Speaker
Description
The talk consists of two parts. Spending the first several minutes, I will talk on ‘Evolution of the PacificSpin Symposium on High Energy Spin Physics’. This symposium was started in Kobe in Japan in 1996. It was only several years after the EMC’s result[1,2] on the proton spin came out. It was before the RHIC experiments started. The aim of the symposium is to further enhance high energy spin physics in the circum-pan-Pacific region. In this symposium, we consider it important 1) to provide sufficient time for discussions among the participants, and 2) to encourage young physicists to participate.
In the rest of the time, I will talk on ‘Evolution of studies of the spin structure of the proton’. The spin structure of the proton has been studied for many years, and the studies are now divided into a few domains. I present an overview of the evolution.
The concept of ‘spin’ was introduced in 1920’s. Spin has been a key element in physics since then. Fermions with spin 1/2 are described as spinors. Spinors have specific properties which are different from vectors and scalars. After the pioneering experiments of polarized deep inelastic scattering (DIS) at SLAC, longitudinally polarized muon beams became available at high energy proton accelerators. Thus, the beam energy increased about 10 times. As the result, the accessible kinematic region in DIS was extended to the small Bjorken-x region. EMC group, using the muon beam up to 200 GeV, reported in 1988 and 1989 [1,2] that the contributions of spins of quarks and anti-quarks to the proton spin are small. The amount of the contributions reported in their 1989 paper was (12 ± 9 ± 14)%. This result motivated many experimental and theoretical studies afterwards. The spin sum rule
1/2 = 1/2 ΔΣ + ΔG + Lq,¯q + LG
suggests the possible contributions from other parts. Here, the right hand side of the equation has terms representing contributions of spins of quarks and anti-quarks, spins of gluons, orbital angular momenta of quarks and anti-quarks and of gluons. As the theoretical framework, the parton model and the scaling property in DIS were proposed in 1960’s. The asymptotic freedom was theoretically discovered in 1973 and QCD was established as a field theory. The factorization theorem was important to interpret DIS and hadron reactions. Scaling violation (Q2-dependence) of the parton distributions and the QCD evolution equation were essential for the developments of this field. The perturbative QCD and the lattice QCD calculations are two approaches to the proton structure.
The transverse single-spin asymmetry had been observed in hadron reactions since many decades. The asymmetry was also observed in DIS about 20 years ago. The analyses in the context of transverse-momentum-dependent parton distributions opened a new aspect of the studies of the parton structure of the proton. It became also possible to observe exclusive productions of photons and mesons in lepton scattering. The generalized parton distributions led to the studies of 3D structure of the proton. The contributions of orbital angular momenta of quarks, antiquarks and gluon could be crucial. Polarized p-p reaction program at the collider RHIC added new insights to the proton spin structure, in particular to the gluon spin contribution. EIC (Electron-Ion-Collider)[3] now in preparation will be the first collider experiments with polarized electron and polarized light ions. The evolution of the studies of the spin structure of the proton will continue with novel experimental technique and advanced theory.
[1] J. Ashman et al., EMC, Phys. Lett. B206 (1988) 364.
[2] J. Ashman et al., EMC, Nucl. Phys. B328 (1989) 1.
[3] R. Abdul Khalek et al., EIC Yellow Report, Nucl. Phys. A1026 (2022) 122447.
