Volume 3, Issue 23 October 2006
Revealing the Invisible of the Universe
Theoretical physicist Hitoshi Murayama at work. (From "Discovering the Quantum Universe: The Role of Particle Colliders," 2006. DOE/NSF High Energy Physics Advisory Panel.)
The questions "Why are we here?" and "Why do we exist?" may sound purely philosophical in nature. Scientists, however, believe answers to these questions lie in the sub-atomic world inhabited by quarks and neutrinos. These fundamental particles were created when the universe was formed and, to theoretical physicists like UC Berkeley's Hitoshi Murayama, they represent clues to life's ultimate mysteries.
"Once you know how these tiny particles work, you can answer how the universe came about," says Hitoshi Murayama, professor in the Department of Physics. According to the laws of physics, our existence would not be possible without some process that was able to create and exploit a minute imbalance in energy. "That little bit of energy is why we are here," says Murayama, who is also a member of the Berkeley Center for Theoretical Physics (BCTP).
Murayama and others believe neutrinos may hold the key to this process and that they played an important role in the evolution of the universe. Neutrinos are produced by radioactive decay, including that generated by nuclear power plants, the sun and exploding stars. Trillions of these invisible, electrically neutral particles pass through the human body every second. For many years, scientists believed neutrinos were weightless entities moving at the speed of light.
In 1998, scientists discovered that neutrinos had a small, but finite mass. The discovery overturned an orderly and elegant view of the universe called the Standard Model, which had held up to more than 30 years' worth of scientific challenge. Now, Murayama and his colleagues have taken on the task of re-thinking the Standard Model in light of the discovery of neutrino mass.
As a theoretical physicist, Murayama works to construct a model that would explain experimental observations of the physical world. These models help make sense of previous research and serve to guide future experiments. The importance of this theoretical work is highlighted by the history of the discovery of neutrinos. Nobel Laureate Wolfgang Pauli first hypothesized their existence in 1930. Scientists did not confirm his theory until 1956. Today, neutrinos are the focus of intense, worldwide scientific scrutiny.
Murayama, for example, is part of an international team of scientists conducting experiments aimed at revealing the fundamental properties of neutrinos. Because they are nearly weightless and electrically neutral, they often pass through the earth without interacting with any atoms. This makes observing them incredibly challenging. "However, once in a while, neutrinos leave traces of themselves behind," Murayama explains.
The history of the universe as seen by particle physicists, beginning with the Big Bang and showing various particles and their relative distribution through to present time. (Courtesy of the Particle Data Group, Lawrence Berkeley National Lab)
In order to observe and capture data on these rare sub-atomic interactions, a multi-national consortium built a neutrino observatory, called KamLAND, beneath an abandoned zinc mine near Kamioka, Japan. (KamLAND stands for Kamioka Liquid-Scintillator Anti-Neutrino Detector.) The observatory consists of a sphere 42 feet in diameter filled with mineral oil located 1 kilometer below the earth.
A flash of light is produced when neutrinos collide with hydrogen protons within the oil. Scientists record these interactions for later study. Murayama travels to Japan once a year to take a shift monitoring the activity within the sphere. The experiment is painstaking work, he says. About only one neutrino-proton collision is recorded per day.
While he is studying the role of neutrinos in the sub-atomic world, Murayama is also studying their place in the universe. Scientists believe there are five thousand neutrinos in every cubic inch of the universe, billion times more than ordinary atoms. "With so many of them, they must have played important role in the birth and evolution of the Universe," Murayama says. Neutrinos are an important part of the "dark side" of the universe, namely 95 percent of the energy of the universe that is not atoms and cannot be seen in telescopes.
Today, Murayama and others continue to design and test models that attempt to encompass all scientific observations of the physical world, the so-called Unified Theory. Murayama is among those eagerly awaiting data generated from experiments to be done at two international facilities, the Large Hadron Collider (LHC), nearing completion and the International Linear Collider (ILC), in planning. LHC and ILC experiments aim to produce dark matter for the first time in the lab, as well as conduct the most detailed studies of the sub-atomic world to date. The data promise to give scientists what they need to finally come up with a Unified Theory that stands the test of time. "We are literally on the verge of better understanding of our place in the universe," Murayama says.