arXiv subjects

Y. Zhu

Publications and source records attributed to Y. Zhu.

At least 19 recordsLinked to original sources

arXiv2026

Optimization studies of silicon remoTES cryogenic calorimeters

The remoTES design, developed within the COSINUS experiment, enables a broader range of materials to be operated as cryogenic calorimeters read out with Transition Edge Sensors (TESs). In this configuration, the TES is fabricated onto a separate chip and thermally coupled to the absorber via a gold (Au) link. The remoTES concept has been successfully tested on various target materials. To further enhance detector performance and to fully exploit the advantages of the remote coupling design a series of optimization studies has been conducted using silicon (Si) absorbers as benchmark. This work presents an evaluation of several measurements aimed at reducing the thermal boundary resistance and enhancing signal transmission across Si remoTES interfaces, specifically from the absorber to the phonon collector and from the phonon collector to the TES. By testing a new Au link design and three distinct phonon collector configurations, Au, copper, and aluminum (Al)/Au, we achieved a baseline resolution of (21.5 +/- 0.3)eV using the Al/Au phonon collector.

physics.ins-det↗

arXiv2026

Fast and periodic propagating disturbances along coronal loops detected with EUI on board Solar Orbiter

Recent high-resolution observations from the Solar Orbiter mission can help detect the indirect signatures of heating at the smallest scales. In this work we measure the properties and investigate the physical origin of propagating disturbances (PDs) in the intensity at the smallest resolvable scales with Solar Orbiter/EUI along coronal loops. We used two sequences of EUI/HRIEUV at high spatial (down to 125 km per pixel) and temporal resolutions (5 s of cadence). We placed slits along 13 active region (AR) coronal loops. We measured the plane-of-sky (PoS) velocities and the intensity perturbation damping along the slits of PDs. We also measured the periodicity of PDs in one slit by using a Fourier analysis. We report the detection of PDs that have high PoS velocities ranging between 500 and 2000 km/s (which we call "fast" PDs). They are only visible in the upper part of the coronal loops. The intensity increase associated with these fast PDs is on the order of 4\% to 8\% of the HRIEUV intensity, and we measured little to no damping of their intensity with distance. We also measured a peak in the Fourier power spectra at 2 min above the 95\% confidence limit that is associated with fast PDs. The fast PDs are detected in the same coronal loops as PDs with a lower PoS velocity (70 to 90 kms/s), which we refer to as "slow" PDs. Unlike the fast PDs, these slow PDs are only visible in the lower part of the coronal loop, and they show clear intensity damping. Slow PDs show properties consistent with slow magneto-acoustic modes or upflows. On the other hand, fast PDs cannot be explained by slow magneto-acoustic modes. Instead, they show properties consistent with fast flows induced by magnetic reconnection, current sheet generated by propagating transverse oscillations, and fast magnetohydrodynamics modes or Alfv\'en waves.

astro-ph.SR↗

arXiv2026

Constraints on Sub-MeV Dark Matter from Solar Reflection with DAMIC-M

The Sun acts as a natural dark matter accelerator. Galactic halo particles scattering in the solar plasma emerge with velocities well beyond the Galactic escape speed, providing a boosted flux that extends the kinematic reach of direct detection experiments into the sub-MeV mass regime. We present constraints on solar-reflected dark matter (SRDM) from the DAMIC-M prototype detector using $\sim$1.3~kg-day of data acquired with silicon skipper charge coupled devices (CCDs) at the Modane Underground Laboratory. Exploiting the spatial diffusion signature of low-energy electron recoils, we derive 90% C.L. upper limits on the DM-electron scattering cross section for both heavy- and ultralight-mediator benchmarks, reaching $\bar{\sigma}_e\sim 3.16 \cdot 10^{-37} \rm{cm^2}$ at $0.1$ MeV. For the ultralight mediator, our limits are competitive with the world-leading constraints, achieved with an integrated exposure of $\sim1.3$~kg-day. These results probe the parameter space between stellar-cooling bounds and terrestrial limits from standard halo searches, a region inaccessible to direct detection experiments relying solely on the standard halo flux.

