Showing papers by "T. J. Sumner published in 2019"
Case Western Reserve University1, Stanford University2, SLAC National Accelerator Laboratory3, University of Wisconsin-Madison4, Imperial College London5, South Dakota School of Mines and Technology6, University of California, Davis7, University of Edinburgh8, University of California, Berkeley9, Lawrence Livermore National Laboratory10, Yale University11, Lawrence Berkeley National Laboratory12, University of Liverpool13, University of Coimbra14, University of South Dakota15, Pennsylvania State University16, Brown University17, University of Rochester18, State University of New York System19, University College London20, University of Maryland, College Park21, University of California, Santa Barbara22, University of Massachusetts Amherst23, University of Sheffield24, Texas A&M University25, California State University, Stanislaus26
TL;DR: Constraints on spin-independent DM-nucleon scattering for DM particles with masses of 0.4-5 GeV/c^{2} using 1.4×10^{4}kgday of search exposure from the 2013 data from the Large Underground Xenon (LUX) experiment are reported.
Abstract: The scattering of dark matter (DM) particles with sub-GeV masses off nuclei is difficult to detect using liquid xenon-based DM search instruments because the energy transfer during nuclear recoils is smaller than the typical detector threshold. However, the tree-level DM-nucleus scattering diagram can be accompanied by simultaneous emission of a bremsstrahlung photon or a so-called "Migdal" electron. These provide an electron recoil component to the experimental signature at higher energies than the corresponding nuclear recoil. The presence of this signature allows liquid xenon detectors to use both the scintillation and the ionization signals in the analysis where the nuclear recoil signal would not be otherwise visible. We report constraints on spin-independent DM-nucleon scattering for DM particles with masses of 0.4-5 GeV/c^{2} using 1.4×10^{4} kg day of search exposure from the 2013 data from the Large Underground Xenon (LUX) experiment for four different classes of mediators. This analysis extends the reach of liquid xenon-based DM search instruments to lower DM masses than has been achieved previously.
115 citations
TL;DR: The Atomic Experiment for Dark Matter and Gravity Exploration (AEDGE) as discussed by the authors is a space experiment using cold atoms to search for ultra-light dark matter and to detect gravitational waves in the frequency range between the most sensitive ranges of LISA and the terrestrial LIGO/Virgo/KAGRA/INDIGO experiments.
Abstract: We propose in this White Paper a concept for a space experiment using cold atoms to search for ultra-light dark matter, and to detect gravitational waves in the frequency range between the most sensitive ranges of LISA and the terrestrial LIGO/Virgo/KAGRA/INDIGO experiments. This interdisciplinary experiment, called Atomic Experiment for Dark Matter and Gravity Exploration (AEDGE), will also complement other planned searches for dark matter, and exploit synergies with other gravitational wave detectors. We give examples of the extended range of sensitivity to ultra-light dark matter offered by AEDGE, and how its gravitational-wave measurements could explore the assembly of super-massive black holes, first-order phase transitions in the early universe and cosmic strings. AEDGE will be based upon technologies now being developed for terrestrial experiments using cold atoms, and will benefit from the space experience obtained with, e.g., LISA and cold atom experiments in microgravity.
This paper is based on a submission (v1) in response to the Call for White Papers for the Voyage 2050 long-term plan in the ESA Science Programme. ESA limited the number of White Paper authors to 30. However, in this version (v2) we have welcomed as supporting authors participants in the Workshop on Atomic Experiments for Dark Matter and Gravity Exploration held at CERN: ({\tt this https URL}), as well as other interested scientists, and have incorporated additional material.
73 citations
TL;DR: In this paper, the Weak Equivalence Principle (WEP) was tested to a precision of $10^{-15} by using two masses of different compositions (titanium and platinum alloys) on a quasi-circular trajectory around the Earth.
