Short answer: No experimentally confirmed theory currently replaces the Standard Model. The leading possibilities—supersymmetry, grand unified theories, extra dimensions, neutrino and hidden-sector models, axions, composite Higgs theories, and string theory—solve different parts of the puzzle and make different predictions. The next successor will have to explain established evidence such as dark matter and neutrino mass while surviving increasingly precise tests.
Why physicists are looking beyond the Standard Model
The Standard Model describes the known quarks, charged leptons, neutrinos as originally formulated, gauge bosons and the Higgs field with extraordinary accuracy. Experiments have repeatedly confirmed its calculations for particle masses, scattering rates and magnetic properties. Yet its minimal form is incomplete. Several observations are not optional clues or philosophical discomforts; they require additional physics.
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Dark matter
Galaxies, gravitational lensing and the evolution of cosmic structure show that most matter does not emit or absorb light in the ordinary way. CERN’s current dark-matter explainer estimates that dark matter makes up roughly 27% of the universe, while ordinary matter contributes about 5%. The Standard Model contains no particle with the required stable, weakly interacting properties. A successful successor must either identify a dark-matter particle or explain the observations through a comparably successful modification of known physics.
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Neutrino oscillation experiments establish that neutrinos change flavor and therefore have nonzero masses. The minimal Standard Model treats them as massless. Adding right-handed neutrinos, a seesaw mechanism, Majorana masses or another neutrino-sector extension can account for the observed behavior, but each option changes the model’s particle content and produces different experimental consequences.
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The matter–antimatter imbalance
The observable universe contains far more matter than antimatter. The Standard Model includes some CP violation, but the known effects are not sufficient to explain the cosmic asymmetry. Proposed solutions include leptogenesis, in which a lepton asymmetry is converted into a baryon asymmetry, and electroweak baryogenesis, which uses processes associated with the Higgs field and the early universe.
Gravity and quantum theory
General relativity describes gravity on large scales, whereas the Standard Model is a quantum field theory for the other interactions. The two frameworks do not form a complete quantum description of gravity. String theory and other quantum-gravity programs attempt to resolve this mismatch, although they operate at energy scales far beyond current accelerators and have not produced a uniquely verified low-energy theory.
The Higgs hierarchy and naturalness problem
Quantum corrections tend to push the Higgs mass toward very high scales. Keeping the observed Higgs mass comparatively light can require delicate parameter adjustment unless a principle or new dynamics intervenes. This hierarchy problem motivates supersymmetry, composite-Higgs models and related ideas. It is a theoretical motivation rather than a direct discovery of new particles, so it must be weighed alongside data.
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“Beyond the Standard Model” is a broad category, not one theory. A useful comparison asks five questions:
- Which gap does it address? A model may target dark matter but say little about gravity, or explain neutrino mass without solving the Higgs hierarchy.
- What new entities does it require? Count particles, fields, dimensions or strong interactions added to the Standard Model.
- What can experiments observe? Look for a mass range, decay pattern, missing energy, displaced vertex, precision deviation or astrophysical signal.
- At what scale should effects appear? Some proposals are testable at the Large Hadron Collider (LHC); others primarily predict phenomena in the early universe or at quantum-gravity energies.
- How much parameter adjustment is accepted? A theory can remain mathematically viable after searches exclude its simplest parameter choices, but its naturalness argument may become weaker.
| Theory family | Main problems addressed | Characteristic tests | Current status |
|---|---|---|---|
| Supersymmetry | Higgs naturalness; possible dark matter; gauge-coupling unification | Partner particles, missing momentum, long-lived particles and displaced decays | No superpartner detected; many non-minimal versions remain viable |
| Grand unified theories | Unification of strong, weak and electromagnetic interactions; sometimes neutrino mass | Proton decay, monopoles, coupling evolution and fermion-mass relations | No proton-decay signal or unique model established |
| Extra dimensions | Modified gravity, hierarchy problem and possible dark matter | Kaluza–Klein states, resonances, missing energy and short-distance gravity tests | No confirmed extra-dimensional particle |
| Neutrino and hidden sectors | Neutrino masses, leptogenesis and dark matter | Oscillation effects, rare decays, beam-dump events and weakly coupled mediators | Active search area with many possible mass scales |
| Axions | Strong-CP problem; possible dark matter | Haloscopes, helioscopes and astrophysical observations | No confirmed axion or axion-like particle |
| Composite Higgs | Higgs naturalness and dynamical electroweak symmetry breaking | Higgs and top-coupling deviations, resonances and partner states | Active hypothesis without a confirmed composite signal |
| String theory | Quantum gravity and possible unification | Indirect consistency, compactification consequences and any distinctive low-energy prediction | Powerful framework, but no uniquely verified vacuum or prediction |
Supersymmetry: partner particles and a lighter Higgs
Supersymmetry, or SUSY, relates fermions to bosons. For each known particle, a superpartner appears in the theory: fermions receive bosonic partners and bosons receive fermionic partners. Loops involving these partners can cancel the largest quantum corrections to the Higgs mass, preserving a light Higgs without extreme fine-tuning. In many SUSY constructions, the lightest new particle is stable and electrically neutral, making it a dark-matter candidate. Supersymmetric particles can also alter the running of gauge couplings in a way that improves unification at high energy.
