arXiv subjects

D. Santos

Publications and source records attributed to D. Santos.

At least 19 recordsLinked to original sources

arXiv2026

Discovery of a star sensitive to the spin of Sgr A*

Residing in the center of the Milky Way, Sgr A* is the closest massive black hole (MBH). Its vicinity has allowed measuring individual stellar orbits around it. The stars act as test particles and probe the gravitational potential around the $4.3 \times 10^6 M_\odot$ MBH. These observations have determined the central mass to sub-percent precision, and the mildly relativistic motions of stars have given access to the dominant relativistic corrections, the gravitational redshift, the transverse Doppler effect, and the prograde precession imposed by the Schwarzschild metric nature of the potential. These effects are of order $\beta^2 = (v/c)^2$ (for velocity $v$ and speed of light $c$). The Kerr metric for a rotating black hole leads to corrections of order $\beta^3$. Here, we report the discovery of a faint main-sequence star ($m_K = 19.3$), S301, on a 8.7-year orbit and with small enough a pericenter distance, such that the star's peak velocity reaches $25000\,$km/s. Within the measurement capabilities of current near-infrared interferometry and future spectroscopy on an extremely large telescope, S301's motion is directly sensitive to the spin of Sgr A*. The high eccentricity of S301 suggests that it is the captured component of a binary that was torn apart via the Hills mechanism.

astro-ph.GA↗

arXiv2026

The GRAVITY young stellar object survey -- XV. The star-disk interaction region of the T Tauri star DO Tau

Protoplanetary disks around young Sun-like stars are the cradles of the vast majority of detected exoplanets. Probing these disks at multiple spatial scales is key to uncovering how planets form. We aim to spatially and spectrally resolve the inner disk and star-disk interaction region of the M0.3 T Tauri star DO Tau by combining two complementary techniques. We used high-resolution near-infrared spectra from CFHT/SPIRou to constrain the magnetospheric star-disk interaction process and optical long-baseline interferometry with ESO VLTI/GRAVITY to determine the sizes of the K-band continuum and Br$\gamma$ line emitting regions. From the SPIRou spectra, we confirmed that this ~0.5 M$_\odot$ star is a strong accretor. The HI and HeI lines exhibit strong variability on a daily timescale, consistent with the burster classification of DO Tau derived from its K2 light curve. We derived an upper limit of 0.35 on the ratio between the magnetospheric truncation radius and the disk corotation radius, indicative of an ordered unstable accretion regime. The size of the Br$\gamma$ line emitting region obtained from GRAVITY is much smaller than the K-band continuum emitting region. This compact Br$\gamma$ emission region ($R_{Br\gamma} \sim$ 0.011 au) suggests that most of the line flux originates from the magnetospheric accretion region and/or from an inner wind close to the magnetosphere-disk interface. The inclination we derived for the inner disk (45-55{\deg}) differs from that of the outer disk inferred from the ALMA continuum (30{\deg}). This points toward a misalignment or warp of the outer disk that may originate from the suspected past encounter with the neighboring HV Tau system.

astro-ph.SR↗

arXiv2026

First Dark Photon Search Results from the Dandelion Experiment

This paper presents the first results from the Dandelion experiment, a directional detection, which searches for 1 meV dark photon dark matter. We use a spherical mirror to convert dark photons into standard millimeter-wavelength photons that can then be detected with an array of 221 Kinetic Inductance Detectors (KIDs) cooled down to 150 mK within the KISS-NIKA camera ( (KIDs Interferometric Spectral Surveyor) and operating between 150 and 350 GHz. We used 1480 minutes of data to search for the signal of dark photons in the KID detectors, which is expected to be modulated due to the Earth's rotation. Our main challenge was to deal with a large background from room temperature and stray-light fluctuations. We used a de-correlation analysis to remove these background fluctuations. Templates of the background fluctuations were constructed from a Principal Component Analysis decomposition of detector measurements outside the expected Field of View trajectory of dark photons. We found that the dark photon signal was consistent with zero, giving a new upper limit on the dark photon's kinetic mixing, $\chi$, with masses between 0.6 meV and 1.4 meV. These are the first constraints on dark photons as a dark matter candidate using an array of KIDs at millimeter wavelength.

astro-ph.CO↗

arXiv2025

Spatially resolved broad line region in a quasar at z=4: Dynamical black hole mass and prominent outflow

