Determination of fundamental QCD parameters and Proton Structure with LHC data

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).

The precision in the fundamental parameters of QCD, like quark masses and the strong coupling plays an immense role in searches for New Physics. As of now, no sign of New States, which would manifest as a clear peak at the mass of new particles, is observed at the LHC. Therefore, the most promising probe of New Physics would be in the search for New Interactions, which could be observed as deviations of momentum or angular distributions from the Standard Model expectation. Being the most powerful method for searches, since probing much higher energies then experimentally available, this is also the most challenging one. It relies to large extent on the understanding of the Standard Model, since the observed final states represent a folded mixture of new physics and the standard model processes, driven by the strong interaction between the quarks and the gluons. Besides the highest possible precision in the description of the proton structure and couplings of the strong interactions, an unbiased method for the search has to be established, to disentangle the new interaction from the effects induced by the strong force.

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. In particular, we concentrate on alternative methods to measure the mass of the top quark at highest possible precision.

We work closely to the leading experts in theory of Quantum Chromodynamics and physics beyond the Standard Model and develop the tools for the first unbiased search for New Physics, performing a combined (SMEFT) fit to different measurements of a variety of processes at the LHC in order to determine simultaneously the couplings of the Standard Model and the effective couplings of a possible new (contact) interaction.

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. Recently, the measurement and the QCD analysis of associated production of W+C at the CMS at 13 TeV was published.

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. We are performing the measurements of the inclusive jet production at the CMS at 13 TeV and are the first group extracting the parton distributions and the strong coupling using the jet production in proton-proton collisions at the perturbative order of QCD beyond NLO. We are working closely with the word-leading theory groups in the subject, as Durham and University of Hamburg/Bejing University.

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.

Activities and open projects

General PDF phenomenology and Tool Development
  • Recommendations for usage of PDFs in the LHC analyses see publication of 2016
  • Review on the Impact of the LHC data on PDFs see publication of 2015
  • Development of XFitter (former HERAFitter) see publication

    Collaboration with other Institutes, Experiments and Theory Groups
  • Inclusive and Semi-Inclusive Constraints on the Parton Distributions at the LHC and the Study of Hard Processes Joined Project of DESY and Universities of Hamburg, Freiburg, Karlsruhe, Mainz and Wuppertal
  • Determination of the proton structure using deep inelastic scattering and proton-proton collisions Dedicated project of Helmholtz association and DESY
  • Proton Structure Collaboration website
  • Scientific exchange program between DESY and Southern Methodist University (Dallas, USA). Contacts P. Nadolsky for CTEQ collaboration, K. Lipka for DESY

    Last updated on 20-11-2017


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