hep-ex↗

arXiv2026

Cosmic Ray Boosted Dark Matter in COSINUS: Modeling and Constraints

Direct detection of nuclear recoils due to sub-GeV dark matter is challenging because of the small kinetic energy of the light dark matter particles. Although limits down to a few hundred MeV have been reached using specially designed low threshold detectors, further improvements are now constrained more by background event rates than by energy thresholds. However, constraints down to sub-MeV dark matter masses can still be obtained through the boosted dark matter framework. In this scenario, high-energy cosmic rays or neutrinos scatter off dark matter particles, imparting additional kinetic energy and boosting them beyond the typical velocities expected from the non-relativistic dark matter halo. These boosted dark matter particles can then be detected even by experiments with higher energy thresholds. In this work, we present a catalog of dark matter - nucleon scattering cross sections corresponding to a heavy mediator limit for spin zero, one half and one dark matter and for scalar and vector mediators with even or odd parity. Based on these results, we present projected constraints on the dark matter - nucleon cross section for the COSINUS experiment, assuming an exposure of 100 kg d, demonstrating the potential sensitivity to sub-GeV boosted dark matter.

hep-ph↗

arXiv2026

JWST Reveals Two Overmassive Black Hole Candidates in Dwarf Galaxies at z $\approx$ 0.7: Pushing Black Hole Searches into the Dwarf-Galaxy Regime

We report the discovery and characterization of two compact galaxies, Pelias and Neleus, at z ~ 0.71 and z ~ 0.75, identified in MACS J0416.1-2403 and GOODS-North. Both exhibit unusual spectral energy distributions (SEDs), with very blue rest-frame UV-optical emission and a steep rise toward near- and mid-infrared wavelengths. JWST/NIRISS and JWST/NIRSpec spectroscopy show strong rest-frame optical lines ([O III] 4959,5007 and Halpha) with extreme equivalent widths (>= 1000 Angstrom), indicating young burst-dominated populations with low metallicities (Z ~ 0.1-0.4 Zsun), low dust attenuation (Av ~ 0.2 mag), and stellar masses of Mstar ~ 10^7 Msun. Nonetheless, JWST/MIRI photometry reveals a strong mid-infrared excess that cannot be explained by stellar populations or star-formation-heated dust alone, requiring a hot-dust component most naturally associated with a deeply embedded active galactic nucleus (AGN). SED modelling yields log10(Lbol [erg/s]) ~ 43.7-44.0, implying black hole masses of log10(MBH [Msun]) ~ 5.7-6.7 under the assumption of Eddington-limited accretion. Given the very low stellar masses of the hosts, this corresponds to black-hole-to-stellar mass ratios of about 6-60%, well above the extrapolation of local scaling relations. The lack of X-ray detections suggests that the accretion may be either heavily obscured or intrinsically X-ray weak. Their SEDs also resemble those of Blue Excess Hot Dust Obscured Galaxies and show the characteristic V-shaped continuum seen in Little Red Dots, although with the inflection occurring at redder wavelengths.

astro-ph.GA↗

arXiv2026

E-XQR-30: Evidence for an Increase in the Ionization State of Metal Absorbers from z~6 to z~2

We investigate the evolution of the ionization state of metal-enriched gas in and around galaxies near the epoch of reionization using a sample of 488 metal absorption systems at 4.3<z<6.3 from the E-XQR-30 survey. We classify the absorption systems based on whether they display only low-ionization absorption (CII, SiII, MgII), only high-ionization absorption (CIV, SiIV), or both. The percentage of low-ionization-only systems decreases from 24% at $z\sim$6 to 2% at $z\sim$4.3, whilst the fraction of high-ionization-only systems increases from 52% to 82%. For mixed absorbers (with both low and high ionization absorption), we use the column density ratios log(N_CII/N_CIV) and log(N_SiII/N_SiIV) to quantify the average ionization as a function of redshift. The log(N_SiII/N_SiIV) ratio does not change significantly over 5$\lesssim z \lesssim$6.3. We combine the E-XQR-30 log(N_CII/N_CIV) measurements with literature measurements at $z\sim$2-4 and find that the log(N_CII/N_CIV) ratio declines by a factor of $\sim$20 between $z\sim$6 and $z\sim$2. To explore possible drivers of this evolution, we run photoionization models of gas slabs illuminated by a uniform UV background at fixed density, metallicity and HI column density. We find that the increase in the ionization state of metal absorbers towards lower redshifts can likely be explained by some combination of 1) an increase in the metallicity of CIV-absorbing gas and 2) a decrease in the typical HI column densities of the absorbing gas, driven by the declining cosmic mean density and a rapid rise in the strength of the UV background during the final stages of reionization.