We present the first near-infrared interferometric data of a QSO at z=4. The K-band observations were performed with GRAVITY+ on the VLTI using all four UTs, detecting a differential phase signal that traces the spatially resolved kinematics for both the \hb\ and \hg\ lines in the broad line region. We fit the two lines simultaneously with an updated model that includes distinct rotating and conical outflowing components. For the best fit model, more than 80\% of the \hi\ line emission from the BLR originates in an outflow with a velocity up to $10^4$ km s$^{-1}$. This is oriented so that our line of sight is along an edge of the conical structure, which produces the prominent blue wing on the line profile. A combination of anisotropic line emission and mid-plane opacity lead to the single-sided phase signal. The model is able to qualitatively match both the outflowing \civ\ line profile and the systemic \oi\ fluorescent emission. The derived black hole mass of $8\times10^8$ M$_\odot$ is the highest redshift black hole mass measurement to date obtained directly from BLR dynamics. It is an order of magnitude lower than that inferred from various single epoch scaling relations, and implies that the accretion is highly super-Eddington. With reference to recent simulations, the data suggest that this QSO is emitting close to its radiative limit in a regime where strong outflows are expected around a polar conical region.

astro-ph.GA↗

arXiv2024

The ionization yield in a methane-filled spherical proportional counter

Spherical proportional counters (SPCs) are gaseous particle detectors sensitive to single ionization electrons in their target media, with large detector volumes and low background rates. The $\mbox{NEWS-G}$ collaboration employs this technology to search for low-mass dark matter, having previously performed searches with detectors at the Laboratoire Souterrain de Modane (LSM), including a recent campaign with a 135 cm diameter SPC filled with methane. While in situ calibrations of the detector response were carried out at the LSM, measurements of the mean ionization yield and fluctuations of methane gas in SPCs were performed using a 30 cm diameter detector. The results of multiple measurements taken at different operating voltages are presented. A UV laser system was used to measure the mean gas gain of the SPC, along with $\mathrm{^{37}Ar}$ and aluminum-fluorescence calibration sources. These measurements will inform the energy response model of future operating detectors.

physics.ins-det↗

arXiv2024

Search for Light Dark Matter with NEWS-G at the Laboratoire Souterrain de Modane Using a Methane Target

The NEWS-G direct detection experiment uses spherical proportional counters to search for light dark matter candidates. New results from a 10 day physics run with a $135\,\mathrm{cm}$ in diameter spherical proportional counter at the Laboratoire Souterrain de Modane are reported. The target consists of $114\,\mathrm{g}$ of methane, providing sensitivity to dark matter spin-dependent coupling to protons. New constraints are presented in the mass range $0.17$ to $1.2\,\mathrm{GeV/c^2}$, with a 90% confidence level cross-section upper limit of $30.9\,\mathrm{pb}$ for a mass of $0.76\,\mathrm{GeV/c^2}$.

hep-ex↗

arXiv2024

Directional detection of keV proton and carbon recoils with MIMAC

Directional detection is the dedicated strategy to demonstrate that DM-like signals measured by direct detectors are indeed produced by DM particles from the galactic halo. The experimental challenge of measuring the direction of DM-induced nuclear recoils with (sub-)millimeter tracks has limited, so far, the maximal directional reach to DM masses around $100~\rm{GeV}$. In this paper, we expose the MIMAC detector to three different neutron fields and we develop a method to reconstruct the direction of the neutron-induced nuclear recoils. We measure an angular resolution better than $16^\circ$ for proton recoils down to a kinetic energy of $4~\rm{keV}$ and for carbon recoils down to a kinetic energy of $5.5~\rm{keV}$. For the first time, a detector achieves the directional measurement of proton and carbon recoils with kinetic energies in the keV range without any restriction on the direction of the incoming particle. This work demonstrates that directional detection is around the corner for probing DM with masses down to $\mathcal{O}(1~\rm{GeV})$.

physics.ins-det↗

arXiv2023

Exploring light dark matter with the DarkSPHERE spherical proportional counter electroformed underground at the Boulby Underground Laboratory

