  We present the new MSHT20 set of parton distribution functions (PDFs) of the
proton, determined from global analyses of the available hard scattering data.
The PDFs are made available at NNLO, NLO, and LO, and supersede the MMHT14
sets. They are obtained using the same basic framework, but the
parameterisation is now adapted and extended, and there are 32 pairs of
eigenvector PDFs. We also include a large number of new data sets: from the
final HERA combined data on total and heavy flavour structure functions, to
final Tevatron data, and in particular a significant number of new LHC 7 and 8
TeV data sets on vector boson production, inclusive jets and top quark
distributions. We include up to NNLO QCD corrections for all data sets that
play a major role in the fit, and NLO EW corrections where relevant. We find
that these updates have an important impact on the PDFs, and for the first time
the NNLO fit is strongly favoured over the NLO, reflecting the wider range and
in particular increased precision of data included in the fit. There are some
changes to central values and a significant reduction in the uncertainties of
the PDFs in many, though not all, cases. Nonetheless, the PDFs and the
resulting predictions are generally within one standard deviation of the MMHT14
results. The major changes are the $u-d$ valence quark difference at small $x$,
due to the improved parameterisation and new precise data, the $\bar d, \bar u$
difference at small $x$, due to a much improved parameterisation, and the
strange quark PDF due to the effect of LHC $W,Z$ data and inclusion of new NNLO
corrections for dimuon production in neutrino DIS. We discuss the
phenomenological impact of our results, and in general find reduced
uncertainties in predictions for processes such as Higgs, top quark pair and
$W,Z$ production at post LHC Run-II energies.
