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Thursday, July 24, 2014

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Findings help explain why matter exists

Princeton physicists are among the leaders of two international research collaborations that are answering one of the most fundamental questions in the universe: Why does matter exist?

They are uncovering a rare but critical loss of symmetry in the behavior of subatomic particles. Physicists are counting on this asymmetry to explain why matter and antimatter particles did not cancel each other after they were created in the Big Bang.

One of the collaborations, based at the Stanford Linear Accelerator, announced July 6 that it had confirmed the asymmetric behavior in subatomic particles called B mesons. The other group, based at a particle accelerator in Japan, reported a similar result July 23.

They are the first new examples since the asymmetry, referred to as charge-parity, or CP, violation, was discovered in 1964 by Princeton physicist Val Fitch and James Cronin of the University of Chicago. That discovery, in a different particle called the K meson, earned Fitch and Cronin the 1980 Nobel Prize in physics.

"After 37 years of searching for further examples of CP violation, physicists now know that there are at least two kinds of subatomic particles that exhibit this puzzling phenomenon, thought to be responsible for the great preponderance of matter in the universe," said Princeton physicist Stewart Smith, who is spokesman for the Stanford collaboration.

"The two groups disagree somewhat on how large the violation is, but the difference can probably be explained as a statistical fluctuation," said Daniel Marlow, a Princeton professor of physics who is one of the leaders of the Japanese experiment, which is based at the KEK particle accelerator near Tokyo, Japan.

Shortly after the Fitch-Cronin discovery, Russian physicist Andrei Sakharov found that CP violation along with other recently discovered phenomena could explain the existence of matter in the universe, which started with equal numbers of matter and antimatter particles.

"Things in the universe tend to be symmetric," said Marlow. "So why is it that the universe consists mainly of matter?"

When a particle of matter and its antimatter counterpart meet, they annihilate each other, leaving nothing but light. Although every particle has an antimatter equivalent, the balance apparently shifted over time. "If it weren't the case we wouldn't exist," said Marlow. "The last person you want to meet is your antimatter counterpart."

The existence of CP violation is important to physicists not just because of its cosmological implications. The original discovery by Fitch and Cronin required physicists to completely rethink several fundamental concepts, including the mistaken conviction that all physical laws would apply equally well with time running forward or backward.

Today, CP violation is a key element in the currently accepted Standard Model of Particle Physics, which describes the interactions of nearly all particles and forces. Indeed, the equations of the Standard Model predict the existence of CP violation in B mesons. To have found otherwise would have required another fundamental rethinking of physical laws.

In searching for further examples, said Fitch, "There has always been this question, 'Is it going to be something that fits in with what happens in the K system or is it totally new?'"

The result that emerged from Stanford was very much in line with the predictions of the standard model, while the KEK result showed somewhat more CP violation than predicted. Fitch said it has been good to see his 1964 result incorporated into standard physics. "It was very satisfying for that to happen, to find that it was not an effect standing alone," he said.

Yet physicists actually have a touch of disappointment about the Stanford results. As it stands, the Standard Model does not have enough CP violation to account for the great preponderance of matter in the universe, Smith said. Physicists are looking for a result that contradicts their current understanding and points to new areas of research.

Both experiments still may produce such contradicting results. As the accelerators pour out more data, scientists will be able to conduct more extensive experiments that involve rarer phenomena. "The Standard Model passes this test, but it is not out of the woods," said Smith.

The CP violation discoveries are the result of giant research programs. Smith is the leader of a collaboration that includes 600 scientists and engineers from around the world. The project began in 1993 with a $200 million upgrade to the Stanford Linear Accelerator and the construction of a $100 million detector that could record the minute and subtle events. The KEK experiment, which started at about the same time, involves nearly 300 people from 45 institutions.

Technicians in Princeton's Elementary Particles shop contributed significantly to the building of the detectors for both experiments.

The experiments compare the radioactive decay of B mesons and their antimatter counterparts, which are referred to as B-bars. Mixing the two particle names and making a reference to a king elephant in children's books, the Stanford experiment has been dubbed Babar. The KEK experiment is called BELLE.

Physicists study B mesons by slamming two particles, an electron and a positron, together at tremendous speeds. The resulting B particles survive for only one or two trillionths of a second before they decay into other particles. But they are moving so fast -- half the speed of light -- that they move a measurable distance, about a tenth of a millimeter, during their short lives. The physicists measure the difference between how far a B and B-bar particles travel before they decay.

But it's not so simple. Only about one in a thousand B particles decay in such a way that it is easy to look for CP violation. The accelerators are tuned in a precise way so they produce many B and B-bar particles -- 32 million pairs in the last year-and-a-half in the case of Babar. The Babar results were based on an analysis of 600 events.

"The accelerators are the real heroes of this game," said Smith. "It's like a fire hose, there is so much data. It's a great problem to have."

Contact: Marilyn Marks (609) 258-3601

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