We present the conceptual design and the physics potential of DarkSPHERE, a proposed 3 m in diameter spherical proportional counter electroformed underground at the Boulby Underground Laboratory. This effort builds on the R&D performed and experience acquired by the NEWS-G Collaboration. DarkSPHERE is primarily designed to search for nuclear recoils from light dark matter in the 0.05--10 GeV mass range. Electroforming the spherical shell and the implementation of a shield based on pure water ensures a background level below 0.01 dru. These, combined with the proposed helium-isobutane gas mixture, will provide sensitivity to the spin-independent nucleon cross-section of $2\times 10^{-41} (2\times 10^{-43})$ cm$^2$ for a dark matter mass of $0.1 (1)$ GeV. The use of a hydrogen-rich gas mixture with a natural abundance of $^{13}$C provides sensitivity to spin-dependent nucleon cross-sections more than two orders of magnitude below existing constraints for dark matter lighter than 1 GeV. The characteristics of the detector also make it suitable for searches of other dark matter signatures, including scattering of MeV-scale dark matter with electrons, and super-heavy dark matter with masses around the Planck scale that leave extended ionisation tracks in the detector.

hep-ex↗

arXiv2023

Directional detection of meV dark photons with Dandelion

This paper presents Dandelion, a new dish antenna experiment searching for dark photons (DPs) with masses around the meV that will start acquiring data by the end of 2023. A spherical mirror acts as a conversion surface between DPs and standard photons that converge to a matrix of 418 Kinetic Inductance Detectors cooled down to 150 mK. A tilt of the mirror at 1 Hz moves the expected signal over the pixels thus enabling a continuous background measurement. The expected signal has two modulations: a spatial modulation providing a directional signature for the unambiguous discovery of a DP, and an intensity modulation allowing the determination of the polarization of the DP. For masses near the meV, the inflationary production of longitudinal and transverse DPs are mutually excluded, thus the polarization determination by Dandelion could shed a new light on the inflation phase of the early universe. A first Dandelion prototype operating for 30 days would improve by more than one order of magnitude the current exclusion limits on DPs at the meV mass scale and would probe this region with an unprecedented discovery potential based on directional detection.

physics.ins-det↗

arXiv2023

The NEWS-G detector at SNOLAB

The New Experiments With Spheres-Gas (NEWS-G) collaboration intends to achieve $\mathrm{sub-GeV/c^{2}}$ Weakly Interacting Massive Particles (WIMPs) detection using Spherical Proportional Counters (SPCs). SPCs are gaseous detectors relying on ionization with a single ionization electron energy threshold. The latest generation of SPC for direct dark matter searches has been installed at SNOLAB in Canada in 2021. This article details the different processes involved in the fabrication of the NEWS-G experiment. Also outlined in this paper are the mitigation strategies, measurements of radioactivity of the different components, and estimations of induced background event rates that were used to quantify and address detector backgrounds.

physics.ins-det↗

arXiv2022

Measurements of the ionization efficiency of protons in methane

The amount of energy released by a nuclear recoil ionizing the atoms of the active volume of detection appears "quenched" compared to an electron of the same kinetic energy. This different behavior in ionization between electrons and nuclei is described by the Ionization Quenching Factor (IQF) and it plays a crucial role in direct dark matter searches. For low kinetic energies (below $50~\mathrm{keV}$), IQF measurements deviate significantly from common models used for theoretical predictions and simulations. We report measurements of the IQF for proton, an appropriate target for searches of Dark Matter candidates with a mass of approximately 1 GeV, with kinetic energies in between $2~\mathrm{keV}$ and $13~\mathrm{keV}$ in $100~\mathrm{mbar}$ of methane. We used the Comimac facility in order to produce the motion of nuclei and electrons of controlled kinetic energy in the active volume, and a NEWS-G SPC to measure the deposited energy. The Comimac electrons are used as reference to calibrate the detector with 7 energy points. A detailed study of systematic effects led to the final results well fitted by $\mathrm{IQF}~(E_K)= E_K^α~/~(β+ E_K^α)$ with $α=0.70\pm0.08$ and $β= 1.32\pm0.17$. In agreement with some previous works in other gas mixtures, we measured less ionization energy than predicted from SRIM simulations, the difference reaching $33\%$ at $2~\mathrm{keV}$

