@article{adamiak_baldonado_kovchegov_melnitchouk_pitonyak_sato_sievert_tarasov_tawabutr_2023, title={Global analysis of polarized DIS and SIDIS data with improved small-x helicity evolution}, volume={108}, ISSN={["2470-0029"]}, DOI={10.1103/PhysRevD.108.114007}, abstractNote={We analyze the world polarized deep-inelastic scattering (DIS) and semi-inclusive DIS (SIDIS) data at low values of $x<0.1$, using small-$x$ evolution equations for the flavor singlet and nonsinglet helicity parton distribution functions (hPDFs). The hPDFs for quarks, antiquarks, and gluons are extracted and evolved to lower values of $x$ to make predictions for the future Electron-Ion Collider (EIC). We improve on our earlier work by employing the more realistic large-$N_c\, \&N_f$ limit of the revised small-$x$ helicity evolution, and incorporating running coupling corrections along with SIDIS data into the fit. We find an anti-correlation between the signs of the gluon and $C$-even quark hPDFs as well as the $g_1$ structure function. While the existing low-$x$ polarized DIS and SIDIS data are insufficient to constrain the initial conditions for the polarized dipole amplitudes in the helicity evolution equations, future EIC data will allow more precise predictions for hPDFs and the $g_1$ structure function for $x$ values beyond those probed at the EIC. Using the obtained hPDFs, we discuss the contributions to the proton spin from quark and gluon spins at small $x$.}, number={11}, journal={PHYSICAL REVIEW D}, author={Adamiak, Daniel and Baldonado, Nicholas and Kovchegov, Yuri V and Melnitchouk, W. and Pitonyak, Daniel and Sato, Nobuo and Sievert, Matthew D. and Tarasov, Andrey and Tawabutr, Yossathorn}, year={2023}, month={Dec} }
 @article{feal_tarasov_venugopalan_2023, title={QED as a many-body theory of worldlines. II. All-order S-matrix formalism}, volume={107}, ISSN={["2470-0029"]}, DOI={10.1103/PhysRevD.107.096021}, abstractNote={In arXiv:2206.04188, we developed a first-quantized worldline formalism for all-order computations of amplitudes in QED. In particular, we demonstrated in this framework an all-order proof of the infrared safety of the Faddeev-Kulish (FK) S-matrix for virtual exchanges in the scattering of charged fermions. In this work, we extend the worldline formalism for both the Dyson and FK S-matrix to consider further the emission and absorption of arbitrary numbers of photons. We show how Low's theorem follows in this framework and derive Weinberg's theorem for the exponentiation of IR divergences. In particular, we extend our all-order proof of the IR safety of the FK S-matrix to both virtual exchanges and real photon emissions. We argue that the worldline approach leads to a modern Wilsonian interpretation of the IR safety of the FK S-matrix and provides a novel template for the treatment of IR divergences in real-time problems. Using Grassmannian integration methods, we derive a simple and powerful result for N-th rank vacuum polarization tensors. Applications of these methods will be discussed in follow-up work.}, number={9}, journal={PHYSICAL REVIEW D}, author={Feal, Xabier and Tarasov, Andrey and Venugopalan, Raju}, year={2023}, month={May} }