arXiv subjects

S. Das

Publications and source records attributed to S. Das.

At least 19 recordsLinked to original sources

arXiv2026

Calibration and Performance of Germanium High Voltage Detectors for SuperCDMS SNOLAB

As SuperCDMS SNOLAB is getting ready to search for low mass dark matter particles, using cryogenic Ge and Si detectors, a set of six of the new SuperCDMS High Voltage (HV) detectors (four Ge and two Si) were tested in the Cryogenic Underground TEst facility (CUTE) at SNOLAB. This provided the first opportunity to gain experience with this new detector type and assess their performance thoroughly under low background conditions. Here we describe the SuperCDMS HV detector concept and discuss some of the newly developed analysis methods and approaches. Focusing on the Ge detectors, we investigate the detector performance under voltage bias (up to 90 V), exercise the low energy (keV to sub-keV range) calibration based on the electron capture peaks generated by the decay of $^{71}$Ge, assess the detector resolution, and demonstrate the unexpected (and encouraging) ability of these detectors to also measure high energy interactions in the hundreds of keV range with good resolution (better than 3% at 356 keV).

physics.ins-det↗

arXiv2026

Fundamental differences in the X-ray accretion properties of low and high-excitation radio galaxies

We characterise the accretion rate properties of radio-detected AGN by combining deep radio and X-ray observations of the Boötes field. We used deep international LOFAR telescope observations to identify 2840 radio-AGN across $0.3 < z \leq 2$, divided into samples that are complete in radio luminosity. We further split this sample into four different classes: radio-quiet AGN (RQ-AGN), high-excitation radio galaxies (HERGs), and low-excitation radio galaxies (LERGs) hosted by star-forming (SF-LERGs) and quiescent galaxies (Q-LERGs). Through performing X-ray stacking, we determined the average X-ray luminosities, $L_{\rm{X, 2-10\,keV}}$ and average specific X-ray luminosities, $λ_{sL_{X}}$ (X-ray luminosity scaled by the stellar mass; a proxy for the accretion rate). We studied how these X-ray properties depend on radio luminosity, stellar mass, and redshift for each of the four AGN classes. We found that the LERGs, regardless of their star-formation activity, show significantly lower $L_{\rm{X, 2-10\,keV}}$ and $λ_{sL_{X}}$ than both HERGs and RQ-AGN across all redshifts. The average X-ray luminosities for the HERGs, RQ-AGN, and SF-LERGs typically increase with redshift, which may be associated with the increased cold gas fractions at earlier times, resulting in more enhanced black hole accretion. The average X-ray luminosities show weak-to-no correlation with radio luminosity at a given redshift, suggesting that the physical processes producing the X-ray and radio emission may not be coupled on spatial and/or temporal scales.

astro-ph.GA↗

arXiv2026

Levitation of a YIG sphere using a magnetic Paul trap - towards strongly coupled quantum magno-mechanics

Magnetic levitation offer passive levitation of massive objects for use in advanced inertia sensors, for the generation of non-classical macroscopic motional states, and towards the table-top testing of low energy gravity with quantum mechanics. Magnons, a quanta of spin wave, couple to many physical quantities and strongly to electromagnetic fields, even at room temperature. In this work we demonstrate the stable trapping of a small YIG sphere using a magnetic Paul trap. We present a classical stability analysis of a magnetic Paul trap and show the stability diagram for all mechanical degrees of freedom. We show that coupling between librational and translational modes changes the stability region. We experimentally levitate the soft magnet yttrium iron garnet at room temperature obtaining Q-factors of $\sim25$ and secular frequencies $15.8$ Hz and $17.2$ Hz. We provide numerical estimates of the achievable enhanced coupling between the center-of-mass motion and excited magnon modes, with a cooperativity above unity despite strong mechanical damping, indicating potential applications in quantum information processing, quantum interconnects and quantum memories.

