How well do we know the gluon polarization in the proton?

Abstract

We perform the first simultaneous global QCD analysis of spin-averaged and spin-dependent parton distribution functions (PDFs), including single jet production data from unpolarized and polarized hadron collisions. We critically assess the impact of SU(3) flavor symmetry and PDF positivity assumptions on the quark and gluon helicity PDFs, and find strong bias from these, particularly on the gluon polarization. The simultaneous analysis allows for the first time extraction of individual helicity-aligned and antialigned PDFs with a consistent treatment of uncertainties.

Snowmass 2021 White Paper: Electron Ion Collider for High Energy Physics

“…15. The analysis is performed within the JAM global QCD analysis framework at NLO, including all existing data on A LL and inclusive jet production from polarized pp scattering at RHIC [90], along with A LL from EIC proton pseudodata (simulated with L = 100 fb −1 , 2.3% normalization uncertainty, and 2% point-by-point uncorrelated systematic uncertainties).…”

Signature of the gluon orbital angular momentum

“…It should be mentioned that there are large uncertainties in our prediction even in the helicity part because currently ∆G(x) is poorly constrained, including even the sign, especially in the small-x region but also in the large-x region. (See a recent discussion [42] on this point.) Besides, nothing is known about L g (x) experimentally at the moment, and our model for L g involves key assumptions (the WW approximation and the use of the double distribution technique) whose validity needs to be investigated.…”

Polarized Antimatter in the Proton from Global QCD Analysis

“…Introduction.-Understanding the detailed decomposition of the proton spin into its constituent quark and gluon helicity and orbital angular momentum components promises to be one of the most significant accomplishments in nuclear and particle physics of this generation [1][2][3]. While the total light quark contributions to the helicity are well determined from polarized inclusive deep-inelastic scattering (DIS) data [4][5][6][7][8][9][10][11][12][13][14][15][16][17], and jet production in polarized pp collisions [18][19][20][21][22][23][24][25] provides constraints on the gluon helicity [26,27], far less is known about the polarization of the antiquark sea. There have been some intriguing hints of a polarized antiquark asymmetry, ∆ū − ∆ d, from polarized semi-inclusive DIS (SIDIS) measurements [28][29][30][31], in analogy with the spinaveraged ū − d asymmetry inferred from unpolarized DIS and Drell-Yan measurements [32][33][34][35][36].…”

“…Instead of relying on reweighting prescriptions and assumptions about PDF positivity or flavor symmetry, here we present a new simultaneous global QCD analysis of unpolarized and polarized PDFs and FFs, including for the first time STAR A W L data, along with data on inclusive and semi-inclusive polarized lepton-nucleon DIS and jet production in polarized pp collisions [27]. The Monte Carlo analysis allows us to more reliably quantify the uncertainties on all distributions, and examine the interplay between the sea asymmetry and parametrizations of FFs.…”

“…Beyond polarized lepton scattering, we describe jet production data in polarized pp collisions from STAR [18][19][20][21][22][23][24] and PHENIX [25], with a cut on the jet transverse momentum of 8 GeV [27]. We also include for the first time single-spin asymmetry A W L data from STAR [44] and A W/Z L from PHENIX [45,46], which provide the most direct constraints on the antiquark polarization.…”