Abstract: The Weak Equivalence Principle (WEP), stating that two bodies of different compositions and/or mass fall at the same rate in a gravitational field (universality of free fall), is at the very foundation of General Relativity. The MICROSCOPE mission aims to test its validity to a precision of $10^{-15}$, two orders of magnitude better than current on-ground tests, by using two masses of different compositions (titanium and platinum alloys) on a quasi-circular trajectory around the Earth. This is realised by measuring the accelerations inferred from the forces required to maintain the two masses exactly in the same orbit. Any significant difference between the measured accelerations, occurring at a defined frequency, would correspond to the detection of a violation of the WEP, or to the discovery of a tiny new type of force added to gravity. MICROSCOPE's first results show no hint for such a difference, expressed in terms of E\"otv\"os parameter $\delta(Ti,Pt)=[-1\pm{}9{\rm (stat)}\pm{}9{\rm (syst)}] \times{}10^{-15}$ (both 1$\sigma$ uncertainties) for a titanium and platinum pair of materials. This result was obtained on a session with 120 orbital revolutions representing 7\% of the current available data acquired during the whole mission. The quadratic combination of 1$\sigma$ uncertainties leads to a current limit on $\delta$ of about $1.3\times{}10^{-14}$.
61 citations
TL;DR: In this article, a model of the complete closed-loop system provides a comprehensive understanding of each component of the in-loop coordinates spectral density, and the stability performance of the spacecraft with respect to its geodesic is extracted as a function of frequency.
Abstract: The science operations of the LISA Pathfinder mission have demonstrated the feasibility of sub-femto-g free fall of macroscopic test masses necessary to build a gravitational wave observatory in space such as LISA. While the main focus of interest, i.e., the optical axis or the x-axis, has been extensively studied, it is also of great importance to evaluate the stability of the spacecraft with respect to all the other degrees of freedom (d.o.f.). The current paper is dedicated to such a study: the exhaustive and quantitative evaluation of the imperfections and dynamical effects that impact the stability with respect to its local geodesic. A model of the complete closed-loop system provides a comprehensive understanding of each component of the in-loop coordinates spectral density. As will be presented, this model gives very good agreement with LISA Pathfinder flight data. It allows one to identify the noise source at the origin and the physical phenomena underlying the couplings. From this, the stability performance of the spacecraft with respect to its geodesic is extracted as a function of frequency. Close to 1 mHz, the stability of the spacecraft on the XSC, YSC and ZSC d.o.f. is shown to be of the order of 5.0×10-15 m s−2 Hz-1/2 for X, 6.0×10-14 m s−2 Hz-1/2 for Y, and 4.0×10-14 m s−2 Hz-1/2 for Z. For the angular d.o.f., the values are of the order of 3×10-12 rad s-2 Hz-1/2 for ΘSC, 5×10-13 rad s-2 Hz-1/2 for HSC, and 3×10-13 rad s-2 Hz-1/2 for ΦSC. Below 1 mHz, however, the stability performances are worsened significantly by the effect of the star tracker noise on the closed-loop system. It is worth noting that LISA is expected to be spared from such concerns, as differential wave-front sensing, an attitude sensor system of much higher precision, will be utilized for attitude control.
34 citations
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TL;DR: M Armanoa, H Audleyb, J Bairdc, P Binetruy d,∗, M Bornb, D Bortoluzzie, E Castellif, A Cavallerif , A Cesarinig , A M Cruiseh, K Danzmannb, M de Deus Silvaa, I Diepholzb, G Dixonh, R Dolesif, L Ferraiolii, V Ferronif, E D Fitzsim
Abstract: Since the 2017 Nobel Prize in Physics was awarded for the observation of gravitational waves, it is fair to say that the epoch of gravitational wave astronomy (GWs) has begun However, a number of interesting sources of GWs can only be observed from space To demonstrate the feasibility of the Laser Interferometer Space Antenna (LISA), a future gravitational wave observatory in space, the LISA Pathfinder satellite was launched on December, 3rd 2015 Measurements of the spurious forces accelerating an otherwise free-falling test mass, and detailed investigations of the individual subsystems needed to achieve the free-fall, have been conducted throughout the mission This overview article starts with the purpose and aim of the mission, explains satellite hardware and mission operations and ends with a summary of selected important results and an outlook towards LISA From the LISA Pathfinder experience, we can conclude that the proposed LISA mission is feasible
31 citations
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TL;DR: The scientific motivation for future space tests of the equivalence principle, and in particular the universality of free fall, at the 10− 17 level or better is presented.
Abstract: We present the scientific motivation for future space tests of the equivalence principle, and in particular the universality of free fall, at the $10^{-17}$ level or better. Two possible mission scenarios, one based on quantum technologies, the other on electrostatic accelerometers, that could reach that goal are briefly discussed.
