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axion like particles ( alps ) and hidden sector photons ( hsp ) belong to the family of weakly interacting sub - ev particles ( wisps ) , which can be probed by `` light shining through the wall '' ( lsw ) experiments , exploiting their weak coupling to ordinary photons .
this allows an indirect proof of the otherwise hidden particles without relying on any cosmological assumptions .
previous lsw experiments have been carried out with laser light at desy ( alps ) , cern ( osqar ) and fermilab ( gammev ) .
the concept of an optical lsw experiment @xcite has been adapted to microwaves as shown in fig .
[ fig : theexp ] .
two identical microwave cavities with a diameter of 140 mm each are positioned in close vicinity to each other .
one serves as emitting cavity and is excited on its resonant frequency with @xmath0 = 50 w of rf power at @xmath1 = 2.9565 ghz by an external microwave source .
theory predicts that microwave photons from the emitting cavity can convert to hsps by kinetic mixing or to alps by the primakoff effect , penetrate the cavity walls and convert back to photons in the detection cavity @xcite . considering current exclusion limits ,
the probability for this process to happen is @xmath2 .
if a small excitation of the detection cavity over the thermal noise level can be observed and if any direct electromagnetic ( em ) crosstalk can be excluded , this could indicate the existence of wisps .
the detecting cavity is placed in a em shielding enclosure , together with a low noise amplifier ( lna ) and a vector signal analyzer ( vsa ) .
the em shielding is a critical aspect of the experiment because it is not possible to distinguish between a signal originating from wisps and direct coupling between the cavities by em leakage .
the nominal electric field strength inside the emitting cavity is 186 kv / m .
the smallest detectable signal in a 12 h run corresponds to an electric field strength of 24 nv / m in the detection cavity .
so the shielding must attenuate the em signal by at least 258 db .
the cavity walls provide 110 db each , which was measured by a calibrated near field probe .
the limiting factor is the contact resistance between the two half - shells .
the additional em shielding box provides 90 db of attenuation .
the overall screening attenuation thus is @xmath3 db , which is impossible to measure in its combined form , but which has been determined shell by shell .
all analog and digital signals are transmitted by fibre optic transceivers , to ensure interference can not couple through the screening layer of coaxial cables .
the vsa is remote controlled by a fibre optic ethernet link . to provide ac power within the shielding box ,
a lowpass feed - trough filter has been constructed for 50 hz mains , attenuating interference in the mhz region and above by @xmath4 90 db
.    the resonant frequency of both cavities must stay close to the system frequency @xmath1 during the measurement run .
we define half the 3  db bandwidth of the cavities ( 65  khz ) as the limit for the maximum amount of allowable drift .
the emitting cavity dissipates 50  w of power by forced air cooling .
it heats up and expands considerably . during a warm up period
the drift of @xmath5  1  mhz is manually compensated by a tuning screw .
after @xmath5  1 h , thermal equilibrium is reached , the cavity is stable and no further tuning adjustments are necessary .
we monitor the reflected power from its coupling port continuously by a directional coupler and a detector diode , as long it is below a certain threshold , the cavity is considered to be on tune .
the resonant frequency of the detecting cavity is monitored by observing a bump in its spectral noise power density . a low noise amplifier ( lna ) , based on a high electron mobility transistor ( hemt ) is directly connected to the detecting cavity , it provides g = 44.7  db gain at @xmath6 = 32.4  k noise temperature .
the noise temperature of the detection cavity is frequency dependent , its maximum is at resonance and equals the physical temperature of @xmath7 = 290  k. this can be observed as a significant peak over the noise floor of the amplifier , allowing us to determine the resonant frequency .
the overall system noise temperature of the complete receiving chain , determined by friis formula is @xmath8 = 32.5  k.    since there is no energy loss associated with the wisp conversion process , the regenerated photons in the detecting cavity appear with the same energy as the photons in the emitting cavity .
thus , the signal which is coupled out from the detecting cavity has the same frequency and bandwidth as the one which is generated on the emitting side . for this reason ,
data processing is a matter of detecting a sinusoidal signal of known frequency in the white background noise .
the vsa records complex samples , representing a bandpass limited signal with up to 25 mhz span , centered at @xmath9 .
the recorded data is evaluated by a python script , estimating the noise power spectrum . after applying a hanning window to reduce the effect of spectral leakage (
if the narrowband signal falls between two bins and no window is used , its amplitude can be reduced by up to 36 % @xcite ) , one fast fourier transform ( fft ) is calculated over the entire time trace , which can be up to several hundred millions of samples long .
the `` fftw '' software library is used to implement the fft efficiently .
r0.4        considering pure white noise as an input signal to the calculation , @xmath10 describes the 3 db bandwidth of one spectral bin .
it depends on the length of the recorded time trace ( @xmath11 ) and the applied window function .
