Probing new physics beyond the SM with transverse spin effects

Although the standard model of particle physics has been experimentally validated with remarkable success, it remains inadequate in addressing all fundamental questions and observed anomalies, such as the muon anomalous magnetic moment and neutron electric dipole moment. Electromagnetic dipole moments, one of the fundamental properties of spinning particles, are frequently employed in particle physics experiments to probe the internal structure of particles. Weak dipole interactions, closely related to their electromagnetic counterparts, hold significant importance in exploring new physics beyond the standard model through precise measurements of their strengths. Within the standard model effective field theory, a model-independent framework to systematically parameterize the unknown physics effect, dipole moments are described by electroweak dipole operators, which serve as critical tools for detecting the quantum effects of new physics beyond the standard model. However, the chirality-flipping nature of dipole interactions results in the absence of interference effects between these interactions and the standard model amplitudes in scattering processes involving light fermions, which severely limits the sensitivity of current experimental approaches to such interactions. Recent studies have demonstrated that the transverse polarization effects of fermions provide a promising avenue for extracting information about dipole interactions, with transverse spin azimuthal asymmetries serving as distinctive and unique observables. These transverse polarization effects can be generated through various mechanisms, such as polarized beams, non-perturbative parton distribution functions, and fragmentation functions. Leveraging this characteristic, transverse polarization effects open up new research directions for detecting signals of new physics at the precision frontier. It pioneers the application of spin physics, particularly transverse polarization effects, to new physics searches, thereby establishing an interdisciplinary connection between novel spin asymmetries and chirality-flipping interactions. It represents a groundbreaking and transformative advancement in electroweak precision physics. This paper presents a comprehensive review of the latest advancements in utilizing fermion transverse polarization effects to search for new physics beyond the standard model. We will focus on discussing the relevant theoretical frameworks, experimental methodologies, and their potential applications in particle physics research. As an example, it shows the recent investigation of dipole interactions and four-fermion operators through different spin-polarized processes at electron-positron and electron-ion colliders, leveraging spin physics and polarization phenomena to build up diverse azimuthal asymmetries. It includes probing electroweak dipole interactions of electron via single transverse spin asymmetries with transversely polarized initial-state electrons at lepton colliders, extending to the detection of both electron and quark dipole interactions at polarized electron-ion colliders, and detecting scalar/tensor four-fermion operators via double transverse spin asymmetries in deep inelastic scattering with transversely polarized electrons and transversity of protons at electron-ion colliders. These new physics effects manifest nontrivial cosine and sine azimuthal angular dependencies absent in the standard model, arising from interference between chirality-flipping interactions and standard model amplitudes with linear dependence on their Wilson coefficients. This novel approach enhances constraints on such chirality-flipping interactions by one to two orders of magnitude without additional theoretical assumptions or contamination from other new physics operators. It also simultaneously constrains the real and imaginary parts of these chirality-flipping couplings, providing new opportunities to probe potential CP violation effects at high energies.