30 citations
European Space Research and Technology Centre1, Max Planck Society2, University of Trento3, fondazione bruno kessler4, University of Urbino5, University of Birmingham6, ETH Zurich7, Spanish National Research Council8, European Space Operations Centre9, Imperial College London10, University of Zurich11
TL;DR: In this article, the LISA Pathfinder thrusters were used to limit and measure the fluctuations in acceleration between two free falling test masses down to sub-femto-g levels.
Abstract: The LISA Pathfinder (LPF) mission has demonstrated the ability to limit and measure the fluctuations in acceleration between two free falling test masses down to sub-femto-g levels. One of the key elements to achieve such a level of residual acceleration is the drag free control. In this scheme the spacecraft is used as a shield against any external disturbances by adjusting its relative position to a reference test mass. The actuators used to move the spacecraft are cold gas micropropulsion thrusters. In this paper, we report in-flight characterization of these thrusters in term of noise and artefacts during science operations using all the metrology capabilities of LISA Pathfinder. Using the LISA Pathfinder test masses as an inertial reference frame, an average thruster noise of ∼0.17 μN/Hz is observed and decomposed into a common (coherent) and an uncorrelated component. The very low noise and stability of the onboard metrology system associated with the quietness of the space environment allowed the measurement of the thruster noise down to ∼20 μHz, more than an order of magnitude below any ground measurement. Spectral lines were observed around ∼1.5 mHz and its harmonics and around 55 and 70 mHz. They are associated with the cold gas system itself and possibly to a clock synchronization issue. The thruster noise-floor exhibits an excess of ∼70% compared to characterization that have been made on ground on a single unit and without the feeding system. However this small excess has no impact on the LPF mission performance and is compatible with the noise budget for the upcoming LISA gravitational wave observatory. Over the whole mission, nominal, and extension, the thrusters showed remarkable stability for both the science operations and the different maneuvers necessary to maintain LPF on its orbit around L1. It is therefore concluded that a similar cold gas system would be a viable propulsion system for the future LISA mission.
28 citations
European Space Agency1, Leibniz University of Hanover2, Paris Diderot University3, University of Trento4, fondazione bruno kessler5, University of Urbino6, University of Birmingham7, ETH Zurich8, UK Astronomy Technology Centre9, Institut de Ciències de l'Espai10, European Space Operations Centre11, Imperial College London12, University of Florida13, University of Zurich14, University of Glasgow15, Polytechnic University of Catalonia16, German Aerospace Center17, Goddard Space Flight Center18, Airbus Defence and Space19
TL;DR: In this paper, the authors report on the temperature measurements throughout mission operations, characterize the thermal environment, estimate transfer functions between different locations, and report temperature stability (and its time evolution) at frequencies as low as 10'|$\mu$|Hz, where typically values around 1'K'Hz^−1'1/2 were measured.
Abstract: LISA Pathfinder (LPF) was a technology pioneering mission designed to test key technologies required for gravitational wave detection in space. In the low frequency regime (milliHertz and below), where space-based gravitational wave observatories will operate, temperature fluctuations play a crucial role since they can couple into the interferometric measurement and the test masses’ free-fall accuracy in many ways. A dedicated temperature measurement subsystem, with noise levels in 10 |$\mu$|K Hz^−1/2 down to 1 mHz was part of the diagnostics unit onboard LPF. In this paper we report on the temperature measurements throughout mission operations, characterize the thermal environment, estimate transfer functions between different locations, and report temperature stability (and its time evolution) at frequencies as low as 10 |$\mu$|Hz, where typically values around 1 K Hz^−1/2 were measured.
23 citations
TL;DR: In this article, the Weak Equivalence Principle (WEP) was tested to a precision of $10^{-15} by using two masses of different compositions (titanium and platinum alloys) on a quasi-circular trajectory around the Earth.
Abstract: The Weak Equivalence Principle (WEP), stating that two bodies of different compositions and/or mass fall at the same rate in a gravitational field (universality of free fall), is at the very foundation of General Relativity. The MICROSCOPE mission aims to test its validity to a precision of $10^{-15}$, two orders of magnitude better than current on-ground tests, by using two masses of different compositions (titanium and platinum alloys) on a quasi-circular trajectory around the Earth. This is realised by measuring the accelerations inferred from the forces required to maintain the two masses exactly in the same orbit. Any significant difference between the measured accelerations, occurring at a defined frequency, would correspond to the detection of a violation of the WEP, or to the discovery of a tiny new type of force added to gravity. MICROSCOPE's first results show no hint for such a difference, expressed in terms of Eotvos parameter $\delta(Ti,Pt)=[-1\pm{}9{\rm (stat)}\pm{}9{\rm (syst)}] \times{}10^{-15}$ (both 1$\sigma$ uncertainties) for a titanium and platinum pair of materials. This result was obtained on a session with 120 orbital revolutions representing 7\% of the current available data acquired during the whole mission. The quadratic combination of 1$\sigma$ uncertainties leads to a current limit on $\delta$ of about $1.3\times{}10^{-14}$.
