One cavity resonating, the other not

March 7, 2010 at 8:50 am Leave a comment

Light does not shine through a wall – unless it mixes with mysterious “dark photons” which can pass through matter. A cute experiment conducted by three physicists at the University of Western Australia, one of them a student, is described in a very recent paper (arXiv:1003.0964):


First Microwave Cavity Experiment Search for Hidden Sector Photons

Here is a sketch of their apparatus:
diagram for the dark photon search experiment

Photons in the microwave range are produced by one resonant cavity, the bottom one in the diagram. Suppose they mix with these hypothetical dark photons. The dark photons have a very very weak interaction with matter (hence the moniker “Hidden Sector”), so they can escape the first resonant cavity. There would be a flux of such dark photons into the second cavity. If the dark photons mix back into regular photons, so to speak, then they will excite the resonance in the second cavity. This will not happen without the participation of dark photons, since the two cavities are optically cut off from each other – in principle, only the dark photons can connect them in a resonant fashion.

The prediction for a signal is given in terms of the ratio of power emitted and received:

Prec/Pemit = χ QemitQrec A (Mdark/Eγ)8

where χ is a theoretical mixing parameter, Q refers to the Q-values of the cavities (the quality of their resonance), A is essentially a geometric acceptance factor, Eγ is the energy of the photon produced in the emitter and Mdark is the mass of the dark photon with which it mixes, hypothetically. So the signal in the receiver is large if there is enough mixing, if the resonance in both cavities is high and narrow, if the geometric acceptance is good, and if the energy of the normal photon is not much larger than the mass of the dark photon with which it mixes, hypothetically.

The two cavities are maintained at a low temperature in a vacuum vessel to reduce thermal noise. Small openings are necessary to allow coils for exciting the resonant mode of the emitter and to detect the resonance of the receiver, and unfortunately they allow some false signal to leak into the receiver, limiting the success of this experiment. Since this is just an exploratory demonstration, this is not a serious shortcoming – the authors indicate that they can improve upon this problem in future designs. Another path for improvement is the design and construction of cavities with a higher Q value, ie, a narrower resonance.

For a demonstration prototype, the results are respectable:

Excluded regions for dark photon searches

Excluded regions for dark photons. The horizontal axis the mass of the dark photon in eV, and the vertical axis is the mixing parameter.

Let’s see what the large version of this apparatus can bring!

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Entry filed under: Particle Physics.

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