quant-ph↗

arXiv2026

Search for the $\boldsymbol{B^0 \to K^0_{\rm S} τ^+ τ^-}$ decay

We present the first search for $B^0 \to K^0_{\rm S} τ^+τ^-$ decays. We look for signal decays in $B^0\bar B^0$ events produced in asymmetric-energy electron-positron collisions. This work uses samples from the Belle and Belle~II detectors, comprising 1.16 billion $Υ(4S)$ events. In $Υ(4S)\to B^0\bar{B}^0$ decays, the non-signal $\bar{B}^0$ meson is fully reconstructed in a hadronic channel. For the signal $B^0$ meson, $τ$-lepton decays into final states with a single charged particle are selected. A multivariate classifier is used to combine several discriminating inputs into a single fit observable. We observe no evidence for the signal and set an upper limit on the branching fraction $\mathcal{B}(B^0\to K^0_{\rm S} τ^+τ^-) < 8.3 \times 10^{-4}$ at the 90\% confidence level. Combining this with the recent measurement of the isospin-partner decay $B^+\to K^+τ^+τ^-$, we determine an upper limit $\mathcal{B}(B\to Kτ^+τ^-) < 5.4\times10^{-4}$ at the 90\% confidence level.

hep-ex↗

arXiv2026

The DESI View of the Faint Radio Source Population in LoTSS DR2

The faint radio-source population includes galaxies dominated by both star formation (SF) and active galactic nuclei (AGN), which are two key processes shaping galaxy evolution. To investigate this population, we probabilistically classified 251,413 radio sources from the second data release of the LOFAR Two-metre Sky Survey (LoTSS DR2) using spectroscopic data from the first release of the Dark Energy Spectroscopic Instrument (DESI DR1). Our classification method includes: (i) the identification of radio excess relative to SF, (ii) the Baldwin, Philips & Terlevich (BPT) diagram, (iii) a modified Mass Excitation (MEx) diagram, and (iv) the [OIII]5007 equivalent width. These are combined with Monte Carlo methods to estimate the probability that each source is a star-forming galaxy (SFG), a radio-quiet AGN (RQ AGN), or a low- or high-excitation radio galaxy (LERG or HERG), allowing various thresholds to be applied depending on science goals. Considering classifications above a 90 per cent probability threshold, we identify 68,820 SFGs, 32,288 RQ AGN, 35,210 LERGs and 3,085 HERGs, representing the largest radio sample to date with high-confidence spectroscopic classifications. Using this sample, we show with higher statistical power than previous studies that LERGs typically accrete below 1 per cent of the Eddington limit and HERGs above it. We also identify a small subset of high-accreting LERGs whose stacked spectra reveal prominent star-forming features, highlighting difficulties in interpreting their accretion properties. Our results demonstrate the power of large spectroscopic samples to characterise the radio-source population, providing a foundation for studies in the SKA era.

astro-ph.GA↗

arXiv2026

Evidence of the Excited X(5)-like Critical-Point Symmetry Structures in 152Sm

The positive-parity structure of 152Sm has been investigated through high-statistics γ-ray spectroscopy following the (150Nd(α,2n)152Sm reaction at Elab = 26 MeV. Several collective structures built on excited 0+ states have been extended through the observation of new levels and γ-ray transitions, and spin-parity assignments have been established using directional-correlation and linear-polarization measurements. Electromagnetic transition strengths (B(E2)), deduced from measured branching ratios and known level lifetimes, reveal pronounced collectivity among the excited configurations. The resulting level scheme provides evidence for a sequence of excited collective bands extending beyond the well-known ground-state and first excited 0+ structures. The excitation energies and transition strengths are examined within the framework of the X(5) critical-point description of the first-order U(5)-SU(3) shape-phase transition. In addition to the established X(5)-like features of the low-lying spectrum, the observed systematics of the higher-lying bands are found to be consistent with excited collective structures exhibiting X(5)-like characteristics. The results provide new constraints on the realization of critical-point behavior in finite nuclei and on the evolution of collectivity in the N=90 region.

