Precision Determination of the Small-x Gluon from Charm Production at LHCb

The small-x gluon in global fits of parton distributions is affected by large uncertainties from the lack of direct experimental constraints. In this Letter, we provide a precision determination of the small-x gluon from the exploitation of forward charm production data provided by LHCb for three different center-ofmass (c.m.) energies: 5 TeV, 7 TeV, and 13 TeV. The LHCb measurements are included in the parton distribution function (PDF) fit by means of normalized distributions and cross-section ratios between data taken at different c.m. values, R-13/7 and R-13/5. We demonstrate that forward charm production leads to a reduction of the PDF uncertainties of the gluon down to x similar or equal to 10(-6) by up to an order of magnitude, with implications for high-energy colliders, cosmic ray physics, and neutrino astronomy.