Project F4: "Cross sections and hadronic interactions in particle- and astroparticle physics"

Uncertainties in the hadron production in high-energy proton-nucleus and nucleus-nucleus collisions are one of the biggest challenges when trying to understand the sources of high-energy cosmic rays [Kampert and Unger, 2012; Batista et al., 2019, Albrecht et al., 2021]. Modern cosmic-ray (CR) observatories like the Pierre Auger Observatory and the Cherenkov Telescope Array observe the direction of extensive air showers (EAS), their energy, and two key observables related to the identity of CRs, the depth of the electromagnetic shower maximum, \(X_{max}\,\), and the number of produced muons, \(N_{\mu}\,\). The identity of the CR nucleus is given by its mass \(A\,\), which is inferred from comparing the measured values of \(X_{max}\) and \(N_{\mu}\) of an EAS with corresponding air shower simulations. These simulations require models that accurately predict forward hadron production in collisions for center-of-mass energies \(\sqrt{s_{nn}}\,\) in the nucleon-nucleon system from a few GeV up to 700 TeV. A reliable extrapolation towards these energies requires high-precision forward measurements at the Large Hadron Collider (LHC). This project will go beyond existing efforts in LHCb to realize its full potential for air shower physics. Microscopically, the simulations depend on the inelastic cross sections, forward particle multiplicity spectra \(dN/d\eta\,\), and the hadron composition in the forward region \(\eta > 0\) at the TeV scale and beyond. We will study identified spectra of light-flavored hadrons at forward rapidities \(2 < y < 5\) in p-p, p-Pb, and, if possible, also in p-O collisions, building on the unique strengths of the LHCb experiment as a forward spectrometer with particle identification. Our measurements will directly address the so-called Muon Puzzle, a discrepancy between the predicted and measured muon production in EAS that starts at the TeV scale and increases with energy. On the side of ultra-high energy CRs (UHECR), we will implement this improved knowledge into the commonly employed hadronic interaction models (EPOS, Sibyll, and possibly QGSJet) and study their effect on EAS observables detected with the upgraded Pierre Auger Observatory, called AugerPrime. A specific aim of AugerPrime is to enhance its particle physics capabilities by improving the electron-muon discrimination in EAS. Due to its multi-hybrid character, providing many simultaneous probes of each individual EAS, AugerPrime will not only be able to test the effect of modifications of the interaction models but will also be able to provide valuable input to the models.

Since the upgraded LHCb experiment, including a new SMOG, and AugerPrime start operating at about the same time, there are unique opportunities to address these timely questions. The LHCb studies will be tightly connected to those in the Auger Observatory in a sophisticated global statistical analysis to validate and improve hadronic interaction models used in EAS simulations. Modifying and improving the interaction models will be done jointly with the model builders, and close cooperations have already been established. As a result of this project, we expect to dramatically decrease the uncertainty in the CR mass composition and the atmospheric lepton flux which is the principal background for astro-neutrino observatories, and overall refine our understanding of QCD at high energies. The ramping-up period of the upcoming LHC Run 3 will offer a timely opportunity for these studies. Later in Run 3, there will be the opportunity to study proton-ion collisions. Using the SMOG2 device, we will further exploit LHCb's unique capability as a fixed-target experiment in an unexplored energy scale around
\(\sqrt{s_{nn}}\) = 0.1 TeV.