Tuesday, April 8, 2014

Today, we know that everything from galaxies to atoms is made from matter called “quarks.” Quarks were first proposed in 1964, when physicists realized their understanding that all matter was composed of protons, neutrons and electrons was insufficient to explain the myriad new particles being discovered.

Quarks—known now to be the building blocks of protons and neutrons—have unusual properties that made it difficult for scientists to believe in them initially. Protons, which contain two quarks with an identical electric charge and one quark with a different electric charge, violate a principle that forbids two quarks to be in the same quantum state. To allow the three quarks to coexist and satisfy this principle, a property with three values was needed.

In 1964, a University of Maryland physicist, O.W. (Wally) Greenberg, proposed that quarks exhibit a property called “color,” coined from the idea that primary colors red, green and blue make white light. Color, which is unrelated to human perception of color, provides three distinct quantum states for quarks to exist in and explains the strong interactions of quarks. Quarks and color, which were experimentally verified in 1973, led to the standard model of particle physics that explains what the world is and what holds it together.

To honor the 50-year anniversaries of the discoveries of quarks and color, the University of Maryland will host a symposium on April 11-12, 2014. The sold-out event, called “50 Years of Quarks & Color,” is expected to draw 90 attendees.

In addition to Nobel laureates François Englert and Frank Wilczek, featured speakers include Greenberg, who proposed that quarks have “color” charges; George Zweig, who proposed the existence of quarks; and Robbert Dijkgraff, director of the Institute for Advanced Study. More than a dozen other history-making physicists will speak at the event.

“With the discovery of quarks and color, our view of the fundamental particles of nature changed. Fifty years ago, we thought protons and neutrons were the most basic particles; today, point-like quarks are the most fundamental particles ever seen even with the most advanced high-energy accelerators available,” said Greenberg, who is a fellow of the American Physical Society.

Englert, of the Université Libre de Bruxelles in Belgium, received the Nobel Prize in Physics in 2013 for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles. His work was recently confirmed through the recent discovery of the predicted fundamental particle called the Higgs boson. Wilczek, of the Massachusetts Institute of Technology, received the Nobel Prize in Physics in 2004 for the discovery that the force between quarks remains strong at long distances and becomes weak at short distances, in contrast to the electric force between charged particles that behaves conversely. This phenomenon in the theory of the strong interaction is called asymptotic freedom

Greenberg has been a full-time faculty member of the University of Maryland’s Department of Physics for 53 years. In the fall of 1964, while a visiting scholar at the Institute for Advanced Study in Princeton, New Jersey, Greenberg published a seminal paper proposing color charge for quarks in the journal Physical Review Letters. Greenberg’s color charge is part of the standard model of particle physics.

In his paper, Greenberg proposed that “color” charge comes in three varieties—metaphorically labeled red, green and blue for the primary colors—and each quark comprising a particle must have a different color. Just as a mix of red, green, and blue light yields white light, a combination of red, green, and blue color charges yields a color-neutral proton or neutron. Quarks change their color when they exchange particles with other quarks. The force between color-charged particles is very strong, but the force only takes place on the really small level of quark interactions, which is why you are not aware of the strong force in your everyday life.

Almost 10 years passed after quarks and color were proposed before researchers determined that quarks were real particles that carried a color charge. The challenge then and still today, according to Greenberg, is that quarks are permanently trapped inside other particles like protons and neutrons, so you can’t bring them out individually to study them.

“Basic science research led to the discovery of quarks and color. This type of research is extremely valuable, and continued basic research in particle physics will ultimately lead to a deeper understanding of the universe, and if history is our guide, also to practical applications” added Greenberg.

--University of Maryland/College of Computer, Mathematical, and Natural Sciences--

50 Years of Quarks and Color

Media Advisory

Reporters are invited to attend. The registration fee is waived for accredited media, who are asked to contact Abby Robinson at abbyr@umd.edu or 301-405-5845 to register in advance.