Forward physics at LHC: Intersections with astrophysics

LHC pp collisions at $\sqrt{s}=14\mathrm{TeV}$ correspond to ${10}^{17}\mathrm{eV}$ in the cosmic ray spectrum, reaching regions (f.i. the “knee”) whose interpretation is still debated, and whose models may be strongly influenced by LHC results on forward hadronic production, for instance as measured with CMS forward calorimeters. Other astrophysics problems that may receive a decisive input from LHC, and in particular HL-LHC, concern the interpretation of dark matter and dark energy in the universe, in correspondence with BSM (Beyond Standard Model) particles that may be detected as LLP (Long-Lived Particles). LLP searches have been generally done up to now at low pseudorapidity η, as exemplified by some CMS recent results. There may be however reasons why detectors in the forward direction of LHC experiments, could be better suited for these searches, at least for some lifetime ranges, due to large relativistic boosts of the produced particles at LHC energies in the forward region. Considering the CMS forward detectors, their role for some of these searches and corresponding signatures are discussed. Additional possibilities of instrumenting specific regions of the LHC tunnel near the interaction point IP5, where CMS is installed, are discussed.