First Demonstration of a Scintillating Xenon Bubble Chamber for Detecting Dark Matter and Coherent Elastic Neutrino-Nucleus Scattering.

A 30-g xenon bubble chamber, operated at Northwestern University in June and November 2016, has for the first time observed simultaneous bubble nucleation and scintillation by nuclear recoils in a superheated liquid. This chamber is instrumented with a CCD camera for near-IR bubble imaging, a solar-blind photomultiplier tube to detect 175-nm xenon scintillation light, and a piezoelectric acoustic transducer to detect the ultrasonic emission from a growing bubble. The time of nucleation determined from the acoustic signal is used to correlate specific scintillation pulses with bubble-nucleating events. We report on data from this chamber for thermodynamic "Seitz" thresholds from 4.2 to 15.0 keV. The observed single- and multiple-bubble rates when exposed to a ^{252}Cf neutron source indicate that, for an 8.3-keV thermodynamic threshold, the minimum nuclear recoil energy required to nucleate a bubble is 19±6 keV (1σ uncertainty). This is consistent with the observed scintillation spectrum for bubble-nucleating events. We see no evidence for bubble nucleation by gamma rays at any of the thresholds studied, setting a 90% C.L. upper limit of 6.3×10^{-7} bubbles per gamma interaction at a 4.2-keV thermodynamic threshold. This indicates stronger gamma discrimination than in CF_{3}I bubble chambers, supporting the hypothesis that scintillation production suppresses bubble nucleation by electron recoils, while nuclear recoils nucleate bubbles as usual. These measurements establish the noble-liquid bubble chamber as a promising new technology for the detection of weakly interacting massive particle dark matter and coherent elastic neutrino-nucleus scattering.

Dark Matter Search Results from the PICO-60 C_{3}F_{8} Bubble Chamber.

New results are reported from the operation of the PICO-60 dark matter detector, a bubble chamber filled with 52 kg of C_{3}F_{8} located in the SNOLAB underground laboratory. As in previous PICO bubble chambers, PICO-60 C_{3}F_{8} exhibits excellent electron recoil and alpha decay rejection, and the observed multiple-scattering neutron rate indicates a single-scatter neutron background of less than one event per month. A blind analysis of an efficiency-corrected 1167-kg day exposure at a 3.3-keV thermodynamic threshold reveals no single-scattering nuclear recoil candidates, consistent with the predicted background. These results set the most stringent direct-detection constraint to date on the weakly interacting massive particle (WIMP)-proton spin-dependent cross section at 3.4×10^{-41} cm^{2} for a 30-GeV c^{-2} WIMP, more than 1 order of magnitude improvement from previous PICO results.

Dark Matter Search Results from the PICO-2L C3F8 Bubble Chamber.

New data are reported from the operation of a 2 liter C3F8 bubble chamber in the SNOLAB underground laboratory, with a total exposure of 211.5 kg days at four different energy thresholds below 10 keV. These data show that C3F8 provides excellent electron-recoil and alpha rejection capabilities at very low thresholds. The chamber exhibits an electron-recoil sensitivity of <3.5×10(-10) and an alpha rejection factor of >98.2%. These data also include the first observation of a dependence of acoustic signal on alpha energy. Twelve single nuclear recoil event candidates were observed during the run. The candidate events exhibit timing characteristics that are not consistent with the hypothesis of a uniform time distribution, and no evidence for a dark matter signal is claimed. These data provide the most sensitive direct detection constraints on WIMP-proton spin-dependent scattering to date, with significant sensitivity at low WIMP masses for spin-independent WIMP-nucleon scattering.

Improved limits on spin-dependent WIMP-proton interactions from a two liter CF3I bubble chamber.

