The impact of LHC jet data on the MMHT PDF fit at NNLO

Harland–Lang, Lucian; Martin, A. D.; Thorne, Robert S.

doi:10.1140/epjc/s10052-018-5710-7

Cited by 46 publications

(72 citation statements)

References 26 publications

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“…Such modifications to gluon PDF are not totally unexpected. The MMHT14 gluon PDF [6], which closely resembles that of CT14, acquires rather similar modifications when confronted with the 7 TeV high-luminosity inclusive jet data [30]. Also, attributed to including 8 TeV differential topquark data, the NNPDF3.1 fit has large-x gluons suppressed compared to CT14 and MMHT14 [31].…”

Section: Proton-proton Dijet Spectra and Ct14 Reweightingmentioning

confidence: 86%

Non-quadratic improved Hessian PDF reweighting and application to CMS dijet measurements at 5.02 TeV

2019

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Hessian PDF reweighting, or "profiling", has become a widely used way to study the impact of a new data set on parton distribution functions (PDFs) with Hessian error sets. The available implementations of this method have resorted to a perfectly quadratic approximation of the initial χ 2 function before inclusion of the new data. We demonstrate how one can take into account the first non-quadratic components of the original fit in the reweighting, provided that the necessary information is available. We then apply this method to the CMS measurement of dijet pseudorapidity spectra in proton-proton (pp) and proton-lead (pPb) collisions at 5.02 TeV. The measured pp dijet spectra disagree with next-to-leading order (NLO) theory calculations using the CT14 NLO PDFs, but upon reweighting the CT14 PDFs, these can be brought to a much better agreement. We show that the needed proton-PDF modifications also have a significant impact on the predictions for the pPb dijet distributions. Taking the ratio of the individual spectra, the proton-PDF uncertainties effectively cancel, giving a clean probe of the PDF nuclear modifications. We show that these data can be used to further constrain the EPPS16 nuclear PDFs and strongly support gluon nuclear shadowing at small x and antishadowing at around x ≈ 0.1. arXiv:1903.09832v2 [hep-ph] 26 Sep 20191 The intricacies of choosing an appropriate value for ∆ χ 2 are outside the scope of this article, see Refs. [17,21,22] for discussion.

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Section: Proton-proton Dijet Spectra and Ct14 Reweightingmentioning

confidence: 86%

Non-quadratic improved Hessian PDF reweighting and application to CMS dijet measurements at 5.02 TeV

2019

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“…In particular, as we will see it is the correlation across different distributions that drives the observed deterioration in fit quality. To investigate the above effect further, we first follow the study of [3] and evaluate the variation in the preferred values of the corresponding systematic shifts in (1) when fitting to the different distributions individually. Any significant tension between these values will then indicate that when fitting the distributions in combination, some deterioration in the fit quality will occur, and moreover that a less restrictive degree of correlation between the distribution for the corresponding shift may be preferred by the data/theory comparison.…”

Section: Understanding the Poor Fit Quality: The Role Of Correlated Smentioning

confidence: 99%

“…It should in particular be emphasised that this issue is not bypassed by fitting to one individual distribution alone, which would artificially mask the issues in fitting the complete dataset, while also risking introducing a bias into the fit, through the particular choice of distribution that is made.Further study of the above effects is clearly essential, and indeed in [16], the source of this poor fit quality was traced to a small subset of dominant two-point systematic uncertainties associated with the choice of Monte Carlo generator or input parameters, used in the data unfolding to the parton-level results which are included in the PDF fit. A rather significant improvement in the fit quality, with a relative stability in the resultant gluon, was then observed by introducing some decorrelation in a given systematic error between distributions, reminiscent of the studies [3,11] for the case of jet production. On the other hand, this study was only performed within the ATLAS PDF framework, fitting to a limited dataset and with a rather…”

