Conveners: Néstor Armesto, Claire Gwenlan and Paul Newman.
Indico web page for meetings: https://indico.cern.ch/category/17307/.
This WP aims to establish the possibilities for the determination of proton and nuclear structure at the LHeC and FCC-eh. It is also concerned with the QCD dynamics at small x and the eventual existence of a new non-linear regime of QCD where parton densities saturate. The complementarity with previous studies at HERA and at the LHC, and in the future at the HL-LHC and the EIC, are also addressed, as well as the implications for hadron colliders, present and future.
The present status of the studies on precision QCD is provided in the 2020 update of the CDR and in recent presentations at conferences. It can be summarised in the following items:
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Collinear parton densities in the proton and nucleus can be disentagled, for all parton species, in a kinematic region that extends that of HERA by 1-2 orders of magnitude down in x and up in Q2 for the proton, and that of fixed target experiments by 3-4 orders of magnitude for nuclei. Nuclear PDFs will be measured without any reference to a proton baseline. All this will be achieved by a combination of neutral and charged current cross section mesurements with additional heavy quark identification.
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The strong coupling constant will be extracted with per mille accuracy from a combination of measurements of inclusive and jet cross sections.
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Inclusive measurements with a large lever-arm in Q2 at small x will allow to clearly establish whether non-linear dynamics are relevant in that kinematic regime or not. This will be achieved by searching for tensions in DGLAP fits which show a different Q2 evolution from non-linear dynamics in this kinematic region.
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Diffractive parton densities will be extracted, for proton and nuclei, in an extended kinematic regime compared to HERA. They will clarify the validity of Regge factorisation, and the quark and gluon content of the colourless exchanges in those collisions.
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Exclusive production of photons and vector mesons will provide 3D information of proton and nuclear structure for quark and gluon distributions, in a small-x region complementary to that accessible at the EIC.
At present, the efforts of the WG focus on:
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the complementarity with the EIC for the extraction of parton densities and the strong coupling constant;
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the complementarity with the HL-LHC on the interplay of PDFs and BSM for the extraction of SMEFT parameters; and
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the possibility of disentagling linear effects beyond DGLAP, namely resummation of logarithms of 1/x, from DGLAP or eventual non-linear dynamics, in inclusive cross sections.