astro-ph.GA↗

arXiv2025

Defect Formation in NaI Crystals: A Novel Pathway to Dark Matter Detection

Sodium iodide (NaI) is a widely used scintillator in direct dark matter searches. In particular, NaI-based cryogenic scintillating calorimeters have emerged as promising candidates, like in the COSINUS experiment, for testing the annually modulating signal reported by DAMA/LIBRA. In this study, we investigate defect formation within NaI crystals and its impact on the dark matter detection signal. Using molecular dynamics simulations and density functional theory techniques, we simulate a DM particle collision on an NaI crystal, focusing on the possible defects formation and their structural and electronic properties. Our analysis includes a detailed study of the electronic states associated with the interstitial atoms and vacancies, the energetic cost of defect formation, and the anisotropic threshold displacement energy. Finally, we highlight the potential to exploit dark matter-induced defects as a novel detection channel, enabled by the introduction of new states within the electronic band gap.

hep-ph↗

arXiv2025

Quenching factors for Na recoils as a function of Tl dopant concentrations in NaI(Tl) crystals

Thallium-doped sodium iodide (NaI(Tl)) scintillation detectors play an important role in the field of direct dark matter (DM) searches. The DAMA/LIBRA experiment stands out for its reported observation of an annually modulating DM-like signal, which is in direct contrast with other results. To accurately calibrate the energies of nuclear recoil signals with electron recoils, precise measurements of the quenching factor of the NaI(Tl) crystals are essential, as the two processes have different scintillation light yield. In this article, we present results of a systematic study carried out by the COSINUS collaboration and Duke University to measure the quenching factor of sodium (Na) recoils as a function of nuclear recoil energy and for differing Thallium (Tl) dopant concentrations in the bulk crystal. Five ultrapure NaI(Tl) crystals, manufactured by the Shanghai Institute for Ceramics, were irradiated with a quasi-monoenergetic neutron beam at the Triangle Universities Nuclear Laboratory, North Carolina, USA. The quenching factor for low nuclear recoil energies of 5-26keV$_{nr}$ was extracted for all 5 crystals. A Tl-dependence could be deduced with a proportional response calibration schema using a $^{241}$Am source. However, this effect was not observed when using a low-energy calibration line from $^{133}$Ba.

physics.ins-det↗

arXiv2025

Daily Modulation Constraints on Light Dark Matter with DAMIC-M

The flux of Hidden Sector particles from the Galactic halo reaching an underground detector can be significantly attenuated by interactions within the Earth for sufficiently large scattering crosssections. This attenuation gives rise to a characteristic daily modulation in the detection rate, due to Earth's rotation. We present results from a search for such a modulation using a 1.257 kg-day dataset collected with the DAMIC-M Low Background Chamber. A model-independent analysis reveals no significant modulation in the 1e- event rate over periods from 1 to 48 h, highlighting the excellent temporal stability of the detector. In a complementary model-dependent analysis, we target the expected daily modulation signature of Hidden Sector particles, with masses in the range [0.53,2] MeV/c2, interacting with electrons via a dark photon mediator. By leveraging the expected temporal evolution of the signal, we set improved constraints on Dark Matter masses below 1.2 MeV/c2, surpassing our previous limits.

hep-ex↗

arXiv2025

Near-continuous tracking of solar active region NOAA 13664 over three solar rotations

Magnetic flux emergence and decay in the Sun span from days to months. However, their tracking is typically limited to about half a solar rotation when relying on single-vantage-point observations. Combining observations from both the Earth-facing and far side of the Sun, we monitored the magnetic and coronal evolution and characterised the non-potentiality of one of the most complex and eruptive regions of the past two decades, over more than three full solar rotations. We used photospheric magnetograms and EUV filtergrams from the Solar Orbiter and the Solar Dynamics Observatory along with flare detections from the GOES and the STIX instrument on board the Solar Orbiter. All images were deprojected into a common coordinate system and merged into a unified dataset. We tracked the evolution of magnetic flux and EUV emission and computed magnetic field parameters from the line-of-sight magnetograms to quantify the region's non-potentiality. We identified the region's initial emergence and followed its evolution through to its decay. The region developed through successive flux emergence episodes over a period of 20 days, reached its peak complexity one month after the first emergence, and gradually decayed over the subsequent two months. Unlike many complex regions, it consistently maintained high levels of non-potentiality for most of its lifetime, sustaining equally strong flaring activity. The derived time series of non-potentiality parameters, the first of their kind, far exceeded the typical 14-day window imposed by solar rotation and were remarkably consistent, exhibiting strong correlation with the flaring activity of the region. Multi-vantage-point observations can significantly improve our understanding of how magnetic flux emerges, evolves, and drives solar activity, beyond the two-week limit imposed by solar rotation on observations along the Sun-Earth line.

