Impact of CMS 5.02 TeV dijet measurements on gluon PDFs - a preliminary view

TL;DR

This paper analyzes preliminary CMS dijet data at 5.02 TeV to evaluate their impact on gluon PDFs, revealing discrepancies with NLO predictions and suggesting significant modifications to nuclear shadowing and antishadowing in PDFs.

Contribution

It introduces a non-quadratic Hessian reweighting method to assess the data's impact, leading to reduced uncertainties and insights into nuclear modifications of gluon PDFs.

Findings

Discrepancies between data and NLO predictions with existing PDFs.

Data supports strong nuclear shadowing and valence-like antishadowing effects.

Uncertainties in nuclear PDFs are significantly reduced after reweighting.

Abstract

We discuss the implications of the preliminary CMS dijet data from 5.02 TeV pp and pPb collisions for gluon PDFs of the proton and nuclei. The preliminary pp data show a discrepancy with NLO predictions using for example the CT14 PDFs. We find that this difference cannot be accommodated within the associated scale uncertainties and debate the possible changes needed in the gluon PDF. A similar discrepancy is found between the CMS pPb data and NLO predictions e.g. with the EPPS16 nuclear modifications imposed on the CT14 proton PDFs. When a nuclear modification ratio of the pp and pPb data is constructed, the uncertainties in the scale choices and in proton PDFs effectively cancel and a good agreement between the data and EPPS16 is found, except in some bins at backward rapidities corresponding to large x of the nucleus. To assess the impact of these data on the EPPS16 nuclear PDFs, we use a non-quadratic extension of the Hessian PDF reweighting method. A significant reduction in EPPS16 uncertainties is obtained with the fit supporting strong nuclear shadowing and valence-like antishadowing for gluons. We also indicate the possible extensions needed in the EPPS16 parametrization at large x.