nucl-ex↗

arXiv2026

Low lying excitations in $^{150}$Pm

The low lying excitations in odd-odd $^{150}$Pm have been studied through proton induced reaction with an array of five Compton suppressed Clover HPGe and one segmented planar Ge detectors. The relative excitation functions for the observed $γ$ rays have been studied using singles data at two beam energies of 8~MeV and 9~MeV. 16 new $γ$ rays and 15 new levels have been placed in the level scheme of $^{150}$Pm based on $γ-γ$ coincidence data. The relative intensities for the observed $γ$ rays have been determined using total and gated projections. Tentative spin-parity assignments were made to few low lying excitations of $^{150}$Pm, using limited angular distribution data and other information. Lifetimes were estimated for two excited levels in this nucleus using using generalized centroid difference analysis, applied in the nanosecond range, with Ge detectors. Large basis shell model and projected shell model calculation were performed to interpret the experimentally observed levels. The present work indicates 1$^-$ ground state, a 2$^-$ state close to the ground state ($\sim$50~keV) and a low lying 6$^-$ isomeric state in this odd-odd nucleus along with emerging band structures developed with two quasiparticle configurations.

nucl-ex↗

arXiv2026

Single Particle Excitations, Band Structures and Octupole Correlation in $^{65}$Zn

The excitation scheme of the $^{65}$Zn ($Z = 30, N = 35$) nucleus has been probed following its population in the $^{63}$Cu($α$,pn) reaction at E$_{beam}$ = 30 MeV and using an array of Compton suppressed HPGe clovers as the detection system. This work has identified several new transitions of the nucleus and have modified the placements of some of the previously known ones. The multipolarities and the electric/ magnetic nature of the observed $γ$-ray rays have been measured, using the conventional methodologies. The spin-parity assignments for the levels have consequently been made; some of the spin-parities are new while others are either validation of the existing values or are modified results based on the present analysis. The experimental level scheme exhibits collective as well as single particle structures. The measured level energies have been compared with those calculated in the framework of the large basis shell model using a model space of $p_{3/2}, f_{5/2}, p_{1/2}, g_{9/2}$ orbitals and two different interactions. The collective excitations of the nucleus were probed through the properties of its band structures and through the calculations of the Total Routhian Surface (TRS) for the associated deformations/ shapes. The results of this study brings out the essential features of evolving structural characteristics and developing collectivity with increasing number of nucleons outside a doubly-magic core and with their occupancy of deformation driving high-$j$ orbitals.

nucl-ex↗

arXiv2026

Measurement of mixing-induced CP violation in the decay $B^0 \to π^0 π^0$

Measurements of charge-parity (CP) violation in quark transitions provide stringent tests of the current theory and sensitive probes of particles or interactions beyond it. We report the first measurement of mixing-induced CP violation in $B^0\toπ^0π^0$ decays. The measurement uses 190 million $Υ(4S)\to B^{0}\overline{B}{}^0$ events collected with the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. A new approach that exploits the quantum entanglement of the $B^{0}\overline{B}{}^0$ pair enables a decay-time-dependent analysis without reconstructing the $B^0 \to π^0π^0$ decay position. We measure the mixing-induced CP-violating coefficient to be $S_{π^{0}π^{0}} = 0.61^{+0.75}_{-0.79}\,(\mathrm{stat}) \pm 0.11\,(\mathrm{syst})$, attaining a precision that would require a data set twenty times as large with a conventional analysis. We also measure the direct CP-violating coefficient to be $C_{π^{0}π^{0}} = 0.05 \pm 0.28\,(\mathrm{stat}) \pm 0.07\,(\mathrm{syst})$. These results substantially improve the constraints from $B \to ππ$ decays on the CP-violating phase $ϕ_2$ of the quark-mixing matrix using far less data than had previously been assumed necessary.

hep-ex↗

arXiv2026

Measurement of time-dependent $CP$ violation parameters in $B^{0} \to K_{S}^{0} π^{0} γ$ decays at Belle and Belle II