physics.ins-det↗

arXiv2022

Solar Kaluza-Klein axion search with NEWS-G

Kaluza-Klein (KK) axions appear in theories with extra dimensions as higher mass, significantly shorter lifetime, excitations of the Peccei-Quinn axion. When produced in the Sun, they would remain gravitationally trapped in the solar system, and their decay to a pair of photons could provide an explanation of the solar corona heating problem. A low-density detector would discriminate such a signal from the background, by identifying the separation of the interaction point of the two photons. The NEWS-G collaboration uses large volume Spherical Proportional Counters, gas-filled metallic spheres with a spherical anode in their centre. After observation of a single axionlike event in a 42 day long run with the SEDINE detector, a $90\%$ C.L. upper limit of $g_{aγγ}<8.99\cdot10^{-13}\,GeV^{-1}$ is set on the axion-photon coupling for a KK axion density on Earth of $n_{a}=4.07\cdot10^{13}\,m^{-3}$ and two extra dimensions of size $R = 1\,eV^{-1}$.

hep-ex↗

arXiv2021

Quenching factor measurements of neon nuclei in neon gas

The NEWS-G collaboration uses Spherical Proportional Counters (SPCs) to search for weakly interacting massive particles (WIMPs). In this paper, we report the first measurements of the nuclear quenching factor in neon gas at \SI{2}{bar} using an SPC deployed in a neutron beam at the TUNL facility. The energy-dependence of the nuclear quenching factor is modelled using a simple power law: $α$E$_{nr}^β$; we determine its parameters by simultaneously fitting the data collected with the detector over a range of energies. We measured the following parameters in Ne:CH$_{4}$ at \SI{2}{bar}: $α$ = 0.2801 $\pm$ 0.0050 (fit) $\pm$ 0.0045 (sys) and $β$ = 0.0867 $\pm$ 0.020 (fit) $\pm$ 0.006(sys). Our measurements do not agree with expected values from SRIM or Lindhard theory. We demonstrated the feasibility of performing quenching factor measurements at sub-keV energies in gases using SPCs and a neutron beam.

physics.ins-det↗

arXiv2020

Copper electroplating for background suppression in the NEWS-G experiment

New Experiments with Spheres-Gas (NEWS-G) is a dark matter direct detection experiment that will operate at SNOLAB (Canada). Similar to other rare-event searches, the materials used in the detector construction are subject to stringent radiopurity requirements. The detector features a 140-cm diameter proportional counter comprising two hemispheres made from commercially sourced 99.99% pure copper. Such copper is widely used in rare-event searches because it is readily available, there are no long-lived Cu radioisotopes, and levels of non-Cu radiocontaminants are generally low. However, measurements performed with a dedicated 210Po alpha counting method using an XIA detector confirmed a problematic concentration of 210Pb in bulk of the copper. To shield the proportional counter's active volume, a low-background electroforming method was adapted to the hemispherical shape to grow a 500-$μ$m thick layer of ultra-radiopure copper to the detector's inner surface. In this paper the process is described, which was prototyped at Pacific Northwest National Laboratory (PNNL), USA, and then conducted at full scale in the Laboratoire Souterrain de Modane in France. The radiopurity of the electroplated copper was assessed through Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Measurements of samples from the first (second) hemisphere give 68% confidence upper limits of <0.58 $μ$Bq/kg (<0.24 $μ$Bq/kg) and <0.26 $μ$Bq/kg (<0.11 $μ$Bq/kg) on the 232Th and 238U contamination levels, respectively. These results are comparable to previously reported measurements of electroformed copper produced for other rare-event searches, which were also found to have low concentration of 210Pb consistent with the background goals of the NEWS-G experiment.

physics.ins-det↗

arXiv2020

Dark Matter Directionality Detection performance of the Micromegas-based $μ$TPC-MIMAC detector

