Every branch of science has its pinnacle of achievement, the thing that every scientist in that field dreams of achieving. For an astronomer, it’s the discovery of a new heavenly body; for a paleontologist, a new species of dinosaur. And for a physicist, it’s the discovery of a new subatomic particle.
University of Saskatchewan particle physicist Chary Rangacharyulu has reached that pinnacle, as part of an international team that recently discovered the first known “pentaquark,” a new sub-atomic particle that could change our understanding of physics and the very early universe.
The search for new sub-atomic particles is a continuation of one of science’s grand quests. The Greek philosopher Democritus first suggested about 400 B.C. that all matter is made of minute particles, indivisible and indestructible, which he called atoms.
For well over two millennia, that’s pretty much where the theory stood. Atoms were considered to be the smallest possible particles, rather like tiny unbreakable billiard balls. But the discovery of the electron in the late 19th century changed that view. It changed further with the discovery of the proton and the neutron over the first three decades of the 20th century. Atoms, far from being indivisible, consisted mostly of empty space, with tiny negatively charged electrons whirling around a larger, very dense nucleus of positively charged protons and un-charged neutrons.
Many of us learned something like that in school–but in the 1960s, new particle accelerators revealed more than 80 new kinds of particles. Clearly a new theory was needed. Two Caltech theorists, Murray Gell-Mann and George Zweig, independently suggested that all the new particles being discovered were actually just different arrangements of a small number of truly elementary particles. Zweig wanted to call these new elementary particles “aces,” but Gell-Mann elected to call them “quarks,” from a line in James Joyce’s novel Finnegan’s Wake: “Three quarks for Muster Mark!” “Quarks” stuck.
There are six “flavours” of quarks: “up,” “down,” “strange,” “charm,” “top,” and “bottom,” each of which has a corresponding anti-quark. (Why “flavours”? Well, by coincidence, quark is also the German word for “cottage cheese”…) Stable particles like protons and neutrons, collectively called “baryons,” are made up of three quarks (two ups and a down, and two downs and an up, respectively). Another family of particles, called “mesons,” consist of one quark and one anti-quark, but they’re short-lived and unstable, vanishing in a fraction of a second.
The theory that describes interactions among quarks is called “Quantum Chromodynamics.” According to it, while some combinations of quarks are impossible, there’s no reason you couldn’t have a particle consisting of five quarks. Physicists have therefore been looking for just such a particle for 30 years, without any luck–until now.
Russian theorists Maxim Polyakov, Dmitri Diakonov and Victor Petrov predicted in 1997 that a pentaquark containing two up quarks, two down quarks and an anti-strange quark should be about 1.5 times as heavy as a proton. Their paper met a lot of skepticism at the time.
But at the urging of the Russian theorists, in 2002 the team that Rangacharyulu is on, led by Takashi Nakano of Osaka University, took another look at results from a 2001 experiment in which they fired high-energy gamma rays at carbon atoms, using a synchrotron located at Japan’s SPring-8 physics lab. They discovered evidence of around 20 particles that fit the Russian parameters for pentaquarks. Each lasted for only a miniscule fraction of a second, then decayed into a meson and a neutron.
Since then, other experimenters around the world have discovered evidence of pentaquarks in their own data from previous experiments, and new experiments have been carried out specifically designed to create these new particles, which have been dubbed “theta-plus.”
Why should anyone care? Because particle physics deals with the fundamental stuff of the universe: 99.9 percent of the mass of everyday objects–including you and me–is made up of quarks. The more we understand about how quarks interact, the more we learn about the basic physical laws that govern the universe. The discovery of theta-plus may not have any “practical” application–but whether it does or not, we have added to our basic understanding of the world around us. That alone is reason enough to be excited.
Professor Rangacharyulu says work in this area has just begun. “There are other combinations of pentaquarks to be found,” he says. And he notes that future work in this field could take place in Saskatoon, where one of the world’s most powerful synchrotrons, the Canadian Light Source, will begin operation next year.
That’s something else to be excited about!
