arXiv subjects

X. Lu

Publications and source records attributed to X. Lu.

At least 19 recordsLinked to original sources

arXiv2026

RF cavity design and beam loading effects in the rectilinear cooling channel for a muon collider

High-gradient normal-conducting RF cavities are crucial components of the rectilinear cooling channel in a muon collider, providing the accelerating field necessary to sustain the rapid six-dimensional ionization cooling essential to achieve high luminosity. To reach this goal, a muon collider requires approximately $10^{12}$ muons per bunch. At such high beam intensities, beam loading induced by bunch-driven wakefields can significantly reduce the cavity accelerating gradient and compromise beam stability. This paper focuses on the conceptual RF design and beam-induced effect analysis of copper cavities with so-called beam windows developed to meet the beam dynamics requirements of the rectilinear muon cooling channel. We first present the design and optimization of the cavities, evaluating their RF performance and power requirements. Thereafter, we analyze cavity wakefields and beam loading effects, proposing a new numerical method for their compensation, which takes the transverse distribution of the beam into account.

physics.acc-ph↗

arXiv2026

The Zero-Age Massive Stellar Population of W49A from VLA Observations

We use all-configuration VLA data at 3.3 cm with a physical resolution of ~2000 AU to infer the embedded zero-age massive stellar population of the W49A protocluster, as traced by its compact, ultracompact (UC), and hypercompact (HC) H II regions. Our method consists of visual source identification, the derivation of stellar ionizing-photon rates from the observed emission measure, and the further derivation of stellar masses and luminosities using state-of-the-art stellar calibrations. Considering the 101 robust detections, maximum-likelihood estimation fitting of the high-mass end (M > 10.9 M_\odot) of the sample yields power-law slopes Gamma >= 2.40 for the logarithmic representation of the stellar initial mass function (IMF): dN/d(log M) proportional to M^{-Gamma}. The result is robust considering different assumptions for the ionizing stellar systems. The slopes remain at Gamma >= 1.88 after correction for optical depth effects at 3.3 cm. Therefore, the inferred distribution of stellar masses presents a clear deficit in the high-mass end as compared to the standard stellar IMF (Gamma = 1.35). We propose that this is due to a shorter lifetime of the radio-detected H II regions produced by higher-mass stars, but evolutionary effects in the mass distribution of star formation within embedded protoclusters cannot be discarded.

astro-ph.GA↗

arXiv2026

Gamma Backgrounds for Experiments at the High Flux Isotope Reactor

This article describes the deployment of a germanium detector at Oak Ridge National Lab's High Flux Isotope Reactor (HFIR) for the purpose of understanding the energy and spatial distribution of the gamma field in the experiment hall where the Precision Reactor Oscillation and Spectrum Experiment (PROSPECT) took data and future neutrino experiments could be located. The sources from both the reactor and the neutron beamlines are described in detail, along with their temporal variations due to reactor power and their spatial variations due to the geometry of the beamlines and building materials in the vicinity. Additionally, a shielding study was performed to assess the amount that backgrounds in tens of keV range can be mitigated. This work helps inform backgrounds for future experiments at reactors such as IBD-based neutrino measurements and CEvNS measurements.

hep-ex↗

arXiv2026

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde VI. The photodissociation region M17SW

