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Apr. 6, 2007 Research Highlight Physics / Astronomy

Subatomic particles lighten up

Fundamental particles lose mass in the presence of dense atomic nuclei

Image showing the inside of spectrometer Figure 1: A view inside the spectrometer to detect the decay products of vector mesons. The centre of the image shows the small copper target where the φ mesons are produced by incident protons. The structures around the target are the detectors for the meson decay products.

Some of the most fundamental particles in nature are protons and neutrons. These particles themselves are composed of elementary particles, the quarks. Three quarks make up each proton and neutron. Another class of particles, the mesons, consists of only two quarks. Different types of mesons are classified according to their constituent quarks. The force that holds protons, neutrons and mesons together is the ‘strong interaction’ between the quarks, which essentially determines the mass of particles.

Researchers at the RIKEN Nishina Center for Accelerator-Based Science in Wako, in collaboration with other Japanese researchers, have now studied the change in mass when a φ meson travels through an atom’s nucleus. The nucleus, full of protons and neutrons, is a place where the forces of the strong interaction are particularly intense. As the strong interaction controls the meson mass, it is expected to change in an atomic nucleus compared to its value in vacuum.

Indeed, such a change has previously been observed for two other meson types, ρ and ω. However, according to Ryotaro Muto from the RIKEN team, “Changes in ρ and φ mesons are very difficult to be distinguished from each other in experiments, whereas the φ meson shows as a narrow resonance that can be examined more clearly”.

Writing in the journal Physical Review Letters 1, the researchers report on changes in φ meson mass they detected in experiments conducted at the 12-GeV Proton-Synchrotron of the KEK-PS facility in Tsukuba. In the experiment, protons collide with nucleons and generate φ mesons. Then, to determine the meson mass, the researchers analyzed the energy released by the meson’s radioactive decay. This mass varies according to where the decay takes place: in vacuum or in a large target nucleus such as copper (Fig. 1). The team found that the mass of the φ meson is actually lighter in the copper nucleus.

What is needed now, explains Muto, is “to obtain a clearer picture on the origin of the meson mass and the role of the strong interaction”. This requires further experiments using heavy ion collisions instead of protons. The team already has plans to carry out such experiments in facilities such as the Japan Proton Accelerator Research Complex (J-PARC). These studies could provide important details to one of the most fundamental questions in nature—the origins of the mass of fundamental particles.

References

  • 1. Muto, R., Chiba, J., En’yo, H., Fukao, Y., Funahashi, H., Hamagaki, H., Ieiri, M., Ishino, M., Kanda, H., Kitaguchi, M. et al. Evidence for in-medium modification of the ϕ meson at normal nuclear density. Physical Review Letters 98, 042501 (2007). doi: 10.1103/PhysRevLett.98.042501

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