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Physical review letters 126(10), 102003 (2021). doi:10.1103/PhysRevLett.126.102003, We present, for the first time, an ab initio calculation of the individual up, down, and strange quark helicity parton distribution functions for the proton. The calculation is performed within the twisted mass clover-improved fermion formulation of lattice QCD. The analysis is performed using one ensemble of dynamical up, down, strange, and charm quarks with a pion mass of 260 MeV. The lattice matrix elements are nonperturbatively renormalized and the final results are presented in the $\overline{MS}$ scheme at a scale of 2 GeV. We give results for $��u^+(x), ��d^+(x) ��u^-(x), ��d^-(x)$, including disconnected quark loop contributions, as well as for $��s^+(x)$ and $��s^-(x)$. For the latter we achieve unprecedented precision compared to the phenomenological estimates., Published by APS, College Park, Md.

Physics letters / B 769, 34-41 (2017). doi:10.1016/j.physletb.2017.03.018, Published by North-Holland Publ., Amsterdam

University of Illinois, Diss., 2009; 121 pp., (2009). doi:10.3204/DESY-THESIS-2009-045, The motivation for this work was to improve upon prior analyses that extracted the quark helicity distributions, ¢q(x), of the proton. These extractions, performed by the HERMES collaboration, were excellent analyses, the most recent of which contained the vast majority of the data analyzed here. \If they were so good, how much could you improve on their results?' one might ask. Well, not to spoil the punch line, but some. The precision was improved modestly and our understanding of the systematic uncertainties was improved more signi¯cantly. The reader is welcome to jump to Chapter 6 and see. The approach that we took however was to try many new things. Not all of these e®orts yielded improvements on the scale we had hoped for, but many interesting new techniques were developed and we were able to produce several related new results along the way. Let me suggest then that rather than thinking of Chapter 6 as the result of this analysis, consider that there are valuable new results and insights scattered throughout this work. Chapter 4 contains several new double-spin asymmetries which are results in their own right. The ph? dependence is plotted for the ¯rst time with HERMES data which is uniquely hadron separated. The hadron charge di®erence asymmetry is presented which, in combination with the quark helicity densities of Chapter 6, can put limits on fragmentation symmetry breaking in semi-inclusive DIS. Additionally, a novel method of unfolding yields (reducing smearing e®ects from detector resolution limitations and QED radiation) was developed and presented here for the ¯rst time which potentially allows new kinds of asymmetries to be constructed which were unavailable before. Also, this chapter describes the method by which the ¯rst ever three dimensionally binned SIDIS double-spin asymmetries were produced. These asymmetries, which will be used as the data inputs for the ¢q(x) extraction, are valuable inputs to world ¯ts being performed by theorists. Chapter 5 further explores this idea of fragmentation symmetry breaking with Monte Carlo studies of fragmentation functions. These studies test assumptions which are frequently made in the interpretation of asymmetries like the hadron charge di®erence of the prior chapter and suggest that these assumptions should be approached with some caution. Also, a technique for tuning and more importantly propagating systematic uncertainty through non-analytic Monte Carlo models, like the Lund-String model which provides an essential input to the ¢q(x) extraction, is developed vi and potentially has widespread applications. That being said, the author hopes you enjoy the techniques developed and results presented in the following chapters., Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

The scattering of a baryon consisting of three massive quarks is investigated in the high energy limit of perturbative QCD. A model of a relativistic proton-like wave function, dependent on valence quark longitudinal and transverse momenta and on quark helicities, is proposed, and we derive the baryon impact factors for two, three and four t-channel gluons. We find that the baryonic impact factor can be written as a sum of three pieces: in the first one a subsystem consisting of two of the three quarks behaves very much like the quark-antiquark pair in gamma* scattering, whereas the third quark acts as a spectator. The second term belongs to the odderon, whereas in the third (C-even) piece all three quarks participate in the scattering. This term is new and has no analogue in gamma* scattering. We also study the small x evolution of gluon radiation for each of these three terms. The first term follows the same pattern of gluon radiation as the gamma*-initiated quark-antiquark dipole, and, in particular, it contains the BFKL evolution followed by the 2-->4 transition vertex (triple Pomeron vertex). The odderon-term is described by the standard BKP evolution, and the baryon couples to both known odderon solutions, the Janik-Wosiek solution and the BLV solution. Finally, the t-channel evolution of the third term starts with a three reggeized gluon state which then, via a new 3-->4 transition vertex, couples to the four gluon (two-Pomeron) state. We briefly discuss a few consequences of these findings, in particular the pattern of unitarization of high energy baryon scattering amplitudes., 36 pages, 16 figures

Double-spin asymmetries of semi-inclusive cross sections for the production of identified pions and kaons have been measured in deep-inelastic scattering of polarized positrons on a polarized deuterium target. Five helicity distributions including those for three sea quark flavors were extracted from these data together with re-analyzed previous data for identified pions from a hydrogen target. These distributions are consistent with zero for all three sea flavors. A recently predicted flavor asymmetry in the polarization of the light quark sea appears to be disfavored by the data.