Abstract: In inelastic collisions at the LHC most of the produced particles have small transverse momentum, less than 2 GeV/c, with high rapidity and large longitudinal momentum fraction Feynman-x (= pL/pbeam). The four large detectors focus on high-pT and high mass and cover the central region (pseudorapidity eta less than about 6). Charged particles with small pT and Feynman-x ~ 0.1 - 0.8 have not been measured at higher energies than square-root(s) = 63 GeV at the CERN ISR. At the LHC we are 200 times higher in square-root(s), i.e. 40,000 times higher in equivalent fixed target beam energy. All studies of cosmic rays, from below the knee to the GZK cut-off, measure showers in the atmosphere and compare with models which differ by more than an order of magnitude in their predictions of leading particle spectra. Anomalies such as an excess of muons could be due to more leading heavy flavours than expected or some other new physics. Neutrinos from hadron decays need to be well understood for the shower energy measurement as well as the atmospheric neutrino ‘background’ in ICECUBE and other projects.
I will describe a spectrometer that can measure these pi/K/p spectra as well as light anti-nuclei in p+p, p+O and O+O collisions at the LHC in Run 4 (2027), 80 - 120 m downstream of ATLAS or CMS. It will include precision tracking, timing and calorimetry and include transition radiation detectors for multi-TeV hadron identification.