In an effort to explain the universe’s behavior fractions of a second after the Big Bang, physicists have invoked short-lived X particles – so named because almost nothing was known about their nature.

Although the conditions that created these particles are long gone from nature, efforts are underway to recreate them using particle accelerators. Producing an X particle from a somewhat similar environment should take us closer to understanding that early universe.

Thousands of physicists from almost 200 research institutions have announced the first detection of an X particle by colliding lead ions together, published in the journal Physical Review Letters. 

The particle X(3872) has been observed before, but conditions meant they decayed too quickly to learn much about them, which is particularly significant since they don’t fit the current quark model.

The paper describes evidence for the production of the X(3872) particle in quark-gluon plasma, a mix of exceptionally hot elementary particles thought to resemble the universe in the first millionths of a second of its existence. Although X-particles are short-lived even in these conditions, it is hoped they will last long enough for better study.

As is the case for much in particle physics, the X particles cannot be detected directly – but the paper claims the expected products from their decay were recorded using CERN’s Large Hadron Collider in 2018. It has taken years to analyze the 13 billion collisions created when ions of lead were smashed together close to the speed of light.

The collisions produce quark-gluon plasma that cools to form an abundance of particles, some of them exotic. X particles are among these, but have been almost impossible to spot among the other, far more numerous, creations.

The team built a machine-learning algorithm to find the daughter particles from X particle decay amid this abundance. "We managed to lower the background by orders of magnitude to see the signal," said Dr Jing Wang of MIT in a statement. The search revealed 100 X(3872) particles, equivalent to finding 100 needles in a truly giant haystack.

“This is just the start of the story,” said MIT’s Dr Yen-Jie Lee. “We’ve shown we can find a signal. In the next few years we want to use the quark-gluon plasma to probe the X particle’s internal structure, which could change our view of what kind of material the universe should produce.”

Quarks are subatomic particles that, in combination, make up most of the universe. The word quark is a reference to the fact they were originally thought to always come in threes, as it comes from the line “The quarks for Muster Mark!” in Finnegans Wake by James Joyce.

However, while the neutrons and protons that make us up are indeed composed of three quarks, subsequent work has revealed the existence of tetraquarks, particles made of four quarks. The authors hope to settle the question of whether X(3872) is a tetraquark or not.

The quest to find X(3872) particles brought together so many authors in the hope measurement of the speed of its decay will reveal how tightly bound X(3872) is. Existing theories allow for a radius ranging from roughly 0.3 femtometers (3*10^-16 meters) to 1.5 fm. Resolving the binding tightness, and therefore the size, for this and other exotic particles, is crucial to refining models of the universe’s early state.