Novel signatures of dark matter in laser-interferometric gravitational-wave detectors H. Grote1, ∗ and Y. V. Stadnik2, 3, y 1Cardiff University, School of Physics and Astronomy, The Parade, CF24 3AA, United Kingdom 2Helmholtz Institute Mainz, Johannes Gutenberg University, 55128 Mainz, Germany 3Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan (Dated: October 1, 2019) Dark matter may induce apparent temporal variations in the physical \constants", including the electromagnetic fine-structure constant and fermion masses. In particular, a coherently oscillating classical dark-matter field may induce apparent oscillations of physical constants in time, while the passage of macroscopic dark-matter objects (such as topological defects) may induce apparent tran- sient variations in the physical constants. In this paper, we point out several new signatures of the aforementioned types of dark matter that can arise due to the geometric asymmetry created by the beam-splitter in a two-arm laser interferometer. These new signatures include dark-matter-induced time-varying size changes of a freely-suspended beam-splitter and associated time-varying shifts of the main reflecting surface of the beam-splitter that splits and recombines the laser beam, as well as time-varying refractive-index changes in the freely-suspended beam-splitter and time-varying size changes of freely-suspended arm mirrors. We demonstrate that existing ground-based experiments already have sufficient sensitivity to probe extensive regions of unconstrained parameter space in models involving oscillating scalar dark-matter fields and domain walls composed of scalar fields. In the case of oscillating dark-matter fields, Michelson interferometers | in particular, the GEO 600 detector | are especially sensitive. The sensitivity of Fabry-Perot-Michelson interferometers, in- cluding LIGO, VIRGO and KAGRA, to oscillating dark-matter fields can be significantly increased by making the thicknesses of the freely-suspended Fabry-Perot arm mirrors different in the two arms. Not-too-distantly-separated laser interferometers can benefit from cross-correlation measurements in searches for effects of spatially coherent dark-matter fields. In addition to broadband searches for oscillating dark-matter fields, we also discuss how small-scale Michelson interferometers, such as the Fermilab holometer, could be used to perform resonant narrowband searches for oscillating dark-matter fields with enhanced sensitivity to dark matter. Finally, we discuss the possibility of using future space-based detectors, such as LISA, to search for dark matter via time-varying size changes of and time-varying forces exerted on freely-floating test masses. I. INTRODUCTION ticles in this case carry very small momenta. Instead, one can take advantage of the fact that low-mass DM parti- While the existence of dark matter (DM) is well es- cles must have large occupation numbers if they comprise tablished from astrophysical and cosmological observa- the observed DM content of the Universe (the average lo- 3 tions, the elucidation of its precise nature remains one of cal cold DM density is given by ρDM ≈ 0:4 GeV=cm [1]) the most important problems in contemporary physics. and look for wavelike and other coherent signatures of Since extensive searches for DM particles of relatively these DM fields. In recent years, a number of novel ideas high masses (e.g., WIMPs) through their possible non- have emerged to search for low-mass DM using precision gravitational effects have not yet produced a strong pos- measurement techniques from the fields of atomic and itive result, in recent years the possibility of searching optical physics; see [2,3] for recent overviews. for low-mass (sub-eV) DM candidates has been receiving DM may induce apparent temporal variations in the increased attention. There are numerous well-motivated physical \constants", including the electromagnetic fine- DM candidates of this type, including the canonical ax- structure constant α, as well as the electron and nucleon arXiv:1906.06193v3 [astro-ph.IM] 29 Sep 2019 ion, axion-like particles and dilatons, which may form masses me and mN , via certain non-gravitational inter- a coherently oscillating classical field and/or stable soli- actions with standard-model (SM) fields [4]. In partic- tonic field configurations known as topological defects ular, a coherently oscillating classical DM field may in- (such as domain walls). Searches for such low-mass DM duce apparent oscillations of physical constants in time candidates via possible particlelike signatures (such as re- [5,6], while the passage of topological defects may induce coils, energy depositions and ionisations) are practically apparent transient variations in the physical constants impossible, since the individual non-relativistic DM par- [7,8]. Some possible effects of such time-varying phys- ical constants in laser interferometers and optical cav- ities, including time-varying changes of solid sizes and laser frequencies, were explored in [5,9]. Several