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BRASS: Broadband Radiometric Axion SearcheS |
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Attempts to detect the elusive dark matter particle are now gradually shifting their focus of attention towards potential light-mass, weakly interacting sub-eV particles (WISP), including axions and hidden photons with masses well below one electron-Volt (eV).1 Recent studies have singled out the range of 10—104 meV (2.4 GHz—2.4 THz) where axions can both reproduce the dark matter and satisfy the critical constraints from cosmology, fundamental physics, and astrophysical observations. 2,3 ● BRASS aims at creating a single experimental framework that can be employed to search for axion dark matter over the entire 10—104 meV range of axion mass. ● BRASS will employ novel experimental approaches and synergies between particle physics experiments and state-of-the-art broadband detection techniques developed in radio astronomy, with the University of Hamburg, the Max-Planck Institute for Radioastronomy in Bonn, the Technical University of Hamburg, and DESY Hamburg working on different aspects of the research. ● During its first phase, BRASS will focus on three specific ranges of mass covering the “sweet spots” expected for the axion dark matter. Subsequent measurements will cover the entire range of mass of interest. |
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Experimental
foundation: BRASS will use the Primakoff process in which axions
can be converted into photons in presence of magnetic field. This approach
was typically realized in narrowband, resonant cavity experiments such as
ADMX4,5. In contrast to
such experiments BRASS relies on a novel, broadband concept in which the
strength of resulting photon signal is proportional to the area , A,
of the surface over which the conversion occurs and the square of the
magnetic field, B, parallel to the surface.6 To provide sufficient
detection sensitivity, the product of B||2A ³ 100 T2 m2 has to be achieved. |
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Basic design of BRASS
BRASS will consist of two or more detection chambers, each outfitted with a permanently magnetized surface and a secondary reflector. The axion-photon conversion will take place near the permanently magnetized surface and the resulting photonic signal will then be focused by secondary reflectors and detected in the receiver room, using interchangeable receiver modules. Signals from the individual BRASS chambers will be combined in the correlator module, increasing the measurement sensitivity and also enable constraining the flow direction of the hypothetical axion dark matter. |
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BRASS
sensitivity: Alongside the B||2A figure
of merit, sensitivity of BRASS measurements will depend on the detection
limits of the receivers and only weakly on the measurement time (µ t1/4). BRASS
will use state-of-the art heterodyne detectors reaching the noise temperature
Tnoise
» 4K and the efficiency of ~2hn/kB at frequencies up to 1
THz. The resulting expected sensitivity of BRASS is compared below to other
completed and planned axion search experiments. |
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Sensitivity
of BRASS search runs of 100 days in duration, compared to other completed
(dark grey shades) and planned (light grey shades) axion
search experiments. The regions of parameter space in which the QCD axion and axion-like particles
(ALP) can represent the dark matter are marked with the pink band and dashed
red line. The “sweet spots” for the axion DM are
shown in darker pink shades. The thick dashed line shows the overall benchmark
sensitivity limits of BRASS measurements, and the yellow shaded areas mark
the particle mass range which will be targeted during the Phase 1 of BRASS
experiments. |
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Components of BRASS: The magnetized surface will be made of small permanent magnets arranged in specific Halbach array7 configurations providing magnetic fields of order of ~1 T. The secondary reflectors will be manufactured to specifications limiting reflection losses to within about 0.5% at 1 THz. The receiver module will comprise low-noise detectors designed and built on the basis of the radioastronomical detectors used for APEX8 and ALMA telescopes. The correlator module will provide instantaneous spectral processing of a 16 GHz bandwidth with a fractional frequency resolution of ≤10-6, based on the digital broadband converters (DBBC3),9 Mark VI recorders, and digital FX (DiFX) correlator10 developed and used for radioastronomical measurements. Research opportunities: Already at its preparatory stage, BRASS provides ample opportunities for engaging in research projects at the MSc and PhD level. Examples of potential research areas include: ● Two-dimensional Halbach arrays for creating large magnetized surfaces
with strong and homogenous magnetic field. ● Optimization and
calibration of the optical system necessary for minimizing optical losses and
ensuring the required accuracy of BRASS measurements. ● Frontend design of
low-noise detectors ensuring the required performance within the Phase 1
bands of BRASS. ● Broadband digitization
of the signal using the DBBC3 technology, streamlining the data throughput of
the experiment. ● Data processing and signal detection, optimizing the methodology for axion signal detection in broadband spectra recorded by BRASS. |
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3.
A. Ringwald, K. Saikawa,
Phys. Rev. D, 93, 085031
(2016) [arXiv:1512.06436]. 4.
R. Bradley, J. Clark,
D. Kinion et al., Rev. Mod. Phys., 75, 777 (2003). 5.
S.J. Asztalos, ADMX Collaboration., Phys. Rev. Lett., 104, 041301 (2010) [arXiv:0910.5914]. 6.
D. Horns, J.
Jaeckel, A. Lindner et al., JCAP, 4,
016 (2013) [arXiv:1212.2970]. 7.
K. Halbach, Nucl.Instrum. and Methods, 169,
1 (1980). 8.
R. Güsten, L. Å.
Nyman, P. Schilke et al., A&A, 454, L13 (2006). 9.
L. Vertatschitsch,
R. Primiani, A. Young et al., PASP,
127, 1226 (2015). 10. A.T. Deller, W.F. Brisken,
C.J. Phillips et al., PASP, 123,
275 (2011) [arXiv:1101.0885]. |
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