Search for Chameleon Scalar Fields with the Axion Dark Matter Experiment

Scalar fields with a “chameleon” property, in which the effective particle mass is a function of its local environment, are common to many theories beyond the standard model and could be responsible for dark energy. If these fields couple weakly to the photon, they could be detectable through the afterglow effect of photon-chameleon-photon transitions. The ADMX experiment was used in the first chameleon search with a microwave cavity to set a new limit on scalar chameleon-photon coupling ${\beta }_{\gamma }$ excluding values between $2\times {10}^9$ and $5\times {10}^{14}$ for effective chameleon masses between 1.9510 and $1.9525\mu \mathrm{eV}$.

Figure 1

Chameleon search procedure.

Figure 2

Predicted excess power in the ADMX experiment from Primakoff conversion of chameleons immediately after turn-on of the first-stage amplifier as a function of chameleon-photon coupling strength ${\beta }_{\gamma }$. Smaller ${\beta }_{\gamma }$ power is limited by coupling strength, larger ${\beta }_{\gamma }$ power is limited by decay time. Noise floor corresponds to 3 K system temperature.

Figure 3

A simulated chameleon signal corresponding to ${\beta }_{\gamma }=2\times {10}^9$ with an effective mass of $1.952\mu \mathrm{eV}$ superimposed on real data. The data is shown as a series of sequential power spectra; for this case the chameleon decay time is long compared to the integration time.

Figure 4

Shaded region: 90% confidence limit excluded parameters for scalar chameleons from ADMX search. Dashed lines: upper and lower exclusion bounds from Ref. 12.