The kinetic temperature structure of the photodissociation region M17SW was mapped using the IRAM 30 m telescope. This mapping employed the para-H2CO triplet (J(KaKc) = 303-202, 322-221, and 321-220) near 218 GHz on a scale of ~0.2 pc. The kinetic temperatures were derived by modeling the average H2CO line ratios (322-221/303-202 + 321-220/303-202) with the RADEX non-local thermodynamic equilibrium approach. These temperatures range from 28 to 181 K with an average of 54.2 +/- 0.3 K at a spatial density of 5.5x10^5 cm^-3. Comparing with the temperature measurements obtained from multiple transitions of NH3 (1,1)-(6,6) and the far infrared (FIR) dust continuum, the H2CO lines show temperatures similar to those measured by NH3 but slightly higher than values derived from FIR observations. The high kinetic temperatures observed from H2CO are associated with the ultracompact H II region UC1, dense clumps, as well as H2O and CH3OH masers, showing a similar distribution as NH3. This indicates that dense gas in the M17SW region is heated by star formation activity. The presence of a significant gas temperature gradient across the M17SW region, as measured by H2CO and NH3, provides direct evidence for gas heated predominantly by radiation emitted from the OB star cluster NGC 6618. On a smaller scale, the dense gas surrounding the dense clumps experiences significant heating from internal protostars and/or young stellar objects. Higher temperatures traced by H2CO are linked to turbulence on a scale of ~0.2 pc. The complex temperature structure of the M17SW region is revealed by H2CO and NH3, which may be attributed to both large-scale external radiative heating and small-scale internal radiative and turbulent heating.

astro-ph.GA↗

arXiv2026

Production and installation of wavelength-shifting reflective light enhancers for the Short-Baseline Near Detector

We report on the design, production, and installation of a wavelength-shifting reflective system on the cathode of the Short-Baseline Near Detector (SBND), a liquid argon time projection chamber located along the Fermilab Booster Neutrino Beam. To increase and homogenize scintillation-light collection, 64 double-sided plates were fabricated from FR4, laminated with specular reflector film and coated with 300 $\mu$g/cm$^2$ of tetraphenyl butadiene (TPB) wavelength shifter using controlled physical vapor deposition. The coating uniformity was validated through dedicated measurements of deposited mass and profilometry studies. Because exposure to ambient blue/UV light could degrade the TPB, protective filtering and controlled storage conditions were implemented during handling and installation. The coated plates were assembled between conductive meshes for high-voltage compatibility and installed in situ during detector integration. This system constitutes the largest TPB-coated area deployed in a neutrino detector. It operates in conjunction with SBND's photon detection system, which consists of photomultiplier tubes and X-ARAPUCAs. Early light-collection measurements show high uniformity and light response across the detector, supporting improved triggering, calorimetry, and position reconstruction in SBND.

physics.ins-det↗

arXiv2026

Molecular Clouds at the Edge of the Galaxy II. Physical properties and scaling relations

The outer Galaxy presents an optimal setting for investigating molecular clouds and star formation in environments with low metallicity. A total of 72 Galactic edge clouds were surveyed using the CO\,(2--1) line with the IRAM\,30\,m telescope, leading to the identification of 112 CO clumps within molecular clouds with linear resolutions of 0.5--0.9\,pc. Parameters such as size, mass, surface density, and velocity dispersion of these CO clumps, derived from CO\,(2--1) observations, exhibit ranges of 0.6--3.4\,pc, 34--8250\,M$_\odot$, 12--1025\,M$_{\odot}$\,pc$^{-2}$, and 0.3--1.7\,km\,s$^{-1}$, respectively. Over the Galactocentric distance range of 14--23\,kpc, no systematic variations are found in these parameters. The velocity dispersion-size relationship of the Galactic edge clumps is modeled as $\sigma_{\rm v}$\,=\,0.69($\pm$0.03)$R_{\rm eff}^{0.36(\pm0.10)}$, indicating that turbulence is present within the Galactic edge clumps, akin to observations in the inner Galactic disk clouds. Furthermore, the luminous mass-size relation of the Galactic edge clumps is described by $M_{\rm lum}$\,=\,196($\pm$17)$R_{\rm eff}^{\,2.18\,(\pm0.26)}$, suggesting the average column density remains almost constant for clouds of different sizes. The virial parameters range from 0.6 to 15.3, with a median value of 2.8\,$\pm$\,0.6, suggesting that most clumps are gravitationally unbound. Furthermore, the virial parameters of our Galactic edge clumps show a decreasing trend with increasing Galactocentric distances, described by an exponential relation $\alpha_{\rm vir}$\,=\,33.0($\pm$\,10.4)\,e$^{-R_{\rm g}/6.7(\pm0.9)}$, consistent with previous results.

