Jing Wang, postdoc at MIT and one of the researchers.

Finding X(3872) in all this, lost in the rainforest of LHC data, is like trying to find a needle in a meadow. Wang and his colleagues developed a method relying on machine learning: they trained an algorithm to find the signature of X(3872), its fingerprints, as it decayed into other particles. After some tweaking, the algorithm found a particle with the mass of X(3872) no less than a hundred times.

The results tell us more about an artifact of the earliest ticks in history. Quark-gluon plasma filled the universe in the first millionths of a second of its life, before what we recognize as matter – molecules, atoms, or even protons or neutrons – had formed.

Lee says that in the future, quarks and gluons from the plasma could be used to break up the particle and see what’s inside.

Some physicists believe that X(3872) could be a four-quark particle: a tetraquark. The subatomic particles we know – typical protons and neutrons – are made up of three quarks; tetraquark particles are strange and generally need high energies to stay together. Over the past decade, physicists have seen other examples tetraquarks in their particle accelerators.

Another possibility is that X(3872) is in fact constructed from mesons. It is another type of subatomic particle made up of two particles: a quark and an antiquark, the antimatter look-alike of the quark. Mesons can sometimes appear fleetingly on Earth, when high-energy cosmic rays collide with typical matter. But no one has ever seen a larger particle made up of several mesons.

It’s exciting, says Lee, because if X(3872) is created from mesons, it’s a sign that the universe was resplendent with such “exotic” particles.

But to learn, they will have to wait for even more data. The LHC is currently three years into its second extended maintenance and upgrade period, aptly called the Long stop 2— and its restart date has been pushed back several times, thanks to COVID. (It’s possible from next month.) After that, there will be more collisions, more quark-gluon plasma, more data to sift through.

“It’s going to be exciting to continue this line of study with a much larger amount of data,” says Lee.

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