Axion Haloscope: Dark Matter Resonant Cavity Search

Haloscope Controls

Magnet field B 8.0 T

Tuning rod position 40%

Loaded Q factor 3.2×10⁴

Adjust the tuning rod or start a scan to search for a resonant excess.

Live Readout

Resonant f_c

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Axion mass m_a

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Signal power P_sig

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SNR (1 s)

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Scan Spectrum

How it works

An axion haloscope (ADMX-style) places a tunable microwave cavity inside a strong static magnetic field B. If dark-matter axions exist, the inverse Primakoff effect lets a photon of the virtual field convert an axion into a real microwave photon only when the cavity's resonant frequency matches the axion's Compton frequency:

E = m_a c² = h f_c
m_a[μeV] ≈ 4.1357 × f_c[GHz]

The cavity supports a TM₀₁₀ mode whose axial electric field follows the Bessel profile E_z(r) ∝ J₀(x₀₁ r/R), x₀₁ ≈ 2.405 — rendered below as the pulsing field grid. A movable tuning rod perturbs this mode and shifts f_c as it sweeps from the cavity's center toward its wall.

The expected conversion power follows the standard haloscope formula (Sikivie 1983; as used by ADMX):

P_sig ∝ g_aγγ² · B² · V · C · Q_L · ρ_a / m_a

Doubling B quadruples the signal; a higher loaded quality factor Q_L both boosts power and narrows the cavity linewidth Δf = f_c/Q_L that must be scanned bin-by-bin. The thermal/quantum noise floor follows the Dicke radiometer relation P_noise = k_B T_sys Δf, so SNR = P_sig/P_noise for a 1-second dwell. A real experiment steps the rod slowly across the whole band, integrating longer at each point, because a true signal only appears in the narrow bin where f_c happens to equal the (unknown) axion mass — reproduced here as the hidden target frequency you can reseed and hunt for.

TM₀₁₀ field |E_z(r)| Tuning rod Solenoid field B

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