Data from the operation of a bubble chamber filled with 3.5 kg of CF3I in a shallow underground site are reported. An analysis of ultrasound signals accompanying bubble nucleations confirms that alpha decays generate a significantly louder acoustic emission than single nuclear recoils, leading to an efficient background discrimination. Three dark matter candidate events were observed during an effective exposure of 28.1 kg day, consistent with a neutron background. This observation provides strong direct detection constraints on weakly interacting massive particle (WIMP)-proton spin-dependent scattering for WIMP masses >20 GeV/c2.

Search for light dark matter in XENON10 data.

We report results of a search for light (≲10 GeV) particle dark matter with the XENON10 detector. The event trigger was sensitive to a single electron, with the analysis threshold of 5 electrons corresponding to 1.4 keV nuclear recoil energy. Considering spin-independent dark matter-nucleon scattering, we exclude cross sections σ(n)>7×10(-42) cm(2), for a dark matter particle mass m(χ)=7 GeV. We find that our data strongly constrain recent elastic dark matter interpretations of excess low-energy events observed by CoGeNT and CRESST-II, as well as the DAMA annual modulation signal.

The novel Mechanical Ventilator Milano for the COVID-19 pandemic.

This paper presents the Mechanical Ventilator Milano (MVM), a novel intensive therapy mechanical ventilator designed for rapid, large-scale, low-cost production for the COVID-19 pandemic. Free of moving mechanical parts and requiring only a source of compressed oxygen and medical air to operate, the MVM is designed to support the long-term invasive ventilation often required for COVID-19 patients and operates in pressure-regulated ventilation modes, which minimize the risk of furthering lung trauma. The MVM was extensively tested against ISO standards in the laboratory using a breathing simulator, with good agreement between input and measured breathing parameters and performing correctly in response to fault conditions and stability tests. The MVM has obtained Emergency Use Authorization by U.S. Food and Drug Administration (FDA) for use in healthcare settings during the COVID-19 pandemic and Health Canada Medical Device Authorization for Importation or Sale, under Interim Order for Use in Relation to COVID-19. Following these certifications, mass production is ongoing and distribution is under way in several countries. The MVM was designed, tested, prepared for certification, and mass produced in the space of a few months by a unique collaboration of respiratory healthcare professionals and experimental physicists, working with industrial partners, and is an excellent ventilator candidate for this pandemic anywhere in the world.

Effect of sterol incorporation on head group separation in liposomes.

Electrophoretic mobilities of multilamellar liposomes of varying composition have been measured to determine the effect of incorporated sterols on surface charge density. Liposomes made from mixtures of zwitterionic egg phosphatidylcholine (PC) and anionic egg phosphatidylglycerol (PG) in varying proportions were shown to have electrophoretic mobilities consistent with the anticipated surface charge density. Incorporation of cholesterol up to 50 mole per cent in the bilayer produced no detectable change in surface charge density. Similar results were obtained for lanosterol and epicoprostanol. These results are interpreted to mean that incorporation of the sterols into the bilayers produced no detectable change (less than 3%) in the spacing of charged phospholipids. It is inferred that sterols are incorporated among the fatty acyl chains of these phospholipid bilayers with little or no displacement of the head groups at the surface.

First results from the XENON10 dark matter experiment at the Gran Sasso National Laboratory.

The XENON10 experiment at the Gran Sasso National Laboratory uses a 15 kg xenon dual phase time projection chamber to search for dark matter weakly interacting massive particles (WIMPs). The detector measures simultaneously the scintillation and the ionization produced by radiation in pure liquid xenon to discriminate signal from background down to 4.5 keV nuclear-recoil energy. A blind analysis of 58.6 live days of data, acquired between October 6, 2006, and February 14, 2007, and using a fiducial mass of 5.4 kg, excludes previously unexplored parameter space, setting a new 90% C.L. upper limit for the WIMP-nucleon spin-independent cross section of 8.8x10(-44) cm2 for a WIMP mass of 100 GeV/c2, and 4.5x10(-44) cm2 for a WIMP mass of 30 GeV/c2. This result further constrains predictions of supersymmetric models.

Limits on spin-dependent WIMP-nucleon cross sections from the XENON10 experiment.

