Progress in the Determination of the Partonic Structure of the Proton

Abstract

We review the current state of the art in the determination of the parton substructure of the nucleon, as expressed in terms of parton distribution functions (PDFs), and probed in high-energy lepton-hadron and hadron-hadron collisions, and we assess their implications for current precision collider phenomenology, in particular at the Large Hadron Collider (LHC). We review the theoretical foundations of PDF determination: the way cross sections are expressed in terms of PDFs using perturbative QCD factorization and evolution, the methodology used to extract PDFs from experimental data, and the way in which different physical processes can be used to constrain different PDFs. We summarize current knowledge of PDFs and the limitations in accuracy currently entailed for the computation of hadron collider processes, in particular at the LHC. We discuss the current main sources of theoretical and phenomenological uncertainties, and the direction of progress towards their reduction in the future.

Precision Determination of the Small- x Gluon from Charm Production at LHCb

“…The determination of the internal structure of the proton, as encoded by the nonperturbative parton distribution functions (PDFs) [1][2][3], has far-reaching implications for many areas in nuclear, particle, and astroparticle physics. A topic that has recently attracted substantial interest is the determination of the gluon PDF at small x, which is of direct relevance for the modeling of soft QCD at the LHC [4], neutrino astronomy [5][6][7][8], and cosmic ray physics [9], as well as for future lepton-proton [10] and proton-proton higher-energy colliders [11].…”

A meta-analysis of parton distribution functions

“…Inclusive jet bin1, LHC calculated on a process-by-process basis for multiple PDF sets and combined at the final stage based on a certain prescription, e.g., the PDF4LHC recommendation [16,27,28] or the replica combination method [29,30]. These approaches, while providing a reasonable uncertainty estimate, handle the PDFs inefficiently and promote lengthy computations.…”

PDFs and LHC data: current and future constraints

“…As far as correlations are concerned, the importance of correctly including the normalisation error and all correlated systematics and the cross correlation among experiments was outlined and backed up in several studies [3,40]. As for the treatment of inconsistent data, this is a topic still under discussion.…”

“…2 the impact of the LHC data in the NNPDF3.0 set is quantified. The latter is at a half-sigma level, both for central values and for uncertainties, and it always leads XLIV International Symposium on Multiparticle Dynamics (ISMD 2014) 07001-p. 3 to an improvement in uncertainties, pointing to the consistency of the LHC data with respect to the bulk of data in global analyses. Central values are mostly affected for quarks at medium and large x, and to a lesser extent for the gluon.…”