We perform a measurement of time-dependent $CP$ violation parameters in $B^{0} \to K_{S}^{0} π^{0} γ$ decays using a dataset of approximately $772 \times 10^6$ and $521 \times 10^6$ $Υ(4S)$ decays collected by the Belle and Belle II experiments, respectively. The measured parameters for the combined dataset in the $K^{*0}(892)$ dominated region ($M_{K_{S}^{0} π^{0}} \in [0.8,1.0] \mathrm{GeV}/c^2$) are $S = 0.09 \pm 0.16 \pm 0.02$ and $C = -0.09 \pm 0.08 \pm 0.04$. For the non-$K^{*0}(892)$ region ($M_{K_{S}^{0} π^{0}} \in [1.0,1.8] \mathrm{GeV}/c^2$), the corresponding values are $S = -0.32 \pm 0.33 \pm 0.09$ and $C = -0.07 \pm 0.17 \pm 0.08$. The first quoted uncertainties are statistical, while the second ones are systematic. These results are consistent with Standard Model predictions and more precise than previous measurements.

hep-ex↗

arXiv2026

Demonstrating CBM Capabilities by $Λ$ Baryon Reconstruction in Ni+Ni Collisions with the mCBM Experiment at SIS18 of GSI/FAIR

The Compressed Baryonic Matter (CBM) experiment at the upcoming Facility for Antiproton and Ion Research (FAIR) is a high-rate fixed-target experiment designed to investigate nuclear matter at extreme baryon densities in relativistic nucleus-nucleus collisions. To enable high-statistics measurements of rare probes, CBM is designed to operate at event rates up to 10 MHz. This necessitates the development of fast and radiation-tolerant detectors, self-triggered front-end electronics, a free-streaming data acquisition architecture, and real-time event reconstruction capabilities. Prototype versions and pre-series productions of the CBM detector systems have been deployed in the mini-CBM demonstrator setup mCBM - an experimental precursor comprising sub-components of all major CBM systems, installed at the SIS18 facility of GSI/FAIR within the FAIR Phase-0 program. In 2024, Ni+Ni collisions at a kinetic beam energy of 1.93 AGeV and an average interaction rate of about 250 kHz were successfully recorded. This dataset enables a detailed evaluation of the operational performance of the detector systems as well as the complete CBM data chain, while the reconstruction of rare $Λ$ baryons serves as a natural benchmark. This paper presents the first results on $Λ$ signal reconstruction with the mCBM experiment, demonstrating the readiness of the detector technologies and the data chain for the upcoming full-scale CBM experiment.

physics.ins-det↗

arXiv2026

The MUSE Target Chamber Post Veto

The Muon Scattering Experiment (MUSE) was developed to address the proton radius puzzle through simultaneous electron-proton and muon-proton scattering using the Paul Scherrer Institute's PiM1 secondary beamline. MUSE uses a large-solid-angle, non-magnetic spectrometer to detect beam particles scattering from a liquid hydrogen cell contained within a vacuum chamber. Due to the large scattering windows, the structural integrity of the chamber is supported by posts located at small scattering angles. While out of the acceptance, particles in the tails of the beam distribution can strike these posts, causing a significant trigger background. We describe the design and performance of the Target Chamber Post Veto (TCPV) detector installed inside the vacuum chamber to remove these background events at the trigger level.

physics.ins-det↗

arXiv2026

Charging-up and reverse charging-up phenomena in a double-mask triple GEM detector

The Gas Electron Multiplier (GEM) detectors are widely used in high-energy physics (HEP) experiments as tracking devices because of their excellent position resolution and to handle high particle rates capability. Charging-up effect is a well known phenomenon in GEM detectors because of the presence of the dielectric medium -- Kapton in the foil. Charging-up of GEM foil takes place when it is exposed to high radiation after application of high voltage. A new phenomenon of reverse charging-up, a complementary behaviour is also observed when the irradiation rate is reduced, where the gain relaxes gradually towards its initial value. In this study, the charging-up and reverse charging-up effects are investigated for a double-mask triple GEM chamber operated with an Argon and Carbon dioxide (70/30) gas mixture. The measurements provide a detailed understanding of the gain variation under irradiation and its stabilisation behaviour. The experimental setup, methodology and results are presented in this article.

physics.ins-det↗

arXiv2026

Long-term stability study of single-mask triple GEM detector: impact of continuous irradiation