XENON10 is an experiment to directly detect weakly interacting massive particles (WIMPs), which may comprise the bulk of the nonbaryonic dark matter in our Universe. We report new results for spin-dependent WIMP-nucleon interactions with 129Xe and 131Xe from 58.6 live days of operation at the Laboratori Nazionali del Gran Sasso. Based on the nonobservation of a WIMP signal in 5.4 kg of fiducial liquid xenon mass, we exclude previously unexplored regions in the theoretically allowed parameter space for neutralinos. We also exclude a heavy Majorana neutrino with a mass in the range of approximately 10 GeV/c2-2 TeV/c2 as a dark matter candidate under standard assumptions for its density and distribution in the galactic halo.

Simultaneous measurement of ionization and scintillation from nuclear recoils in liquid xenon for a dark matter experiment.

We report the first measurements of the absolute ionization yield of nuclear recoils in liquid xenon, as a function of energy and electric field. Independent experiments were carried out with two dual-phase time-projection chamber prototypes, developed for the XENON dark matter project. We find that the charge yield increases with decreasing recoil energy, and exhibits only a weak field dependence. These results are the first unambiguous demonstration of the capability of dual-phase xenon detectors to discriminate between electron and nuclear recoils down to 20 keV, a key requirement for a sensitive dark matter search.

Electron spin resonance studies on interaction of complement proteins with erythrocyte membranes.

Sheep erythrocytes have been spin labeled with 5-, 12-, and 16-nitroxystearic acid in order to investigate complement-induced changes in the physical state of the lipid bilayer. Formation of osmotic lesions in the membrane causes an increase in the fluidity of the membrane which overcomes the decrease in membrane fluidity caused by the interaction of the complement proteins. A decrease in membrane fluidity is observed only when complement-lysed membranes are resealed or when complement proteins react with isosmolar ghosts that do not undergo osmotic lysis. The decrease in bulk fluidity of the membrane is first observed when C8 binds to the membranes bearing C5b67 and is enhanced upon the subsequent binding of C9. The decrease in membrane fluidity shown by the electron spin resonance spectra of spin-labeled fatty acids suggests that certain of the complement proteins penetrate the membrane and interact with hydrophobic regions of the lipid bilayer.

Stimulation of cell proliferation and polyphosphoinositide metabolism in Saccharomyces cerevisiae GL7 by ergosterol.

The effect of ergosterol on cell division and phospholipid metabolism was investigated in Saccharomyces cerevisiae strain GL7, a sterol and unsaturated fatty acid auxotroph. Cells growing poorly on cholesterol were stimulated to grow more rapidly by supplementing the medium with 100 ng of ergosterol per ml. Within 10 min after ergosterol addition to cells prelabeled with 32Pi or [3H]inositol the isotope content of the polyphosphoinositides increases markedly followed by an equally striking and rapid decrease. Subsequently upon continuous labeling, 32P incorporation into phosphatidylinositol and, to a lesser degree, other phospholipids increased. Finally 3h after ergosterol addition the growth rate increased. Only stimulation of the first process, i.e. polyphosphoinositide metabolism, upon ergosterol addition is resistant to inhibition by cycloheximide.

Coordinate regulation of unsaturated phospholipid, RNA, and protein synthesis in Mycoplasma capricolum by cholesterol.

The effect of cholesterol, epicoprostanol, and phosphatidylcholine on phospholipid, RNA, and protein synthesis was investigated in the sterol auxotroph Mycoplasma capricolum. Cells growing poorly on lanosterol were stimulated to grow more rapidly by supplementing the medium with either 2 micrograms of cholesterol or 2.2 micrograms of egg phosphatidylcholine per ml. In such cells cholesterol caused a sequential stimulation of phospholipid, RNA, and protein synthesis. Enhanced oleate incorporation into phospholipid occurred early; the rates of RNA and protein synthesis increased later. In cells supplemented with phosphatidylcholine only RNA and protein syntheses were enhanced. The addition of 2 micrograms of epicoprostanol per ml to cells growing on lanosterol promptly inhibited the rate of unsaturated phospholipid synthesis and subsequently the rate of growth. Inhibition of both processes was relieved by supplying 2 micrograms of cholesterol or 2.2 micrograms of phosphatidylcholine per ml along with the inhibitory sterol. The results suggest that cholesterol in small amounts exerts a positive regulatory effect and epicoprostanol exerts a negative one on unsaturated phospholipid synthesis and, in turn, that RNA and protein synthesis are coordinately controlled with phospholipid synthesis. The previously reported phenomenon of sterol synergism and the postulated novel role of sterols in membranes.

