PHYSICAL REVIEW D 97, 043002 (2018)
Searching for axion stars and Q-balls with a terrestrial magnetometer network
D. F. Jackson Kimball,1,* D. Budker,2,3,4,5 J. Eby,6,7 M. Pospelov,8,9 S. Pustelny,10 T. Scholtes,11 Y. V. Stadnik,2,3 A. Weis,11 and A. Wickenbrock2 1Department of Physics, California State University—East Bay, Hayward, California 94542-3084, USA 2Johannes Gutenberg-Universität Mainz, 55128 Mainz, Germany 3Helmholtz Institut Mainz, 55099 Mainz, Germany 4Department of Physics, University of California at Berkeley, Berkeley, California 94720-7300, USA 5Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 6Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA 7Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, Illinois 60510, USA 8Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia V8P 1A1, Canada 9Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2J 2W9, Canada 10Institute of Physics, Jagiellonian University, 30-059 Kraków, Poland 11Physics Department, University of Fribourg, CH-1700 Fribourg, Switzerland
(Received 23 October 2017; published 7 February 2018)
Light (pseudo-)scalar fields are promising candidates to be the dark matter in the Universe. Under certain initial conditions in the early Universe and/or with certain types of self-interactions, they can form compact dark-matter objects such as axion stars or Q-balls. Direct encounters with such objects can be searched for by using a global network of atomic magnetometers. It is shown that for a range of masses and radii not ruled out by existing observations, the terrestrial encounter rate with axion stars or Q-balls can be sufficiently high (at least once per year) for a detection. Furthermore, it is shown that a global network of atomic magnetometers is sufficiently sensitive to pseudoscalar couplings to atomic spins so that a transit through an axion star or Q-ball could be detected over a broad range of unexplored parameter space.
DOI: 10.1103/PhysRevD.97.043002
I. INTRODUCTION searches for continuous oscillatory signals generated by the axion/ALP dark matter background assuming that terres- A host of astrophysical and cosmological measurements trial detectors are bathed in a continuous dark-matter flux, suggest that over 80% of all matter in the Universe is dark see for example Refs. [13–19]. matter [1–3]. In order to elucidate the nature of dark matter, However, it is possible that instead of a roughly uniform terrestrial experiments seek to measure non-gravitational distribution throughout the halo, ALPs could be concen- interactions of dark matter with standard-model particles trated in compact objects. For example, the mass-energy and fields. However, extrapolation from the ≳1 kpc associated with the Universe’s dark sector (dark matter and distances associated with astrophysical observations to partially dark energy) could be stored primarily in topo- particle-physics phenomena accessible to laboratory-scale logical defects such as domain walls, strings or monopoles experiments leaves open a vast number of plausible [20–22]. Another plausible scenario is that initial inhomo- theoretical possibilities worth exploring. geneities in the galactic dark matter distribution enable A well-motivated hypothesis is that a substantial fraction gravity or self-interactions [23] to generate bound clumps of dark matter consists of ultralight bosons such as axions “ ” – – – or stars composed of ALPs [24 37]. A prominent, [4 6] or axionlike particles (ALPs [7 9]) with masses closely related example of a compact, composite dark- 2 ≲ 10 mac eV. Such ultralight bosons will have a large matter object is the Q-ball [38–42] or Q-star, a non- number density in the galaxy and thus their phenomenol- topological soliton of a light scalar field [40,41]. In this ogy is well described by a classical field. In this scenario, work, we are primarily interested in ALP stars or Q-stars the mass-energy associated with dark matter is primarily (collectively referred to as soliton stars [40]) with radii stored in coherent oscillations of the dark-matter field R ≫ R (the radius of Earth). Under conditions where the – E [10 13]. There are a number of proposed and ongoing attractive interactions between ALPs are sufficiently strong so that most of the dark-matter mass takes the form of *[email protected] soliton stars, instead of being bathed in a continuous
