Molecular Clouds as Multi-Messenger Sources: Cosmic Rays, Gamma Rays, and Neutrinos from Galactic Hadronic Interactions

Molecular clouds are among the most promising multimessenger sources in the Galaxy. When illuminated by cosmic rays accelerated in nearby sources such as supernova remnants, young massive stellar clusters, or pulsar wind nebulae, these dense gas reservoirs can act as hadronic interaction targets, ranging from optically thin emitters to partially calorimetric systems depending on their density, size, and the transport properties of cosmic rays within them, converting a fraction of the cosmic-ray energy into gamma rays and neutrinos through proton-proton interactions and the subsequent decay of charged and neutral pions. In this review, we discuss the physical mechanisms underlying these interactions, including the pion-decay spectral signature, the nuclear enhancement factor, and the energy-dependent diffusion of cosmic rays within and around molecular clouds. We survey the observational landscape from GeV to ultra-high energies: the Fermi-LAT detection of the pion-decay spectral break in supernova remnant-cloud systems, the H.E.S.S. measurement of a 1/r cosmic-ray density profile in the Central Molecular Zone, establishing the Galactic center as a PeVatron candidate, and the LHAASO observations of sub-PeV and PeV emission from illuminated complexes such as W51, the Cygnus superbubble, and the orphan source LHAASOJ2108+5157. We examine the implications of the recent IceCube detection of high-energy neutrinos from the Galactic plane at 4.5σ significance and discuss theoretical predictions for the neutrino flux from giant molecular cloud populations. Finally, we assess the prospects for next-generation multimessenger studies with CTAO and SWGO in the gamma-ray band and with IceCube-Gen2 and KM3NeT/ARCA in the neutrino channel, highlighting the synergies between these facilities that will be decisive for identifying the sources of Galactic cosmic rays up to and beyond the PeV regime.