Physicists find major clue to matter’s biggest mystery

Researchers have nailed down an elusive quantity called baryon number, which may be responsible for the cosmic mismatch between matter and antimatter

Why is there something rather than nothing? Philosopher Martin Heidegger called this “the first of all questions,” and it has vexed scholars and theologians alike throughout history.

Science has yet to find an answer, either—but researchers have narrowed it down. What’s clear is that the big bang churned out infinitesimally more baryonic matter —the protons and neutrons that glom together as atomic nuclei—than it did antimatter. This is strange because matter and antimatter annihilate each other when they interact—and today whenever physicists turn energy into particles, antiparticle doppelgangers emerge in equal numbers. So, all things being equal, matter and antimatter in the hot, dense primordial universe should have reacted together to simply poof out of existence. Total annihilation would be the norm—and we shouldn’t be here. Somehow, though, this process instead left behind a miniscule excess of matter, forged in the first split second of time, which became the source of essentially everything we’re made of and all that we can see.

Physicists have now found a new clue about the source of this mismatch by studying quarks and gluons—the building blocks of baryonic matter. Nature, it seems, is set askew not through either quarks or gluons alone but via their intermingling. The research, which appears today in Science , used the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, N.Y. (RHIC was permanently shuttered earlier this year to make way for a new, better particle collider at Brookhaven.)

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“This is one of the most significant results achieved by the RHIC program,” says Dmitri Kharzeev, a physicist at Stony Brook University, who was not involved with the new paper but is mentioned in its acknowledgments. “It reshapes our understanding of baryon structure and how baryonic matter emerged.”

Physicists create baryonic matter all the time by smashing subatomic particles together with all the power their equipment can muster. And every time they do, the same corresponding amount of antimatter pops out, too. The net “baryon number”—the total number of baryonic particles (like protons) they create minus the number of antiparticles—is always zero.

If the early universe obeyed this rule, our modern cosmos would be devoid of substance, just energy fields rippling eternally through emptiness. But try as they might, physicists can’t seem to break the symmetry. “No experiment has ever observed a violation of it,” says Prithwish Tribedy, a physicist at Brookhaven National Laboratory, who worked on the new result.

When two protons collide, the baryon number (particles minus antiparticles) is two. The protons explode in a blaze of new particles rapidly created and destroyed, but the difference remains two until the end.