A Local Perspective on Life in Tsukuba, Ibaraki, Japan.

Research in Tsukuba

The Case of the Disappearing Mass

According to theory, some particles have a different mass in empty space than they do in an atomic nucleus. In the 19 January Physical Review Letters, researchers describe evidence for such an effect in a new particle–one made of “strange” quarks, rather than the ordinary quarks found in protons and neutrons. The findings may help guide theorists trying to understand how the masses of larger particles arise from their internal quarks and force fields


So far the effect has shown up in particles containing up and down quarks, including protons, neutrons, and two exotic mesons called the rho and the omega. But the results have been controversial because they conflict with some other experiments that suggested no mass reduction at all. Now researchers at KEK, the High Energy Accelerator Research Organization in Tsukuba, Japan, led by Hideto En’yo of RIKEN, a Japanese research institute, hope they have swayed some critics by observing the effect in the phi meson, which is made of a strange and an anti-strange quark. The mass reduction is less dramatic for these heavier quarks, but by collecting data from more collisions, and making more precise measurements of the products flying out of these collisions, the team found the 3.4 percent reduction in mass predicted by theory.

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