the hanning window broadens the frequency response by a factor of @xmath12 .
the average available noise power @xmath13 decreases proportionally with the length of the time trace @xmath11 . if a signal is observed , it will appear as a single line in the spectrum with constant power .
the signal to noise ration should thus improve linearly with @xmath11 .
for this to hold true , all oscillators must be synchronized and stay stable in frequency relative to each other within a tolerance given by @xmath14 .
synchronization is achieved by a 10 mhz signal , which all instruments use as frequency reference .
long - term frequency drifts in the order of few mhz can not be avoided during the @xmath15  h measurement time .
however , with synchronization , they will affect all oscillators in the same way and cancel out in the measurement result .
this way , each bin of the synchronized fft effectively corresponds to a lock - in amplifier , tuned to the bin s center frequency . to proof the setup is stable over 12 h , a test run has been conducted . for the evaluation
, its time trace has been truncated to different lengths . in fig
[ fig : signoise ] , the expected linear decrease of the average noise floor can be observed .
a narrowband signal has been introduced by provoking electromagnetic leakage .
once the signal emerges from the noise floor , it stays relatively constant in power .
the signal peak never exceeded the width of two bins in the spectrum , which is the minimum dictated by the hanning window , showing that the frequency - locking with a relative accuracy in the @xmath16hz range has been achieved between all the involved oscillators .
a 11.5 h measurement run for hsps has been carried out in march 2012 . no narrowband signal was detected and the data was interpreted as an exclusion result .
details can be found in @xcite .
we improved over the most sensitive exclusion limits from cosmic microwave background measurements , in the energy range of @xmath17ev .    for axion measurements ,
both cavities need to be placed in a strong magnetic field .
thus modification of the setup was necessary and a smaller secondary shielding box has been constructed from non magnetic stainless steel .
it contains the detecting cavity , the lna and a analog optical link with 4.5 ghz bandwidth .
the optical fibre allows signal transmission from the secondary to the primary shielding box , containing the optical receiver and the vsa .
the cavities were tuned to the tm@xmath18 mode , at 1.76 ghz , coupling well to axions @xcite . the hardware transfer function from the cavity port , through all cables , the lna , and
the optical link has been carefully determined by a network analyzer to be 60.0 db at 2.95 ghz and 58.7 db at 1.76 ghz , allowing to normalize the measured noise spectra .    in june 2012
a magnet was available during a one week timeslot .
the normal conducting dipole magnet is regularly used for material testing at cern and has a aperture of @xmath5  100  x  50  x  30  cm .
it provides an average field strength of 0.51 t , which has been confirmed by a calibrated hall probe .
we were able to record 3 measurement runs of 4 h , 4 h and 6 h length .
the first run showed em leakage .
a weak spot could be narrowed down with a near - field probe to the gasket between lid and flange of the enclosure and mitigated by copper mesh .
no signal and only noise was observed in the second and third run ( see fig .  [
fig : exclsignal ] ) .
the data of the second run was translated to an exclusion limit for axions and is shown in fig .
[ fig : excldetail ] and fig .
[ fig : excloverview ] as the red trace . in the third run , @xmath5 7 mm@xmath19 of cr@xmath20o@xmath21 crystals
have been placed on the bottom of each cavity .
one may speculate if cr@xmath20o@xmath21 interacts with axions because of its magneto - electric properties @xcite .
however , despite a 2% reduction in q factor , no surprising effects were observed .
although current exclusion limits could not be challenged , we gained experience on our measurement setup and successfully operated it within a magnet for the first time . this allows to utilize a stronger , superconducting magnet in a next step .
we are planning to work together with an external partner , where a superconducting magnet with a wide enough aperture for our measurement setup is available .
the projected sensitivity with this magnet ( 4.5 t ) and the current setup in a 8 h run is shown in fig .
[ fig : excloverview ] as the blue trace .
+ the authors would like to thank r.  jones , e.  jensen and the be department management for encouragement and support .
thanks to the organizers of the patras workshop for a very enjoyable and inspiring conference .
supported by the wolfgang - gentner - programme of the bundesministerium fr bildung und forschung ( bmbf ) .
j.  jaeckel , a.  ringwald , _
`` a cavity experiment to search for hidden sector photons '' _ , physics b659 2008 g. heinzel et al . , `` _ _ spectrum and spectral density estimation by the discrete fourier transform ( dft ) , including a comprehensive list of window functions and some new flat - top windows _ _ '' , max - planck - institut fr gravitationsphysik 02/2002
to detect or exclude the existence of hidden sector photons or axion like particles , a table - top `` microwaves shining through the wall '' experiment has been set up at cern . 
 an overview of the experimental layout is given , the technical challenges involved are reviewed and the measurement procedure including data - evaluation and its results to date are shown .