astro-ph.GA↗

arXiv2026

Measurement of coherent elastic neutrino nucleus scattering on germanium by COHERENT

The COHERENT collaboration reports the most precise measurement of the coherent elastic neutrino-nucleus scattering cross section to date. This measurement was performed with COHERENT's germanium detector array, Ge-Mini, at the Spallation Neutron Source at Oak Ridge National Laboratory. A cumulative exposure of $4.68\times10^{22}$ protons on target yielded a total number of observed counts of $124^{+14}_{-12}$ and a flux-averaged cross section of $1.00 \pm 0.10 \mathrm{(statistical)} \pm 0.10 \mathrm{(systematic)}$ relative to the standard-model expectation of $5.9\times10^{-39} \mathrm{cm}^2$. The well-understood energy and timing distributions of the neutrino source allow for independent measurements of muon- and electron-neutrino scattering rates. This information is used to improve constraints on non-standard neutrino interactions mediated by heavy particles.

hep-ex↗

arXiv2026

Fast Attention-Based Simplification of LiDAR Point Clouds for Object Detection and Classification

LiDAR point clouds are widely used in autonomous driving and consist of large numbers of 3D points captured at high frequency to represent surrounding objects such as vehicles, pedestrians, and traffic signs. While this dense data enables accurate perception, it also increases computational cost and power consumption, which can limit real-time deployment. Existing point cloud sampling methods typically face a trade-off: very fast approaches tend to reduce accuracy, while more accurate methods are computationally expensive. To address this limitation, we propose an efficient learned point cloud simplification method for LiDAR data. The method combines a feature embedding module with an attention-based sampling module to prioritize task-relevant regions and is trained end-to-end. We evaluate the method against farthest point sampling (FPS) and random sampling (RS) on 3D object detection on the KITTI dataset and on object classification across four datasets. The method was consistently faster than FPS and achieved similar, and in some settings better, accuracy, with the largest gains under aggressive downsampling. It was slower than RS, but it typically preserved accuracy more reliably at high sampling ratios.

cs.CV↗

arXiv2026

The COHERENT Experiment: 2026 Update

The COHERENT experiment measures neutrino-induced recoils from coherent elastic neutrino-nucleus scattering (CEvNS) with multiple nuclear targets at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL), USA. Several successful CEvNS measurements have been achieved in recent years with tens-of-kg detector masses, with a CsI scintillating crystal, a liquid argon single-phase detector, and high-purity germanium spectrometers. For the next phase, COHERENT aims at high-statistics detection of CEvNS events for precision tests of the standard model of particle physics, and to probe new physics beyond-the-standard model. Percent-level precision can be achieved by lowering thresholds, reducing backgrounds, and by scaling up the detector masses. It goes hand in hand with benchmarking the neutrino flux from the SNS. Further detectors will measure CEvNS in additional nuclei, including lighter target nuclei such as sodium and neon, to continue to test the expected neutron-number-squared dependence of the cross section. COHERENT can furthermore study charged-current and neutral-current inelastic neutrino-nucleus cross sections on various nuclei at neutrino energies below $\sim$50 MeV. Many of these cross sections have never been measured before, but are critical input for the interpretation of core-collapse supernova detection in large-scale neutrino experiments such as DUNE, Super-K, Hyper-K, and HALO.

hep-ex↗

arXiv2025

YOLO-CIANNA: Galaxy detection with deep learning in radio data: II. Winning the SKA SDC2 using a generalized 3D-YOLO network

