Physicists discovered a rare “X” particle from the beginning of the universe

A blue and gray tube carrying accelerated protons in a tunnel at CERN in Switzerland.

A corridor of the Large Hadron Collider at CERN, in Switzerland, in February 2020.
Photo: VALENTIN FLAURAUD / AFP (Getty Images)

Researchers at CERN’s Large Hadron Collider in Switzerland recently discovered a vanishingly rare particle that they believe was around the beginning of the universe. The particle – called an X-particle for now because no one knows exactly what it is – was produced by colliding billions of heavy ions inside the famous particle accelerator.

The team at CMS Collaboration, which collects data from the LHC’s Compact Muon Solenoid, smashed heavy lead atoms together at temperatures of about 5.5 trillion degrees Celsius (9.9 trillion degrees Fahrenheit). The team’s results are published and Physical Review Letters.

Physicists theorize that matter in the earliest moments of the universe after the Big Bang was a plasma made of subatomic quarks and gluons crammed together in an overheated soup. (Only when the plasma cooled down several microseconds after the Big Bang did the well-known protons and neutrons take shape, paving the way for much more massive forms of matter.) But before the material cooled down, some of these quarks and gluons collided to form more enigmatic particles. , which physicists call X particles.

X-particles are rare today because the universe is no longer as dense or as hot, but like Krishna Rajagopal, a particle physicist at MIT who is not affiliated with the latest research, said in 2010: “If you’re interested in the properties of the microsecond ancient universe, the best way to study it is not by building a telescope, it’s by building an accelerator.”

The team was able to identify 100 X particles of a specific mass, called X (3872), that survived for about a sixty-tenth of a second before decaying. X (3872) was first found in 2003 by Good cooperation via bump-hunting, which is when scientists identify an unexpected amount of mass or energy in their system.

“The X (3872) is a strange animal,” said Patrick Koppenburg, a physicist at the Dutch National Institute of Subatomic Physics and a member of the LHCb team at CERN, in an email to Gizmodo. “I was at Belle when it was discovered and I remember we stared at the little bump without understanding what was going on.”

A blurry image shows the cylindrical magnet used in the central part of the Large Hadron Collider.

The magnetic core of the Large Hadron Collider’s Compact Muon Solenoid magnet in March 2007.
Photo: FABRICE COFFRINI / AFP (Getty Images)

Last year, Koppenberg’s team at LHCb discovered a new species of tetraquark. Like the X (3872), the tetraquark had a fleeting lifespan – probably just over a quintillionth of a second. Although other exotic particles appear and disappear at the LHC, X (3872) is the first X particle detected in the quark gluon plasma generated there.

The team behind the new study was able to mimic the conditions of the early universe by accelerating 13 billion ions. When the particles collided, they produced thousands of short-lived charged particles. Yen-Jie Lee, en physicist at MIT and a co-author of the new research, told Gizmodo that it is possible that other X-particles may be in the latest data, but the researchers did not have a good way of selecting them based on background noise.

“The first heavy ion in ‘Run 3’ starts at the end of this year and we hope to be able to accumulate more data with the lead-lead collision runs in Run 3 and Run 4,” Lee wrote in an email. “With a much larger dataset, we will be able to determine the magnitude of the improvement in X production in the quark soup and gain more insight into its internal structure.”

The identity of X (3872) is still uncertain. The team believes that the particle could be a type of loosely bound mesonic molecule (two subatomic particles called mesons bound via the strong force) or a tetraquark, a type of hadron composed of four quarks glued together. “So far, mesonic molecules have not yet been definitively observed, and X (3872) is a good candidate,” said Jing Wang, a physicist at MIT who led the analysis of the new data., in an email to Gizmodo. “If X (3872) turns out to be a mesonic molecule, we show that in the early universe there must be different kinds of mesonic molecules in addition to the ordinary hadrons.”

“The more I look at the data, the more I am convinced that the X is a superposition of a molecule and a harmonium state,” said Koppenberg. Describes the idea of superposition further, he noted: “Our brain fails to represent these things. … There is no such thing as one or the other in quantum mechanics. If you can not distinguish between two things, then the truth must be both at the same time.”

Perhaps the coming runs of the LHC will finally resolve the identity of the X (3872). Of course it would has an actual name and will no longer be considered an X particle.

More: CERN physicists have discovered a batch of new exotic particles

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