Designed for the discovery of the Higgs boson and physics beyond the Standard Model, the LHC is a QCD machine operating in a new energy regime. The LHC experiments are confronted with unprecedented rates of events with multiple jets and vector bosons, as well as top quarks. The full discovery potential therefore depends on reliable predictions for signal and background processes. Measurements of various observables in different regions of phase space are required in order to extract complementary information on the fundamental parameters of the theory. Most involved observables are interconnected by their sensitivity to the proton structure, expressed via parton distribution functions (PDFs).

Collaboration

Members of the group belong to ATLAS and CMS groups at DESY and contribute significantly to development and support of XFitter and are involved in determination of QCD parameters and parton disitribution functions using DIS and proton collision data. This work is realized in close collaboration with theorists at DESY, University of Hamburg and Southern Methodist University in Dallas. The project group is the origin and the core of the PROSA collaboration.

Computing tools, developed in the scope of the project :

XFitter QCD analysis framework

The open-source program XFitter 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.

Difftop

DiffTop is the Fortran-based package which allows the user to calculate the differential and total cross section for heavy-quark pair production at hadron colliders in One-particle inclusive (1PI) kinematics. The cross sections are calculated in perturbative QCD at approximate next-to-next-to-leading order (approx.NNLO) by using methods of threshold resummation beyond the leading logarithmic accuracy.

Scientific publications in peer-reviwed journals with major contributions by the project members:

S. Alekhin, K. Daum, K. Lipka, S. Moch, Phys. Lett. B 718 (2012) 550 Determination of the charm-quark mass in the MS-bar scheme using charm production data from deep inelastic scattering at HERA,

S. Alekhin, J. Bluemlein, K. Daum, K. Lipka, S. Moch, Phys.Lett. B 720 (2013) 172-176 Precise charm-quark mass from deep-inelastic scattering,

G. Aad et al. [ATLAS Collaboration] Phys.Rev.Lett. 109 (2012) 012001, Determination of the strange quark density of the proton from ATLAS measurements of the W->l nu and Z->ll cross sections ,

B. Malaescu, P. Starovoitov, Eur.Phys.J. C72 (2012) 2041, Evaluation of the Strong Coupling Constant alpha_s Using the ATLAS Inclusive Jet Cross-Section Data ,

S. Chatrchyan et al. [CMS Collaboration], Phys. Lett. B 728 (2013) 496, Determination of the top-quark pole mass and strong coupling constant from the t t-bar production cross section in pp collisions at sqrt(s) = 7 TeV ,

M. Guzzi et al, Eur. Phys. J. A49 (2013) 35, Massive neutral gauge boson production as a probe of nuclear modifications of parton distributions at the LHC ,

J. Gao, M. Guzzi, P. Nadolsky, Eur. Phys. J. C73 (2013) 2541, Charm quark mass dependence in a global QCD analysis ,

S. Chatrchyan et al. [CMS Collaboration], Phys. Rew. D 90 (2014) 032004, Measurement of the muon charge asymmetry in inclusive pp to WX production at sqrt(s) = 7 TeV and an improved determination of light parton distribution functions ,

S. Alekhin et al., Phys. Rew. D. 91, 094002 (2015) Determination of Strange Sea Quark Distributions from Fixed-target and Collider Data ,

J. Gao, M. Guzzi et al, Phys. Rev. D 89, 033009 (2014), The CT10 NNLO Global Analysis of QCD ,

G. Aad et al. [ATLAS Collaboration], JHEP1406, 112 (2014), Measurement of the low-mass Drell-Yan differential cross section at sqrt(s) = 7 TeV using the ATLAS detector ,

M. Guzzi, P. M. Nadolsky, B. Wang, Phys. Rev. D 90, 014030 (2014), Nonperturbative contributions to a resummed leptonic angular distribution in inclusive neutral vector boson production

P. Belov et al. [XFitter Collaboration], Eur. Phys. J. C74,10, 3039 (2014) Parton distribution functions at LO, NLO and NNLO with correlated uncertainties between orders ,

M. Guzzi, K. Lipka, S.-O. Moch, DESY-14-077, JHEP 1501 (2015) 082, Top-quark pair production at hadron colliders: differential cross section and phenomenological applications with DiffTop ,

S. Alekhin et al. [XFItter Group], arXiv:1410.4412, accepted by Eur. Phys. J. HERAFitter, Open Source QCD Fit Project ,

O. Zenaiev et al. [PROSA Collaboration], arXiv:1503.04581, in review by Eur. Phys. J. Impact of heavy-flavour production cross sections measured by the LHCb experiment on parton distribution functions at low x .

Experiment-reviewed public results by the project members:

[CMS Collaboration] Determination of the top-quark mass from the mlb distribution in dileptonic ttbar events at sqrt(s) = 8 TeV (CMS-PAS-TOP-14-014)

[PROSA Collaboration] Impact of the LHCb measurements of forward charm and beauty production on PDFs (PROSA-14-001)

[CMS Collaboration] W differential lepton charge asymmetry at 7 TeV and QCD analysis of W charge asymmetry and W+c (CMS-SMP-12-021)

[CMS Collaboration] W differential lepton charge asymmetry at 8 TeV (CMS-SMP-14-022)

[ATLAS Collaboration] Studies of theoretical uncertainties on the measurement of the mass of the W boson at the LHC (ATL-PHYS-PUB-2014-015)

[CMS Collaboration] Study of the underlying event, b-quark fragmentation and hadronization properties in tbart events (CMS-PAS-TOP-13-007)

[CMS Collaboration] Projected improvement of the accuracy of top-quark mass measurements at the upgraded LHC (CMS-PAS-FTR-13-017)

[ATLAS Collaboration] Measurement of the high-mass Drell-Yan differential cross-section in pp collisions at sqrt{s} =7 TeV with the ATLAS detector (ATLAS-CONF-2012-159)

[ATLAS Collaboration] (ATL-CONF-2012-128) Measurement of the inclusive jet cross section in pp collisions at sqrt(s) = 2.76 TeV and comparison to the inclusive jet cross section at sqrt(s) = 7 TeV using the ATLAS detector

[CMS Collaboration] First Determination of the Strong Coupling Constant from the ttbar Cross Section (CMS-PAS-12-022)

Members