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2025 EN
A. Semakin · J. Ahokas · O. Hanski
+8 more
We present a design and performance tests of an intense source of coldhydrogen atoms for loading large magnetic traps. Our source is based on acryogenic dissociator of molecular hydrogen at 0.6 K followed by a series ofthermal accommodators at 0.5, 0.2 and 0.13 K with inner surfaces covered by asuperfluid helium film. All components are thermally anchored to correspondingstages of a dilution refrigerator. The source provides a continuous flux of7$\cdot$$10^{13}$ H atoms/s in a temperature range of 130-200 mK. We havesuccessfully used the source for loading a large Ioffe-Pritchard magnetic traprecently built in our laboratory [arXiv:2108.09123 or Rev. Sci. Instr. 93 (2),023201 (2022)]. Calorimetric measurements of the atomic recombination heatallow reliable determination of the atomic flux and H gas density in the trap.We have tested the performance of the source and loading of H atoms into thetrap at various configurations of the trapping field, reducing the magneticbarrier height to 75% and 50% of the nominal value of 0.8 T (0.54 K) as well asat the open configuration of the trap at its lower end, when the atoms are incontact with the trapping cell walls covered by a superfluid helium film. Inthe latter case, raising the trapping cell temperature to 200-250 mK, thelow-field seeking atoms at densities exceeding 10$^{11}$ cm$^{-3}$ can bestored for the time over 1000 s, sufficiently long for experiments on precisionspectroscopy of cold H gas.
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2025 EN
Otto Hanski · Tom Kiilerich · Sampsa Ahopelto
+4 more
We present experimental results of characterization of Siliconphotomultipliers (SiPM) in a temperature range from 90~mK to 40~K. Two SiPMs,one from ONSEMI and one from Hamamatsu Photonics were tested. Operating voltageranges, dark count rates, afterpulsing effects and photon detectionefficiencies (PDE) were determined with illumination by 275 and 470~nm lightfed into the cryostat via an optical fiber. A cryogenic shutter provided a truedark condition, where thermal radiation from room temperature is shielded andthe thermal excitations in the chips are frozen. A second tunneling breakdownwas observed at this condition, which substantially limits the operatingvoltage range for the temperatures 20-30 K. Below $\sim$5 K, both SiPMs recoverto an operating over-voltage range of 3-5 V. We found the chips functionthrough the entire tested temperature range, and are capable of withstandingthermal cycling with no major performance degradation.
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2025 EN
Jason A. S. Hunt · Eugene Vasiliev
The Gaia mission has triggered major developments in the field of Galacticdynamics in recent years, which we discuss in this review. The structure andkinematics of all Galactic components - disc, bar/bulge and halo - are nowmapped in great detail not only in the Solar neighbourhood, but across a largepart of the Milky Way. The dramatic improvements in the coverage and precisionof observations revealed various disequilibrium processes, such asperturbations in the Galactic disc and the deformations of the outer halo,which are partly attributed to the interaction with satellite galaxies. Theknowledge of the gravitational potential at all scales has also advancedconsiderably, but we are still far from having a consistent view on the keyproperties of the Galaxy, such as the bar pattern speed or the mass profile andshape of the dark halo. The complexity and interplay of several dynamicalprocesses makes the interpretation of observational data challenging, and it isfair to say that more theoretical effort is needed to fully reap the fruit ofthe Gaia revolution.
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2025 EN
Diego Barberena
We present a fluctuating $N$ formalism, based on second-quantization, todescribe large $N$ vector models from field theory using Hamiltonian methods.We first present the method in the simpler setting of a quantum mechanicalsystem with quartic interactions, and then apply these techniques to the $O(N)$model in $2+1$ and $3+1$ dimensions. We recover various known results, such asthe gap equation determining the ground state of the system, the presence ofbound states at negative coupling and the leading order contribution tocritical exponents, and provide an interpretation of the large $N$ pathintegral saddle point as a Bose-Einstein condensate of extended objects in thepresence of a non-local interaction. In the large $N$ limit, this formalismleads naturally to a description of elementary $O(N)$ symmetric excitations interms of bilocal fields, which are at the basis of $\text{AdS}_4/\text{CFT}_3$studies of the $O(N)$ model and Vasiliev gravity.
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2025 EN
Matthew A. Taylor · Behzad Tahmasebzadeh · Solveig Thompson
+18 more
The integral-field unit mode of the Near-Infrared Spectrograph (NIRSpec+IFU)mounted on the James Webb Space Telescope has now enabled kinematic studies ofsmaller and less massive compact stellar systems in which to search for centralmassive black holes (BHs) than ever before. We present here the first suchdetection using NIRSpec+IFU in its highest resolution (R~2700) mode. We reporta $3\sigma$ detection of a central black hole with mass ${\calM}_{BH}=2.2\pm1.1\times10^6\,M_\odot$ in UCD736 orbiting within the Virgogalaxy cluster based on Schwarzschild's modeling of the 1D kinematic profile.The presence of such a massive BH strongly argues against a globular clusterorigin of this UCD, and rather suggests a tidally stripped formation route froma former $\gtrsim10^9\,M_\odot$ dwarf galaxy host. Two other methods produceresults consistent with Schwarzschild's modelling, but can only provideupper-limits on ${\cal M}_{BH}$. This represents the detection of a BH in themost compact ($r_h\approx15\,{\rm pc}$) stellar system to date, with a ${\calM}_{BH}$ corresponding to ~9 percent of the system's stellar mass, roughly inline with previously reported UCD BH detections and comparable to the BHdetected in the compact elliptical galaxy NGC4486B.
