  We discuss a CMS eXtension for Studying Energetic Neutrinos (CMS-XSEN).
Neutrinos at the LHC are abundant and have unique features: their energies
reach out to the TeV range, and the contribution of the {\tau} flavour is
sizeable. The measurement of their interaction cross sections has much physics
potential. The pseudorapity range 4<|{\eta}|<5 is of particular interest since
leptonic W decays provide an additional contribution to the neutrino flux from
b and c production. A modest detector of 4.1x$10^{27}$ nucleons/cm$^{2}$,
placed in the LHC tunnel, 25 m from the interaction point, around the focusing
magnet (Q1) closest to CMS, can cover that region. The hadronic calorimeter HF
and the CMS forward shield will protect it from the debris of pp collisions.
With a luminosity of 300/fb, foreseen for the LHC run in the years 2021-2023,
the detector can observe over a thousand {\tau} neutrino interactions, and a
hundred TeV-neutrino interactions of all flavours. Several backgrounds are
considered. A major source can be prompt muons from the interaction point.
However, the CMS magnetic field and the structure of the Forward Shield make
the estimation of their flux in the location of interest uncertain. Besides,
machine induced backgrounds are expected to vary rapidly while moving along and
away from the beam line. We propose to acquire experience during the 2018 LHC
run by a brief test with a small Neutrino Experiment Demonstrator, based on
nuclear emulsions.
