arXiv subjects

R. Saha

Publications and source records attributed to R. Saha.

12 recordsLinked to original sources

arXiv2024

Formal deformations and extensions of `twisted' Lie algebras

The interplay between derivations and algebraic structures has been a subject of significant interest and exploration. Inspired by Yau's twist and the Leibniz rule, we investigate the formal deformation of twisted Lie algebras by invertible derivations, herein referred to as "InvDer Lie". We define representations of InvDer Lie, elucidate cohomology structures of order 1 and 2, and identify infinitesimals as 2-cocycles. Furthermore, we explore central extensions of InvDer Lie, revealing their intricate relationship with cohomology theory.

math.RA↗

arXiv2023

Multi-Messenger Measurements of the Static Structure of Shock-Compressed Liquid Silicon at 100 GPa

Ionic structure of high pressure, high temperature fluids is a challenging theoretical problem with applications to planetary interiors and fusion capsules. Here we report a multi-messenger platform using velocimetry and \textit{in situ} angularly and spectrally resolved X-ray scattering to measure the thermodynamic conditions and ion structure factor of materials at extreme pressures. We document the pressure, density, and temperature of shocked silicon near 100 GPa with uncertainties of 6%, 2%, and 20%, respectively. The measurements are sufficient to distinguish between and rule out some ion screening models.

hep-ex↗

arXiv2020

Euclid preparation: X. The Euclid photometric-redshift challenge

Forthcoming large photometric surveys for cosmology require precise and accurate photometric redshift (photo-z) measurements for the success of their main science objectives. However, to date, no method has been able to produce photo-$z$s at the required accuracy using only the broad-band photometry that those surveys will provide. An assessment of the strengths and weaknesses of current methods is a crucial step in the eventual development of an approach to meet this challenge. We report on the performance of 13 photometric redshift code single value redshift estimates and redshift probability distributions (PDZs) on a common set of data, focusing particularly on the 0.2--2.6 redshift range that the Euclid mission will probe. We design a challenge using emulated Euclid data drawn from three photometric surveys of the COSMOS field. The data are divided into two samples: one calibration sample for which photometry and redshifts are provided to the participants; and the validation sample, containing only the photometry, to ensure a blinded test of the methods. Participants were invited to provide a redshift single value estimate and a PDZ for each source in the validation sample, along with a rejection flag that indicates sources they consider unfit for use in cosmological analyses. The performance of each method is assessed through a set of informative metrics, using cross-matched spectroscopic and highly-accurate photometric redshifts as the ground truth. We show that the rejection criteria set by participants are efficient in removing strong outliers, sources for which the photo-z deviates by more than 0.15(1+z) from the spectroscopic-redshift (spec-z). We also show that, while all methods are able to provide reliable single value estimates, several machine-learning methods do not manage to produce useful PDZs. [abridged]

astro-ph.GA↗

arXiv2017

Fine Structure Constant and Fractal Potential

The variation of fine structure constant with the variation of velocity of light has been studied considering contribution to the permeability and permittivity of the vacuum and incorporating contribution of fractal potential originated from the non-differentiability of fractal fluctuation. The time variation of $α$ is found to depend on the complex velocity field characteristics of fractal nature of space time.

physics.gen-ph↗

arXiv2013

Deformation in 28Si* produced via 16O + 12C reaction

The energy spectra of the $α$ particles emitted in the reactions $^{16}$O (7-10 MeV/nucleon) + $^{12}$C have been measured in the center of mass angular range of 25$^\circ$ $\lesssim θ_{c.m.} \lesssim$ 70$^\circ$. The experimental energy spectra have been compared with those obtained from the statistical model calculation with "deformability" parameters predicted by rotating liquid drop model (RLDM) and also fitted the same with optimized "deformability" parameters, which are quite different from the respective RLDM values. The data have also been found to be explained quite well using "frozen" deformation approximation, where the "deformability" parameters have been kept fixed at RLDM values of the parent nucleus throughout the decay process. The effective radius in the latter case is smaller compared to that obtained using the optimized parameters; however, in both cases, the deformations (effective radii) are larger than the corresponding RLDM values. So, considering the uncertainties in the estimation of actual compound nucleus deformations, it can, only qualitatively, be said that equilibrium orbiting, which is similar to particle evaporation in time scale, could also be one of the contributing factors for the observed deformation.

nucl-ex↗

arXiv2012

Coupled Phonons, Magnetic Excitations and Ferroelectricity in AlFeO3: Raman and First-principles Studies

