arXiv subjects

Hung-Liang Lai

Publications and source records attributed to Hung-Liang Lai.

13 recordsLinked to original sources

arXiv2014

The CT10 NNLO Global Analysis of QCD

We present next-to-next-to-leading order (NNLO) parton distribution functions (PDFs) from the CTEQ-TEA group. The CT10NNLO PDF fit is based on essentially the same global data sets used in the CT10 and CT10W NLO PDF analyses. After exploring the goodness of the fits to the HERA combined data and the Tevatron jet data, we present various predictions at NNLO accuracy for both existing and forthcoming precision measurements from the CERN Large Hadron Collider. The range of variations in the gluon distribution introduced by correlated systematic effects in inclusive jet production is also examined.

hep-ph↗

arXiv2012

General-mass treatment for deep inelastic scattering at two-loop accuracy

We present a next-to-next-to-leading order (NNLO) realization of a general quark mass scheme (S-ACOT-$χ$) in deep inelastic scattering and explore the impact of NNLO terms on heavy-quark structure functions $F_{2,L}^{c}(x,Q)$. An amended QCD factorization theorem for DIS is discussed that validates the S-ACOT-$χ$ scheme to all orders in the QCD coupling strength. As a new feature, kinematical constraints on collinear production of heavy quarks that are crucial near the heavy-quark threshold are included in the amended factorization theorem. An algorithmic procedure is outlined for implementing this scheme at NNLO by using mass-dependent and massless results from literature. At two loops in QCD cut diagrams, the S-ACOT-$χ$ scheme reduces scale dependence of heavy-quark DIS cross sections as compared to the fixed-flavor number scheme.

hep-ph↗

arXiv2012

MEKS: a program for computation of inclusive jet cross sections at hadron colliders

EKS is a numerical program that predicts differential cross sections for production of single-inclusive hadronic jets and jet pairs at next-to-leading order (NLO) accuracy in a perturbative QCD calculation. We describe MEKS 1.0, an upgraded EKS program with increased numerical precision, suitable for comparisons to the latest experimental data from the Large Hadron Collider and Tevatron. The program integrates the regularized patron-level matrix elements over the kinematical phase space for production of two and three partons using the VEGAS algorithm. It stores the generated weighted events in finely binned two-dimensional histograms for fast offline analysis. A user interface allows one to customize computation of inclusive jet observables. Results of a benchmark comparison of the MEKS program and the commonly used FastNLO program are also documented.

hep-ph↗

arXiv2012

Progress in CTEQ-TEA PDF analysis

Recent developments in the CTEQ-TEA global QCD analysis are presented. The parton distribution functions CT10-NNLO are described, constructed by comparing data from many experiments to NNLO approximations of QCD.

hep-ph↗

arXiv2011

Heavy-flavor contributions at NNLO in CTEQ PDF analysis

We discuss an NNLO realization of the general mass scheme S-ACOT-$χ$ for treatment of heavy-flavour production in neutral current deep-inelastic scattering. Practical implementation of the NNLO calculation is illustrated on the example of semi-inclusive structure functions $F_{2 c}(x,Q)$ and $F_{L c}(x,Q)$.

hep-ph↗

arXiv2011

The PDF4LHC Working Group Interim Report

This document is intended as a study of benchmark cross sections at the LHC (at 7 TeV) at NLO using modern parton distribution functions currently available from the 6 PDF fitting groups that have participated in this exercise. It also contains a succinct user guide to the computation of PDFs, uncertainties and correlations using available PDF sets. A companion note, also submitted to the archive, provides an interim summary of the current recommendations of the PDF4LHC working group for the use of parton distribution functions and of PDF uncertainties at the LHC, for cross section and cross section uncertainty calculations.

hep-ph↗

arXiv2011

CT10 parton distributions and other developments in the global QCD analysis

We summarize several projects carried out by the CTEQ global analysis of parton distribution functions (PDFs) of the proton during 2010. We discuss a recently released CT10 family of PDFs with a fixed and variable QCD coupling strength; implementation of combined HERA and Tevatron lepton asymmetry data sets; theoretical issues associated with the analysis of $W$ charge asymmetry in PDF fits; PDFs for leading-order shower programs; and constraints on new color-octet fermions from the hadronic data.