19 citations
TL;DR: In this article, the authors report results from an extensive set of measurements of the \b{eta}decay response in liquid xenon, derived from high-statisticscalibration data from injected sources of both $^{3}$H and $^{14}$C in the LUXdetector.
Abstract: We report results from an extensive set of measurements of the \b{eta}-decay
response in liquid xenonThese measurements are derived from high-statistics
calibration data from injected sources of both $^{3}$H and $^{14}$C in the LUX
detector The mean light-to-charge ratio is reported for 13 electric field
values ranging from 43 to 491 V/cm, and for energies ranging from 15 to 145
keV
18 citations
TL;DR: In this paper, the photoelectric emission properties of 4.6 × 4. 6 cm2 gold plated surfaces, representative of those used in typical satellite applications with a film thickness of 800 nm, and measured surface roughnesses between 7 and 340 nm.
Abstract: Many applications require charge neutralization of isolated test bodies, and this has been successfully done using photoelectric emission from surfaces which are electrically benign (gold) or superconducting (niobium). Gold surfaces nominally have a high work function (∼5.1 eV) which should require deep UV photons for photoemission. In practice, it has been found that it can be achieved with somewhat lower energy photons with indicative work functions of (4.1–4.3 eV). A detailed working understanding of the process is lacking, and this work reports on a study of the photoelectric emission properties of 4.6 × 4.6 cm2 gold plated surfaces, representative of those used in typical satellite applications with a film thickness of 800 nm, and measured surface roughnesses between 7 and 340 nm. Various UV sources with photon energies from 4.8 to 6.2 eV and power outputs from 1 nW to 1000 nW illuminated ∼0.3 cm2 of the central surface region at angles of incidence from 0° to 60°. Final extrinsic quantum yields in the range 10 ppm–44 ppm were reliably obtained during 8 campaigns, covering a period of ∼3 years but with intermediate long-term variations lasting several weeks and, in some cases, bake-out procedures at up to 200 °C. Experimental results were obtained in a vacuum system with a baseline pressure of ∼10−7 mbar at room temperature. A working model, designed to allow accurate simulation of any experimental configuration, is proposed.
Goddard Space Flight Center1, Marshall Space Flight Center2, University of Michigan3, Johns Hopkins University4, The Catholic University of America5, European Space Agency6, Leibniz University of Hanover7, Paris Diderot University8, Imperial College London9, University of Rome Tor Vergata10, University of Trento11, Airbus Defence and Space12, University of Birmingham13, Istituto Nazionale di Fisica Nucleare14, ETH Zurich15, UK Astronomy Technology Centre16, Institut de Ciències de l'Espai17, INAF18, University of Urbino19, European Space Operations Centre20, University of Zurich21, University of Glasgow22, Polytechnic University of Catalonia23, University of Florence24, California Institute of Technology25
TL;DR: In this article, a new set of data obtained from direct measurements of momentum transfer to a spacecraft from individual particle impacts is presented, which can be used to detect impacts and measure properties such as the transferred momentum, direction of travel, and location of impact on the spacecraft.
Abstract: The zodiacal dust complex, a population of dust and small particles that pervades the solar system, provides important insight into the formation and dynamics of planets, comets, asteroids, and other bodies. We present a new set of data obtained from direct measurements of momentum transfer to a spacecraft from individual particle impacts. This technique is made possible by the extreme precision of the instruments flown on the LISA Pathfinder spacecraft, a technology demonstrator for a future space-based gravitational wave observatory. Pathfinder employed a technique known as drag-free control that achieved rejection of external disturbances, including particle impacts, using a micropropulsion system. Using a simple model of the impacts and knowledge of the control system, we show that it is possible to detect impacts and measure properties such as the transferred momentum, direction of travel, and location of impact on the spacecraft. In this paper, we present the results of a systematic search for impacts during 4348 hr of Pathfinder data. We report a total of 54 candidates with transferred momenta ranging from 0.2 to 230 μNs. We furthermore make a comparison of these candidates with models of micrometeoroid populations in the inner solar system, including those resulting from Jupiter-family comets (JFCs), Oort Cloud comets, Halley-type comets, and asteroids. We find that our measured population is consistent with a population dominated by JFCs, with some evidence for a smaller contribution from Halley-type comets, in agreement with consensus models of the zodiacal dust complex in the momentum range sampled by LISA Pathfinder.
