Establishing the Higgs-boson physics and the potential of the LHC for further discoveries, in particular in high-energy run at 13 and 14 TeV, depend severely on the precision of the Standard Model predictions. The latter is limited by the knowledge of the proton structure, yet unsolved phenomenological questions in the QCD analyses and accuracy of measured quark masses and strong coupling constant. Significant improvement in precision of these fundamental quantities can be only achieved in a joined effort of the experimental and the theory groups, involved in the analysis and interpretation of the LHC measurements, in particular sensitive to the QCD parameters and the proton structure, expressed by parton distribution functions (PDFs). DESY CMS group is strongly involved in the analysis of those Standard Model processes at the LHC, which have particular sensitivity to quark and gluon distribtuions in the proton, strong coupling constant and heavy quark masses.
PDFs and QCD parameters at the LHC
Several LHC analyses have already being included in the QCD analysis: production of Z-boson, W-production and charge asymmetry, top-quark pair production, associated production of W+charm and events with jets.
Production of W and Z bosons at the LHC is in particular sensitive to u- and d- quark distributions in the proton. These are of particular importance for predictions of e.g. single top-quark production: different assumptions on ratio of u to d quarks in the proton results in an almost 50% uncertainty in prediction for single-top production cross section at 14 TeV.
By including measurements of associated W+charm production in the QCD analysis, we get direct probe of strange quark in the proton - a unique measurement! The only possible access to the strange quarks in the sea was possible at earlier fixed target experiments using neutrinos, but these measurements have large corrections. Also, the information about the correlations of the experimental uncertainties is lost.
Inclusive and multi-jet production is in particular sensitive to the value of strong coupling constant and using these data in the global analysis can help reducing the correlation between the gluon distribution and the strong coupling.
Top-quark pair production at the LHC probes the gluon distribution at high x, and heavy mass of top quark allows theorists to calculate the process up to next-to-next-to-leading order QCD, which is not yet done for e.g. jet production. Also, the unambiguously defined mass of the top quark can be obtained using the top-pair cross sections measured at the LHC.
The results and questions of relevance, in particular the correlation of experimental uncertainties, are discussed within ATLAS and CMS Experiments in the PDF Forums.
Read here more about PDF Forum in CMS (CMS internal)
XFitter QCD Analysis Tool
The global QCD analysis of different data sets involves usage of tools, based on or interfaced to different theoretical calculations. Quite some phenomenology is involved in the development of a tool for QCD analysis or PDF fit. The QCD analyses of the LHC data are performed within ATLAS and CMS collaborations using the general QCD platform XFitter, one of the projects strongly supported at DESY. XFitter is an open-source program, which provides the basis for comparisons of different theoretical approaches and can be used for direct tests of the impact of new experimental data in the QCD analyses through coherent treatment of the data and theory calculations. Originally developed by the H1 and ZEUS collaborations and used for determination of HERAPDF sets, XFitter has been extended to the LHC experiments and to the implementation of different phenomenological approaches. The package includes tools Applgrid, fastNLO, Hathor, NNPDF-reweighting, QCDNUM, Difftop etc. DESY CMS members contribute significantly to support and further development of the program.
Related activities and person power associated to the topic
Last updated on 20-11-2017