  LHCSpin aims at installing a polarized gas target in front of the LHCb
spectrometer, bringing, for the first time, polarized physics to the LHC. The
project will benefit from the experience achieved with the installation of an
unpolarized gas target at LHCb during the LHC Long Shutdown 2. LHCb will then
become the first experiment simultaneously running in collider and fixed-target
mode with polarized targets, opening a whole new range of explorations to its
exceptional spectrometer.
  LHCSpin will offer a unique opportunity to probe polarized quark and gluon
parton distributions in nucleons and nuclei, especially at high $x$ and
intermediate $Q^2$, where experimental data are still largely missing. Beside
standard collinear parton distribution functions (PDFs), LHCSpin will make it
possible to study multidimensional polarized parton distributions that depend
also on parton transverse momentum.
  The study of the multidimensional partonic structure of the nucleon,
particularly including polarization effects, can test our knowledge of QCD at
an unprecedented level of sophistication, both in the perturbative and
nonperturbative regime. At the same time, an accurate knowledge of hadron
structure is necessary for precision measurements of Standard Model (SM)
observables and discovery of physics beyond the SM.
  Due to the intricate nature of the strong interaction, it is indispensable to
perform the widest possible suite of experimental measurements. It will be
ideal to have two new projects complementing each other: a new facility for
polarized electron-proton collisions and a new facility for polarized
proton-proton collisions. LHCSpin stands out at the moment as the most
promising candidate for the second type of project, going beyond the kinematic
coverage and the accuracy of the existent experiments, especially on the
heavy-quark sector.