Effect of cholesterol on phospholipid, RNA, and protein synthesis in Mycoplasma capricolum.

The regulatory role of cholesterol on phospholipid, RNA and protein synthesis was investigated in Mycoplasma capricolum. The addition of 2 micrograms/ml of cholesterol to cells growing slowly on lanosterol (10 micrograms/ml) caused an early stimulation of unsaturated phospholipid synthesis followed by a later stimulation of RNA and protein synthesis. Epicoprostanol, a cholesterol antagonist, sequentially inhibited these three processes. Phospholipid served as the acyl donor for the acylation of membrane proteins. Membrane-bound cholesterol correlated with a decrease in the amount of [3H]amino acids and an increase in the amount of [3H]oleate found in two membrane proteins (MW 78 kDa and 26 kDa). Taken together, the results suggest that a small amount of membrane-associated cholesterol serves as a signal for membrane biogenesis and, in turn, macromolecular synthesis and cell growth.

Phospholipids as acyl donors to membrane proteins of Mycoplasma capricolum.

Mycoplasma capricolum, a procaryotic sterol and fatty acid auxotroph, contains a large number of membrane proteins covalently modified by both saturated and unsaturated fatty acids (Dahl, C.E., Dahl, J.S., and Bloch, K. (1983) J. Biol. Chem. 258, 11814-11818). Pulse-chase experiments show that the radioactivity in the fatty acid moieties of the acyl proteins increases rather than decreases during a 4.5-h incubation period suggesting that a large intracellular pool of metabolites such as phospholipid serves as the donor for protein acylation. We find that cells incubated for 4 h in a growth medium containing [3H]palmitate-labeled phosphatidylglycerol or 2-[3H]palmitoyl dipalmitoylphosphatidylcholine show the same labeling pattern as cells incubated for 4 h in a complete growth medium with [3H]palmitate. Exogenously added phospholipids are not hydrolyzed to free fatty acid during the labeling period. Acylation of proteins is inhibited in cells treated with chloramphenicol showing that there is no pool of proacyl protein in the cell. Labeling of membrane proteins also occurs with [3H]glycerol. Glycerol is incorporated primarily into the same proteins as oleate suggesting that acylation by unsaturated fatty acid may involve a protein bound diglyceride moiety. Palmitate, on the other hand, appears to bind to other sites along the polypeptide chain in addition to the diglyceride moiety.

Acylated proteins in Acholeplasma laidlawii.

The covalent modification of membrane proteins by long-chain fatty acids was determined in two strains of Acholeplasma laidlawii by one-dimensional gel electrophoresis of radiolabeled membranes. Of the more than 50 membrane polypeptides detected, approximately 30 were labeled with [3H]palmitate, whereas covalent binding of [3H]oleate to membrane proteins could not be demonstrated. We suggest that in these wall-less bacteria, membrane protein acylation with saturated fatty acids may serve to ensure the structural integrity of the membrane.

T and B epitope determination and analysis of multiple antigenic peptides for the Schistosoma mansoni experimental vaccine triose-phosphate isomerase.