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confidence: 98%

Differential top quark pair production at the LHC: Challenges for PDF fits

Bailey¹,

Harland–Lang²

2020

Eur. Phys. J. C

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We present the results of a PDF fit to differential top quark production within the MMHT framework. We in particular consider ATLAS data in the lepton + jet and dilepton channels and CMS data in the lepton + jet channel, at 8 TeV. While the fit quality to the ATLAS dilepton data is good, for the CMS case we see some issues in achieving a good fit quality for certain distributions. However, we focus on the ATLAS lepton + jet data, for which correlations of the statistical and systematic errors are provided across the four relevant distributions for PDF determination, namely p t T , M tt , y t and y tt . We find severe difficulties in fitting these distributions simultaneously, with particular sensitivity to the precise degree of correlation taken between the dominant two-point MC uncertainties in the data. We investigate the effect of some reasonable decorrelation of these uncertainties, finding the impact on the fit quality to be significant and the resultant gluon not negligible. This is in particular found to be larger than the effect of including NNLO QCD and NLO EW corrections in the top quark pair production cross section on the fit, motivating a closer understanding of the physics underlying these errors sources and in particular the uncertainty on the degree of correlation in them.The determination of proton structure via the parton distribution functions (PDFs) is an integral part of the LHC physics programme [1,2]. In particular, as we enter the high precision LHC era, in terms of both theory and experiment, a detailed control over all uncertainties associated with the PDFs is essential. An area of particular phenomenological relevance is the determination of the gluon PDF at high x, which can for example play an important role in predictions for new heavy BSM states via gluon-initiated production, but until more recently has suffered from a relative lack of direct constraints, resulting in rather large PDF uncertainties. However, with the advent of high precision LHC data at higher masses and transverse momenta this situation has in principle changed rather dramatically. As discussed in [3][4][5], the inclusion of inclusive jet, Z boson transverse momentum, and differential top quark pair production in PDF fits places important constraints on the high x gluon that were lacking in earlier PDF fits. In all cases, these benefit from cutting edge high precision theory calculations at NNLO in QCD [6][7][8][9].On the other hand, the inclusion of such processes in PDF fits is not without issues. In the case of jet production, it is well known [3, 10, 11] that the ATLAS jet production data at 7 and 8 TeV cannot be well fit across all rapidity bins. For the Z boson transverse momentum distribution, issues with fitting the 7 TeV (normalized) data [12], and the CMS 8 TeV data [13] in the highest rapidity bin have been reported [5], while more generally in this study an additional source of uncorrelated error, assumed to be due to residual theoretical uncertainties and possible underestimated experimental errors, h...

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“…The factorization has been applied successfully at collider and fixed target experiments. In particular, it is possible to consistently extract universal parton distribution functions (PDFs) within global analyses from different processes and experiments [4][5][6][7][8]. These phenomenological results support the validity of QCD factorization in proton-proton (p + p) collisions and the universality of PDFs, which ensures the predictive power of the approach.…”

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confidence: 85%

Factorization of Jet Cross Sections in Heavy-Ion Collisions

et al. 2019

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We propose a new phenomenological approach to establish QCD factorization of jet cross sections in the heavy-ion environment. Starting from a factorization formalism in proton-proton collisions, we introduce medium modified jet functions to capture the leading interaction of jets with the hot and dense QCD medium. A global analysis using a Monte Carlo sampling approach is performed in order to reliably determine the new jet functions from the nuclear modification factor of inclusive jets at the LHC. We find that gluon jets are significantly more suppressed due to the presence of the medium than quark jets. In addition, we observe that the jet radius dependence is directly related to the relative suppression of quark and gluon jets. Our approach may help to improve the extraction of medium properties from data.

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The impact of LHC jet data on the MMHT PDF fit at NNLO

Cited by 46 publications

References 26 publications

Non-quadratic improved Hessian PDF reweighting and application to CMS dijet measurements at 5.02 TeV

Non-quadratic improved Hessian PDF reweighting and application to CMS dijet measurements at 5.02 TeV

Differential top quark pair production at the LHC: Challenges for PDF fits

Factorization of Jet Cross Sections in Heavy-Ion Collisions

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