How did matter survive the winner-take-all confrontation with antimatter? Physicists at IIT are exploring this question, and the results may open a new chapter in physics.

By now, the notion of antimatter has become so popularly enshrined, it could as easily show up in a children’s game as in a physics lecture. “I’m matter, you’re antimatter,” one playmate might declare, and we all know the catastrophic results should the pair tempt fate and touch each other.

Theory suggests that nature produced exactly equal amounts of matter and antimatter in the first turbulent microseconds of creation. Following the inevitable annihilations just after the Big Bang, there should have been nothing left—no matter, no antimatter.

But here we are.

The quest for a solution to the puzzle has lured IIT high energy physicists to study a perplexing yet foundational issue known as CP violation. In addition to refining our knowledge of particle behavior, such research may help to explain nature’s preference for matter—a subtle favoritism essential for the universe we inhabit.

The IIT Department of Physics benefits not only from an outstanding faculty, but from the school’s proximity to Fermilab’s Tevatron Collider, one of the most powerful instruments for investigating nature on the tiniest scale, located in Batavia, Ill.

In 1975, Daniel Kaplan, then an eager graduate student, joined the team of Leon Lederman, who directed Fermilab’s  momentous investigations leading to the discovery of the bottom quark. (In 1988, Lederman won the Nobel prize for earlier work on neutrinos.) More recently, Kaplan and Lederman teamed up with Ray Burnstein and Howard Rubin—all now at IIT—forming a strong quartet to collaborate on other experiments, several bearing critically on the behavior of antimatter. 

Cosmic Origins
Like many scientific curiosities—black holes, relativity, or the existence of genes—antimatter was hypothesized before it was actually observed. In 1928, the physicist Paul Dirac attempted to reconcile two cornerstones of twentieth century physics: special relativity and quantum theory. His mathematical result implied the existence of an elusive mirror-reality, where weird companions to the familiar particles of matter could be found. These antiparticles were believed to have similar properties (like mass and spin) to their matter mates, but would carry opposite charges and other characteristics.

At the time of Dirac’s insight, no one had yet seen an antiparticle, but all that changed in 1932. Although accelerators had yet to be invented, physicists were able to study cosmic rays—high-energy particles streaming toward earth from space. These observations led C. D. Anderson to discover the electron’s antiparticle (now known as the positron). Other antiparticles also began to emerge. Antimatter—no longer restricted to the realm of theory—became a fact of life.

Today, most antimatter is confined to the pages of sci-fi novels or the tunnels of powerful accelerators. To all appearance, our universe seems to have been emptied of the stuff. But it wasn’t always so. Antimatter enjoyed a brief, violent reign at the very beginning of time. The civil war of particles and antiparticles liberated in the Big Bang should have left a condition almost unworthy of the term universe—a structureless (and surely, lifeless) ocean of radiation, with everything else falling victim to mutual annihilation.

Instead, it seems, something very different took place. For every billion antiparticles, a billion and one particles of matter were produced during the period of so-called baryogenesis. The trifling excess of matter paved the way for a cosmos hospitable to both stars and starfish.

Nature’s curious irregularity, however, was deeply unsettling to physicists, long convinced that antimatter behavior was indistinguishable from the behavior of normal matter and that nature on the tiniest levels operated in a strictly even-handed manner.

What’s the Matter with Antimatter? 
Physicists speak of three fundamental symmetries in the particle world. These are known as Charge (C), Parity ( P), and Time (T). Charge symmetry implies that if a particle is changed into its antiparticle (a proton into an antiproton, let’s say) its behavior should be identical. P, or parity symmetry, assumes that left and right could be interchanged—the world reflected in a mirror will be indistinguishable from ours. Time symmetry (T) demands that the direction of time be reversible.

So C, P, and T, these fundamental aspects of matter, ought to retain their pleasing symmetry. Kaplan asks, “If you happened to live in an antiworld, how would you know?” The answer is, you wouldn’t. At least, this was the long-cherished assumption.

But is the world/anti-world symmetry truly perfect to the last detail? Nature, it turns out, has a mischievous side.

Parity symmetry was the first sacred cow to be slain, when in 1956–57 C. N. Yang and T. D. Lee proposed (and Chien-Shiung Wu experimentally proved) it was occasionally violated. The combination of Charge and Parity (or CP), however, was still assumed to be a fundamental, inviolable symmetry in nature.

The Mirror Shatters
Today, we know CP symmetry is also sometimes broken. The news came in 1964, with the experiments of James Cronin and Val Fitch, who were able to demonstrate slight CP asymmetry in a particular class of particles known as kaons. The verdict caused both consternation and intrigue in the physics world, but the broad implication was clear: the worlds of matter and antimatter are not symmetric.

The most exciting consequence of CP violation is that it offers the first solid clue to the puzzling dominance of matter in the cosmos. This tantalizing possibility accounts for the tremendous interest CP asymmetry has generated among scientists working at opposite extremes in terms of scale.

In one domain, particle physicists investigate the most minute constituents of matter over infinitesimally short time frames. In another domain, cosmologists preoccupied with the origins of the universe explore the consequences of CP violation, pondering immense expanses of time and space.