Schistosoma mansoni triose-phosphate isomerase (TPI), a glycolytic enzyme, was originally identified as the target of the mAb M.1 that conferred protection when the antibody was administered in vivo. In this study we increase the evidence that schistosome TPI is a potential vaccine Ag by showing that it also a potent inducer of IL-2 and IFN-gamma production (Th1 responses), driving production of these cytokines in the same cell populations of infected animals that have high Th2 responses directed at other parasite egg Ag. With the goal of synthetic peptide vaccine design, rTPI was used to determine specific T and B cell epitopes recognized by two strains of mice representing high and moderate responders (C57Bl/6J and CBA/J). All selected epitopes were from nonconserved regions of TPI and thus parasite-specific. We then defined minimal size immunoreactive epitopes and synthesized four-armed multiple antigenic peptides (MAP) consisting of T and B cell epitopes that could be recognized by both strains of mice in the same molecule. Characterization of the immunoreactivity of the MAP showed that higher antibody recognition of the MAP was attained when the B cell epitope was placed on the amino termini relative to the T cell epitope, whereas equivalent immunoreactivity occurred for the T cell epitopes when located at either position. Most interesting was the finding that one of the minimal T cell epitopes, when incorporated into the MAP, required enlarging to retain immunoreactivity. Finally we showed that both the full-length TPI molecule and the final version of the MAP were immunogenic to T cells in naive animals and induced cross-recognition in the form of IL-2 and IFN-gamma production.

Proteolipid formation in Mycoplasma capricolum. Influence of cholesterol on unsaturated fatty acid acylation of membrane proteins.

Mycoplasma capricolum, a procaryotic sterol and fatty acid auxotroph was grown on media supplemented with [3H]palmitate or [3H]oleate. The isolated bacterial membranes were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Of the more than 50 membrane polypeptides revealed by Coomassie blue staining, approximately 25 were labeled with [3H]palmitate and only about 6 were labeled with [3H]oleate. Exhaustive delipidation of the membranes with chloroform:methanol did not alter the labeling pattern. Treatment of delipidated membranes by mild alkaline hydrolysis released up to 71% of the [3H]palmitate and 93% of the [3H]oleate. The data suggest that numerous membrane proteins of M. capricolum are covalently modified by acylation with saturated and unsaturated fatty acids. Cerulenin, a specific inhibitor of fatty acid synthesis had no effect on the labeling of mycoplasma membrane proteins by either [3H]palmitate or [3H]oleate. A small amount of membrane-associated cholesterol previously shown to stimulate sequentially the synthesis of unsaturated phospholipid, RNA, and protein (Dahl, J. S., and Dahl, C. E. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 692-696) specifically enhances the acylation of certain proteolipids by oleate but not by palmitate.

Effect of cholesterol on macromolecular synthesis and fatty acid uptake by Mycoplasma capricolum.

The rates of protein and lipid synthesis of Mycoplasma capricolum were essentially synchronous during growth and depended on the sterol supplement in the media increasing in the order cholesterol (0.5 microgram/ml) < lanosterol (10 microgram/ml) < lanosterol (10 microgram/ml) + cholesterol (0.5 microgram/ml) < cholesterol (10 microgram/ml). The effect of lanosterol plus low cholesterol on macromolecular synthesis was synergistic. Whereas protein and lipid synthesis were brought virtually to a halt by cholesterol starvation, DNA synthesis continued for about 8 h. Increasing the palmitate and elaidate concentrations 4-fold in the lanosterol-supplemented media raised the growth rate even in the absence of the small amount of cholesterol (0.5 microgram/ml) needed otherwise for the synergistic effect on growth. Studies of the kinetics of fatty acid uptake by resting cells showed that the apparent Km (17 microM) of oleate uptake in lanosterol-grown cells was specifically lowered to 3 microM, a value equal to that seen in cholesterol-grown cells, by the inclusion of a synergistic amount of cholesterol in the growth media. By contrast, the apparent Km for palmitate uptake was the same (2 microM) for all three cell types. The results are consistent with the membrane cholesterol serving in a dual role, one as a bulk component and another more specific function involving the regulation of unsaturated fatty acid uptake and thereby phospholipid biosynthesis.