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Abstract:
Over the years, a classic picture has been established from comprehensive experiments: nucleons such as the proton and neutron are composed of quarks tightly bound together, while atomic nuclei are further formed from those nucleons, held together by relatively weak interactions. Since 1990s, a series of experiments found that the deuteron, which is formed by a proton and a neutron, exhibits an anomalously large cross section in GeV-scale high-energy interactions. This phenomenon has been interpreted as the presence of the SU(2) flavour asymmetry, meaning that there are more anti-down quarks than anti-up quarks in the proton. This finding suggests that sea quarks are generated via flavour-asymmetric mechanisms. In this talk, I report an analysis of a series of new measurements recently obtained from pure proton interactions which are free from those nuclear assumptions. The analysis of the new proton data unexpectedly shows that the anti-down quark component is rather consistent with the anti-up quark inside the proton. This result provides evidence not only that quarks are generated via a symmetric mechanism, but also that the nuclear assumptions underlying the classic picture may break down at high energy scale, meaning that both the structure of the proton and the structure of high energy nuclei could differ fundamentally from our established understanding.
Biography:
Siqi Yang is an experimental particle physicist from University of Science and Technology of China. He has been focusing on the electroweak interactions at TeV-hardron colliders. Before 2020, his physical interest was the precise measurement on the electroweak symmetry breaking, which is essential both to the global test of the standard model and to the indirect constraint on the potential new physics. However, in the past years he realized that the precision of the measurements were severely limited by the uncertainty induced from the PDF of the quarks in the initial state. Then, his research focus gradually shifted to the fields of proton structure and nuclear structure. Although the experimental measurements still rely on the electroweak production, the physics goal is no longer electroweak itself. Instead, he aims to exploit the unique properties of electroweak symmetry breaking to launch new studies of proton structure and nuclear structure at hadron colliders.
Host: Prof. Wei Wang
Alternative online link：https://meeting.tencent.com/dm/k8RFBvrBtXO4
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