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Particles and interactions

Particles and interactions

Dramatic differences found in matter and antimatter

03 Aug 2004 Belle Dumé

The international BaBar collaboration working at the Stanford Linear Accelerator Center (SLAC) in the US has found the most dramatic evidence to date for the difference between matter and antimatter. The experiment has found that the number of decays observed for a class of particles known as B mesons is 13% higher than for their antiparticle equivalents -- anti-B mesons ( The results could help explain why the universe is dominated by matter, rather than anti-matter, or why it does not contain equal amounts of both.

Cosmologists believe that equal amounts of matter and anti-matter were created in the big bang. If matter and anti-matter particles were exact opposites of each other, they should have annihilated to leave only photons. However, the existence of our matter-dominated universe suggests that matter and anti-matter underwent different processes after the big bang. To account for this excess of matter, the Standard Model of particle physics predicts that matter and anti-matter decay at slightly different rates. Known as charge-parity (CP) violation, this effect was first seen indirectly in kaons in 1964.

Three years ago, the BaBar team — and independently the Belle collaboration at the KEK laboratory in Japan — discovered indirect CP violation in another family of particles, known as B mesons, for the first time. BaBar produces large numbers of B and anti-B mesons by firing a beam of positrons into a beam of electrons. B mesons and their antiparticles are very short lived and decay quickly into other, lighter particles, such as kaons and pions. By measuring these decay rates extremely accurately, BaBar was able to show that B mesons decay slightly slower than their antiparticle equivalents.

Now, BaBar scientists have observed “direct” CP violation by analyzing the decays of more than 200 million pairs of B and anti-B mesons. Among the many ways that the mesons can decay, they were looking for rare events that turn B mesons into K+pi pairs and anti-B mesons into Kpi+ pairs. Theory suggests that the two events should have the same odds of happening — and hence that one should get equal numbers of pairs of K+pi and Kpi+ pairs. However, the BaBar scientists discovered that this was not the case — they found 910 K+pi– pairs but only 696 Kpi+ pairs. In other words, there was a big difference in the number of matter and antimatter decays.

Although direct CP violation has been observed before in experiments on kaons, the new observation with B mesons is much stronger. The new measurement is a result of the improved performance of SLAC’s PEP-II accelerator and the efficiency of the BaBar detector, which is now able to detect about 98% of collisions. “This observation is a significant step forward in assembling the pieces of the puzzle of matter versus antimatter in the universe,” says SLAC director Jonathan Dorfan.

“We have observed a clear, strong signal for asymmetrical behavior of matter and antimatter resulting from the direct CP violation mechanism,” adds James Olsen of Princeton University, one of the leaders of the analysis.

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