A study has been carried out to evaluate the performance stability of Gas Electron Multiplier (GEM) chamber prototypes in the laboratory using $^{55}$Fe radiation source with Argon and CO$_2$ gas mixture. This research focuses on the characterisation of the GEM detector's gain, efficiency (count rate with radioactive source), and energy resolution under varying operational conditions. A patch on the detector has been subjected to continuous and absolutely uninterrupted radiation for about 98 days. The gain and energy resolution of the detector are measured along with the ambient parameters temperature (t), pressure (p) and relative humidity (RH). In addition to that, the long-term behaviour of the count rate with a strong radioactive source are also studied. This work is very relevant for Micro Pattern Gaseous Detectors (MPGD) such as GEM before installing on large experiment. The experimental setup, methodology, and results are presented in this article.

physics.ins-det↗

arXiv2026

Solution of variable order fractional differential equations using Homotopy Analysis Method

In the present article an endeavor is made to solve the variable order fractional diffusion equations using a powerful method viz., Homotopy Analysis method. It is demonstrated how the method can be used while solving approximately two types of variable order fractional diffusion equations having physical importance. Numerical simulation results show that the method is reliable and effective for solving fractional order diffusion equations even when the order of the derivative is varying with respect to space or time or both or it is dependent upon some other parameters.

math.GM↗

arXiv2026

Measurement of the $CP$ asymmetry in $D^0\toπ^+π^-π^0$ decays at Belle II

We measure the time- and phase-space-integrated $CP$ asymmetry $A_{CP}$ in $D^0\toπ^+π^-π^0$ decays reconstructed in $e^+e^-\to c\bar c$ events collected by the Belle II experiment from 2019 to 2022. This sample corresponds to an integrated luminosity of 428 fb$^{-1}$. We require $D^0$ mesons to be produced in $D^{*+}\to D^0π^+$ decays to determine their flavor at production. Control samples of $D^0\to K^-π^+$ decays are used to correct for reconstruction-induced asymmetries. The result, $A_{CP}(D^0\toπ^+π^-π^0)=(0.29\pm0.27\pm0.13)\%$, where the first uncertainty is statistical and the second systematic, is the most precise result to date and is consistent with $CP$ conservation.

hep-ex↗

arXiv2026

Search for low-mass electron-recoil dark matter using a single-charge sensitive SuperCDMS-HVeV Detector

We present constraints on low-mass dark matter electron scattering and absorption interactions using a SuperCDMS high-voltage eV-resolution (HVeV) detector. Data were taken underground in the NEXUS facility located at Fermilab with an overburden of 225 meters of water equivalent. The experiment benefits from the minimizing of luminescence from the printed circuit boards in the detector holder used in all previous HVeV studies. A blind analysis of $6.1\,\mathrm{g\cdot days}$ of exposure produces exclusion limits for dark matter-electron scattering cross sections for masses as low as $1\,\mathrm{MeV}/c^2$, as well as on the photon-dark photon mixing parameter and the coupling constant between axionlike particles and electrons for particles with masses $>1.2\,\mathrm{eV}/c^2$ probed via absorption processes.

hep-ex↗

arXiv2026

A generalized inner product-based wave scattering from an underwater source in a compressible ocean

Motivated by applications to underwater explosions and volcanic eruptions, this paper considers the evolution of an initial pressure disturbance in the ocean, including effects due to the dynamic and static compression of water and the free surface. In order to solve the equations of motion of a linear compressible ocean, a special inner product is introduced, which allows us to apply self-adjoint operator theory. What results is a Hilbert space in which the acoustic-gravity modes are orthogonal in the generalised sense. This allows the time-domain evolution of the free surface and subsurface pressure field resulting from an initial disturbance to be calculated. Our simulations show initial radial propagation of the pressure pulse and subsequent reflection from the water surface and the rigid ocean floor, eventually leading to horizontal propagation away from the source point. The solutions with and without the inclusion of the static compression are compared, and the effect of static compression is shown to be small but not negligible.

physics.flu-dyn↗