We present new parton distribution functions (PDFs) at next-to-next-to-leading order (NNLO) from the CTEQ-TEA global analysis of quantum chromodynamics. These differ from previous CT PDFs in several ...respects, including the use of data from LHC experiments and the new D0 charged-lepton rapidity asymmetry data, as well as the use of a more flexible parametrization of PDFs that, in particular, allows a better fit to different combinations of quark flavors. Predictions for important LHC processes, especially Higgs boson production at 13 TeV, are presented. These CT14 PDFs include a central set and error sets in the Hessian representation. For completeness, we also present the CT14 PDFs determined at the LO and the NLO in QCD. Besides these general-purpose PDF sets, we provide a series of (N)NLO sets with various alpha sub()svalues and additional sets in general-mass variable flavor number schemes, to deal with heavy partons, with up to three, four, and six active flavors
We present the new parton distribution functions (PDFs) from the CTEQ-TEA collaboration, obtained using a wide variety of high-precision Large Hadron Collider (LHC) data, in addition to the combined ...HERA I + II deep-inelastic scattering dataset, along with the datasets present in the CT14 global QCD analysis. New LHC measurements in single-inclusive jet production with the full rapidity coverage, as well as production of Drell-Yan pairs, top-quark pairs, and high-pT Z bosons, are included to achieve the greatest sensitivity to the PDFs. The parton distributions are determined at next-to-leading order and next-to-next-to-leading order, with each of these PDFs accompanied by error sets determined using the Hessian method. Fast PDF survey techniques, based on the Hessian representation and the Lagrange multiplier method, are used to quantify the preference of each data set to quantities such as αs(mZ), and the gluon and strange quark distributions. We designate the main resulting PDF set as CT18. The ATLAS 7 TeV precision W / Z data are not included in CT18, due to their tension with other datasets in the global fit. Alternate PDF sets are generated including the ATLAS precision 7 TeV W / Z data (CT18A), a new scale choice for low- x DIS data (CT18X), or all of the above with a slightly higher choice for the charm mass (CT18Z). Theoretical calculations of standard candle cross sections at the LHC (such as the g g fusion Higgs boson cross section) are presented.
The strong force which binds hadrons is described by the theory of quantum chromodynamics (QCD). Determining the character and manifestations of QCD is one of the most important and challenging ...outstanding issues necessary for a comprehensive understanding of the structure of hadrons. Within the context of the QCD parton picture, the parton distribution functions (PDFs) have been remarkably successful in describing a wide variety of processes. However, these PDFs have generally been confined to the description of collinear partons within the hadron. New experiments and facilities provide the opportunity to additionally explore the transverse structure of hadrons which is described by generalized parton distributions (GPDs) and transverse-momentum-dependent parton distribution functions (TMD PDFs). In our previous report Lin et al. (2018), we compared and contrasted the two main approaches used to determine the collinear PDFs: the first based on perturbative QCD factorization theorems, and the second based on lattice-QCD calculations. In the present report, we provide an update of recent progress on the collinear PDFs, and also expand the scope to encompass the generalized PDFs (GPDs and TMD PDFs). We review the current state of the various calculations, and consider what new data might be available in the near future. We also examine how a shared effort can foster dialog between the PDF and lattice-QCD communities, and yield improvements for these generalized PDFs.
We propose a new observable for the measurement of the forward–backward asymmetry
(
A
FB
)
in Drell–Yan lepton production. At hadron colliders, the
A
FB
distribution is sensitive to both the ...electroweak (EW) fundamental parameter
sin
2
θ
W
, the weak mixing angle, and the parton distribution functions (PDFs). Hence, the determination of
sin
2
θ
W
and the updating of PDFs by directly using the same
A
FB
spectrum are strongly correlated. This correlation would introduce large bias or uncertainty into both precise measurements of EW and PDF sectors. In this article, we show that the sensitivity of
A
FB
on
sin
2
θ
W
is dominated by its average value around the
Z
pole region, while the shape (or gradient) of the
A
FB
spectrum is insensitive to
sin
2
θ
W
and contains important information on the PDF modeling. Accordingly, a new observable related to the gradient of the spectrum is introduced, and demonstrated to be able to significantly reduce the potential bias on the determination of
sin
2
θ
W
when updating the PDFs using the same
A
FB
data.
