Monte Carlo sampling variant of the DSSV14 set of helicity parton densities

Daniel De Florian,Gonzalo Agustín Lucero,Rodolfo Sassot,Marco Stratmann,Werner Vogelsang

doi:10.1103/physrevd.100.114027

Abstract

We implement a Monte Carlo sampling strategy to extract helicity parton densities and their uncertainties from a reference set of longitudinally polarized scattering data, chosen to be that used in the DSSV14 global analysis. Instead of adopting the simplest possible functional forms for the helicity parton distributions and imposing certain restrictions on their parameter space in order to constrain them, we employ redundant, flexible parametrizations and fit them to a large number of Monte Carlo replicas of the existing data. The optimum fit and its uncertainty estimates are then assumed to be given by the statistical average of the obtained ensemble of replicas of helicity parton densities and their corresponding variance, respectively. We compare our results to those obtained by the traditional fitting approach and to the uncertainty estimates derived with the robust Lagrange multiplier method, finding good agreement. As a first application of our new set of replicas, we discuss the impact of the recent STAR dijet data in further constraining the elusive gluon helicity density through the reweighting method.

Highlights

AND MOTIVATIONThe precise determination of parton distribution functions (PDFs) is a key ingredient to establish the validity and accuracy of perturbative QCD factorization and the assumed parton density universality and, of our current understanding of the nucleon structure and the strong interactions at the most fundamental level as expressed in term of quarks, antiquarks, and gluons [1]
The EMC result was later on confirmed by similar experiments at SLAC, DESY, CERN, and JLAB, and complemented with semi-inclusive deep-inelastic scattering (DIS) (SIDIS) measurements in order to pin down how the different quark and antiquark species are polarized individually [3]
We study the same set of polarized scattering data utilized in the well-known and frequently used DSSV14 analysis [5], a global fit at next-to-leading order (NLO) accuracy of DIS and SIDIS data together with results on the hadroproduction of jets and neutral pions in polarized proton-proton collisions from BNL-Relativistic Heavy Ion Collider (RHIC)

Summary

INTRODUCTION

The precise determination of parton distribution functions (PDFs) is a key ingredient to establish the validity and accuracy of perturbative QCD factorization and the assumed parton density universality and, of our current understanding of the nucleon structure and the strong interactions at the most fundamental level as expressed in term of quarks, antiquarks, and gluons [1]. [9,29] are highly interested in exploring the uncharted low-x domain of helicity PDFs, we will provide extrapolations of our ensemble of replicas beyond the kinematic region where data faithfully constrain them To this end, we supplement the Monte Carlo sampling approach with information coming from the Lagrange multiplier method. The latter allows one to estimate the uncertainty of any observable dependent on the PDFs or of the PDFs themselves within any given confidence level limit and under the assumption of a given functional form We use this extra information to generate a set of ten pseudodata points uniformly distributed in logarithmic scale between x 1⁄4 10−6 and x 1⁄4 10−3, i.e., outside the range spanned by actual data, with a Gaussian error distribution around the result of the DSSV14 best fit for the gluon helicity distribution, with variances corresponding to the 68% C.L. limit estimated in the Lagrange multiplier method as discussed below. ; ð8Þ and where N denotes the number of data points in the nth data set under consideration

RESULTS

REWEIGHTING APPLIED TO DIJET DATA

CONCLUSIONS