SciPost Submission Page

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:
Date submitted: 2019-07-03
Submitted by: Harland-Lang, Lucian
Submitted to: SciPost Physics
Domain(s): Theoretical
Subject area: High-Energy Physics - Phenomenology


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:
Editor-in-charge assigned

Submission & Refereeing History

Submission 1906.10127v1 on 3 July 2019

Reports on this Submission

Anonymous Report 1 on 2019-7-23 Invited Report


1. Gives a reasonable estimate of the effect of a future LHeC on the uncertainties of Parton Distribution Functions
2. Relates the work to other studies with critical comment
3. Relates the work to BSM and SM cross sections of current interest
4. Most points are made very clearly


1. Does not deal with realistic systematic uncertainties- but to be fair this is close to impossible with future projections
2. LHeC pseudo-data are used which may be superseded soon- but that is not anything the authors could control
2. The odd point is obscure


This paper makes as assessment of the likely impact of a future LHeC on the uncertainties on parton distribution functions PDFs using similar procedures to a recent paper which assessed the impact of the HL-LHC (by an overlapping set of authors). It is a valuable contribution to the debate. It also addresses most of the criticisms that could be made of such a study within the paper itself and as such it is a pleasure to read. I particularly enjoyed the discussion of appropriate tolerances.
Even if the LHeC pseudodata used are supeseded the paper makes a contribution in comparing methods currently used to assess their significance, which will be useful in future studies.
I recommend publication and have only a few minor comments to make under requested changes, which the authors may wish to consider.

Requested changes

1. 1 TeV proton data are mentioned in context of extending the kinematic range, would these not also be useful for the measurement of FL and hence the gluon PDF--please comment.
2. Add to references 31 and 32 a reference to arXIV:1906.01884 on the same topic.
3. The use of open charm and beauty production and exclusive vector meson production to constrain the low-x gluon is made. The theoretical status of these processes is not on the same level as the cross sections used and I think this should be pointed out.
4. On page 15 it becomes a little confusing when it is stated that 'when the LHeC pseduo-data are generated with this more restrictive HERAPDF2.0 parametrisation one is making strong assumptions about the future'. Surely the LHeC pseudo-data are whatever they are independent of whether ot not PDF4LHC15 or HERAPDF2.0 are being used for the profiling exersize. The strong assumption is that the HERAPDF parametrisation will be adequate to describe future LHeC data. Can the authors please clarify, or re-phrase?
5. In Fig 5.5 an attempt is made to separte the role of the input data set and the parametrisation. This is quite interesting but the model and parametrisation variations for the HERAPDF are not used. It would be interesting to see this figure when they are used. However since this is not the most important point of the paper, I do not insist.

  • validity: high
  • significance: high
  • originality: high
  • clarity: high
  • formatting: excellent
  • grammar: excellent

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