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Proton-proton collisions at the LHC generate a high-intensity collimated beam of neutrinos in the forward (beam) direction, characterised by energies of up to several TeV. The recent observation of LHC neutrinos by FASER ν and SND@LHC signifies that this previously overlooked particle beam is now available for scientific investigation. Here we quantify the impact that neutrino deep-inelastic scattering (DIS) measurements at the LHC would have on the parton distributions (PDFs) of protons and heavy nuclei. We generate projections for DIS structure functions for FASER ν and SND@LHC at Run III, as well as for the FASER ν 2, AdvSND, and FLArE experiments to be hosted at the proposed Forward Physics Facility (FPF) operating concurrently with the High-Luminosity LHC (HL-LHC). We determine that up to one million electron-neutrino and muon-neutrino DIS interactions within detector acceptance can be expected by the end of the HL-LHC, covering a kinematic region in x and Q 2 overlapping with that of the Electron-Ion Collider. Including these DIS projections in global (n)PDF analyses, specifically PDF4LHC21, NNPDF4.0, and EPPS21, reveals a significant reduction in PDF uncertainties, in particular for strangeness and the up and down valence PDFs. We show that LHC neutrino data enable improved theoretical predictions for core processes at the HL-LHC, such as Higgs and weak gauge boson production. Our analysis demonstrates that exploiting the LHC neutrino beam effectively provides CERN with a “Neutrino-Ion Collider” without requiring modifications in its accelerator infrastructure.

A bstract Searches for new physics in the top quark sector are of great theoretical interest, yet some powerful avenues for discovery remain unexplored. We characterize the expected statistical power of the LHC dataset to constrain the single production of heavy top partners T decaying to a top quark and a photon or a top quark and a gluon. We describe an effective interaction which could generate such production, though the limits apply to a range of theoretical models. We find sensitivity to cross sections in the 10 2 − 10 5 fb range, for T masses between 300 and 1000 GeV, depending on decay mode.

A bstract The first direct detection of neutrinos at the LHC not only marks the beginning of a novel collider neutrino program at CERN but also motivates considering additional neutrino detectors to fully exploit the associated physics potential. As the existing forward neutrino detectors are located underground, it is interesting to investigate the feasibility and physics potential of neutrino experiments located at the surface-level. A topographic desk study is performed to identify all points at which the LHC’s neutrino beams exit the earth. The closest location lies about 9 km east of the CMS interaction point, at the bottom of Lake Geneva. Several detectors to be placed at this location are considered, including a water Cherenkov detector and an emulsion detector. The detector designs are outlined at a conceptual level, and projections for their contribution to the LHC forward neutrino program and searches for dark sector particles are presented. However, the dilution of the neutrino flux over distance reduces the neutrino yield significantly, necessitating large and coarse detector designs. We identify the experimental challenges to be overcome by future research, and conclude that at present the physics potential of surface-level detectors is limited in comparison to ones closer to the interaction point, including the proposed Forward Physics Facility.

A bstract Proton-proton collisions at energy-frontier facilities produce an intense flux of high-energy light particles, including neutrinos, in the forward direction. At the LHC, these particles are currently being studied with the far-forward experiments FASER/FASER ν and SND@LHC, while new dedicated experiments have been proposed in the context of a Forward Physics Facility (FPF) operating at the HL-LHC. Here we present a first quantitative exploration of the reach for neutrino, QCD, and BSM physics of far-forward experiments integrated within the proposed Future Circular Collider (FCC) project as part of its proton-proton collision program (FCC-hh) at s ≃ 100 TeV. We find that 10 9 electron/muon neutrinos and 10 7 tau neutrinos could be detected, an increase of several orders of magnitude compared to (HL-)LHC yields. We study the impact of neutrino DIS measurements at the FPF@FCC to constrain the unpolarised and spin partonic structure of the nucleon and assess their sensitivity to nuclear dynamics down to x ∼ 10 − 9 with neutrinos produced in proton-lead collisions. We demonstrate that the FPF@FCC could measure the neutrino charge radius for ν e and ν μ and reach down to five times the SM value for ν τ . We fingerprint the BSM sensitivity of the FPF@FCC for a variety of models, including dark Higgs bosons, relaxion-type scenarios, quirks, and millicharged particles, finding that these experiments would be able to discover LLPs with masses as large as 50 GeV and couplings as small as 10 − 8 , and quirks with masses up to 10 TeV. Our study highlights the remarkable opportunities made possible by integrating far-forward experiments into the FCC project, and it provides new motivation for the FPF at the HL-LHC as an essential precedent to optimize the forward physics experiments that will enable the FCC to achieve its full physics potential.

