Particle physics

Physics beyond the Standard Model

Extensions such as supersymmetry or grand unification address limits of the Standard Model; they remain research hypotheses, not experimental discoveries.

The Standard Model succeeds but lacks quantum gravity and does not explain dark matter, neutrino masses, or questions such as the mass hierarchy and cosmic matter–antimatter asymmetry. Beyond-Standard-Model (BSM) physics studies extensions; theoretical appeal is not experimental evidence.

\mathcal{L}_{\mathrm{eff}}=\mathcal{L}_{\mathrm{SM}}+\sum_{d>4}\frac{c_d}{\Lambda^{d-4}}\mathcal{O}_d

Definition: Effective field theory

Below a new-physics scale $\Lambda$ , unknown effects can be parameterized by higher-dimension operators suppressed by powers of $\Lambda$ . This is a systematic way to search for deviations without specifying the full underlying theory.

Compare an illustrative hypothetical nuclear-recoil spectrum with background while changing WIMP mass and detector threshold. Curves are not real data or evidence of detection.

From hypothesis to test

A WIMP is a hypothetical massive, weakly interacting particle. Direct searches look for nuclear recoils in underground detectors; indirect searches seek annihilation or decay products in astrophysical environments; colliders look for missing momentum. Each method depends on signal, background, and cross-channel assumptions.

Example: Detector threshold

If a detector accepts recoils only above $E_{th}=5$ keV, what happens to the softer recoil signal below threshold?

Solution

It is not recorded by an ideal threshold selection. Lowering the threshold may recover signal events but can make backgrounds and noise more important.

One way to compare BSM hypotheses is to place them in a common effective-field-theory framework. Each new operator predicts relations among observables, and its coefficient is constrained jointly by colliders, rare decays, or precision measurements. A null result excludes a parameter region rather than every model; an observed deviation must be checked for the same pattern in other channels.

A credible extension must recover the Standard Model's tested low-energy predictions while identifying measurements that could distinguish it. Effective field theory enables comparisons without assuming a particular new particle; searches for resonances, missing energy, and precision deviations are complementary strategies. With no evidence selecting one scenario, results are reported as limits or model tests.

What does a WIMP search limit with no signal mean?

What signal do direct WIMP searches typically seek in a detector?

References

T.D. Lee (1981). Particle Physics and Introduction to Field Theory