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How Strange is the Proton?

To ascertain the "strangeness" property of the proton, an international collaboration including IRFU has produced, starting from protons, other particles containing a "strange" quark and characterized them. These high-precision measurements carried out at CERN in Geneva will lead to a better understanding of the contribution of strange quarks to the nucleon's spin. 

Published on 23 October 2017


The protons and neutrons in the atomic nucleus each possess three "up" or "down" quarks—and actually many more, as innumerable quark-antiquark pairs are continuously made and unmade inside the nucleus. Yet the physicists suspect that they also conceal a part of strange quarks that remains to be determined in order to correctly interpret the properties of nucleons.

To study this nucleon's stealth property, they are taking a side road: nucleon fragmentation. By bombarding a target of protons with a muon (a particle likened to a "heavy" electron), they produce particles comprising two quarks from protons, including one that is strange (kaons). This allows the researchers to access information on the strange quark (positively charged kaon) or on the strange antiquark (negatively charged kaon).

On IRFU's initiative, the COMPASS international collaboration (Common Muon and Proton Apparatus for Structure and Spectroscopy) at CERN has performed high precision measurements using detectors developed by IRFU (Micromegas microstructures detectors and large drift chambers in particular) to identify and characterize these kaons. The scientists were then able to deduce constraints on the structure of the proton in relation to strange quarks and on the mechanisms underlying the production of particles comprised of strange quarks.

Strange quarks are suspected to significantly contribute to the nucleon's spin. This study should lead to progress on this issue.

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