Despite decades of extraordinary experiments, neutrinos remain profoundly mysterious. These open questions sit at the frontier of physics — some may reshape our understanding of the universe itself.
Neutrinos might be Majorana fermions — particles identical to their own antiparticles — unlike every other known fermion. If true, a process called neutrinoless double beta decay (0νββ) would be allowed, violating lepton number conservation. Its discovery would reveal the nature of neutrino mass, potentially explain why the universe is made of matter, and confirm leptogenesis as the origin of the matter-antimatter asymmetry.
Oscillation experiments only measure mass-squared differences, not absolute values. We know Δm²₂₁ ≈ 7.53×10⁻⁵ eV² and |Δm²₃₁| ≈ 2.51×10⁻³ eV², but the lightest mass eigenstate could be essentially zero. KATRIN directly measures the electron antineutrino mass via tritium β-decay kinematics, currently achieving a bound of <0.45 eV (90% CL), targeting 0.2 eV sensitivity.
We do not know whether the heaviest mass eigenstate (ν₃) is largely ν_τ (normal ordering: m₃ ≫ m₁,m₂) or mostly ν_e (inverted: m₁ ≈ m₂ ≫ m₃). This seemingly technical question determines strategies for neutrinoless double beta decay, affects cosmological constraints, and is critical for understanding why neutrino masses are so small. DUNE, Hyper-K, and JUNO should all resolve this within the next decade.
The observable universe contains vastly more matter than antimatter, yet the Big Bang should have created equal amounts. CP violation in the lepton sector — different oscillation probabilities for neutrinos vs. antineutrinos — could be the origin, via the leptogenesis mechanism: early-universe decays of heavy Majorana neutrinos produced a lepton asymmetry, later converted into a baryon asymmetry. T2K's hint of δCP ≈ −π/2 is tantalising. DUNE aims for a 5σ determination.
The LSND experiment (1995–2001) and later MiniBooNE reported oscillation signals inconsistent with three-flavour physics, suggesting a fourth neutrino species with Δm² ~ 1 eV² that doesn't interact via the weak force — a "sterile" neutrino. Sterile neutrinos are dark matter candidates and could explain reactor and gallium anomalies. However, MicroBooNE (2022) disfavoured a simple electron-neutrino appearance explanation for MiniBooNE, leaving the picture confused.
IceCube detects neutrinos at energies up to petaelectronvolts arriving from beyond our galaxy. Their primary sources remain largely unidentified. Confirmed associations include the blazar TXS 0506+056 and the Seyfert galaxy NGC 1068. Other candidates include tidal disruption events, gamma-ray bursts, starburst galaxies, and flat-spectrum radio quasars. The multi-messenger picture is slowly emerging but the dominant acceleration mechanism remains open.
The Antarctic Impulsive Transient Antenna (ANITA) balloon experiment detected two anomalous upward-going radio pulses consistent with an ultrahigh-energy particle (E > 10¹⁸ eV) emerging from the Earth — something the Standard Model cannot explain, since a neutrino of that energy should be absorbed. Proposed explanations include τ-neutrino conversion, sterile neutrino mixing, supersymmetric particles, and even dark matter. Follow-up ground arrays have not confirmed the signal.
The answers could explain everything from the origin of matter to the nature of dark matter.
If CP violation in the lepton sector is large, leptogenesis could explain why the universe has more matter than antimatter — one of the deepest unanswered questions in cosmology. δ_CP = −π/2 would be a powerful hint.
Sterile neutrinos with keV-scale masses are a leading dark matter candidate. They would be produced in the early universe, gravitationally cluster, and be nearly undetectable — fitting dark matter's observational profile.
Understanding why neutrino masses are so incredibly tiny (10¹² times lighter than the top quark) may reveal physics far beyond the Standard Model — possibly at the GUT scale (10¹⁶ GeV), far beyond any collider's reach.