astro-ph.SR↗

arXiv2025

First Production of Skipper-CCD Modules for the DAMIC-M Experiment

The DAMIC-M experiment will search for sub-GeV dark matter particles with a large array of silicon skipper charge-coupled devices (CCDs) at the Modane Underground Laboratory (LSM) in France. After five years of development, we recently completed the production of 28 CCD modules at the University of Washington, each consisting of four 9-megapixel skipper CCDs. Material screening and background controls were implemented to meet stringent radio-purity targets, while extensive testing was employed to select science-grade CCDs for the modules and confirm their excellent performance after fabrication. Further testing at LSM will select 26 of these modules (${\sim}$350 g active mass) to be installed and operated in the DAMIC-M detector in early 2026.

physics.ins-det↗

arXiv2025

A New Method of Deriving Doppler Velocities for Solar Orbiter SPICE

This paper presents a follow-up to previous work on correcting PSF-induced Doppler artifacts in observations by the SPICE spectrograph on Solar Orbiter. In a previous paper, we demonstrated correction of these artifacts in the $y-\lambda$ plane with PSF Regularization, treating the forward problem with a method based on large sparse matrix inversion. It has since been found that similar apparent artifacts are also present in the $x-\lambda$ direction, i.e., across adjacent slit positions. This is difficult (although not impossible) to correct with the previous matrix inversion method due to the time variation between slit positions. We have therefore devised a new method which addresses both $x-\lambda$ and $y-\lambda$ artifacts simultaneously by applying wavelength dependent shifts at each $x-y$ plane of the spectral cube. This paper demonstrates the SPICE data issue, describes the new method, and shows a comparison with the previous one. We explore the time variation of the correction parameters for the SPICE data and show a clear orbit dependence. The results of the method are significantly higher quality derived Doppler signals, which we estimate at less than $\sim$ 5 km/s uncertainty for brighter lines in the absence of other systematics. Furthermore, we show the new SPICE polar observation results as a demonstration. The correction codes are written in Python, publicly available on GitHub, and can be directly applied to SPICE level 2 datasets.

astro-ph.SR↗

arXiv2025

Imaging-spectroscopy diagnosis of the giant sloshing spiral in the Virgo cluster with the Einstein Probe Follow-up X-ray Telescope

We performed deep X-ray observations of the Virgo cluster using the Einstein Probe Follow-up X-ray Telescope (EP-FXT) with a total exposure of 295 ks. Leveraging the large field of view (FoV) and low particle background of EP-FXT, the image reveals a giant spiral feature connecting the cold fronts in the northwest and southeast, forming a coherent structure consistent with earlier results from XMM-Newton and Suzaku. We also present two-dimensional maps of temperature, metallicity, and entropy across the Virgo Cluster, covering a FoV of approximately 28.5 arcmin. These maps clearly show a spiral structure with high density, low temperature, high metallicity, and low entropy. The results support a scenario where the spiral morphology arises from gas sloshing driven by a minor merger. Additionally, EP-FXT temperature measurements agree well with XMM-Newton data within uncertainties.

astro-ph.HE↗

arXiv2025

COSINUS -- a model-independent challenge of the DAMA/LIBRA dark matter claim with cryogenic NaI detectors operated in a new low-background facility

Low-temperature detectors are a powerful technology for dark matter search, offering excellent energy resolution and low energy thresholds. COSINUS is the only experiment that combines scintillating sodium iodide (NaI) crystals with an additional phonon readout at cryogenic temperatures, using superconducting sensors (remoTES), alongside the conventional scintillation light signal. Via the simultaneous phonon and scintillation light detection, a unique event-by-event particle identification is enabled. This dual-channel approach allows for a model-independent cross-check of the long-standing DAMA/LIBRA signal with a moderate exposure of a few hundred kg d, while completely avoiding key systematic uncertainties inherent to scintillation-only NaI-based searches. COSINUS built and commissioned a dedicated low-background cryogenic facility at the LNGS underground laboratories. Data taking with eight NaI detector modules (COSINUS1$\pi$ Run1) is planned to begin in late 2025.

