Elliot Leader

The goal of this study is to check which, if any, of the published versions of the pion and kaon fragmentation functions (FFs) is compatible with the STAR data on semi-inclusive pion and kaon production in proton–proton collisions, and on the basis of this analysis to make reliable predictions for the [ ] spectra of the pions and kaons in inclusive pion and kaon production at the future NICA proton–proton collider. The calculations are carried out in next-to-leading order (NLO) of perturbative quantum chromodynamics (pQCD), using the well-tested CTEQ6 parton distributions. We consider the following pion and kaon FFs — DSEHS-14 [D. de Florian, R. Sassot, M. Epele, R. J. Hernández-Pinto and M. Stratmann, Phys. Rev. D 91, 014035 (2015), 1410.6027], DSEHS-17 [D. de Florian, M. Epele, R. J. Hernandez-Pinto, R. Sassot and M. Stratmann, Phys. Rev. D 95, 094019 (2017), 1702.06353], LSS-15 [E. Leader, A. V. Sidorov and D. B. Stamenov, Phys. Rev. D 93, 074026 (2016), 1506.06381], HKNS-07 [M. Hirai, S. Kumano, T. H. Nagai and K. Sudoh, Phys. Rev. D 75, 094009 (2007), hep-ph/0702250] and AKK-08 [S. Albino, B. A. Kniehl and G. Kramer, Nucl. Phys. B 803, 42 (2008), 0803.2768]. Our analysis shows that within the experimental errors all tested sets of FFs provide a good fit to STAR data at the c.m. energy [ ], and the best ones are both LSS-15 and DSEHS-14 for pions and DSEHS-17 for kaons. From comparison of the LO and NLO results it is clear that the latter fit data much better, specially in the region of small [ ]. The NLO cross-sections are also less scale-[ ] dependent, where [ ], than the LO ones. In order to make predictions for NICA energies, we compare the NLO pQCD results with the existing experimental BES STAR data on semi-inclusive hadron production in the most peripheral Au+Au collisions where the nuclear effects can be neglected. The comparison for lower energy scales, like at NICA, shows that a purely pQCD approach is inadequate and suggests the necessity to take into account also higher-order effects of initial-state soft-gluon radiation. Nevertheless, these data on the [ ] spectra of [ ], [ ] and also the ratios [ ] and [ ] seem to favor LSS-15 and DSEHS-14 FFs for pions and DSEHS-17 for kaons, similarly as at the energy scale [ ][ ]GeV.

The covariant quantization of QED and QCD requires the introduction of subsidiary gauge-fixing and ghost fields and it is crucial to understand the role of these in the energy-momentum tensor, and in the momentum and angular momentum operators. These issues were discussed by E. Leader [ Phys. Rev. D 83, 096012 (2011)], and certain key results from this study, namely that the subsidiary and ghost fields do not contribute to the physical matrix elements of the canonical or Bellinfante momentum and angular momentum, were utilized in the major review of angular momentum by E. Leader and C. Lorce [ Phys. Rep., 541, 163 (2014)]. B. Damski [ Phys. Rev. D 104, 085003 (2021)] has rightly criticized as incorrect the derivation of these results for the QED case by Leader (2011) and has given explicit expressions for the contribution of the subsidiary fields to the physical matrix elements of the QED momentum and angular momentum. We show, however, that the key results of Leader (2011), mentioned above, which are utilized by Leader and Lorce (2014), are unaffected by Damski’s criticism and that his expressions for the contribution from the subsidiary fields, in fact, vanish.

The polarized strange quark puzzle concerns the fact that the polarized strange quark density extracted from polarized inclusive deep inelastic scattering data is significantly negative, whereas it is zero or slightly positive when extracted from a combined analysis of polarized semi-inclusive and inclusive deep inelastic data. SU(3) flavor symmetry, which, it is generally accepted, is not an exact symmetry, plays an important role in the inclusive analysis, and all the extracted polarized quark densities depend, to some extent, on the level of symmetry breaking introduced. But by far the most sensitive to the breaking is the strange quark density. In this paper we present a NLO QCD analysis of the world data on polarized inclusive DIS data on protons, neutrons and deuterons, including the final JLAB CLAS/EG1b data on the proton and deuteron, and study the sensitivity of the strange quark polarization to the breaking of flavor SU(3).

We extract the valence part of the nucleón Boer-Mulder (BM) function from the new COMPASS data on the unpolarized cosϕ_h and cos2ϕ_h asymmetries in SIDIS on deuteron for producing the hadron h and its antiparticle h ¯ at azimuthal angle ϕ_h . Our results, obtained with only standard assumption of factorization of the x_B (z_h ) and transverse momentum dependences, differ significantly from the presently published data on the BM function, obtained using a model assumption of proportionality to the better known Sivers function. This suggests that the published results on the BM function should be reconsidered.

The spin crisis arose some 30 years ago following results of a European Muon Collaboration experiment on polarized DIS. Since then many people have pointed out that the arguments involved are too naive, yet papers continue to appear devoted to "the spin crisis". We explain the subtleties involved in giving a precise statement of the spin sum rule and clarify why there is really no crisis.