Particles seen emerging from empty space for first time
Particle collisions inside RHIC’s Solenoidal Tracker STAR detector, known as STAR
Brookhaven National Laboratory
A pair of rare particles produced in high-energy proton collisions could provide the clearest evidence yet that mass can emerge from empty space. This discovery could shed light on one of the biggest puzzles in physics: how particles acquire their mass.
According to quantum chromodynamics (QCD) – widely considered our best theory for describing the strong force that binds quarks inside protons and neutrons – even a perfect vacuum is not truly empty. Instead, it is filled with short-lived disturbances in the underlying energy of space that flicker and disappear, known as virtual particles. Among them are quark-antiquark pairs.
Under normal conditions, these ephemeral couples disappear almost as soon as they appear. But if enough energy is injected into the vacuum, QCD predicts that they can be transformed into real, detectable particles with measurable mass.
Now the STAR Collaboration – an international team of physicists working at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York – has observed this process for the first time.
The team smashed high-energy protons in a vacuum, producing a jet of particles. Some of these particles should be quark-antiquark pairs taken directly from the vacuum itself, but quarks can never exist alone and immediately combine into composite particles.
Fortunately for the team, these particular particles contain a clue to their origins. Quarks and antiquarks are born with their spins correlated – a shared quantum alignment inherited from the vacuum.
The researchers found that this connection persists even after quarks and antiquarks become part of larger particles called hyperons, which decay in less than a tenth of a billionth of a second. The detection of these spin-aligned hyperons following proton collisions allowed the researchers to confirm that the quarks they contain came from the vacuum.
“This is the first time we have seen the whole process,” says STAR collaboration member Zhoudunming Tu.
“I am very happy to see this measure,” says Daniel Boer of the University of Groningen in the Netherlands, who was not involved in the work. He says many mysteries remain about quarks, such as why they can’t exist on their own. “That’s what makes this experience particularly interesting.”
You believe this work opens a new avenue for directly examining the properties of a vacuum, allowing scientists to study how particles acquire mass. QCD theory predicts that quarks gain more weight by interacting with the vacuum itself, but how they do this is unclear.
Alessandro Bacchetta, from the University of Pavia in Italy, says the result is not yet definitive, because reconstructing events from particle collisions can be complex. Researchers must first comprehensively rule out other possibilities that could have led to the same signal, he says.
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