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The experimental picture is a constraint, not a final verdict. A 2025 review reported that no supersymmetric particle had been detected. LHC searches have ruled out substantial regions of the simplest weak-scale models, especially those with relatively light, strongly interacting partners and straightforward decay chains. They do not eliminate every realization: compressed spectra, unusual decay modes, very heavy partners and models with long-lived particles can be harder to detect. ATLAS continues to search for displaced decays and other non-minimal signatures.
What would count as a SUSY discovery?
Researchers would need a statistically compelling excess that is reproduced in independent channels and matches a superpartner spectrum. Missing transverse momentum alone is not sufficient because neutrinos and detector effects can create the same signature. Confirmation would require measuring masses, decay relationships and interaction strengths, then checking whether the proposed spectrum also supplies a viable dark-matter history and improves the Higgs-naturalness problem.
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Grand unified theories: one force at high energy
Grand unified theories (GUTs) enlarge the Standard Model’s gauge symmetry so that the strong, weak and electromagnetic forces emerge from a single interaction at a much higher energy. Quarks and leptons may occupy common multiplets, creating relationships between their charges, masses or mixing parameters. Many GUTs also incorporate heavy right-handed neutrinos, linking unification to the origin of neutrino masses and leptogenesis.
The classic experimental prediction is proton decay. If quarks and leptons share unified multiplets, extremely rare processes can convert a proton into lighter particles. The decay channel and lifetime depend on the symmetry group, particle spectrum and mechanism that breaks the unified symmetry. No proton-decay signal has established a GUT, and different models can suppress the rate below current reach. Magnetic monopoles and precise measurements of gauge-coupling evolution provide additional, but model-dependent, tests.
Extra dimensions and brane-world models
Extra-dimensional theories add spatial dimensions that are compactified, warped or otherwise hidden at accessible energies. Particles may be confined to a four-dimensional brane while gravity propagates through a larger space, or all fields may possess higher-dimensional modes. The resulting Kaluza–Klein excitations could appear as heavier copies of familiar particles. Warped geometries can change how the Higgs hierarchy is generated.
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At colliders, searches look for resonances from excited modes, or missing transverse momentum if gravitons or other states escape into hidden dimensions. Short-distance measurements of gravity test whether the inverse-square law changes at very small separations. Some constructions also provide stable dark-matter candidates. No confirmed extra-dimensional particle has been observed, so geometry, compactification scale and predicted spectrum remain decisive differences among models.
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Neutrino extensions and hidden sectors
Right-handed and sterile neutrinos
Adding right-handed neutrinos permits ordinary Dirac masses, while very heavy states can generate tiny observed masses through a seesaw mechanism. If neutrinos are Majorana particles, a neutrino is its own antiparticle and lepton-number-violating processes such as neutrinoless double-beta decay become possible. The masses and mixings of heavy states can also create a lepton asymmetry that later becomes the universe’s baryon asymmetry through leptogenesis.
Portals to hidden particles
A hidden sector can contain particles that carry no Standard Model charge and interact through a weak “portal,” such as a new gauge boson, scalar mixing or neutrino mixing. Dark photons, light scalars and other mediators may produce displaced decays, missing energy or rare meson and lepton decays. CERN’s SND@LHC program searches for weakly interacting particles not predicted by the Standard Model, including candidates that could contribute to dark matter. Beam-dump experiments and fixed-target facilities are particularly useful when the new particles are light and long-lived rather than heavy enough for direct LHC production.
Axions and the strong-CP problem
Quantum chromodynamics allows a term that would violate CP symmetry, yet measurements show that this violation is extraordinarily small. The Peccei–Quinn mechanism promotes the problematic parameter to a dynamical field whose associated particle is the axion. Depending on its mass and cosmic abundance, the axion can also be dark matter.
Axions are difficult to find because their interactions can be extremely weak. Resonant haloscopes search for conversion of dark-matter axions into photons in a magnetic field. Helioscopes look for axions produced in the Sun, while astrophysical observations constrain energy loss and conversion in stars or magnetic environments. These experiments probe parameter ranges largely inaccessible to colliders, making axions complementary to accelerator searches rather than a competing LHC program.
