Decades-long quest reveals new details about antimatter


It goes often not to mention that protons, the positively charged particles of matter at the centers of atoms, are part of antimatter.

We learn in school that a proton is a beam of three elementary particles called quarks – two “up” quarks and one “down” quark, whose electric charges (+2/3 and −1/3, respectively) are combine to give the proton its charge of +1. But this simplistic picture glosses over a much stranger and unresolved story.

In reality, the interior of the proton swirls with varying numbers of six types of quarks, their oppositely charged antimatter counterparts (antiquarks) and “gluon” particles that bind others together, transform into them, and multiply easily. . Somehow, the maelstrom vortex ends up perfectly stable and superficially simple – mimicking, in some ways, a trio of quarks. “How it all works is frankly something of a miracle,” said Donald Geesaman, nuclear physicist at Argonne National Laboratory in Illinois.

Thirty years ago, researchers discovered a striking feature of this “sea of ​​protons”. Theorists expected it to contain a uniform distribution of different types of antimatter; instead, antiquarks seem to outnumber antiquarks significantly. Then, a decade later, another group saw signs of puzzling variations in the antiquark’s down-to-up report. But the results were at the limit of the sensitivity of the experiment.

So, 20 years ago, Geesaman and a colleague, Paul Reimer, embarked on a new experiment to investigate. This experiment, called SeaQuest, is finally over, and researchers report their findings in the newspaper Nature. They measured the proton’s internal antimatter in greater detail than ever before, finding that there are, on average, 1.4 antiquarks down for every antiquark in place.

Illustration: Samuel Velasco / Quanta Magazine

The data immediately supports two theoretical models of the sea of ​​protons. “This is the first real evidence to support these models that have come out,” Reimer said.

The first is the “pion cloud” model, a decades-old popular approach that emphasizes the proton’s tendency to emit and reabsorb particles called pions, which belong to a group of particles called mesons. The other model, the so-called statistical model, treats the proton as a container filled with gas.

Future planned experiments will help researchers choose between the two images. But regardless of the model that fits, SeaQuest’s hard data on the proton’s internal antimatter will be immediately useful, especially for physicists who smash protons together at near the speed of light in the Large Collider hadrons from Europe. When they know exactly what’s in the colliding objects, they can better navigate through collision debris looking for evidence of new particles or effects. Juan Rojo of the VU University of Amsterdam, which helps analyze the LHC data, said the SeaQuest measurement “could have a big impact” on the search for new physics, which is currently “limited by our knowledge. the structure of the proton, in particular its antimatter content. . “

Company of Three

For a brief period about half a century ago, physicists thought they had sorted the proton.

In 1964, Murray Gell-Mann and George Zweig independently proposed what has been called the quark model – the idea that protons, neutrons, and related rarer particles are bundles of three quarks (like Gell- Mann doubled them), while pions and other mesons consist of a quark and an antiquark. The diagram made sense of the cacophony of particles pulverized from high-energy particle accelerators, as their charge spectrum could all be constructed from two- and three-part combos. Then, around 1970, researchers at Stanford’s SLAC accelerator seemed triumphantly confirm the quark model when they shot electrons at high speed at protons and saw the electrons ricochet off the objects inside.



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