As the scientific exploitation of the Square Kilometre Array (SKA) approaches, there is a need for new advanced data analysis and visualization tools capable of processing large high-dimensional datasets. In this study, we aim to generalize the YOLO-CIANNA deep learning source detection and characterization method for 3D hyperspectral HI emission cubes. We present the adaptations we made to the regression-based detection formalism and the construction of an end-to-end 3D convolutional neural network (CNN) backbone. We then describe a processing pipeline for applying the method to simulated 3D HI cubes from the SKA Observatory Science Data Challenge 2 (SDC2) dataset. The YOLO-CIANNA method was originally developed and used by the MINERVA team that won the official SDC2 competition. Despite the public release of the full SDC2 dataset, no published result has yet surpassed MINERVA's top score. In this paper, we present an updated version of our method that improves our challenge score by 9.5%. The resulting catalog exhibits a high detection purity of 92.3%, best-in-class characterization accuracy, and contains 45% more confirmed sources than concurrent classical detection tools. The method is also computationally efficient, processing the full ~1TB SDC2 data cube in 30 min on a single GPU. These state-of-the-art results highlight the effectiveness of 3D CNN-based detectors for processing large hyperspectral data cubes and represent a promising step toward applying YOLO-CIANNA to observational data from SKA and its precursors.

astro-ph.GA↗

arXiv2025

Updated baseline design for HALHF: the hybrid, asymmetric, linear Higgs factory

Particle physicists aim to construct a electron-positron Higgs factory as the next major particle collider. However, the high associated costs motivate the development of more affordable collider designs. Plasma-wakefield acceleration is a promising technology to this end. HALHF is a proposal for a Higgs factory that utilizes beam-driven plasma-wakefield acceleration to accelerate electrons to high energy with high gradient, while using radio-frequency acceleration to accelerate positrons to a lower energy. This asymmetry sidesteps a major difficulty in plasma acceleration: that of accelerating positrons with high efficiency and quality. Since publication, several challenges were identified in the original baseline design. We summarize the updated baseline design, which addresses these challenges, and describe the parameter- and cost-optimization process used to arrive at this design.

physics.acc-ph↗

arXiv2025

Neutrinos from Stored Muons (nuSTORM)

The Neutrinos from Stored Muons, nuSTORM, facility has been designed to deliver a definitive neutrino-nucleus scattering programme using beams of ,{\nu}e- and ,{\nu}{\mu}- from the decay of muons confined within a storage ring. The facility is unique, it will be capable of storing {\mu} beams with a central momentum of between 1 GeV/c and 6 GeV/c and a momentum spread of 16%. This specification will allow neutrino-scattering measurements to be made over the kinematic range of interest to the DUNE and Hyper-K collaborations. At nuSTORM, the flavour composition of the beam and the neutrino-energy spectrum are both precisely known. The storage-ring instrumentation will allow the neutrino flux to be determined to a precision of 1% or better. With its existing proton-beam infrastructure, CERN is uniquely well-placed to implement nuSTORM. A summary of the proposed implementation of nuSTORM at CERN is presented below.

hep-ex↗

arXiv2025

Molecular Clouds at the Edge of the Galaxy I. Variation of CO J=2-1/1-0 Line Ratio

The Galactic edge at Galactocentric distances of 14\,--\,22\,kpc provides an ideal laboratory to study molecular clouds in an environment that is different from the solar neighborhood, due to its lower gas density, lower metallicity, and little or no perturbation from the spiral arms. Observations of CO\,($J$\,=\,2--1) spectral lines were carried out towards 72 molecular clouds located at the Galactic edge using the IRAM\,30\,m telescope. Combined with CO\,($J$\,=\,1--0) data from the MWISP project, we investigate the variations of $R_{21}$ across these Galactic edge clouds, with $R_{21}$ representing CO(2-1)/CO(1-0) integrated intensity ratios. These are found to range from 0.3 to 3.0 with a mean of 1.0\,$\pm$\,0.1 in the Galactic edge clouds. The proportions of very low ratio gas (VLRG; $R_{21}$\,<\,0.4), low ratio gas (LRG; 0.4\,$\le$\,$R_{21}$\,<\,0.7), high ratio gas (HRG; 0.7\,$\le$\,$R_{21}$\,<\,1.0), and very high ratio gas (VHRG; $R_{21}$\,$\ge$\,1.0) are 6.9\%, 29.2\%, 26.4\%, and 37.5\%, respectively, indicating a significant presence of high $R_{21}$ ratio molecular gas within these regions. In our Galaxy, the $R_{21}$ ratio exhibits a gradient of initial radial decline followed by a high dispersion with increasing Galacticentric distance and a prevalence for high ratio gas. There is no apparent systematic variation within the Galactocentric distance range of 14 to 22\,kpc. A substantial proportion of HRG and VHRG is found to be associated with compact clouds and regions displaying star-forming activity, suggesting that the high $R_{21}$ ratios may stem from dense gas concentrations and recent episodes of star formation.