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Composite Higgs and new strong dynamics
Composite-Higgs models treat the Higgs boson as a bound state of a new strong interaction, analogous in broad principle to how quarks form hadrons. A composite origin can explain why the Higgs is light relative to a much higher compositeness scale through an approximate symmetry. The same dynamics often predicts heavier partner resonances, especially partners associated with the top quark, and measurable deviations in Higgs or top couplings.
Precision measurements are essential because the new strong sector may be too heavy to produce directly. Small changes in Higgs production, decay rates or electroweak observables can reveal compositeness before a resonance is found. Current sources identify composite Higgs models as active hypotheses, not as theories supported by a confirmed composite signal.
String theory and quantum gravity
String theory replaces point particles with one-dimensional extended objects whose vibrational modes appear as different particles. A massless spin-2 mode behaves like a graviton, allowing gravity and gauge interactions to be described within one mathematical framework. Consistent versions generally require extra dimensions; compactifying those dimensions can produce four-dimensional particles and forces.
The central limitation is empirical selection. String theory admits many possible compactifications and low-energy realizations, often called vacua. Without a unique, experimentally verified prediction at accessible energies, it is better described as a framework for quantum gravity and unification than as an established replacement for the Standard Model. Any claim that string theory has already been confirmed overstates the evidence.
What experiments can decide
LHC searches
The LHC tests new resonances, missing transverse momentum, long-lived particles, displaced vertices and precision properties of the Higgs, top quark and other known states. Run 3 began in July 2022. The absence of a simple excess narrows the parameter space but does not test every mass, coupling or decay pattern. Analyses must account for compressed spectra, invisible decays and particles that travel measurable distances before decaying.
Neutrino, flavor and precision measurements
Long-baseline neutrino experiments measure oscillation probabilities, mass ordering and CP violation in the lepton sector. Rare flavor-changing decays and electric-dipole-moment searches can reveal heavy particles indirectly. A tiny deviation repeated across experiments may be more informative than a single high-energy event because loop effects can expose particles too heavy to produce directly.
Dark-matter and proton-decay searches
Direct-detection experiments look for nuclear recoils from dark-matter scattering. Indirect searches seek annihilation or decay products in cosmic rays, gamma rays or neutrinos. Proton-decay detectors test a defining GUT prediction. Null results reduce allowed interaction strengths and lifetimes but cannot rule out candidates that interact too weakly or decay too rarely.
Cosmology and astrophysics
The cosmic microwave background, galaxy formation, stellar evolution and gravitational-wave observations constrain relic abundances, extra radiation, phase transitions and compact objects. These observations are indispensable for models whose decisive effects occurred in the early universe rather than inside a collider.
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CERN’s future-collider planning emphasizes precision Higgs measurements alongside opportunities to discover new particles. A cleaner Higgs factory could detect small deviations in couplings, while a higher-energy collider could extend the direct search for heavy resonances and partners. Neither machine guarantees a discovery; their value lies in testing complementary signatures and reducing the space of viable models.
What would qualify as a genuine replacement?
A successor would need more than one unexplained bump. The strongest case would combine a reproducible discovery or anomaly with a quantitatively successful framework that explains multiple shortcomings—such as dark matter, neutrino mass or the matter–antimatter imbalance—without spoiling the Standard Model’s precision successes. Independent experiments should measure related properties, and the theory should make additional predictions that can fail.
This standard matters because many BSM models can be adjusted to fit one observation. A durable replacement must instead connect observations that currently appear unrelated: for example, a neutrino-mass mechanism that predicts leptogenesis, or a stable new particle whose relic abundance and scattering rate agree with cosmology and direct detection. Until such a network of evidence appears, “beyond the Standard Model” describes a research program rather than a single accepted theory.
How to interpret the landscape
Supersymmetry is attractive for naturalness and dark matter but has lost its simplest weak-scale parameter space after null searches. GUTs offer an elegant force-unification picture, with proton decay as their clearest test. Extra dimensions modify gravity and particle spectra, while neutrino and hidden-sector models can target very small masses and couplings. Axions address strong CP and may explain dark matter through experiments unlike collider searches. Composite Higgs theories replace an elementary scalar with new strong dynamics. String theory tackles the deepest unification problem but currently lacks a unique verified low-energy prediction.
The evidence therefore favors breadth rather than a declared winner. Progress is most likely to come from combining collider, neutrino, dark-matter, flavor, precision, proton-decay and cosmological data. Each result can eliminate classes of models, reveal a new particle or show that a supposedly separate problem shares a common origin.
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