hep-ph↗

arXiv2010

Constraints on color-octet fermions from a global parton distribution analysis

We report a parton distribution function (PDF) analysis of a complete set of hadron scattering data, in which a color-octet fermion (such as a gluino of supersymmetry) is incorporated as an extra parton constituent along with the usual standard model constituents. The data set includes the most up-to-date results from deep-inelastic scattering and from jet production in hadron collisions. Another feature is the inclusion in the fit of data from determinations of the strong coupling alpha_s(Q) at large and small values of the hard scale Q. Our motivation is to determine the extent to which the global PDF analysis may provide constraints on the new fermion, as a function of its mass and alpha_s(M_Z), independent of assumptions such as the mechanism of gluino decays. Based on this analysis, we find that gluino masses as low as 30 to 50 GeV may be compatible with the current hadronic data. Gluino masses below 15 GeV (25 GeV) are excluded if alpha_s(M_Z) varies freely (is equal to 0.118). At the outset, stronger constraints had been anticipated from jet production cross sections, but experimental systematic uncertainties, particularly in normalization, reduce the discriminating power of these data.

hep-ph↗

arXiv2010

New parton distributions for collider physics

We extract new parton distribution functions (PDFs) of the proton by global analysis of hard scattering data in the general-mass framework of perturbative quantum chromodynamics. Our analysis includes new theoretical developments together with the most recent collider data from deep-inelastic scattering, vector boson production, and single-inclusive jet production. Due to the difficulty in fitting both the DO Run-II W lepton asymmetry data and some fixed-target DIS data, we present two families of PDFs, CT10 and CT10W, without and with these high-luminosity W lepton asymmetry data included in the global analysis. With both sets of PDFs, we study theoretical predictions and uncertainties for a diverse selection of processes at the Fermilab Tevatron and the CERN Large Hadron Collider.

hep-ph↗

arXiv2010

Uncertainty induced by QCD coupling in the CTEQ global analysis of parton distributions

We examine the dependence of parton distribution functions (PDFs) on the value of the QCD coupling strength $α_{s}(M_{Z})$. We explain a simple method that is rigorously valid in the quadratic approximation normally applied in PDF fitting, and fully reproduces the correlated dependence of theoretical cross sections on $α_s$ and PDF parameters. This method is based on a statistical relation that allows one to add the uncertainty produced by $α_s$, computed with some special PDF sets, in quadrature with the PDF uncertainty obtained for the fixed $α_s$ value (such as the CTEQ6.6 PDF set). A series of four CTEQ6.6AS PDFs realizing this approach, for $α_s$ values in the interval $0.116 \leq α_{s}(M_{Z}) \leq 0.120$, is presented. Using these PDFs, the combined $α_{s}$ and PDF uncertainty is assessed for theoretical predictions at the Fermilab Tevatron and Large Hadron Collider.

hep-ph↗

arXiv2010

Parton Distributions for Event Generators

In this paper, conventional Global QCD analysis is generalized to produce parton distributions optimized for use with event generators at the LHC. This optimization is accomplished by combining the constraints due to existing hard-scattering experimental data with those from anticipated cross sections for key representative SM processes at LHC (by the best available theory) as joint input to the global analyses. The PDFs obtained in these new type of global analyses using matrix elements calculated in any given order will be best suited to work with event generators of that order, for predictions at the LHC. This is most useful for LO event generators at present. Results obtained from a few candidate PDF sets (labeled as CT09MCS, CT09MC1 and CT09MC2) for LO event generators produced in this way are compared with those from other approaches.

hep-ph↗

arXiv1999

Effects of threshold resummation

We investigate effects of threshold resummation of logarithmic corrections $\ln N$ in Mellin space quantitatively. Threshold resummation leads to enhancement of next-to-leading-order QCD predictions for jet production at large jet transverse energy, which is in the trend indicated by experimental data. We show that this enhancement is completely determined by the behavior of threshold resummation at small $N$, the region where hierachy among different powers of $\ln N$ is lost and current next-to-leading-logarithm resummation is not reliable. Our analysis indicates that more accurate threshold resummation formalism should be developed in order to obtain convincing predictions.

hep-ph↗

arXiv1998

Origin of the $k_T$ smearing in direct photon production

We show that the Sudakov factor from the resummation of double logarithms $\ln(s/k_T^2)$ contained in the distribution functions is responsible for the $k_T$ smearing mechanism employed in the next-to-leading-order QCD ($αα_s^2$) calculations of direct photon production. $s$ is the center-of-mass energy, and $k_T$ the transverse momentum carried by a parton in a colliding hadron. This factor exhibits the appropriate $s$-dependent Gaussian width in $k_T$, such that our predictions are in good agreement with experimental data.

hep-ph↗