Researchers collect mysterious, mysterious paries

Scientists at Osaka University were part of a particle accelerator experiment that produced a strange, highly unstable particle, and determined its mass. This could contribute to a better understanding of the inner workings of superdense neutron stars.

The Standard Model of particle physics states that most particles are made up of groups of just six types of fundamental entities called quarks. However, there are still many unsolved mysteries, one of which is Λ (1405), a strange but fleeting lambda resonance. It was previously thought to be a specific combination of three quarks – up, down and strange – and gaining insights into its composition could help reveal information about the very dense matter in neutron stars.

Now, researchers from Osaka University were part of a team that has successfully synthesized Λ (1405) for the first time by combining K with^– Meson and proton and determine their combined mass (mass and width). The letter K^– A meson is a negatively charged particle that contains a strange quark and an antiquark.

Schematic illustration of the reaction used to synthesize Λ (1405) by fusing a K- (green circle) with a proton (dark blue circle), which occurs within a deuteron nucleus. Credit: Hiroyuki Nomi

The most common proton that makes up the matter we are used to has two up quarks and one down quark. The researchers showed that it is best to think of Λ(1405) as a temporary binding state for K.^– The meson and the proton, in contrast to the three-quark excited state.

In a study recently published in Physics letters b, the group describes an experiment they conducted at the J-PARC accelerator. K^– The mesons were fired at a deuterium target, each containing one proton and one neutron. In a successful reaction, A.J^– The meson ejected the neutron, which then fused with the proton to produce the desired Λ (1405). Formation of a bound state of K^– The meson and the proton were only possible because the neutron carried some energy,” says study author Kentaro Inoue.

called the odd baryon Λ (1405) and a schematic illustration of the evolution of matter. Credit: Hiroyuki Nomi

One aspect that was puzzling scientists about Λ (1405) was its very light overall mass, despite containing a strange quark, which is roughly 40 times as heavy as the up quark. During the experiment, the team of researchers was able to successfully measure the complex mass of Λ (1405) by observing the behavior of the decay products.

(Top) Measured reaction cross-section. The horizontal axis is the K- and proton collision bounce energy converted to mass value. Large reaction events occur at mass values ​​lower than the sum of the masses of the K- and proton, which itself indicates the presence of Λ(1405). Data measured by scattering theory were reproduced (solid lines). (Bottom) K distribution^– and the amplitude of proton scattering. When squared, these correspond to the reaction cross-section and are generally complex numbers. The calculated values ​​correspond to the measured data. When the real part (solid line) exceeds zero, the value of the imaginary part reaches its maximum value. This is a typical distribution of the resonant state, and defines the complex mass. Arrows indicate the real part. Credit: 2023, Hiroyuki Nomi, pole position Λ (1405) measured in d (K^–n) πΣ reactions, Physics letters b

“We expect that advances in this type of research will lead to a more accurate description of the super-dense matter at the core of the core[{” attribute=””>neutron star,” says Shingo Kawasaki, another study author. This work implies that Λ(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.

Reference: “Pole position of Λ(1405) measured in d(K−,n)πΣ reactions” by S. Aikawa, S. Ajimura, T. Akaishi, H. Asano, G. Beer, C. Berucci, M. Bragadireanu, P. Buehler, L. Busso, M. Cargnelli, S. Choi, C. Curceanu, S. Enomoto, H. Fujioka, Y. Fujiwara, T. Fukuda, C. Guaraldo, T. Hashimoto, R.S. Hayano, T. Hiraiwa, M. Iio, M. Iliescu, K. Inoue, Y. Ishiguro, S. Ishimoto, T. Ishikawa, K. Itahashi, M. Iwai, M. Iwasaki, K. Kanno, K. Kato, Y. Kato, S. Kawasaki, P. Kienle, Y. Komatsu, H. Kou, Y. Ma, J. Marton, Y. Matsuda, Y. Mizoi, O. Morra, R. Murayama, T. Nagae, H. Noumi, H. Ohnishi, S. Okada, Z. Omar, H. Outa, K. Piscicchia, Y. Sada, A. Sakaguchi, F. Sakuma, M. Sato, A. Scordo, M. Sekimoto, H. Shi, K. Shirotori, D. Sirghi, F. Sirghi, K. Suzuki, S. Suzuki, T. Suzuki, K. Tanida, H. Tatsuno, A.O. Tokiyasu, M. Tokuda, D. Tomono, A. Toyoda, K. Tsukada, O. Vazquez-Doce, E. Widmann, T. Yamaga, T. Yamazaki, H. Yim, Q. Zhang and J. Zmeskal, 20 December 2022, Physics Letters B.
DOI: 10.1016/j.physletb.2022.137637

The study was funded by the Japan Society for the Promotion of Science, Ministry of Education, Culture, Sports, Science and Technology.