clock- ∗Electronic address: groteh@cardiff.ac.uk cavity comparison experiments searching for DM-induced yElectronic address: [email protected] time-varying physical constants have been conducted re- 2 cently [10{12]. Experiments of this type are mainly sen- sitive to oscillation frequencies up to ∼ Hz (equivalently timescales down to ∼ s). However, we would also like to ETMY l precisely probe even higher oscillation frequencies (audio- band frequencies and beyond), which too are interesting from the point of view of current astrophysical observa- tions. Ly R=50% In this work, we propose new ways of searching for AR DM with laser interferometers. A two-arm laser inter- ferometer is typically used to detect small changes in the Laser difference of the optical path lengths in the two arms Lx of the interferometer. Since the two arms of an inter- BS ferometer are practically equal in terms of optical path PRM ETMX length, time-varying arm length fluctuations and changes SRM in the laser frequency due to a homogeneous DM field are common mode, and their effects are therefore strongly PD suppressed in the output signal. There is, however, a geometric asymmetry created by the beam-splitter in a FIG. 1: Simplified layout of a dual-recycled Michelson inter- two-arm laser interferometer. We point out that the ferometer, such as GEO 600 [13, 14] and the Fermilab holome- beam-splitter and arm mirrors of an interferometer, if ter [15, 16]. Dual recycling denotes the combination of power freely suspended, can produce differential optical-path- recycling and signal recycling. PRM: power recycling mirror; length changes if one or more of the physical constants BS: beam-splitter of thickness l; ETMX, ETMY: end arm mirrors (test masses); SRM: signal recycling mirror; PD: pho- of nature vary in time (and space). A non-zero output todetector. The inset shows the beam routing through the signal, namely a phase difference between the two arms beam-splitter. The beam-splitting surface typically has a of the interferometer, can arise in several ways. If the power reflectivity of R = 50%. The opposing face of the DM field is homogeneous across the entire interferometer, beam-splitter, denoted by AR, is anti-reflective coated. For then the main observable effect will generally arise from clarity, we have omitted the single folding of the arms in the freely-suspended beam-splitter. A freely-suspended GEO 600, as well as the second co-located interferometer of beam-splitter would experience time-vaying size changes the Fermilab holometer; furthermore, the Fermilab holometer about its centre-of-mass, thus shifting back-and-forth the does not have a signal recycling mirror SRM. main reflecting surface that splits and recombines the laser beam (see the inset in Fig.1). Additionally, (gen- ETMY erally smaller) time-varying changes in the refractive in- dex of the beam-splitter would change the optical path length across the beam-splitter. On the other hand, if the DM field is inhomogeneous over an interferometer, then Ly substantial observable effects may also arise from time- varying size changes of the freely-suspended arm mirrors (see Figs.1 and2). In some situations, the output sig- nal can be significantly enhanced if the arm mirrors have ITMY different physical characteristics (in particular, different Laser BS thicknesses). Lx Laser interferometry has been optimised over decades to develop ultra-sensitive gravitational-wave detectors, PRM ITMX ETMX which have recently been employed spectacularly to di- rectly observe gravitational waves on Earth for the first SRM time [20, 21]. Additionally, smaller-scale interferom- eters have more recently been utilised to search for PD non-gravitational-wave phenomena, such as quantum- geometry effects that may arise at the Planck scale FIG. 2: Simplified layout of a dual-recycled Fabry-Perot- [15, 16]. Future space-based laser-interferometric Michelson interferometer, such as Advanced LIGO [17], VIRGO [18] and KAGRA [19]. PRM: power recycling mir- gravitational-wave detectors, such as LISA [22], are cur- ror; BS: beam-splitter; ITMX, ETMX, ITMY, ETMY: arm rently under development. In this paper, we explore mirrors (test masses); SRM: signal recycling mirror; PD: pho- novel signatures of DM in ground- and space-based laser todetector. The arm mirrors (test masses) are separated by interferometers. We estimate the sensitivities of these de- the distances Lx and Ly, which are 4 km in the case of LIGO. tectors to the physical parameters of models of DM con- sisting of a coherently oscillating classical field or domain walls. Searches for coherently oscillating classical DM fields share similarities with searches for continuous as 3 well as for stochastic gravitational waves, while searches II. THEORY AND EFFECTS OF for domain-wall objects share similarities with searches DARK-MATTER-INDUCED VARYING for gravitational-wave bursts. Based on our estimates, PHYSICAL \CONSTANTS" we emphasise that existing laser interferometers, partic- ularly the GEO 600 Michelson interferometer [13, 14], al- A.