PDF《MPP-2016-141》

MPP-2016-141 Dielectric Haloscopes: A New Way to Detect Axion Dark Matter Allen Caldwell,1 Gia Dvali,1, 2,

3 B? ela Majorovits,1 Alexander Millar,1 Georg Ra?elt,1 Javier Redondo,1,

4 Olaf Reimann,1 Frank Simon,1 and Frank Ste?en1 (The MADMAX Working Group)

1 Max-Planck-Institut f¨ ur Physik (Werner-Heisenberg-Institut), F¨ ohringer Ring 6,

80805 M¨ unchen, Germany

2 Ludwig-Maximilians-Universit¨ at, Theresienstra?e 37,

80333 M¨ unchen, Germany

3 CCPP, New York University, New York, NY 10003, USA

4 University of Zaragoza, P.

Cerbuna 12,

50009 Zaragoza, Spain We propose a new strategy to search for dark matter axions in the mass range of 40C400 ?eV by introducing dielectric haloscopes, which consist of dielectric disks placed in a magnetic ?eld. The changing dielectric media cause discontinuities in the axion-induced electric ?eld, leading to the generation of propagating electromagnetic waves to satisfy the continuity requirements at the interfaces. Large-area disks with adjustable distances boost the microwave signal (10C100 GHz) to an observable level and allow one to scan over a broad axion mass range. A sensitivity to QCD axion models is conceivable with

80 disks of

1 m2 area contained in a

10 Tesla ?eld. INTRODUCTION The nature of dark matter (DM) is one of the most en- during cosmological mysteries. One prime candidate, the axion, arises from the PecceiCQuinn (PQ) solution to the strong CP problem, the absence of CP violation in quan- tum chromodynamics (QCD). The CP violating QCD phase θ is e?ectively replaced by the axion ?eld whose potential is minimal at θ =

0 [1C3]. Thus θ dynamically relaxes towards zero regardless of its initial conditions, satisfying the neutron electric dipole moment constraints θ 10?11 [4]. Tiny relic oscillations with a frequency given by the axion mass ma around θ =

0 persist, acting as cold DM [5C9]. If DM is purely axionic, its local galac- tic density ρa = (fama)2 θ2 0/2 ?

300 MeV/cm3 implies θ ? θ0 cos(mat) at the Earth, with θ0 ? 4*10?19 . While these oscillations could be detected, the main challenge is to scan over a huge frequency range as ma is unknown. However, cosmology can guide our search. Causality implies that at some early time θ is uncorrelated between patches of causal horizon size. We consider two cosmo- logical scenarios depending on whether cosmic in?ation happens after (A) or before (B) that time. In Scenario A, one patch is in?ated to encompass our observable universe while smoothing θ to a single initial value θI. The cosmic axion abundance depends on both θI and ma, so the DM density can be matched for any ma allowed by astrophysical bounds [10] for a suitable θI. In Scenario B, the axion abundance is given by the average over random initial conditions and the decay of accompanying cosmic strings and domain walls. Freed from the uncertainty in the initial conditions, Scenario B provides a concrete prediction ma ?

100 ?eV [11, 12], although with some theoretical uncertainty [13]. Searches based on cavity resonators in strong mag- netic ?elds (Sikivie'

s haloscopes [14]) such as ADMX [15], ADMX HF [16] or CULTASK [17] are optimal for ma FIG. 1. A dielectric haloscope consisting of a mirror and several dielectric disks placed in an external magnetic ?eld Be and a receiver in the ?eld-free region. A parabolic mirror (not shown) could be used to concentrate the emitted power into the receiver. Internal re?ections are not shown.

10 ?eV. Much lower values of ma can be explored by nu- clear magnetic resonance techniques like CASPER [18] or with LC circuits [19, 20]. The mass range favoured in Scenario B is untouched by current experiments, and for cavity haloscopes will remain so for the foreseeable future. While ?fth-force ex- periments [21] could search this region, they would not directly reveal the nature of DM. We present here a new concept to cover this important gap, capable of discov- ering ?