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2025 EN
A. A. Tarusov · K. A. Ushakov · M. A. Vasiliev
A class of higher-spin gauge theories on $AdS_4$ associated with variousCoxeter groups $\mathcal{C}$ is analyzed at the linear order. For a general$\mathcal{C}$, a solution corresponding to the $AdS_4$ space and the form ofthe free unfolded equations are established. A disentanglement criterion hasbeen formulated for Coxeter HS modules. The shifted homotopy technique isuplifted to the general Coxeter HS models. In case of the Coxeter group $B_2$classification of unitary HS modules and a consistent truncation to them aredetermined, the dynamical content is discussed briefly.
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2025 EN
M. A. Vasiliev
Nonlinear field equations for the supersymmetric higher-spin gauge theorydescribing totally symmetric bosonic and fermionic massless fields along withhook-type bosonic fields of all spins in any space-time dimension arepresented. One of the novel features of the proposed formalism is that the$osp(1,2)$ invariance and factorisation conditions are formulated within theBRST formalism, that greatly simplifies the form of nonlinear HS equations. Tomatch the list of vertices found by Metsaev, higher-spin gauge theory isanticipated to possess an infinite number of independent coupling constants. Aconjecture that these coupling constants result from the locality restrictionson the elements of the factorisation ideal is put forward.
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2025 EN
Yu. M. Zinoviev
We elaborate on the partially massless spin 5/2 supermultiplet, whichcontains partially massless spin 5/2, massless and partially massless spin 2,as well as massless spin 3/2. We consider the global supertransformationsconnecting partially massless spin 5/2 to its two possible superpartners,massless and partially massless spin 2, and make them local by switching theinteraction with the massless gravitino. We use a frame-like gauge-invariantformalism to describe free fields and the Fradkin-Vasiliev formalism toconstruct interactions, Due to the presence of the Stueckelberg fields in thegauge-invariant description of massive and partially massless fields, we faceambiguities related to field redefinitions. We use this freedom to simplifycalculations. At the same time, we demonstrate how these ambiguities can beresolved using unfolded equations.
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2025 EN
Ekaterina Kondrateva · Sandzhi Barg · Mikhail Vasiliev
Accurate and reproducible brain morphometry from structural MRI is criticalfor monitoring neuroanatomical changes across time and across imaging domains.Although deep learning has accelerated segmentation workflows, scanner-inducedvariability and reproducibility limitations remain-especially in longitudinaland multi-site settings. In this study, we benchmark two modern segmentationpipelines, FastSurfer and SynthSeg, both integrated into FreeSurfer, one of themost widely adopted tools in neuroimaging. Using two complementary datasets - a 17-year longitudinal cohort (SIMON) anda 9-site test-retest cohort (SRPBS)-we quantify inter-scan segmentationvariability using Dice coefficient, Surface Dice, Hausdorff Distance (HD95),and Mean Absolute Percentage Error (MAPE). Our results reveal up to 7-8% volumevariation in small subcortical structures such as the amygdala and ventraldiencephalon, even under controlled test-retest conditions. This raises a keyquestion: is it feasible to detect subtle longitudinal changes on the order of5-10% in pea-sized brain regions, given the magnitude of domain-inducedmorphometric noise? We further analyze the effects of registration templates and interpolationmodes, and propose surface-based quality filtering to improve segmentationreliability. This study provides a reproducible benchmark for morphometricreproducibility and emphasizes the need for harmonization strategies inreal-world neuroimaging studies. Code and figures: https://github.com/kondratevakate/brain-mri-segmentation
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2025 EN
Anisa Bajkova · Anton Smirnov · Vadim Bobylev
A new method for determining the nature of the orbital motion (chaotic orregular) of globular clusters in the central region of the Galaxy with a radiusof 3.5 kpc, which are most affected by the bar, is proposed. The method isbased on calculating the orbital power spectrum as a function of time andcalculating the entropy of the power spectrum as a measure of orbital chaos.The sample includes 45 globular clusters. To form the 6D phase space requiredfor integrating the orbits, the most accurate astrometric data to date from theGaia satellite (Vasiliev \& Baumgardt, 2021) were used, as well as new refinedaverage distances (Baumgardt \& Vasiliev, 2021). Orbits of globular clustersare obtained in a non-axisymmetric potential with a bar in the form of atriaxial ellipsoid embedded in an axisymmetric potential, traditionally used byus to construct orbits of globular clusters, described in detail in the paperby Bajkova et al. (2023a). The following, most realistic, bar parameters areadopted: mass $10^{10} M_\odot$, length of the major semi-axis 5 kpc, angle ofrotation of the bar axis 25$^o$, angular velocity of rotation 40 km s$^{-1}$kpc$^{-1}$. A list of 23 globular clusters with regular dynamics and 22globular clusters with chaotic dynamics is determined. The correlation of theobtained classification of globular clusters with the classification obtainedby us using other methods in the work of Bajkova et al. (2024a) was determined.