2470-0010=2018=97(4)=043002(10) 043002-1 © 2018 American Physical Society D. F. JACKSON KIMBALL et al. PHYS. REV. D 97, 043002 (2018) dark-matter flux, terrestrial detectors will instead witness pumped atomic magnetometers [60] located at stations transient events when Earth passes through the soliton throughout the world (nine additional new stations are under stars [43]. construction [61]). The magnetometric sensitivitypffiffiffiffiffiffi of existing GNOME sensors is δB ≈ 100 fT= Hz over a II. TERRESTRIAL SENSOR NETWORKS TO bandwidth of ≈100 Hz. The GNOME is primarily sensitive SEARCH FOR TRANSIENT SIGNALS FROM to exotic interactions of electrons and protons [62]. A next- BOSONIC DARK MATTER generation Advanced GNOME is under development that 3 – Dark-matter fields consisting of ALPs are generically will use alkali- He comagnetometers [63 66] and will be predicted to interact, albeit feebly, with the intrinsic spins of primarily sensitive to neutron interactions [62]. Advanced δ ≈ elementary particles [47,48]. The Global Network of Optical GNOMEpffiffiffiffiffiffi sensors will have effective sensitivities of B Magnetometers to search for Exotic physics (GNOME) 1 fT= Hz to “pseudomagnetic fields” caused by ALP collaboration [22,49] is presently conducting a search for interactions over a similar bandwidth [63,64]. The transient spin-dependent interactions that might arise, for GNOME magnetometers are located within multilayer example, if Earth passes through a compact dark-matter magnetic shields to reduce the influence of external mag- object. While a single atomic-magnetometer system could in netic noise and perturbations, while still maintaining sensi- principle detect such transient events, in practice it is tivity to exotic spin-dependent interactions [67]. Each difficult to confidently distinguish a true signal heralding GNOME sensor also uses auxiliary unshielded magnetom- new physics from “false positives” induced by occasional eters and sensors, such as accelerometers and gyroscopes, to abrupt changes of magnetometer operational conditions. To measure relevant environmental conditions, enabling the veto false positive events, suppress noise, and effectively exclusion of data with known systematic issues. The signals characterize true exotic transient signals, the GNOME from the GNOME sensors are recorded with accurate timing consists of an array of magnetometers widely distributed using a custom GPS-disciplined data acquisition system over Earth’s surface. Crucially, the geographically distrib- [68] and have a characteristic temporal resolution of ≲10 ms uted array of magnetometers enables consistency checks (determined by the magnetometer bandwidths). based on the relative timing and amplitudes of transient signals, suppressing the stochastic background. Data analy- III. OVERVIEW sis is based on proven techniques developed by the Laser One of the most important questions at the outset of our Interferometer Gravitational Wave Observatory (LIGO) considerations is whether it is theoretically plausible that collaboration [50,51] to search for similar correlated “burst” Earth would encounter a soliton star over the course of an signals from a worldwide network of gravitational-wave observational period of ∼1 year. Certainly (and fortu- detectors. It was demonstrated that these techniques can be nately!), stars composed of ordinary matter are so dilute adapted to analyze GNOME data in Ref. [49]. within our galaxy that collisions are extraordinarily infre- A complementary experimental approach is being pur- quent on human time scales, and one might be concerned sued to search for massive compact dark-matter objects whether this is also true of soliton stars for any reasonable using atomic clocks as sensors rather than atomic magne- parameters. This topic is addressed in Sec. IV. In Sec. V we tometers [52]. While atomic magnetometers are sensitive to consider whether encounters with soliton stars having the fields that couple to