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[Research] New research deepens mystery of particle generation in proton collisions

Figure: Transverse (left-right) asymmetry (shown in the vertical axis) around the zero-degree rapidly increases and reaches nearly the same size as the ones at the large angle (transverse momentum). The horizontal axis shows the ratio of longitudinal momentum of neutral pion to the maximum-allowed value. As shown by ■▲▼, the 5 – 10 % transverse asymmetry was observed even at the small transverse momentum as 0.1 GeV.c.

A group of researchers including scientists from the RIKEN Nishina Center for Accelerator-Based Science, University of Tokyo, Nagoya University, and the Japan Atomic Energy Agency (JAEA) used the spin-polarized Relativistic Heavy Ion Collider at Brookhaven National Laboratory in the United States to show that in polarized proton-proton collisions, the neutral pions in the very forward area of collisions—where direct interactions involving quarks and gluons are not applicable—still have a large degree of left-right asymmetry. This finding suggests that the previous consensus regarding the generation of particle in such collisions need to be reevaluated. Learn more about the result at RIKEN website (Press Release). Prof. Yoshitaka Itow, KMI, is actively involved in the project.

This was totally an unexpected result. Originally we planned to measure the transverse spin asymmetry of very forward neutrons at RHICf, but not of neutral pions. We just tried to check it and eventually found it was surprisingly large. This is what sometimes happens in experimentally science. Transverse asymmetry for neutral pions from polarized proton collisions has been well-established phenomena. But people think it would be only observed in the larger angle (transverse momentum) region, because quarks and gluons account for the phenomena. The observed asymmetry, however, is in the lower angle (transverse momentum) region. It’s puzzling and needs further understanding. KMI, Nagoya university has been playing a central role in LHCf and RHICf to study very forward particle production, which is relevant for air-shower development of high energy cosmic rays. Spin physics in this filed may give a new essential hint for understanding forward physics.

(Prof. Yoshitaka Itow, KMI)


Paper Information

H. Kim et al. [RHICf Collaboration], “Transverse single-spin asymmetry for very forward neutral pion production in polarized p + p collisions at  = 510 GeV,” Physical Review Letters


DOI: 10.1103/PhysRevLett.124.252501


Related Links

Press Release (Riken website)

Behind the scenes (Japanese) 

RHICf Experiment webpage

LHCf Experiment webpage