A bstract The first FASER search for a light, long-lived particle decaying into a pair of photons is reported. The search uses LHC proton-proton collision data at s = 13 . 6 TeV collected in 2022 and 2023, corresponding to an integrated luminosity of 57 . 7 fb − 1 . A model with axion-like particles (ALPs) dominantly coupled to weak gauge bosons is the primary target. Signal events are characterised by high-energy deposits in the electromagnetic calorimeter and no signal in the veto scintillators. One event is observed, compared to a background expectation of 0 . 44 ± 0 . 39 events, which is entirely dominated by neutrino interactions. World-leading constraints on ALPs are obtained for masses up to 300 MeV and couplings to the Standard Model W gauge boson, g aWW , around 10 − 4 GeV − 1 , testing a previously unexplored region of parameter space. Other new particle models that lead to the same experimental signature, including ALPs coupled to gluons or photons, U(1) B gauge bosons, up-philic scalars, and a Type-I two-Higgs doublet model, are also considered for interpretation, and new constraints on previously viable parameter space are presented in this paper.

The Large Hadron Collider (LHC) has delivered a wealth of valuable measurements which have proven crucial in our understanding of the Universe. To further exploit the physics output of the LHC, a far-forward physics program has developed which has painted a more complete picture of the collisions at the most energetic particle collider. In this thesis, we demonstrate how the forward neutrino program can further our understanding of the Standard Model. In particular, forward measurements can yield an improved understanding of quantum chromodynamics (QCD), which paves the way for refined measurements at central experiments at the LHC, for cosmic-ray observatories, and neutrino telescopes. We also show that the forward region is where a number of beyond the Standard Model particles are best probed, including very weakly coupled GeV-scale dark matter. Finally, we consider the vast physics potential of far-forward physics at the 100 TeV Future Circular Collider, and the unique neutrino and new physics opportunities that it can yield.

We study properties of a hypothetical scalar particle, \\(\\), which is a color octet and an electroweak singlet. At hadron colliders, \\(\\) is pair produced through its QCD coupling to gluons, so that its mass determines the cross section. It decays at tree level into \\(q q\\) through dimension-5 operators, and at one loop into gluons. Thus, the main LHC signature of \\(\\) is a pair of dijets of equal invariant mass. The CMS search in this channel shows a \\(3.6\\) excess over the QCD background for a dijet mass \\(M_jj 0.95\\) TeV, which can be due to \\(\\): its production cross section (65 fb for a real scalar) and the acceptance of the CMS event selection applied to \\(p p \\! (q q)(q q)\\) yield a rate consistent with the excess. Furthermore, the shape of the \\(d/d M_jj\\) signal is in agreement with the CMS result. Given the data-driven background fit performed by CMS, we find that a complex scalar fits better the data than a real scalar. Besides the pair of dijets, testable LHC signals include a trijet-dijet topology, a \\(t t\\) pair plus a dijet resonance, as well as final states involving a Higgs, \\(W\\) or \\(Z\\) boson plus jets.

The FASER experiment was designed to study long-lived dark sector particles and neutrinos traveling in the forward direction at the LHC. Neutrinos are predominantly produced from meson decays, which also result in an intense energetic flux of muons in the forward direction regularly observed by FASER. So far, these muons are treated only as backgrounds to neutrino and new physics studies, and extensive effort is required to suppress them. In this study, we consider the opposite scenario and use muons produced in the forward direction to produce new muonphilic scalars, which can then be searched for at the FASER detector. To minimize the backgrounds for this search, we make use of an upgraded preshower component, which is expected to be installed at FASER before the end of Run 3, and is capable of spatially resolving two energetic photons. We find that FASER, and its upgrade, FASER2 can probe currently unconstrained regions of parameter space, including regions that can potentially explain the \\((g-2)_\\) anomaly. This highlights the physics opportunities that the intense TeV muon beam at the LHC can bring.

Particle physics theories, such as those which explain neutrino flavor mixing, arise from a vast landscape of model-building possibilities. A model's construction typically relies on the intuition of theorists. It also requires considerable effort to identify appropriate symmetry groups, assign field representations, and extract predictions for comparison with experimental data. We develop an Autonomous Model Builder (AMBer), a framework in which a reinforcement learning agent interacts with a streamlined physics software pipeline to search these spaces efficiently. AMBer selects symmetry groups, particle content, and group representation assignments to construct viable models while minimizing the number of free parameters introduced. We validate our approach in well-studied regions of theory space and extend the exploration to a novel, previously unexamined symmetry group. While demonstrated in the context of neutrino flavor theories, this approach of reinforcement learning with physics software feedback may be extended to other theoretical model-building problems in the future.