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Probing Proton Structure at the Large Hadron electron Collider

by Rabah Abdul Khalek, Shaun Bailey, Jun Gao, Lucian Harland-Lang, Juan Rojo

Submission summary

As Contributors: Lucian Harland-Lang
Arxiv Link: https://arxiv.org/abs/1906.10127v2
Date submitted: 2019-09-05
Submitted by: Harland-Lang, Lucian
Submitted to: SciPost Physics
Domain(s): Theoretical
Subject area: High-Energy Physics - Phenomenology

Abstract

For the foreseeable future, the exploration of the high-energy frontier will be the domain of the Large Hadron Collider (LHC). Of particular significance will be its high-luminosity upgrade (HL-LHC), which will operate until the mid-2030s. In this endeavour, for the full exploitation of the HL-LHC physics potential an improved understanding of the parton distribution functions (PDFs) of the proton is critical. The HL-LHC program would be uniquely complemented by the proposed Large Hadron electron Collider (LHeC), a high-energy lepton-proton and lepton-nucleus collider based at CERN. In this work, we build on our recent PDF projections for the HL-LHC to assess the constraining power of the LHeC measurements of inclusive and heavy quark structure functions. We find that the impact of the LHeC would be significant, reducing PDF uncertainties by up to an order of magnitude in comparison to state-of-the-art global fits. In comparison to the HL-LHC projections, the PDF constraints from the LHeC are in general more significant for small and intermediate values of the momentum fraction x. At higher values of x, the impact of the LHeC and HL-LHC data is expected to be of a comparable size, with the HL-LHC constraints being more competitive in some cases, and the LHeC ones in others. Our results illustrate the encouraging complementarity of the HL-LHC and the LHeC in terms of charting the quark and gluon structure of the proton.

Current status:
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Reports on this Submission

Anonymous Report 2 on 2019-9-12 Invited Report

Report

The authors have addressed all comments of the previous report in a satisfactory manner. I recommend publication.

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Anonymous Report 1 on 2019-9-9 Invited Report

Report

I am now happy that this paper be published. I note a few 'typos' in requested changes below--basically the legends in Figs5.2,5.4 and 5.5,5.6 will not be obvious to a non-expert, or indeed if these figures are used alone, not with their companions 5.1and 5.3.
I am sure the authors will wish to correct this. I don't need to see the paper again.

I have one general comment and it concerns the authors' reply to referee 2.
The authors rightly note that T=1 may not even be applicable to HERA, with which I can- at least partially- agree, and from this deduce that it also would not be applicable to the LHeC, with which I do not agree.
The point is that at the time that HERA was running there was still a focus on finding new physics there and although measurements related to PDFs were considered very important, they were not the paramount aim. Hence insufficient attention was paid to the full consistency of methods of deriving systematic uncertainties both within and between the experiments. This is why it took until 2015 to disentangle all this information to the best of our ability- and some discrepancies remain. In the case of the LHeC the DIS cross sections needed for PDFs will/would be the paramount aim. We have learnt a lot from HERA and we would have accords as to how to handle systematics. The data will/would be analysed in a consistent manner across the whole kinematic plane, and from year to year. Hence T=1 really could be the correct tolerance.

Requested changes

Figs 5.2 and 5.4 have both lost the +LHeC from the second appearance of HERAPDF in their legends. Just compare to 5.1 and 5.3 to see what I mean.
In Figs 5.5 and 5.6 (and I thank the authors for aving added 5.6 as I requested..) I think the green line at unity is now the PDF4LHC profiling result (it was the pre-profling result in may previous figures) for comparison to the HERAPDF profling results. But the legend does not actually specify this. I think it needs to so that the figure can be used 'stand-alone'.

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