A dual-phase liquid xenon time projection chamber (the technology behind XENON1T/XENONnT, LZ and PandaX) is the world's most sensitive instrument for direct WIMP dark-matter searches. A particle interacting in the liquid produces two signals:
S1 — prompt scintillation photons (178 nm) seen instantly by top/bottom PMT arrays
S2 — ionization electrons drift up in a uniform field E, cross the liquid surface
and are amplified into proportional light in the gas phase
The drift time between S1 and S2 gives the interaction depth directly:
z = H − v_d(E) · Δt_drift (v_d rises with drift field E)
The key discrimination physics: a nuclear recoil (a WIMP or a neutron bouncing off a xenon nucleus) deposits energy in a dense track with a high exciton-to-ion ratio and heavy electron–ion recombination, which pushes energy into S1 light and starves S2 charge. An electron recoil (a gamma or beta from ordinary radioactivity) ionizes more sparsely, recombines far less, and yields much more S2 charge per unit S1 light:
n_total = E_recoil / W (W ≈ 13.7 eV per quantum in liquid xenon)
S1 ∝ n_excitons + r·n_ions S2 ∝ (1 − r)·n_ions · survival(drift)
r = recombination fraction — higher for NR, and falls as drift field E rises
Plotting log₁₀(S2/S1) versus S1 for many events separates two bands: WIMPs and neutrons cluster in a low, "nuclear-recoil band," while the much larger flux of gamma/beta backgrounds sits in a higher "electron-recoil band." Real experiments set an NR-band acceptance cut and count how many background events leak below it — that leakage rate is what ultimately limits how light a WIMP mass a detector can probe.
- Particle type — switch between a WIMP-like nuclear recoil and a gamma-ray electron-recoil background.
- Recoil energy — sets how many scintillation/ionization quanta the interaction produces.
- Interaction depth — where in the drift column the event occurs; deeper events take longer to drift and lose more charge to electronegative impurities.
- Drift field — a stronger field speeds up electron drift and suppresses recombination, widening the separation between the two bands.