European Space Agency1, Leibniz University of Hanover2, Imperial College London3, University of Urbino4, University of Florence5, Paris Diderot University6, University of Trento7, fondazione bruno kessler8, University of Birmingham9, ETH Zurich10, University of L'Aquila11, UK Astronomy Technology Centre12, Institut de Ciències de l'Espai13, European Space Operations Centre14, University of Zurich15, University of Glasgow16, INAF17, Polytechnic University of Catalonia18, Goddard Space Flight Center19
TL;DR: In this article, the energy dependence of three Forbush decreases is studied and reported in the LISA Pathfinder (LPF) mission orbiting around the Lagrange point L1 at 1.5 × 106 km from Earth.
Abstract: Non-recurrent short-term variations of the galactic cosmic-ray (GCR) flux above 70 MeV n−1 were observed between 2016 February 18 and 2017 July 3 on board the European Space Agency LISA Pathfinder (LPF) mission orbiting around the Lagrange point L1 at 1.5 × 106 km from Earth. The energy dependence of three Forbush decreases is studied and reported here. A comparison of these observations with others carried out in space down to the energy of a few tens of MeV n−1 shows that the same GCR flux parameterization applies to events of different intensity during the main phase. FD observations in L1 with LPF and geomagnetic storm occurrence are also presented. Finally, the characteristics of GCR flux non-recurrent variations (peaks and depressions) of duration <2 days and their association with interplanetary structures are investigated. It is found that, most likely, plasma compression regions between subsequent corotating high-speed streams cause peaks, while heliospheric current sheet crossing causes the majority of the depressions.
TL;DR: In this article, the authors present a new set of data obtained using a novel technique: direct measurements of momentum transfer to a spacecraft from individual particle impacts, made possible by the extreme precision of the instruments flown on the LISA Pathfinder spacecraft, a technology demonstrator for a future space-based gravitational wave observatory.
Abstract: The zodiacal dust complex, a population of dust and small particles that pervades the Solar System, provides important insight into the formation and dynamics of planets, comets, asteroids, and other bodies. Here we present a new set of data obtained using a novel technique: direct measurements of momentum transfer to a spacecraft from individual particle impacts. This technique is made possible by the extreme precision of the instruments flown on the LISA Pathfinder spacecraft, a technology demonstrator for a future space-based gravitational wave observatory that operated near the first Sun-Earth Lagrange point from early 2016 through Summer of 2017. Using a simple model of the impacts and knowledge of the control system, we show that it is possible to detect impacts and measure properties such as the transferred momentum (related to the particle's mass and velocity), direction of travel, and location of impact on the spacecraft. In this paper, we present the results of a systematic search for impacts during 4348 hours of Pathfinder data. We report a total of 54 candidates with momenta ranging from 0.2$\,\mu\textrm{Ns}$ to 230$\,\mu\textrm{Ns}$. We furthermore make a comparison of these candidates with models of micrometeoroid populations in the inner solar system including those resulting from Jupiter-family comets, Oort-cloud comets, Hailey-type comets, and Asteroids. We find that our measured population is consistent with a population dominated by Jupiter-family comets with some evidence for a smaller contribution from Hailey-type comets. This is in agreement with consensus models of the zodiacal dust complex in the momentum range sampled by LISA Pathfinder.
European Space Agency1, Leibniz University of Hanover2, Paris Diderot University3, University of Trento4, fondazione bruno kessler5, University of Urbino6, University of Birmingham7, ETH Zurich8, UK Astronomy Technology Centre9, Institut de Ciències de l'Espai10, European Space Operations Centre11, Imperial College London12, University of Florida13, University of Zurich14, University of Glasgow15, Polytechnic University of Catalonia16, Goddard Space Flight Center17
TL;DR: The differential acceleration noise measurement presented here with the free-fall mode agrees with noise measured with the continuous actuation scheme, representing an important and independent confirmation of the LPF result.