astro-ph.GA↗

arXiv2025

Machine Learning for Single-Ended Event Reconstruction in PROSPECT Experiment

The Precision Reactor Oscillation and Spectrum Experiment, PROSPECT, was a segmented antineutrino detector that successfully operated at the High Flux Isotope Reactor in Oak Ridge, TN, during its 2018 run. Despite challenges with photomultiplier tube base failures affecting some segments, innovative machine learning approaches were employed to perform position and energy reconstruction, and particle classification. This work highlights the effectiveness of convolutional neural networks and graph convolutional networks in enhancing data analysis. By leveraging these techniques, a 3.3\% increase in effective statistics was achieved compared to traditional methods, showcasing their potential to improve analysis performance. Furthermore, these machine learning methodologies offer promising applications for other segmented particle detectors, underscoring their versatility and impact.

physics.data-an↗

arXiv2024

Disruption of a massive molecular cloud by a supernova in the Galactic Centre: Initial results from the ACES project

The Milky Way's Central Molecular Zone (CMZ) differs dramatically from our local solar neighbourhood, both in the extreme interstellar medium conditions it exhibits (e.g. high gas, stellar, and feedback density) and in the strong dynamics at play (e.g. due to shear and gas influx along the bar). Consequently, it is likely that there are large-scale physical structures within the CMZ that cannot form elsewhere in the Milky Way. In this paper, we present new results from the Atacama Large Millimeter/submillimeter Array (ALMA) large programme ACES (ALMA CMZ Exploration Survey) and conduct a multi-wavelength and kinematic analysis to determine the origin of the M0.8$-$0.2 ring, a molecular cloud with a distinct ring-like morphology. We estimate the projected inner and outer radii of the M0.8$-$0.2 ring to be 79" and 154", respectively (3.1 pc and 6.1 pc at an assumed Galactic Centre distance of 8.2 kpc) and calculate a mean gas density $> 10^{4}$ cm$^{-3}$, a mass of $\sim$ $10^6$ M$_\odot$, and an expansion speed of $\sim$ 20 km s$^{-1}$, resulting in a high estimated kinetic energy ($> 10^{51}$ erg) and momentum ($> 10^7$ M$_\odot$ km s$^{-1}$). We discuss several possible causes for the existence and expansion of the structure, including stellar feedback and large-scale dynamics. We propose that the most likely cause of the M0.8$-$0.2 ring is a single high-energy hypernova explosion. To viably explain the observed morphology and kinematics, such an explosion would need to have taken place inside a dense, very massive molecular cloud, the remnants of which we now see as the M0.8$-$0.2 ring. In this case, the structure provides an extreme example of how supernovae can affect molecular clouds.

astro-ph.GA↗

arXiv2024

Excitation and spatial study of a prestellar cluster towards G+0.693-0.027 in the Galactic centre

