Summary of WG6: Spin physics

In the framework of quantum chromodynamics (QCD), parton distribution functions (PDFs) quantify how the momentum and spin of a hadron are divided among its quark and gluon constituents. Two main approaches exist to determine PDFs. The first approach, based on QCD factorization theorems, realizes a QCD analysis of a suitable set of hard-scattering measurements, often using a variety of hadronic observables. The second approach, based on first-principle operator definitions of PDFs, uses lattice QCD to compute directly some PDF-related quantities, such as their moments. Motivated by recent progress in both approaches, in this document we present an overview of lattice-QCD and globalanalysis techniques used to determine unpolarized and polarized proton PDFs and their moments. We provide benchmark numbers to validate present and future lattice-QCD calculations and we illustrate how they could be used to reduce the PDF uncertainties in current unpolarized and polarized global analyses. This document represents a first step towards establishing a common language between the two communities, to foster dialogue and to further improve our knowledge of PDFs.The detailed understanding of the inner structure of nucleons is an active research field with phenomenological implications in high-energy, hadron, nuclear and astroparticle physics. Within quantum chromodynamics (QCD), information on this structure -specifically on how the nucleon's momentum and spin are divided among quarks and gluons -is encoded in parton distribution functions (PDFs).There exist two main methods to determine PDFs. 1 The first method is the global QCD analysis [3][4][5][6][7][8][9][10][11][12]. It is based on QCD factorization of physical observables, i.e. the fact that a class of hard-scattering cross-sections can be expressed as a convolution between short-distance, perturbative, matrix elements and long-distance, nonperturbative, PDFs. By combining a variety of available hard-scattering experimental data with state-of-the-art perturbative calculations, complete PDF sets, including the gluon and various combinations of quark flavors, are currently determined for protons, in both the unpolarized [13][14][15][16][17] and the polarized [18][19][20][21] case.Recent progress in global QCD analyses has been driven, on the one hand, by the increasing availability of a wealth of high-precision measurements from Jefferson Lab, HERA, RHIC, the Tevatron and the LHC and, on the other hand, by the advancement in perturbative calculations of QCD and electroweak (EW) higher-order corrections. Parton distributions are now determined with unprecedented precision, in many cases at the few-percent level. A paradigmatic illustration of this progress is provided by both the unpolarized and polarized gluon PDFs, which were affected by rather large uncertainties until recently, due to the limited experimental information available. In the unpolarized case, the gluon PDF is now constrained quite accurately from small to large x thanks to the inclusion of processes such a...

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“…3 in Refs. [226] and [8,221]). The NNDPF procedure and the standard (adopted by DSSV) have already been outlined in Sec.…”

Section: Polarized Pdfsmentioning

confidence: 99%

Parton distributions and lattice QCD calculations: A community white paper

Lin

Nocera

Olness

et al. 2018

Progress in Particle and Nuclear Physics

267

222

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“…The double-helicity asym-metries for jet and π 0 production are directly sensitive to the gluon helicity distribution over a small range of x, because of the dominance of gluon-gluon and quarkgluon initiated subprocesses in the kinematic range accessed by PHENIX at the RHIC [125]. In recent helicity PDF fits [3,7,107,126], the RHIC measurements on the double-longitudinal spin asymmetry in the production of hadrons [127,128] and inclusive jet production in pp collisions [129], as well as single-longitudinal spin asymmetry measurements in the production of W ± bosons [130][131][132], have already been used. These data can increase sensitivity to the sign information of gluon density in present and future pQCD helicity PDF fits.…”

Section: Polarized Pdfs In the High-precision Era Of Collider Phymentioning

confidence: 99%

“…The EIC is expected to open up the kinematic domain to significantly lower values of x (x ≈ 10 −4 ) in centerof-mass energy to ∼ 104 GeV 2 , reducing significantly the uncertainty on the contributions from the unmeasured small-x region. The EIC will likely be the only facility to study the spin structure of the proton with the highest precision [132][133][134][135][136][137][138][139][140].…”

Section: Polarized Pdfs In the High-precision Era Of Collider Phymentioning

confidence: 99%

Nucleon spin structure functions at NNLO in the presence of target mass corrections and higher twist effects

Khanpour

Monfared

Tehrani³

2017

Phys. Rev. D

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We extract polarized parton distribution functions (PPDFs), referred to as "KTA17," together with the highly correlated strong coupling αs from recent and up-to-date g1 and g2 polarized structure functions world data at next-to-next-to-leading order in perturbative QCD. The stability and reliability of the results are ensured by including nonperturbative target mass corrections as well as higher-twist terms which are particularly important at the large-x region at low Q 2 . Their role in extracting the PPDFs in the nucleon is studied. Sum rules are discussed and compared with other results from the literature. This analysis is made by means of the Jacobi polynomials expansion technique to the DGLAP evolution. The uncertainties on the observables and on the PPDFs throughout this paper are computed using standard Hessian error propagation which served to provide a more realistic estimate of the PPDFs uncertainties.

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Summary of WG6: Spin physics

Cited by 2 publications

References 52 publications

Parton distributions and lattice QCD calculations: A community white paper

Parton distributions and lattice QCD calculations: A community white paper

Nucleon spin structure functions at NNLO in the presence of target mass corrections and higher twist effects

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