  The differential cross section for inclusive particle production as a
function of energy in proton-proton collisions at a center-of-mass energy of 13
TeV is measured in the very forward region of the CMS detector. The measurement
is based on data collected with the CMS apparatus at the LHC, and corresponds
to an integrated luminosity of 0.35 inverse microbarns. The energy is measured
in the CASTOR calorimeter, which covers the pseudorapidity region -6.6 < eta <
-5.2. The results are given as a function of the total energy deposited in
CASTOR, as well as of its electromagnetic and hadronic components. The spectra
are sensitive to the modeling of multiparton interactions in pp collisions, and
provide new constraints for hadronic interaction models used in collider and in
high energy cosmic ray physics.
