A joint Fermilab/SLAC publication

Fermilab and symmetry breaking

10/08/08

Yesterday we reported on the 2008 Nobel Prize in Physics. Here is an article that connects Tuesday’s Nobel Prize with the work done at the Department of Energy's Fermi National Accelerator Laboratory in Batavia, Ill. See this story for information about SLAC's connection to the Nobel Prize.

Fermilab and symmetry breaking

Yoichiro Nambu, Makoto Kobayashi and Toshihide Maskawa won the 2008 Nobel Prize in Physics for their work on symmetry breaking in the world of elementary particles and forces. The prize recognizes the pioneering development of a picture of nature that has had a major impact on physics at Fermilab and at other laboratories around the world. Nambu’s formulation of symmetry breaking allows physicists to explain why there is matter in the universe, while the work of Kobayashi and Maskawa provides the theoretical tools to explain why the universe contains no antimatter.

When physicists discuss symmetries, they refer to things that appear identical. Symmetry breaking is a way of explaining why things look different from each other. An example is gravity. Skaters have no problem gliding in any direction on an ice rink. But if they jump up, gravity pulls them back down. Gravity breaks the symmetry between left and right motion and up and down motion. Every time you jump up, you rediscover gravity through symmetry breaking.

Yoichiro Nambu incorporated what is known as spontaneous symmetry breaking into the theory of elementary particles to explain why different particles have different masses--why nature allows massive quarks and electrons and massless photons. When Weinberg, Glashow, and Salam earned the Nobel Prize in 1979 for unifying the electromagnetic and weak forces into the electroweak force, they needed to explain why the force carrier of the electromagnetic force, the photon, was massless, while the force carriers of the weak force were very heavy. The concept of spontaneous symmetry breaking allowed them to overcome this difficulty. Without this broken symmetry, matter as we know it would not exist. All particles would be massless and moving at the speed of light.

From the combination of  spontaneous symmetry breaking and electroweak unification comes an exciting prediction: a new particle called the Higgs boson. The Higgs is the hammer that breaks the symmetry and gives different particles different masses.  Currently, close to a thousand physicists from around the world are searching for the Higgs boson in collisions produced by the Tevatron accelerator at Fermilab.

Makoto Kobayashi and Toshihide Maskawa formulated a mechanism to explain symmetry breaking between matter and antimatter, typically called CP violation. In 1963, Nicola Cabibbo made a modification to Enrico Fermi’s theory of the weak interactions to explain the observed decay rates of heavy quarks into light quarks. Cabibbo showed that the strength of the weak force differs for different quarks. Kobayashi and Maskawa expanded upon these ideas and made further modifications to allow for a difference in the interactions between quarks and those between antiquarks. The combined picture, the Cabbibo-Kobayashi-Maskawa matrix, describes all the interactions of quarks.

The CKM picture of the universe allows for the difference in the behavior of matter and antimatter that allowed matter to survive after the Big Bang, while antimatter disappeared from the universe. It also tells us that there must be more sources of CP violation beyond those already discovered. Recent results from Tevatron experiments at Fermilab may point to new sources of symmetry breaking in the interactions of Bs mesons.

Besides helping to explain the absence of antimatter, Kobayashi and Maskawa’s theories make many other predictions that experimental discoveries at Fermilab have shown to be true. The KM mechanism predicted the existence of a third generation of elementary particles. Of the particles in this third generation, 75 percent have been discovered at Fermilab: the bottom quark, the top quark, and the tau neutrino.

In 1964, Cronin and Fitch discovered CP violation in K mesons. The KM mechanism, formulated to explain Cronin and Fitch’s discovery, also predicted still further differences in the way matter and antimatter relate to each other. This led to the discovery of a new type of CP violation in kaons at Fermilab and to the discovery of matter-antimatter oscillations in Bs mesons at the Tevatron. Another prediction pointed to a new way to produce heavy particles, a mechanism also discovered at Tevatron experiments through the observation of single top quarks.

As in the case of matter generation, physicists expect to find the Higgs boson at the heart of matter–antimatter asymmetry. With a lot of work and a little luck, the Higgs boson may be the next great discovery at Fermilab to grow out of the work of these latest Nobel laureates and many other great physicists of our times.

--Brendan Casey, Joe Lykken, and Judy Jackson

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