physics.ins-det↗

arXiv2025

COSINUS model-independent sensitivity to the DAMA/LIBRA dark matter signal

COSINUS is a dark matter direct detection experiment using NaI crystals as cryogenic scintillating calorimeters. If no signal is observed, this will constrain the dark matter scattering rate in sodium iodide. We investigate how this constraint can be used to infer that the annual modulation signal observed in the DAMA/LIBRA experiment cannot originate from dark matter nuclear recoil events, independently of the dark matter model. We achieve this by unfolding the DAMA modulation spectrum to obtain the implied unquenched nuclear recoil spectrum, which we then compare to the expected COSINUS sensitivity. We find that assuming zero background in the signal region, a 1$\sigma$, 2$\sigma$ or 3$\sigma$ confidence limit exclusion can be obtained with 57, 130 or 250 kg day of exposure, respectively. A simple background model indicates that in the presence of background, the exposure requirements may increase by $\sim30\%$.

hep-ph↗

arXiv2025

Antiferromagnetic Chiral Bobber Formation and Topological Proximity Effect in MnBi2Te4

With topological materials being billed as the key to a new generation of nanoelectronics via either functional real-space topological structures (domain walls, skyrmions etc.) or via momentum-space topology (topological insulators), tailored and controllable topological properties are of paramount significance, since they lead to topologically protected states with negligible dissipation, enabling stable and non-volatile information processing. Here, we report on the evolution of topological magnetic textures in the proximity of other topological defects, i.e., antiferromagnetic domain walls in the topological insulator MnBi2Te4. The transition from the antiferromagnetic ground state to a canted antiferromagnetic state at finite magnetic fields is accompanied by the formation of chiral bobbers - bulk-terminated topological defects adjacent to the domain walls in this system, leading to a topological proximity effect.

cond-mat.mes-hall↗

arXiv2025

Probing Benchmark Models of Hidden-Sector Dark Matter with DAMIC-M

We report on a search for sub-GeV dark matter (DM) particles interacting with electrons using the DAMIC-M prototype detector at the Modane Underground Laboratory. The data feature a significantly lower detector single $e^-$ rate (factor 50) compared to our previous search, while also accumulating a ten times larger exposure of $\sim$1.3 kg-day. DM interactions in the skipper charge-coupled devices (CCDs) are searched for as patterns of two or three consecutive pixels with a total charge between 2 and 4 $e^-$. We find 144 candidates of 2 $e^-$ and 1 candidate of 4 $e^-$, where 141.5 and 0.071, respectively, are expected from background. With no evidence of a DM signal, we place stringent constraints on DM particles with masses between 1 and 1000 MeV/$c^2$ interacting with electrons through an ultra-light or heavy mediator. For large ranges of DM masses below 1 GeV/c$^2$, we exclude theoretically-motivated benchmark scenarios where hidden-sector particles are produced as a major component of DM in the Universe through the freeze-in or freeze-out mechanisms.

hep-ex↗

arXiv2024

Not Just a Dot: the complex UV morphology and underlying properties of Little Red Dots

We analyze 99 photometrically selected Little Red Dots (LRDs) at $z \approx 4-8$ in the GOODS fields, leveraging ultra-deep JADES NIRCam short-wavelength (SW) data. We examine the morphology of 30 LRDs, while the remaining 69 appear predominantly compact, with sizes $\leq 400$ pc and no extended components even in stacked SW images. However, their unresolved nature may partly reflect current depth limitations, which could prevent the detection of faint diffuse components. Among the 30 morphologically analyzed LRDs, 50% show multiple associated components, while the rest exhibit highly asymmetric structures, despite appearing as single sources. This diversity in rest-frame UV morphologies may point to interactions or strong internal feedback. We find median stellar masses of $\log_{10}(M_{\star}/M_{\odot}) = 9.07_{-0.08}^{+0.11}$ for pure stellar models with $A_{V} \approx 1.16^{+0.11}{-0.21}$ mag, and $\log{10}(M_{\star}/M_{\odot}) = 9.67^{+0.17}{-0.27}$ for models including AGNs with $A{V} \approx 2.74^{+0.55}_{-0.71}$ mag, in line with recent studies suggesting higher masses and dust content for AGN-fitted LRDs. NIRSpec spectra are available for 15 sources, six of which are also in the morphological sample. Broad H$\alpha$ is detected in 40% (FWHM = 1200-2900 km/s), and one source shows broad H$\beta$ emission. Emission line ratios indicate a composite nature, consistent with both AGN and stellar processes. Altogether, these results suggest that LRDs are a mixed population, and their rest-frame UV morphology reflects this complexity. Morphological studies of larger samples could provide a new way to understand what drives their properties and evolution.

astro-ph.GA↗