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Despite its extraordinary success, the Standard Model leaves fundamental questions unanswered: the origin of neutrino masses, the nature of dark matter, the matter-antimatter asymmetry, the hierarchy problem (why the Higgs mass is so much lighter than the Planck scale), the strong CP problem, the pattern of fermion masses and mixing angles, and the absence of quantum gravity. BSM physics encompasses the theoretical frameworks and experimental searches aimed at addressing these shortcomings.
The Standard Model's incompleteness is established by observation, not just theoretical preference. Neutrino oscillations, dark matter, and the baryon asymmetry are three experimental facts that require new physics. The theoretical motivations -- the hierarchy problem, the strong CP problem, the flavor puzzle, the cosmological constant problem, and quantum gravity -- add urgency but are less definitive (the SM could simply be fine-tuned).
The hierarchy problem has driven much of BSM model building. If the Standard Model is valid up to the Planck scale (~1019 GeV), the Higgs mass requires cancellation between the bare mass and radiative corrections at the level of one part in 1034. Three broad classes of solutions have been proposed: (1) supersymmetry introduces partner particles for every SM particle, whose loop contributions cancel the quadratic divergences; (2) composite Higgs models replace the fundamental scalar with a bound state of a new confining interaction, analogous to pions in QCD; (3) extra dimensions lower the fundamental gravitational scale from the Planck scale to the TeV scale, eliminating the large hierarchy. The LHC has not found evidence for any of these, pushing the parameter space of each framework and prompting reconsideration of the naturalness criterion.
Experimental BSM searches at the LHC cover an enormous range of signatures. Direct searches look for resonances (new particles decaying to known particles, producing bumps in invariant mass distributions), missing energy (dark matter or other invisible particles produced in association with jets, photons, or W/Z), displaced vertices (long-lived particles traveling millimeters to meters before decaying), and anomalous production rates (deviations from SM predictions in precision observables). The null results from Run 1 and Run 2 have excluded many natural BSM scenarios: squarks and gluinos below ~2 TeV, Z' bosons below ~5 TeV, and certain dark matter mediators below ~2 TeV.
The future BSM program combines direct searches at the HL-LHC and potential future colliders (FCC-hh at 100 TeV, muon collider) with indirect searches through precision measurements (Higgs coupling deviations, electroweak precision, flavor anomalies, g-2, EDMs) and dedicated experiments (dark matter direct detection, neutrinoless double beta decay, axion searches, beam dump experiments for light weakly-coupled particles). The breadth of the search program reflects the theoretical uncertainty about where and how new physics will appear.