spins (such as the pseudoscalar requisite characteristics for detection by the GNOME or interaction associated with ALPs [47,48]), atomic clocks other similar terrestrial detector networks might be ruled are sensitive to fields that effectively alter the values of out by other observations (e.g., the stability of lunar and fundamental constants, such as the fine-structure constant, planetary orbits in our solar system, lunar laser ranging, through scalar interactions. An encounter with a soliton star “ ” gravimeter data, gravitational microlensing studies, etc.). that has such scalar interactions would manifest as a glitch Next, in Sec. VI, we investigate the parameter space over propagating through an atomic-clock network, such as the which the GNOME is sensitive to soliton-star transits given global positioning system (GPS) [52]: clocks would become the GNOME’s technical characteristics (sensitivity, band- desynchronized as Earth passes through the soliton star. The width, etc.). Finally, in Sec. VII we investigate the range of GPS.DM collaboration is analyzing data from the GPS fundamental parameters corresponding to characteristic satellites and have recently produced the first constraints on masses and radii of soliton stars necessary for sufficiently such events [53]. There have also been recent proposals to frequent terrestrial encounters as a way of evaluating the search for transient signals generated by exotic physics plausibility of this scenario. using networks of laser/maser interferometers [54,55], resonant-bar detectors [56–58], and pulsar timing [59]. IV. TERRESTRIAL ENCOUNTERS Here we analyze the prospects for observing a transient WITH SOLITON STARS signal from an encounter with a soliton star using a terrestrial detector network, with the GNOME as a concrete example. To determine the soliton-star parameter space to which Presently, the GNOME consists of six dedicated optically a terrestrial detector network is sensitive, we begin by
043002-2 SEARCHING FOR AXION STARS AND Q-BALLS WITH … PHYS. REV. D 97, 043002 (2018) assuming a uniform local distribution of soliton stars. The For context, the most massive soliton stars considered here characteristic relative velocity of our solar system with correspond to the average mass of a comet. respect to other objects in the galaxy is given by the local In principle, the acceleration due to the gravitational virial velocity v ∼ 10−3c. Thus in order for soliton stars to force from an encounter with a soliton star offers another be detectable with the GNOME during a 1-year observa- avenue for detection. However, the peak acceleration felt tional period, the mean-free-path length L between soliton during an encounter would be stars must be ≲L ¼ 10−3 ly, where max GM πGρ L g ≈ ≈ DM max ≈ 3 × 10−16 cm=s2; ð4Þ 1 2 a R2 c2 ≈ ≈ Mc ≲ ð Þ L 2 2 Lmax: 1 nπR ρ πR −19 3 2 DM or 3 × 10 g (where g ≈ 10 cm=s is the acceleration due ’ Here n is the number density of soliton stars, R is the to Earth s gravity). This is far smaller than even the best characteristic soliton-star radius, the local dark-matter accelerometers could conceivably measure [77]. The tidal 3 effects of such a soliton-star encounter on gravitational- energy density is ρ ≈ 0.4 GeV=cm [69–72], and M DM wave observatories such as LIGO are orders of magnitude is the characteristic soliton-star mass. We assume that the below LIGO’s strain sensitivity, and in any case would bulk of the dark matter is in the form of soliton stars, so that tend to be excluded from detection by LIGO because ρ ≈ nMc2. This establishes an upper limit on R since the DM they would not generate tensor effects on the two inter- concept of a compact dark-matter object only makes sense 6 ferometer arms. Moreover, the inverse time of the passage, if R ≪ L ≈ 10 R , where R is Earth’s radius. In turn, −3 −2 max E E 10 c=R ≲ 10 Hz, is in the frequency domain where this gives an upper limit on the soliton star mass based on 2 54 −12 LIGO has poor sensitivity due to seismic noise. Eq. (1), Mc ≪ 10 eV ≈ 10 M⊙, where M⊙ is the mass of the Sun. VI. SENSITIVITY TO SOLITON-STAR TRANSITS V. EXISTING CONSTRAINTS Having established that sufficiently frequent