Differential Top Quark Pair Production at the LHC: Challenges for PDF Fits

TL;DR

This paper investigates the challenges of fitting parton distribution functions (PDFs) using differential top quark pair production data from the LHC, highlighting issues with data correlations and their impact on gluon distribution modeling.

Contribution

It demonstrates the sensitivity of PDF fits to data correlation assumptions and explores the effects of decorrelating uncertainties on the fit quality and gluon distribution.

Findings

Fitting ATLAS lepton + jet data is challenging due to correlation issues.

Decorrelation of uncertainties significantly improves fit quality.

Uncertainty on data correlations impacts the gluon PDF more than NNLO QCD and NLO EW corrections.

Abstract

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.