Directional Dark Matter Detection (DDMD) can open a new signature for Weakly Massive Interacting Particles (WIMPs) Dark Matter. The directional signature provides in addition, an unique way to overcome the neutron and neutrino backgrounds. In order to get the directional signature, the DDM detectors should be sensitive to low nuclear energy recoils in the keV range and have an angular resolution better than $20^{\circ}$. We have performed experiments with low energy ($<30\,\mathrm{keV}$) ion beam facilities to measure the angular distribution of nuclear recoil tracks in a MIMAC detector prototype. In this paper, we study angular spreads with respect to the electron drift direction ($0^{\circ}$ incident angle) of Fluorine nuclear tracks in this low energy range, and show nuclear recoil angle reconstruction produced by a monoenergetic neutron field experiment. We find that a high-gain systematic effect leads to a high angular resolution along the electron drift direction. The measured angular distribution is impacted by diffusion, and space charge or ion feedback effects, which can be corrected for by an asymmetry factor observed in the flash-ADC profile. The estimated angular resolution of the $0^{\circ}$ incident ion is better than $15^{\circ}$ at $10$ keV kinetic energy and agrees with the simulations within $20$%. The distributions from the nuclear recoils have been compared with simulated results based on a modified Garfield++ code. Our study shows that protons would be a more adapted target than heavier nuclei for DDMD of light WIMPs. We demonstrate that directional signature from the Galactic halo origin of a Dark Matter WIMP signal is experimentally achievable, with a deep understanding of the operating conditions of a low pressure detector with its diffusion mechanism.

physics.ins-det↗

arXiv2020

Track length measurement of $^{19}$F$^+$ ions with the MIMAC Dark Matter directional detector prototype

Weakly Interacting Massive Particles (WIMPs) are one of the most preferred candidate for Dark Matter. WIMPs should interact with the nuclei of detectors. If a robust signal is eventually observed in direct detection experiments, the best signature to confirm its Galactic origin would be the nuclear recoil track direction. The MIMAC collaboration has developed a low pressure gas detector providing both the kinetic energy and three-dimensional track reconstruction of nuclear recoils. In this paper we report the first ever observations of $^{19}$F nuclei tracks in a $5$ cm drift prototype MIMAC detector, in the low kinetic energy range ($6$-$26$ keV), using specially developed ion beam facilities. We have measured the recoil track lengths and found significant differences between our measurements and standard simulations. In order to understand these differences, we have performed a series of complementary experiments and simulations to study the impact of the diffusion and eventual systematics. We show an unexpected dependence of the number of read-out corresponding to the track on the electric field applied to the $512\ \mathrm{μm}$ gap of the Micromegas detector. We have introduced, based on the flash-ADC observable, corrections in order to reconstruct the physical 3D track length of the primary electron clouds proposing the physics behind these corrections. We show that diffusion and space charge effects need to be taken into account to explain the differences between measurements and standard simulations. These measurements and simulations may shed a new light on the high-gain TPC ionization signals in general and particularly at low energy.

physics.ins-det↗

arXiv2019

Precision laser-based measurements of the single electron response of SPCs for the NEWS-G light dark matter search experiment

Spherical Proportional Counters (SPCs) are a novel gaseous detector technology employed by the NEWS-G low-mass dark matter search experiment for their high sensitivity to single electrons from ionization. In this paper, we report on the first characterization of the single electron response of SPCs with unprecedented precision, using a UV-laser calibration system. The experimental approach and analysis methodology are presented along with various direct applications for the upcoming next phase of the experiment at SNOLAB. These include the continuous monitoring of the detector response and electron drift properties during dark matter search runs, as well as the experimental measurement of the trigger threshold efficiency. We measure a mean ionization energy of $\mathrm{W}=27.6\pm0.2~\mathrm{eV}$ in $\mathrm{Ne + CH_4}$ $(2\%)$ for 2.8 keV X-rays, and demonstrate the feasibility of performing similar precision measurements at sub-keV energies for future gas mixtures to be used for dark matter searches at SNOLAB.

physics.ins-det↗

arXiv2018

Bifurcations in the theory of current transfer to cathodes of dc discharges and observations of transitions between different modes

General scenarios of transitions between different spot patterns on electrodes of dc gas discharges and their relation to bifurcations of steady-state solutions are analyzed. In the case of cathodes of arc discharges, it is shown that any transition between different modes of current transfer is related to a bifurcation of steady-state solutions. In particular, transitions between diffuse and spot modes on axially symmetric cathodes, frequently observed in the experiment, represent an indication of the presence of pitchfork or fold bifurcations of steady-state solutions. Experimental observations of transitions on cathodes of dc glow microdischarges are analyzed and those potentially related to bifurcations of steady-state solutions are identified. The relevant bifurcations are investigated numerically and the computed patterns are found to conform to those observed in the course of the corresponding transitions in the experiment.

physics.plasm-ph↗