  A combination is presented of all inclusive deep inelastic cross sections
previously published by the H1 and ZEUS collaborations at HERA for neutral and
charged current $e^{\pm}p$ scattering for zero beam polarisation. The data were
taken at proton beam energies of 920, 820, 575 and 460 GeV and an electron beam
energy of 27.5 GeV. The data correspond to an integrated luminosity of about 1
fb$^{-1}$ and span six orders of magnitude in negative four-momentum-transfer
squared, $Q^2$, and Bjorken $x$. The correlations of the systematic
uncertainties were evaluated and taken into account for the combination. The
combined cross sections were input to QCD analyses at leading order,
next-to-leading order and at next-to-next-to-leading order, providing a new set
of parton distribution functions, called HERAPDF2.0. In addition to the
experimental uncertainties, model and parameterisation uncertainties were
assessed for these parton distribution functions. Variants of HERAPDF2.0 with
an alternative gluon parameterisation, HERAPDF2.0AG, and using
fixed-flavour-number schemes, HERAPDF2.0FF, are presented. The analysis was
extended by including HERA data on charm and jet production, resulting in the
variant HERAPDF2.0Jets. The inclusion of jet-production cross sections made a
simultaneous determination of these parton distributions and the strong
coupling constant possible, resulting in $\alpha_s(M_Z)=0.1183 \pm 0.0009
{\rm(exp)} \pm 0.0005{\rm (model/parameterisation)} \pm 0.0012{\rm
(hadronisation)} ^{+0.0037}_{-0.0030}{\rm (scale)}$. An extraction of
$xF_3^{\gamma Z}$ and results on electroweak unification and scaling violations
are also presented.
