AMS-02 Positron Excess: Charge-Sign Spectrometer

Beam Settings

Particle energy 10.0 GeV

Spectrometer field B 1.4 T

e⁺ positron (bends up) e⁻ electron (bends down)

Live Readout

Rigidity R = pc/Ze

10.0 GV

Bend radius r

24 m

Particles counted

Measured e⁺ fraction

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Positron Fraction vs Energy

secondary-production model AMS-02 / PAMELA fit

your Monte Carlo bins

How it works

A magnetic spectrometer like AMS-02 (mounted on the ISS since 2011) never "sees" a charge — it infers it from geometry. A charged particle of momentum p crossing a field B curves with radius:

r [m] = p [GeV/c] / (0.3 · Z · B [T])
Rigidity  R = pc / (Ze)  ≈ E  for ultra-relativistic leptons

The permanent magnet's field direction is fixed and known, so the *sign* of the curvature (up vs down in this view) directly reads off the sign of the charge — silicon tracker planes on either side of the magnet measure the trajectory precisely enough to separate positrons (e⁺) from the ~1000× more common electrons (e⁻) and cosmic-ray protons.

Doing this for millions of particles gives the positron fraction f(E) = Φ_e⁺ / (Φ_e⁺ + Φ_e⁻). Ordinary "secondary" positrons, knocked loose when cosmic-ray protons hit interstellar gas, predict a fraction that falls smoothly with energy. Instead, AMS-02 (confirming an earlier PAMELA result) measured f(E) turning around and rising from about 8 GeV up to roughly 200–300 GeV before flattening — an excess with no confirmed source, debated between dark-matter annihilation and nearby pulsars such as Geminga.

Particle energy — sets the rigidity of injected particles and where they land on the graph below.
Field B — a stronger field tightens the curvature radius (visually exaggerated here for clarity; the readouts use the real formula).
Inject / Stream — fires particles whose e⁺:e⁻ mix is drawn from the real AMS-02 fit at that energy; watch the orange Monte Carlo dots converge onto the blue measured curve as more particles accumulate in each bin.

AMS-02–style spectrometer · ISS orbit Beam: 10.0 GeV

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