Abstract: We report on the results of the LISA Pathfinder (LPF) free-fall mode experiment, in which the control force needed to compensate the quasistatic differential force acting on two test masses is applied intermittently as a series of "impulse" forces lasting a few seconds and separated by roughly 350 s periods of true free fall. This represents an alternative to the normal LPF mode of operation in which this balancing force is applied continuously, with the advantage that the acceleration noise during free fall is measured in the absence of the actuation force, thus eliminating associated noise and force calibration errors. The differential acceleration noise measurement presented here with the free-fall mode agrees with noise measured with the continuous actuation scheme, representing an important and independent confirmation of the LPF result. An additional measurement with larger actuation forces also shows that the technique can be used to eliminate actuation noise when this is a dominant factor.
TL;DR: In this paper, the presence of free charge on isolated proof-masses, such as those within space-borne gravitational reference sensors, causes a number of spurious forces which will give rise to associated acceleration noise.
Abstract: The presence of free charge on isolated proof-masses, such as those within space-borne gravitational reference sensors, causes a number of spurious forces which will give rise to associated acceleration noise. A complete discusssion of each charge induced force and its linear acceleration noise is presented. The resulting charge acceleration noise contributions to the LISA mission are evaluated using the LISA Pathfinder performance and design. It is shown that one term is largely dominant but that a full budget should be maintained for LISA and future missions due to the large number of possible contributions and their dependence on different sensor parameters.
TL;DR: In this paper, the authors present methods and techniques for creating and improving a model that reproduces the scintillation and ionization response of a dual-phase liquid and gaseous xenon time-projection chamber.
Abstract: We report here methods and techniques for creating and improving a model that reproduces the scintillation and ionization response of a dual-phase liquid and gaseous xenon time-projection chamber. Starting with the recent release of the Noble Element Simulation Technique (NEST v2.0), electronic recoil data from the $\beta$ decays of ${}^3$H and ${}^{14}$C in the Large Underground Xenon (LUX) detector were used to tune the model, in addition to external data sets that allow for extrapolation beyond the LUX data-taking conditions. This paper also presents techniques used for modeling complicated temporal and spatial detector pathologies that can adversely affect data using a simplified model framework. The methods outlined in this report show an example of the robust applications possible with NEST v2.0, while also providing the final electronic recoil model and detector parameters that will used in the new analysis package, the LUX Legacy Analysis Monte Carlo Application (LLAMA), for accurate reproduction of the LUX data. As accurate background reproduction is crucial for the success of rare-event searches, such as dark matter direct detection experiments, the techniques outlined here can be used in other single-phase and dual-phase xenon detectors to assist with accurate ER background reproduction.
TL;DR: In this paper, the energy dependence of three Forbush decreases (FDs) is studied and compared with others carried out in space down to the energy of a few tens of MeV n$^{-1}$.
Abstract: Non-recurrent short term variations of the galactic cosmic-ray (GCR) flux above 70 MeV n$^{-1}$ were observed between 2016 February 18 and 2017 July 3 aboard the European Space Agency LISA Pathfinder (LPF) mission orbiting around the Lagrange point L1 at 1.5$\times$10$^6$ km from Earth. The energy dependence of three Forbush decreases (FDs) is studied and reported here. A comparison of these observations with others carried out in space down to the energy of a few tens of MeV n$^{-1}$ shows that the same GCR flux parameterization applies to events of different intensity during the main phase. FD observations in L1 with LPF and geomagnetic storm occurrence is also presented. Finally, the characteristics of GCR flux non-recurrent variations (peaks and depressions) of duration $<$ 2 days and their association with interplanetary structures are investigated. It is found that, most likely, plasma compression regions between subsequent corotating high-speed streams cause peaks, while heliospheric current sheet crossing cause the majority of the depressions.
TL;DR: A working model, designed to allow accurate simulation of any experimental configuration, is proposed for photoelectric emission from gold surfaces, and results were obtained in a vacuum system with a baseline pressure of ∼10-7 mbar at room temperature.
