IIT physicists join the international race to find the ingredients of our existence.
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.
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.
A New Standard?
IIT physicists have undertaken several grand projects, hoping to shed new light on the mysteries of CP asymmetryin the subatomic realm. At Fermilab, the HyperCP experiment brought the IIT physics team together with scientists from nine other institutions. Their study involves hyperons, short-lived particles built out of three subunits known as quarks.
“Today, we know that everything is made of quarks and leptons,” Kaplan explains. “And there isn’t just one kind of lepton, we’ve discovered six kinds of leptons and six kinds of quarks.” These 12 tiny pieces and the four forces of nature that act on them are a blueprint for designing reality.
Elegant in its simplicity, the Standard Model has been a triumph for science, consistent with every observation ever made regarding subatomic particles. For all this, however, the Standard Model cannot be a complete description.
One of the key shortcomings of the model is that it fails to explain how all the little building blocks—the quarks and leptons—acquire their mass. An elusive particle known as the Higgs Boson (sometimes dubbed “The God Particle”) is believed to fill this gap.
The trouble is, the Higgs is so elusive that no one has seen one yet. It is now one of the most coveted prizes in any area of physics, and new experiments at both CERN’s LHC and Fermilab’s Tevatron Collider appear within striking distance of the momentous discovery. If the Higgs is there, we should know fairly soon. The discovery could provide the final jewel in the crown of the Standard Model, helping to answer many particle/antiparticle quandaries, including the mysteries of CP violation.
In Fermilab’s HyperCP experiment, the IIT team helped design and build one of the highest-rate spectrometers in the world, capable of detecting 100,000 particle events per second. The device allowed for the collection of a staggering amount of data—120 terabytes worth (or roughly 25 times the information on all the websites on the entire Internet).
One intriguing though yet-mysterious result of HyperCP involved a rare decay sequence of the so-called Sigma-plus (∑+) hyperon. An intermediary particle may have been involved in this peculiar occurrence, which was observed on just three fleeting occasions. If the sequence was more than a statistical fluke, there is a chance the HyperCP group picked up the scent of the reclusive Higgs. Unfortunately, false sightings abound in this tricky arena. Physicists contend it’s just too soon to know.
And so the search for the origins of matter domination continue. While substantial matter/antimatter asymmetry so far remains absent in the quark sector, perhaps it will appear among lighter particles—the leptons, specifically, in the enigmatic interactions of neutrinos.
One of the most intriguing topics in high-energy physics these days, neutrinos were once thought to be lackluster particles of zero mass. They didn’t seem to do a whole lot or to interact much with their lively particle neighbors. But recent insights have dragged this tiny entity into the limelight.
The sun produces neutrinos in fantastic abundance—some 100 billion or so pass effortlessly through your hand each second as you read this magazine. We now know there are actually three types, or flavors, of neutrino: the electron neutrino, tau neutrino, and muon neutrino. Further, these flavors have an odd tendency to change form—one into the other—in a process dubbed oscillation.
Today, two ambitious neutrino projects are luring IIT physicists to Europe and Asia. Kaplan and Rubin are collaborating on a project in eastern France known as Double Chooz, while Chris White—an IIT neutrino authority and veteran of the HyperCP team—is investigating neutrinos at a Chinese facility known as Daya Bay. Rather than using accelerators to generate the particles to be studied, both Double Chooz and Daya Bay will examine oscillation properties among antineutrinos streaming from the cores of nuclear reactors.
The three neutrino flavors exist together, in a mixture of states, as White explains: “One would imagine that if you have an electron-type neutrino that it would have a well-defined mass. It turns out that’s not the case. Instead of this subatomic particle being one thing, it’s really three things at the same time. It’s just one of those wonderfully funny properties of quantum mechanics that allows this to actually be true.”
Both Double Chooz and Daya Bay hope to measure the degree of neutrino oscillation between the electron- and tau-type neutrinos with high precision—a first step before evaluations of CP asymmetry can be carried out.
Prospects of fresh insight into the neutrino’s peculiar properties as well as early intimations of physics beyond the Standard Model have energized the physics community. IIT scientists are on the leading edge of this research, and the race is now on between Double Chooz’s rapid start-up capability and economic efficiency, and Daya Bay’s superior thermal output (which produces more neutrinos to study).
Rubin speaks enthusiastically of the French project, pointing out that Double Chooz expects to provide a first measurement by 2009. Daya Bay promises to nail down the value with still greater precision, hopefully close enough to determine if a CP violating component can exist.
Daya Bay also will be something of a political milestone—the largest basic science collaboration between the United States and China in history. The project’s principal U.S. investigator, Kam-Biu Luk, savors Daya Bay’s potential: “This is very exciting and important, and I would love to be part of the team that finds out why we are here,” he recently told Symmetry Magazine. Concerning the elusive riddle of antimatter’s disappearing act, Luk declares, “It’s a good thing, because I don’t have to worry about shaking hands with a friend and being annihilated,” adding, “It’s the reason that everyone and everything exists.”
Back to the Future
Studies of CP violation continue to occupy inquisitive minds and powerful machines. Soon, the Large Hadron Collider comes online, and further down the road a still more awesome device, the International Linear Collider. IIT researchers are already assisting in the planning stages of this 35 kilometer goliath, as new research inches ever closer to a final account of the matter/antimatter enigma.
The coming decade in physics is shaping up to be one of tremendous achievement. A few of the field’s most intricate puzzle pieces may soon be moved into place. With an eye toward the future of experimentation, Kaplan considers the road ahead: “Opinions and hunches are fine, but experiments are the only way to be sure. Nature may have new surprises in store.”
Richard Harth is a writer based in New Orleans.