= Solution
The <Standard Model> is a <gauge theory> with group $SU(3)_c\times SU(2)_L\times U(1)_Y$, matter <quarks> and <leptons>, gauge bosons and a <Higgs field>. It combines <Quantum chromodynamics> with the <electroweak interaction>. Its successes do not make its field content a complete account of nature. Established missing phenomena should be distinguished from theoretical questions about its parameters.
<neutrino oscillation> demonstrates nonzero mass-squared differences and mixing of flavor states. The minimal renormalizable <Standard Model> has no <right-handed neutrino> and no <neutrino mass> term. The atmospheric experiment compared zenith-angle distributions of muon-like and electron-like events in a large water <Cherenkov detector>. A direction-dependent muon-neutrino deficit, with different path lengths through Earth and comparison samples, discriminated against a uniform flux-normalization error.<Evidence for oscillation of atmospheric neutrinos by Y. Fukuda and others>[Abstract and zenith-angle distributions, Physical Review Letters 81, pp. 1562–1567 (1998)]{ref} One extension adds <right-handed neutrinos> and small Dirac Yukawa couplings; another has a <Majorana mass term> and <seesaw mechanism>, with light eigenvalue of order $m_D^2/M$. Oscillations alone do not decide whether <neutrinos> are <Majorana neutrinos>.
<dark matter> is inferred from gravitational observations such as galactic motions and <gravitational lensing>; <Standard Model> particles do not supply the required dominant cold component. Active <neutrinos> contribute some matter but cannot constitute all the cold <dark matter>. A new stable weakly interacting particle, an <axion>, or other hidden-sector states are possible explanations, with different scattering, production and astrophysical signals. \b[Dark-matter evidence does not identify a particular new particle.] The institutional account distinguishes gravitational evidence from proposed particle candidates.<Dark matter by CERN>[Gravitational evidence and candidate-particle discussion]{ref}
The matter-antimatter imbalance poses a different problem. Dynamical <baryogenesis> needs <Sakharov conditions>, including baryon-number violation, <CP violation> and departure from <thermal equilibrium>. Although the <Standard Model> contains relevant ingredients in principle, they do not generate the observed asymmetry in its conventional thermal history with measured parameters. Additional CP phases, a stronger electroweak transition, or <leptogenesis> from heavy <Majorana neutrinos> are possible extensions. Each needs a quantitative mechanism.
<gravity> is absent as a dynamical quantum interaction, motivating <quantum gravity> and unification. The <hierarchy problem> asks why the Higgs <mass> scale is small compared with much higher scales when scalar <masses> receive sensitive quantum corrections. <supersymmetry> can cancel particular contributions between partner fields if its breaking pattern is suitable; compositeness gives another interpretation of the Higgs sector. Neither motivation proves that the corresponding new particles exist. The <Strong CP problem> asks why the allowed <QCD> CP-violating parameter is so small; an <axion> mechanism is one possible answer.
A <grand unified theory> embeds the separate gauge factors in a larger group and can relate couplings or <fermion> representations, sometimes predicting <proton decay>. Its tests differ from dark-matter or neutrino-mass searches. \b[A useful extension must explain a specified deficiency, preserve successful low-energy predictions and supply distinguishable experimental consequences.] Attractive new symmetries alone are not evidence.
Back to article page
