Astronomy & Celestial Mechanics
the sky, computed
Concept

Charged current — where it appears

The weak interaction mediated by the W boson, in which a neutrino turns into its partner charged lepton. At solar energies only electron neutrinos can do this, so a charged-current rate counts the electron flavour alone.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Also named here as detector threshold, neutral current, solar neutrino — the same set of essays touches all of them, so they are one junction rather than several.

Seven sources over twelve decades of flux, and one threshold below the one that matters. The Sun's neutrino spectrum at the Earth, with the continua drawn per unit energy and the two monoenergetic lines as spikes at their own energies. The pp reaction supplies 91 per cent of all of them and its endpoint is at 0.4233 MeV. The vertical lines are experimental thresholds, and they are the figure's argument: only gallium sits below that endpoint. Chlorine, which produced the deficit and held it for twenty years, could not see a single pp neutrino — it counted ⁷Be and ⁸B, which are a rare branch of a rare branch, together under a per cent of the total — and the water detectors that followed were higher still. So the discrepancy that eventually turned out to be a property of the neutrino was measured, for two decades, using the least representative one per cent of the flux available. The total drawn here is 6.54·10¹⁰ cm⁻² s⁻¹, and it is checkable without any stellar model at all: every completed chain turns four protons into helium, releases 26.73 MeV of which 0.59 leaves as neutrinos, and emits two neutrinos — so the solar constant of 1361 W m⁻² fixes the number at 6.5·10¹⁰ cm⁻² s⁻¹, within 0.6 per cent of the sum of the model's own branches. The dominant flux is a consequence of the Sun shining and of nothing else.

The only thing that leaves the centre

A photon made in the Sun's core takes a hundred thousand years to get out and arrives thermalised past recognition. A neutrino takes 2.3 seconds and arrives unchanged, so its flux is the fusion rate now — which is why a factor of three could not be absorbed by any adjustment to the Sun.

stars · Solar neutrinos The rise in survival, and what two detectors see of it. The survival probability of ⁸B electron neutrinos against neutrino energy (grey), and what two detectors measure of it against the energy of the electron they record: the ratio of the counted spectrum to the one expected without conversion, for electron scattering in water (blue) and for charged-current capture on deuterium (orange), both smeared by a water detector's energy resolution. Between 3 and 10 MeV the survival probability rises by 0.122. Scattering shows a rise of 0.044 over the same recoil energies, 36 per cent of it; capture on deuterium shows 0.063, 52 per cent.

A rise a detector shrinks to a tilt

Between 10 and 3 MeV the chance that a solar neutrino arrives as it left should rise by a third of its value, and no detector has clearly seen it. Counting is not the obstacle: a large water detector has many times the exposure the rise needs. What hides it is that scattering shows only a third of the rise, spread over the spectrum as a tilt of a few per cent, and that half of the evidence for the tilt sits where the radioactive background is and the other half where an error in the energy scale looks the same.

stars · Solar neutrinos

Named alongside it

The objects these essays reach for when they reach for this one.

Detector thresholdNeutral currentSolar neutrinoCore temperatureEnergy scaleFlavourHelioseismologyLuminosity constraintMsw effectNeutrino fluxNeutrino oscillationPp chain

All concepts