encounters ON SOLITON-STAR TRANSITS with soliton stars are both possible (Sec. IV) and not ruled out by existing observations (Sec. V), the next question is Do existing observations rule out frequent encounters whether the GNOME has sufficient sensitivity to detect a with soliton stars of this size? Searches for gravitational transient event resulting from a terrestrial encounter with microlensing due to MAssive Compact Halo Objects −7 such a soliton star. At this point, we adopt a more specific (MACHOs) constrain their masses to be ≲10 M⊙ theoretical model for estimates, namely the Q-star model of [73]. It was shown that objects of any size with masses Refs. [39–42], although it turns out that our conclusions are −10 ≲10 M⊙ would not create measurable consequences for quite generic for soliton stars of the considered sizes the Solar system or Earth-Moon dynamics [74]. Recent regardless of the details of the attractive interactions limits on primordial black holes from gravitational femto- holding the ALPs together (so long as they are sufficiently lensing (light deflection of ∼10−15 arcseconds [75])of strong). gamma-ray bursts show that objects with masses in the −16 −13 range 10 M⊙ to 10 M⊙ do not compose a dominant A. Q-stars fraction of dark matter [76]. The gravitational femtolensing In the model described in Refs. [39–42], the Q-stars arise constraint from gamma-ray bursts rules out the most as a consequence of a particle-antiparticle asymmetry of a massive soliton stars considered above, so the observatio- complex scalar field and its self-interaction. Consider a nally allowed range of soliton-star masses that could be region of space where a complex scalar field oscillates encountered during a one year search is at angular frequency ω (not necessarily the Compton −16 frequency), M ≲ 10 M⊙: ð2Þ að r;tÞ¼eiωtϕð rÞ: ð5Þ To simplify the analysis of the GNOME data, we assume that the size of an encountered dark matter object is much Such a configuration possesses a conserved additive ’ ≫ larger than Earth s radius, i.e., R RE: in this case, all quantum number Q (where each individual ALP has charge GNOME sensors would register a transient signal within Q ¼ 1), in which case [39] ∼ 2 the time T RE=v it takes for Earth to pass through the Z surface of the dark-matter object. For concreteness, we ω ∼ 10 Q ¼ jϕð rÞj2d3 r: ð6Þ assume Rmin RE, and thus for the soliton star radii we ℏ2c3 have [From now on, we shall refer to að r;tÞ as a generalized 10 ≲ ≲ 106 ð Þ RE R RE: 3 ALP field.] The necessary conditions for the appearance of
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Q-stars are that Q ≠ 0 averaged over the whole space, and a coupling of the ALP field to the intrinsic spins of standard self-interaction potential UðϕÞ possessing at least two model fermions [82]. The gradient of a real-valued ALP field distinct minima at ϕ ¼ 0 and at ϕ ¼ ϕ0 [39–42]. If initially can couple to the spin Si of a particle i through a non- there exist regions of space with different energy vacua, relativistic Hamiltonian (the so-called linear interaction) regions where ϕ¼ϕ0 can deform but not disappear entirely ℏ because of the conserved charge Q. Furthermore, UðϕÞ is ¼ c S ∇ ð Þ ’ ϕ Hlin;i i · a: 11 nonzero in the Q-star s transitional surface region where flin;i goes from ϕ0 to 0 [40,41]; Uðϕ0Þ in the interior of the Q- S ℏ star may also be nonzero [39], but this is not required Here i is in units of and flin;i (having dimensions of [40,41] and so for simplicity we set Uðϕ0Þ¼0 here. Thus energy) is related to the coupling constant for the considered the Q-star possesses a potential energy per unit surface area particle i, and can be different for electrons, neutrons, and of σ, where σ is a constant depending on the particular protons [22]. We treat the coupling constant flin;i, properties of UðϕÞ [78]. The total energy of a Q-star with apart from experimental and observational limits, as a free volume V and surface area A is parameter. In a theory with one real-valued ALP field, interactions E ¼ ℏωQ þ σA; ð7Þ with standard model fermions result from the Lagrangian density given by the coupling of the space-time derivative where each ALP within the Q-star contributes a quantum of of the ALP field a to fermion axial-vector currents, energy ℏω. To minimize the energy of the field configu- ration while conserving the charge Q, we express ω in 1 L ∝ ∂ ψ¯ γ γ ψ ð Þ terms of Q using Eq. (6), μa × i μ 5 i; 12 flin;i
