Press Release


Researchers at Osaka University participate in a particle accelerator experiment that creates an exotic, highly unstable particle and measures its mass, which may help explain the interior of ultra-dense neutron stars

Astral Alchemy

  The Standard Model of particle physics tells us that most particles we observe are made up of combinations of just six types of fundamental entities called quarks. However, there are still many mysteries, one of which is an exotic, but very short-lived, Lambda resonance known as ∧(1405) . For a long time, it was thought to be a particular excited state of three quarks-up, down, and strange-and understanding its internal structure may help us learn more about the extremely dense matter that exists in neutron stars.

  Now, investigators from Osaka University were part of a team that succeeded in synthesizing ∧(1405) for the first time by combining a K- meson and a proton and determining its complex mass (mass and width) . The K- meson is a negatively charged particle containing a strange quark and an up antiquark. The much more familiar proton that makes up the matter that we are used to has two up quarks and a down quark. The researchers showed that ∧(1405) is best thought of as a temporary bound state of the K- meson and the proton, as opposed to a three-quark excited state.

  In a study published recently in Physics Letters B, the group describe the experiment they carried out at the J-PARC accelerator. K- mesons were shot at a deuterium target, each of which had one proton and one neutron. In a successful reaction, a K- meson kicked out the neutron, and then merged with the proton to produce the desired ∧(1405) . "The formation of a bound state of a K- meson and a proton was only possible because the neutron carried away some of the energy," says an author of the study, Kentaro Inoue One of the aspects that had been perplexing scientists about ∧(1405) was its very light overall mass, even though it contains a strange quark, which is nearly 40 times as heavy as an up quark. During the experiment, the team of researchers was able to successfully measure the complex mass of ∧(1405) by observing the behavior of the decay products.

  "We expect that progress in this type of research can lead to a more accurate description of ultra-high-density matter that exists in the core of a neutron star." says Shingo Kawasaki, another study author. This work implies that ∧(1405) is an unusual state consisting of four quarks and one antiquark, making a total of 5 quarks, and does not fit the conventional classification in which particles have either three quarks or one quark and one antiquark. This research may lead to a better understanding of the early formation of the Universe, shortly after the Big Bang, as well as what happens when matter is subject to pressures and densities well beyond what we see under normal conditions.


Fig. 1 The exotic baryon called ∧(1405) and a schematic illustration of the evolution of matter
Credit: Hiroyuki Noumi


Fig. 2 Schematic illustration of the reaction used to synthesize ∧(1405) by fusing a K- (green circle) with a proton (dark blue circle) , which takes place inside a deuteron nucleus
Credit: Hiroyuki Noumi


Fig. 3 (Top) Measured reaction cross-section. The horizontal axis is the K- and proton collision recoil energy converted into a mass value. Large reaction events occur at mass values lower than the sum of the K- and proton masses, which itself suggests the existence of ∧(1405). The measured data were reproduced by scattering theory (solid lines) . (Bottom) Distribution of K- and proton scattering amplitudes. When squared, these correspond to the reaction cross-section, and are generally complex numbers. The calculated values match with the measured data. When the real part (solid line) crosses 0, the value of the imaginary part reaches its maximum value. This is a typical distribution for a resonance state, and determines the complex mass. The arrows indicate the real part.
Credit: 2023, Hiroyuki Noumi, Pole position of Λ(1405) measured in d(K^-,n)π ∑ reactions, Physics Letters B

  The article, "Pole position of ∧(1405) measured in d(K-,n)π ∑ reactions," was published in Physics Letters B at DOI:

  The current work was performed by an international research collaboration, E31, involving scientists from Research Center for Nuclear Physics (RCNP), Osaka University together with RIKEN, KEK, JAEA, J-PARC, Tohoku University, INFN (Italy), SMI (Austria) and others.

  Research representives:
  Prof. Hiroyuki Noumi, RCNP, Osaka University/IPNS, KEK
  Dr. Fuminori Sakuma, RIKEN Cluster for Pioneering Research, RIKEN
  Dr. Tadashi Hashimoto, Advanced Science Research Center, JAEA
  Prof. Hiroaki Ohnish, Research Center for Electron Photon Science, Tohoku University
  Prof. Catalina Curceanu, Laboratori Nazionali di Frascati, INFN
  Prof. Johannes Zmeskal, Stefan-Mayer-Institut für subatomare Physik