CERN’s Axion Solar telescope (CAST) has been running since 2003 searching for the axion: a hypothetical particle introduced in 1978 as a consequence of the Peccei-Quinn mechanism aiming to solve the strong CP problem, i.e why is the neutron electric dipole moment limit some ten orders of magnitude smaller than expected from QCD. CAST’s detection technique is based on the inverse Primakoff effect, dictating that inside a strong electric or magnetic field an axion can couple to a virtual photon producing a detectable photon. During its operation, CAST has progressively put world-class limits on the axion-photon coupling for a wide range of axion masses. The latest result published in 2017 in Nature Physics, sets an upper limit on the axion-photon coupling strength of gaγγ < 0.66 x 10-10 GeV-1 (95% CL) for all axions with masses below 0.02 eV.
The latest solar axion exclusion plot.
In addition to offering a solution to the strong CP problem, axions in the mass range around 1 - 100 μeV are also good cold dark matter candidates, as they are non-baryonic and can be produced in sufficient abundance during the Big Bang. Given that the dark matter density within our Milky Way halo is expected to be ρΑ=0.45 GeV/cm3, dark matter axions would have a local number density of the order of 1014 / cm3 but remaining nearly undetectable due to their very feeble coupling.
Today, the most promising experiments for detecting dark matter axions are still the so-called “Axion Haloscopes”, based on the Sikivie haloscope technique, which have the potential to detect them around the μeV mass region. Haloscopes consist of high-Q microwave cavities immersed in a strong magnetic field. The axion-to-photon conversion rate in a region of space where a strong magnetic field is present, is further enhanced if the outgoing photon, is detected in a microwave cavity resonating to the frequency of the axion mass. The operation of such RF cavities involves the appropriate choice of available electromagnetic modes, and, as the axion mass is unknown, a tuning mechanism for adjusting the resonant frequencies and thus covering a wide range of axion masses is indispensable.
The CAST superconducting magnet provides a dipole magnetic field of 9T, and has a twin-bore geometry ( length, diameter) into which the rectangular design of the cavities used for dark matter axion searches has to fit. The on-resonance axion conversion efficiency in a microwave cavity, and thus its ability to search for dark matter axions, increases with the magnetic field squared (B2), the cavity quality factor Q (the ratio of the cavity stored-energy to its power loss per cycle), the volume V of the cavity, and the so-called geometry factor C determined by the direction of the external magnetic dield and the cavity mode, with the dependence is given by the following formula where ρΑ stands for the axion field density and mA the axion mass:
Early this autumn the CAST team successfully completed the installation and commissioning of two varieties of cavities, the RADES and CAST-CAPP sub-detectors, one in each bore of the CAST dipole magnet (Figure 1), making CAST the only experiment at CERN looking for the direct detection of dark matter axions. RADES consists of a 1m long “alternating irises” stainless-steel cavity able to search for dark matter axions around 34 μeV.
Figure 1a: CAST experiment and close up photo of the twin bores of the CASt magnet where the RADES and CAST-CAPP RF cavities are installed.
Figure 1b. Installation of CAST-CAPP RF cavities on one of the two bores of CAST magnet
On the other hand, the CAST-CAPP sub-detector, whose latest results are presented here, consists of four tunable stainless-steel cavities 25mm x 23mm x 390mm electroplated with 30μm of copper. The most crucial element of CAST-CAPP, its delicate tuning mechanism, consists of 2 parallel sapphire plates activated by a piezoelectric motor through a locomotive mechanism delivering a tuning resolution of better than 100 Hz
in stable conditions (Figure 2). The current maximum tuning range is of the order of 400 MΗz corresponding to axions masses between 21-23 μeV.
Figure 2: The tuning mechanism of CAST-CAPP detector (left) and the stepper motor providing the movement through the locomotive mechanism (right).
Furthermore, since CAST’s sensitivity increases with the cavity volume, in order to increase the effective volume, the “phase-matching” technique has been applied. The concept is to get simultaneous read-outs from several frequency-matched cavities and then combine them coherently. This improves the signal-to-noise ratio linearly with the number of cavities. This technically challenging concept has also been achieved with CAST-CAPP for three coherent cavities in data-taking conditions (Figure 3).
Figure 3: Three phase-matched cavities of CAST-CAPP (left) and a presentation of complex spectrum mode of a spectrum analyzer during data-taking with a calibration peak (right).
In addition to searches for conventional axions, CAST-CAPP introduced for the first time in axion dark matter research, the “fast resonant scanning technique”. Thanks to the fast scanning mechanism CAST-CAPP detector is also sensitive to dark matter axion tidal or cosmological streams as well as to theoretically motivated axion mini-clusters. A quite wide axion mass range can be already scanned within a time period of a few hours to eventually take advantage of streaming dark matter towards the Earth. The axion flux enhancement due to gravitational focusing by the Sun can be up to 108 and in the ideal case as high as 1011 (Figure 4). Apparently, the faster the scanning the shorter axion bursts can be detected. The current maximum scanning speed of CAST-CAPP is 5 MHz/ 30 sec and therefore it's full tuning range of 400 MHz can be covered with one hour.
Figure 4: Schematic view of the flow of a putative slow-moving stream being gravitationally focused by the Sun towards the Earth.
At the same time, an alternative wideband scanning technique has been established for the first time, which is based on an out of resonance scanning, abandoning the resonance enhancement factor Q . This could be more than compensated by a temporally large axion flux enhancement. The minimum scanning period for this technique is 10 min for the maximum 300 MHz usable range and is currently done automatically for several hours.
During the latest 2019 data taking run (12/09 - 01/12) a total of 280 MHz frequency range with 200 kHz steps has been scanned with the “fast scanning” method with a total data acquisition time of using all four cavities. Moreover, using the phase-matching technique with three of the cavities, we successfully covered a frequency range of 64 MHz using steps of 200 kHz over a total data acquisition time of 18.4 d. With the present performance and a total of 26.5 d of data analyzed CAST-CAPP has excluded DM halo axions in the parameter space above the reference KSVZ line (Figure 5). If during these measurements there was an axion stream or axion cluster (in this frequency range) with a density enhancement of 103 passing by, we would have seen it.
Figure 5: Exclusion plot assuming galactic halo dark matter axions (green) and with enhancement due to streaming (other colours) (left). Perspectives for future longtime measurements assuming present and improved cavity performance (right).
These first successful runs have demonstrated the discovery potential of CAST as an axion dark matter antenna. We are now ready for continued stable running, aiming to cover a bigger frequency range and at the same time increase the sensitivity as the acquisition time will be bigger. Possible upgrades could include an enhancement of the piezoelectric motors for faster tuning in a broader frequency range with less heat dissipation, minimization of the heat dissipation of the cryogenic amplifiers through different bias conditions and better cooling of the cavities, and a fully automatic and user independent DAQ system and storage. Ongoing R&D in IBS/CAPP-Korea could increase the cavity’s Q-factor up to 106 (almost a factor of 100), by transforming them into superconducting ones using YBCO tape on their inner surface. CAST is ready to extend its sensitive axion searches, if the current run is extended into 2020.