2 3 Q ω ¼ ℏ c ; ð Þ where ψ represents the fermion field and γμ and γ5 are ϕ2 8 i 0V Dirac matrices. For a complex-valued field a forming Q-stars, such a form of L is inconsistent with the Uð1Þ and thus Q symmetry in the a sector. In that case, the interactions Q2 would have to be bilinear in a. A possible form of such an E ¼ ℏ3c3 þ σA: ð9Þ ð1Þ 2 interaction consistent with U Q can then be ϕ0V
1 Thus the energy is minimized when the Q-star assumes a L ∝ ∂μða aÞ ψ¯ γμγ5ψ ; ð Þ ð Þ2 × i i 13 spherical shape which minimizes A and maximizes V. fquad;i Minimizing E with respect to R, one arrives at the total mass-energy of the Q-star: or alternatively 1 2 5 10π 2 2 3 ¼ ¼ ℏω ¼ ω ϕ ð Þ L ∝ i½a ∂μa − ð∂μa Þa × ψ¯ γμγ5ψ : ð14Þ E Mc Q 3 0R ; 10 ð Þ2 i i 2 3ℏc fquad;i where in the last step we have substituted the relationships The nonrelativistic Hamiltonian corresponding to the sec- from Eqs. (6) and (8). ond case [Eq. (14)] is proportional to the gradient of the Note that it is energetically favorable for ALPs to remain square of the field (the so-called quadratic interaction): within the Q-star if ω ≲ ωa, where ωa is the ALP Compton frequency, since ALPs inside the Q-star have energy ℏω ℏ ¼ c S ½ ∇ − ð∇ Þ ð Þ Hquad;i 2 i · i a a a a ; 15 while those outside the Q-star have energy ℏωa. The values ð Þ 2 2 2 2 fquad;i of ω and ωa are proportional to ∂ U=∂ϕ at the respective potential minima inside (ϕ ¼ ϕ0) and outside (ϕ ¼ 0) the while for the first case [Eq. (13)] the corresponding Q-star and can thus be different [39,42]. The condition combination of scalar fields is ∇jaj2. ω ≲ ω a ensures stability of the Q-star with respect to Astrophysical observations disfavor ALPs with nucleon radiative decay via ALP emission. In the described sce- ≲ 109 couplings flin;i GeV [84] and electron couplings nario, the oscillating ALP field exists only within the ≲ 1010 flin;i GeV [85,86]. Astrophysical constraints on Q-stars and thus evades detection by terrestrial experiments the quadratic interaction are less stringent: so far ALPs searching for a uniform dark-matter field. ≲ 104 with fquad;i GeV are disfavored [22,87]. In order to understand the effect of such an interaction on B. Couplings of axions/ALPs to atomic spins atomic spins during the transit of Earth through a soliton The GNOME is sensitive to encounters with such star, we need to understand the behavior of ∇a during the Q-stars (and axion/ALP stars in general) through the transit. In principle, there are two components to such a
043002-4 SEARCHING FOR AXION STARS AND Q-BALLS WITH … PHYS. REV. D 97, 043002 (2018) gradient term: the first one is related to a gradient of the “envelope” ϕðrÞ and it would exist even in the limit of vanishing relative velocity. The second effect is due to a combination of the time-dependence of að r;tÞ [Eq. (5)] and a nonzero relative velocity between the soliton star and the detector. Based on Eqs. (1) and (10), we can approxi- mate the ALP field amplitude ϕ0 as a step-function with a value inside the Q-star given by:
3ℏc3 ρ L ϕ2 ≈ DM : ð16Þ 0 10π ω2R For the time being, we neglect terms associated with ∇ϕðrÞ, while the relative motion creates a nonzero oscillating spin- dependent energy shift. The amplitude of the oscillating FIG. 1. Estimated parameter space probed by the Advanced gradient of a is given by GNOME (dotted line, light blue fill) for the linear interaction of neutron spins with an ALP star, assuming that the mean-free-path −3 ωv length for terrestrial encounters with ALP stars is L ¼ 10 ly j∇aj ≈ ϕ0; ð17Þ ¼ 10−3 c2 and v c. The solid line and green fill represent existing astrophysical constraints on spin-dependent ALP interactions where v ∼ 10−3c is the relative velocity between the soliton with nucleons [84]. The sensitivity of the existing GNOME is star and the terrestrial detectors, and similarly slightly below the level of the astrophysical constraints.