Abstract: Many applications require charge neutralisation of isolated test bodies and this has been successfully done using photoelectric emission from surfaces which are electrically benign(gold) or superconducting (niobium). Gold surfaces nominally have a high work function ($\sim 5.1$\,eV)which should require deep UV photons for photoemission. In practice it has been found that it can be achieved with somewhat lower energy photons with indicative work functions of ($ 4.1-4.3$\,eV). A detailed working understanding of the process is lacking and this work reports on a study of the photoelectric emission properties of 4.6x4.6 cm^2 gold plated surfaces, representative of those used in typical satellite applications with a film thickness of 800 nm, and measured surface roughnesses between 7 and 340 nm. Various UV sources with photon energies from 4.8 to 6.2 eV and power outputs from 1 nW to 1000 nW, illuminated a ~0.3 cm^2 of the central surface region at angles of incidence from 0 to 60 degrees.
Final extrinsic quantum yields in the range 10 ppm to 44 ppm were reliably obtained during 8 campaigns, covering a ~3 year period, but with intermediate long-term variations lasting several weeks and, in some cases, bake-out procedures at up to 200 C. Experimental results were obtained in a vacuum system with a baseline pressure of ~10^{-7} mbar at room temperature. A working model, designed to allow accurate simulation of any experimental configuration, is proposed.
TL;DR: In this article, two novel methods, tested by LISA Pathfinder, were used to measure the gravitational constant G for the first time in space, using electrostatic suspension forces to measure a change in acceleration due to a displaced source mass.
Abstract: We present two novel methods, tested by LISA Pathfinder, to measure the gravitational constant G for the first time in space. Experiment 1 uses electrostatic suspension forces to measure a change in acceleration of a test mass due to a displaced source mass. Experiment 2 measures a change in relative acceleration between two test masses due to a slowly varying fuel tank mass. Experiment 1 gave a value of G=6.71±0.42(×10-11) m3 s-2 kg-1 and experiment 2 gave 6.15±0.35(×10-11) m3 s-2 kg-1, both consistent with each other to 1σ and with the CODATA 2014 recommended value of 6.67408±0.00031(×10-11) m3 s-2 kg-1 to 2σ. We outline several ideas to improve the results for a future experiment, and we suggest that a measurement in space would isolate many terrestrial issues that could be responsible for the inconsistencies between recent measurements.
TL;DR: In this paper, the development and availability of new innovative propulsion techniques in the 2040s, that will allow us to fly a spacecraft outside the Solar System (at 150 AU and more) in a reasonable amount of time, in order to directly probe our (gravitational) Solar System neighborhood and answer pressing questions regarding the dark sector (dark energy and dark matter).
Abstract: We speculate on the development and availability of new innovative propulsion techniques in the 2040s, that will allow us to fly a spacecraft outside the Solar System (at 150 AU and more) in a reasonable amount of time, in order to directly probe our (gravitational) Solar System neighborhood and answer pressing questions regarding the dark sector (dark energy and dark matter). We identify two closely related main science goals, as well as secondary objectives that could be fulfilled by a mission dedicated to probing the local dark sector: (i) begin the exploration of gravitation's low-acceleration regime with a man-made spacecraft and (ii) improve our knowledge of the local dark matter and baryon densities. Those questions can be answered by directly measuring the gravitational potential with an atomic clock on-board a spacecraft on an outbound Solar System orbit, and by comparing the spacecraft's trajectory with that predicted by General Relativity through the combination of ranging data and the in-situ measurement (and correction) of non-gravitational accelerations with an on-board accelerometer. Despite a wealth of new experiments getting online in the near future, that will bring new knowledge about the dark sector, it is very unlikely that those science questions will be closed in the next two decades. More importantly, it is likely that it will be even more urgent than currently to answer them. Tracking a spacecraft carrying a clock and an accelerometer as it leaves the Solar System may well be the easiest and fastest way to directly probe our dark environment.
TL;DR: In this paper, the authors report on the search for two-neutrino double electron capture in the Large Underground Xenon (LUX) experiment, in a total of 27769.5~kg-days.
Abstract: Two-neutrino double electron capture is a process allowed in the Standard Model of Particle Physics. This rare decay has been observed in $^{78}$Kr, $^{130}$Ba and more recently in $^{124}$Xe. In this publication we report on the search for this process in $^{124}$Xe and $^{126}$Xe using the full exposure of the Large Underground Xenon (LUX) experiment, in a total of of 27769.5~kg-days. No evidence of a signal was observed, allowing us to set 90\% C.L. lower limits for the half-lives of these decays of $2.0\times10^{21}$~years for $^{124}$Xe and $1.9\times10^{21}$~years for $^{126}$Xe.