This Letter reports the results from a haloscope search for dark matter axions with masses between 2.66 and 2.81 μeV. The search excludes the range of axion-photon couplings predicted by plausible models of the invisible axion. This unprecedented sensitivity is achieved by operating a large-volume haloscope at subkelvin temperatures, thereby reducing thermal noise as well as the excess noise from the ultralow-noise superconducting quantum interference device amplifier used for the signal power readout. Ongoing searches will provide nearly definitive tests of the invisible axion model over a wide range of axion masses. DOI: 10.1103/PhysRevLett.120.151301 Axions are particles predicted to exist as a consequence of the Peccei-Quinn solution to the strong-CP problem [1][2][3] and could account for all of the dark matter in our Universe [4][5][6]. While there exist a number of mechanisms to produce axions in the early Universe [4,[7][8][9] that allow for a wide range of dark matter axion masses, current numerical and analytical studies of QCD typically suggest a preferred mass range of 1-100 μeV for axions produced after cosmic inflation in numbers that saturate the Lambda-CDM (cold dark matter) density [10][11][12][13][14]. The predicted coupling between axions and photons is model dependent; in general, axions with dominant hadronic couplings as in the Kim-Shifman-Vainshtein-Zakharov (KSVZ) model [15,16] are predicted to have an axion-photon coupling roughly 2.7 times larger than that of the Dine-FischlerSrednicki-Zhitnitsky (DFSZ) model [17,18]. Because the axion-photon coupling is expected to be very small, Oð10 −17 -10 −12 GeV −1 Þ over the expected axion mass range, these predicted particles are dubbed invisible axions [4].The most promising technique to search for dark matter axions in the favored mass range is the axion haloscope [19] consisting of a cold microwave resonator immersed in a strong static magnetic field. In the presence of this magnetic field, the ambient dark matter axion field produces a volume-filling current density oscillating at frequency f ¼ E=h, where E is the total energy consisting mostly of the axion rest mass with a small kinetic energy addition. When the resonator is tuned to match this frequency, the current source delivers power to the resonator in the form of microwave photons which can be detected with a low-noise microwave receiver. To date, a number of axion haloscopes have been implemented. All had noise levels too high to detect the QCD axion signal [20][21][22][23][24][25][26][27][28][29][30] in an experimentally realizable time. Previous versions of the Axion Dark Matter eXperiment (ADMX) [24][25][26][27][28][29] achieved sensitivity to the stronger KSVZ couplings in the ð1.91-3.69Þ-μeV mass range. ADMX has since been improved to utilize a dilution refrigerator to obtain a significantly lower system noise temperature, drastically increasing its sensitivity. We present here results from the first axion experiment to have sensitivity to the more weakly coupled DFSZ axion ...

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This paper reports on a cavity haloscope search for dark matter axions in the galactic halo in the mass range 2.81-3.31 µeV . This search excludes the full range of axion-photon coupling values predicted in benchmark models of the invisible axion that solve the strong CP problem of quantum chromodynamics, and marks the first time a haloscope search has been able to search for axions at mode crossings using an alternate cavity configuration. Unprecedented sensitivity in this higher mass range is achieved by deploying an ultra low-noise Josephson parametric amplifier as the first-stage signal amplifier.Axions are a hypothesized particle that emerged as a result of the Peccei-Quinn solution to the strong CP problem [1][2][3]. In addition, axions are a leading darkmatter candidate that could explain 100% of the darkmatter in the Universe [4][5][6][7][8]. There are a number of mechanisms for the production of dark-matter axions in the early Universe [5,6,9,10]. For the case where U PQ (1) becomes spontaneously broken after inflation, cosmological constraints suggest an axion mass on the scale of 1 µeV or greater [11][12][13][14][15][16]. Two benchmark models for the axion are the Kim-Shifman-Vainshtein-Zakharov (KSVZ) [17,18] and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) [19,20] models. Of the two, the DFSZ model is especially compelling because of its grand unification properties [19].The Axion Dark Matter eXperiment (ADMX) searches for dark-matter axions using an axion haloscope [21], which consists of a microwave resonant cavity inside a magnetic field. In the presence of an external magnetic field, axions inside the cavity can convert to photons with frequency f = E/h, where E is the total energy of the axion, including the axion rest mass energy, plus a small kinetic energy contribution. The power expected from the conversion of an axion into microwave photons in the ADMX experiment is extremely low, O(10 −23 W ), requiring the use of a dilution refrigerator and an ultra low-noise microwave receiver to detect the photons.In limits set in a previous paper, ADMX became the only axion haloscope to achieve sensitivity to both benchmark axion models for axion masses between 2.66 and 2.81 µeV [22]. This paper reports on recent operations which extend the search for axions at DFSZ sensitivity to 2.66-3.31 µeV .The ADMX experiment consists of a 136-liter cylindrical copper-plated cavity placed in a 7.6-T field produced by a superconducting solenoid magnet. The magnet and cavity configuration are similar to the configuration described in Ref. [23,24]. A magnetic field-free region above the cavity is maintained by a counter-wound bucking magnet above the cavity. Field sensitive receiver components, such as a Josephson parametric amplifier (JPA) and circulators, are located there, and the JPA is protected by additional passive magnetic shielding.The resonant frequency of the cavity is set by two copper tuning rods that run parallel to the axis of the cavity and can be positioned between near the center of the cavity and the...

Results are reported from a joint analysis of Phase I and Phase II data from the Sudbury Neutrino Observatory. The effective electron kinetic energy threshold used is T eff = 3.5 MeV, the lowest analysis threshold yet achieved with water Cherenkov detector data. In units of 10 6 cm −2 s −1 , the total flux of active-flavor neutrinos from 8 B decay in the Sun measured using the neutral current (NC) reaction of neutrinos on deuterons, with no constraint on the 8 B neutrino energy spectrum, is found to be NC = 5.140 +0.160 −0.158 (stat) +0.132 −0.117 (syst). These uncertainties are more than a factor of 2 smaller than previously published results. Also presented are the spectra of recoil electrons from the charged current reaction of neutrinos on deuterons and the elastic scattering of electrons. A fit to the Sudbury Neutrino Observatory data in which the free parameters directly describe the total 8 B neutrino flux and the energy-dependent ν e survival probability provides a measure of the total 8