Z-boson Transverse-Momentum Resummation Improves PDF Precision

A new study has investigated the impact of small-transverse-momentum resummation for neutral-current Drell-Yan lepton-pair production on the determination of collinear parton distribution functions (PDFs). Researchers focused on measurements of the Z-boson transverse-momentum spectrum performed by the ATLAS and CMS collaborations at the LHC at centre-of-mass energies of 8 and 13 TeV, including them in PDF fits based on the NNPDF methodology. This analysis is crucial for refining our understanding of the internal structure of protons, a fundamental aspect for theoretical predictions in high-energy physics. Theoretical predictions were computed at next-to-next-to-leading order (NNLO) in perturbative quantum chromodynamics, supplemented with small-transverse-momentum resummation corrections at next-to-next-to-next-to-leading logarithmic accuracy obtained with the RadISH tool. Missing higher-order uncertainties were accounted for through a theory covariance matrix constructed from renormalisation and factorisation scale variations. The study revisited the treatment of 8 TeV data, replacing fixed-order predictions used in previous analyses with numerically stable NNLO calculations and removing additional numerical uncertainties introduced in earlier fits. The results indicate that resummation improves the description of Z boson transverse-momentum data and is essential for ensuring the overall consistency of PDF fits. Nevertheless, it does not conclusively support extending the fitted kinematic region to transverse momenta below a few tens of GeV. The impact of resummation on PDFs is moderate, primarily leading to a stabilisation of the gluon PDF in a kinematic region of relevance for LHC phenomenology. Furthermore, the study highlights that the treatment of correlations among theoretical uncertainties may play a central role in PDF fits to measurements with percent- and sub-percent-level precision.