A
bstract
We investigate the possibility of a (sizable) nonperturbative contribution to the charm parton distribution function (PDF) in a nucleon, theoretical issues arising in its interpretation, ...and its potential impact on LHC scattering processes. The “fitted charm” PDF obtained in various QCD analyses contains a process-dependent component that is partly traced to power-suppressed radiative contributions in DIS and is generally different at the LHC. We discuss separation of the universal component of the nonperturbative charm from the rest of the radiative contributions and estimate its magnitude in the CT14 global QCD analysis at the next-to-next-to leading order in the QCD coupling strength, including the latest experimental data from HERA and the Large Hadron Collider. Models for the nonperturbative charm PDF are examined as a function of the charm quark mass and other parameters. The prospects for testing these models in the associated production of a Z boson and a charm jet at the LHC are studied under realistic assumptions, including effects of the final-state parton showering.
A
bstract
Understanding how sea quarks behave inside a nucleon is one of the most important physics goals of the proposed Electron-Ion Collider in China (EicC), which is designed to have a 3.5 GeV ...polarized electron beam (80% polarization) colliding with a 20 GeV polarized proton beam (70% polarization) at instantaneous luminosity of 2 × 10
33
cm
−
2
s
−
1
. A specific topic at EicC is to understand the polarization of individual quarks inside a longitudinally polarized nucleon. The potential of various future EicC data, including the inclusive and semi-inclusive deep inelastic scattering data from both doubly polarized electron-proton and electron-
3
He collisions, to reduce the uncertainties of parton helicity distributions is explored at the next-to-leading order in QCD, using the Error PDF Updating Method Package (
e
P
ump
) which is based on the Hessian profiling method. We show that the semi-inclusive data are well able to provide good separation between flavour distributions, and to constrain their uncertainties in the
x >
0
.
005 region, especially when electron-
3
He collisions, acting as effective electron-neutron collisions, are taken into account. To enable this study, we have generated a Hessian representation of the DSSV14 set of PDF replicas, named DSSV14H PDFs.
Determinations of the proton’s collinear parton distribution functions (PDFs) are emerging with growing precision due to increased experimental activity at facilities like the Large Hadron Collider. ...While this copious information is valuable, the speed at which it is released makes it difficult to quickly assess its impact on the PDFs, short of performing computationally expensive global fits. As an alternative, we explore new methods for quantifying the potential impact of experimental data on the extraction of proton PDFs. Our approach relies crucially on the Hessian correlation between theory-data residuals and the PDFs themselves, as well as on a newly defined quantity-the sensitivity-which represents an extension of the correlation and reflects both PDF-driven and experimental uncertainties. This approach is realized in a new, publicly available analysis package PDFSense, which operates with these statistical measures to identify particularly sensitive experiments, weigh their relative or potential impact on PDFs, and visualize their detailed distributions in a space of the parton momentum fraction x and factorization scale μ. This tool offers a new means of understanding the influence of individual measurements in existing fits as well as a predictive device for directing future fits toward the highest impact data and assumptions. Along the way, many new physics insights can be gained or reinforced. As one of many examples, PDFSense is employed to rank the projected impact of new LHC measurements in jet, vector boson, and tt¯ production and leads us to the conclusion that inclusive jet production at the LHC has a potential for playing an indispensable role in future PDF fits. These conclusions are independently verified by preliminarily fitting this experimental information and investigating the constraints they supply using the Lagrange multiplier technique.
In an earlier publication, we introduced the software package Error PDF Updating Method Package (epump) that can be used to update or optimize a set of parton distribution functions (PDFs), including ...the best-fit PDF set and Hessian eigenvector pairs of PDF sets (i.e., error PDFs), and to update any other set of observables, in the Hessian approach. Here, we validate the epump program with a detailed comparison against a full global analysis, and we demonstrate the potential of epump by presenting selected phenomenological applications relevant to the Large Hadron Collider. For example, we use the package to estimate the impact of the recent LHC data of the measurements of W and Z boson and top quark pair differential distributions on the CT14HERA2 PDFs.
A
bstract
We explore connections between two common methods for quantifying the uncertainty in parton distribution functions (PDFs), based on the Hessian error matrix and Monte-Carlo sampling. CT14 ...parton distributions in the Hessian representation are converted into Monte-Carlo replicas by a numerical method that reproduces important properties of CT14 Hessian PDFs: the asymmetry of CT14 uncertainties and positivity of individual parton distributions. The ensembles of CT14 Monte-Carlo replicas constructed this way at NNLO and NLO are suitable for various collider applications, such as cross section reweighting. Master formulas for computation of asymmetric standard deviations in the Monte-Carlo representation are derived. A correction is proposed to address a bias in asymmetric uncertainties introduced by the Taylor series approximation. A numerical program is made available for conversion of Hessian PDFs into Monte-Carlo replicas according to normal, log-normal, and Watt-Thorne sampling procedures.
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