Using a data sample corresponding to an integrated luminosity of 2.0 fb−1, collected by the LHCb experiment, the production of the (1) state in proton–proton collisions at a centre-of-mass energy of √=13 TeV is studied in the rapidity range 2.0<<4.5 and in the transverse momentum range 6.5<T<14.0GeV. The cross-section for prompt production of (1) mesons relative to that of the / meson is measured using the ⎯⎯⎯ decay mode and is found to be (1)//=1.69±0.15±0.10±0.18. The quoted uncertainties are, in order, statistical, systematic and due to uncertainties on the branching fractions of the /→⎯⎯⎯ and →⎯⎯⎯ decays. The prompt (1) production cross-section is determined to be (1)=1.26±0.11±0.08±0.14μb, where the last uncertainty includes that on the / meson cross-section. The ratio of the branching fractions of -hadron decays to the (1) and / states is measured to be →/→/=0.48±0.03±0.03±0.05, where the last uncertainty is due to those on the branching fractions of the /→⎯⎯⎯ and →⎯⎯⎯ decays. The difference between the / and (1) masses is also determined to be 113.0±0.7±0.1MeV, which is the most precise single measurement of this quantity to date