Star formation in the central molecular zone (CMZ) is suppressed with respect to that of the Galactic disk, and this is likely related to its high turbulent environment. This turbulence impedes the potential detection of prestellar cores. We present the temperature, density, and spatial structure of the CMZ molecular cloud G+0.693-0.027, which has been proposed to host a prestellar cluster in the Sgr B2 region. We analysed multiple HC$_{3}$N rotational transitions that were observed with the IRAM 30m, APEX, Yebes 40m, and GBT radio telescopes, together with SMA+APEX spatially resolved maps. The spectral shape of HC$_{3}$N lines shows three velocity components: a broad component with a line width of 23 km s$^{-1}$ (C1), and two narrow components with line widths of 7.2 and 8.8 km s$^{-1}$ (C2 and C3). This suggests that a fraction of the molecular gas in this cloud is undergoing turbulence dissipation. From a non-local thermodynamic equilibrium analysis we have found H$_{2}$ densities of 2$\times$10$^{4}$ cm$^{-3}$, 5$\times$10$^{4}$ cm$^{-3}$, and 4$\times$10$^{5}$ cm$^{-3}$ and kinetic temperatures of 140 K, 30 K, and 80 K for C1, C2, and C3, respectively. The spatially resolved maps confirm that the colder and high-density condensations C2 and C3, which peak in the 70-85 km s$^{-1}$ velocity range, are embedded in a more diffuse and warmer gas (C1). The larger-scale structure of the Sgr B2 region shows a hole at 40-50 km s$^{-1}$ that is likely due to a small cloud that shocked the Sgr B2 region and is spatially related with a massive cloud at 60-80 km s$^{-1}$. We propose that the impacting small cloud sequentially triggered the formation of Sgr B2(M), (N), and (S) and the condensations in G+0.693-0.027 during its passage. Based on the analysis of the masses of the two condensations and on the virial parameters, C2 might expand, while C3 might further fragment or collapse.

astro-ph.GA↗

arXiv2024

ALMA-IMF XV: The core mass function in the high-mass star-formation regime

The stellar initial mass function (IMF) is critical to our understanding of star formation and the effects of young stars on their environment. On large scales, it enables us to use tracers such as UV or Halpha emission to estimate the star formation rate of a system and interpret unresolved star clusters across the universe. So far, there is little firm evidence of large-scale variations of the IMF, which is thus generally considered universal. Stars form from cores and it is now possible to estimate core masses and compare the core mass function (CMF) with the IMF, which it presumably produces. The goal of the ALMA-IMF large program is to measure the core mass function at high linear resolution (2700 au) in 15 typical Milky Way protoclusters spanning a mass range of 2500 to 32700 Msun. In this work, we used two different core extraction algorithms to extract about 680 gravitationally bound cores from these 15 protoclusters. We adopt per core temperature using the temperature estimate from the PPMAP Bayesian method. A power-law fit to the CMF of the sub-sample of cores above the 1.64 Msun completeness limit, 330 cores, through the maximum likelihood estimate technique yields a slope of 1.97 +/- 0.06, significantly flatter than the 2.35 Salpeter slope. Assuming a self-similar mapping between the CMF and the IMF, this result implies that these 15 high-mass protoclusters will generate atypical IMFs. This sample is the largest to date produced and analysed self-consistently, derived at matched physical resolution, with per-core temperature estimates and cores as massive as 150 Msun. We provide the raw source extraction catalogues and the source derived size, temperature, mass, and spectral indices in the 15 protoclusters.

astro-ph.GA↗

arXiv2024

First Order Preemptive Ising-nematic Transition in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$

Employing inelastic X-ray scattering and neutron scattering techniques, we observed nematic and magnetic phase transitions with distinct characters in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$. Upon cooling, the nematic order undergoes a strongly first-order phase transition followed by a second-order magnetic transition at $T_{\textrm{N}}$ $\approx$ 34.6 K. The temperature difference between these two phase transitions is $\sim$ 1 K. The observed phenomenon can be attributed to a distinctive first-order preemptive Ising-nematic transition, a characteristic unique to a quasi-two-dimensional scenario marked by strong out-of-plane spatial anisotropy due to weak coupling. Our studies establish K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$ as the first material in the family of iron pnictides and chalcogenides that possesses a nematic tricritical point preceding the magnetic one upon decreasing nematic coupling.

cond-mat.str-el↗