We determine the nature of coupled phonons and magnetic excitations in AlFeO3 using inelastic light scattering from 5 K to 315 K covering a spectral range from 100-2200 cm-1 and complementary first-principles density functional theory-based calculations. A strong spin-phonon coupling and magnetic ordering induced phonon renormalization are evident in (a) anomalous temperature dependence of many modes with frequencies below 850 cm-1, particularly near the magnetic transition temperature Tc ~ 250 K, (b) distinct changes in band positions of high frequency Raman bands between 1100-1800 cm-1, in particular a broad mode near 1250 cm-1 appears only below Tc attributed to the two-magnon Raman scattering. We also observe weak anomalies in the mode frequencies at ~ 100 K, due to a magnetically driven ferroelectric phase transition. Understanding of these experimental observations has been possible on the basis of first-principles calculations of phonons spectrum and their coupling with spins.

cond-mat.str-el↗

arXiv2008

Light charged particle emission from hot $^{32}$S$^{*}$ formed in $^{20}$Ne + $^{12}$C reaction

Inclusive energy distributions for light charged particles ($p, d, t$ and $α$) have been measured in the $^{20}$Ne (158, 170, 180, 200 MeV) + $^{12}$C reactions in the angular range 10$^{o}$ -- 50$^{o}$. Exclusive light charged particle energy distribution measurements were also done for the same system at 158 MeV bombarding energy by in-plane light charged particle -- fragment coincidence. Pre-equilibrium components have been separated out from proton energy spectra using moving source model considering two sources. The data have been compared with the predictions of the statistical model code CASCADE. It has been observed that significant deformation effects were needed to be introduced in the compound nucleus in order to explain the shape of the evaporated $d, t$ energy spectra. For protons, evaporated energy spectra were rather insensitive to nuclear deformation, though angular distributions could not be explained without deformation. Decay sequence of the hot $^{32}$S nucleus has been investigated through exclusive light charged particle measurements using the $^{20}$Ne (158 MeV) + $^{12}$C reaction. Information on the sequential decay chain has been extracted through comparison of the experimental data with the predictions of the statistical model. It is observed from the present analysis that exclusive light charged particle data may be used as a powerful tool to probe the decay sequence of hot light compound systems.

nucl-ex↗

arXiv2007

Characterization of fragment emission in ^{20}Ne (7 - 10 MeV/nucleon) + ^{12}C reactions

The inclusive energy distributions of the complex fragments (3 $\leq$ Z $\leq$ 7) emitted from the bombardment of ^{12}C by ^{20}Ne beams with incident energies between 145 and 200 MeV have been measured in the angular range 10$^{o} \leq θ_{lab} \leq$ 50^{o}. Damped fragment yields in all the cases have been found to be the characteristic of emission from fully energy equilibrated composites. The binary fragment yields are compared with the standard statistical model predictions. Enhanced yields of entrance channel fragments (5 $\leq$ Z $\leq$ 7) indicate the survival of orbiting-like process in ^{20}Ne + ^{12}C system at these energies.

nucl-ex↗

arXiv2006

A re-analysis of the three-year WMAP temperature power spectrum and likelihood

We analyze the three-year WMAP temperature anisotropy data seeking to confirm the power spectrum and likelihoods published by the WMAP team. We apply five independent implementations of four algorithms to the power spectrum estimation and two implementations to the parameter estimation. Our single most important result is that we broadly confirm the WMAP power spectrum and analysis. Still, we do find two small but potentially important discrepancies: On large angular scales there is a small power excess in the WMAP spectrum (5-10% at l<~30) primarily due to likelihood approximation issues between 13 <= l <~30. On small angular scales there is a systematic difference between the V- and W-band spectra (few percent at l>~300). Recently, the latter discrepancy was explained by Huffenberger et al. (2006) in terms of over-subtraction of unresolved point sources. As far as the low-l bias is concerned, most parameters are affected by a few tenths of a sigma. The most important effect is seen in n_s. For the combination of WMAP, Acbar and BOOMERanG, the significance of n_s =/ 1 drops from ~2.7 sigma to ~2.3 sigma when correcting for this bias. We propose a few simple improvements to the low-l WMAP likelihood code, and introduce two important extensions to the Gibbs sampling method that allows for proper sampling of the low signal-to-noise regime. Finally, we make the products from the Gibbs sampling analysis publically available, thereby providing a fast and simple route to the exact likelihood without the need of expensive matrix inversions.

astro-ph↗

arXiv2006

Study of Dissipative Collisions of $^{20}$Ne ($\sim$7-11 MeV/nucleon) + $^{27}$Al

The inclusive energy distributions of complex fragments (3 $\leq$Z $\leq$ 9) emitted in the reactions $^{20}$Ne (145, 158, 200, 218 MeV) + $^{27}$Al have been measured in the angular range 10$^{o}$ - 50$^{o}$. The fusion-fission and the deep-inelastic components of the fragment yield have been extracted using multiple Gaussian functions from the experimental fragment energy spectra. The elemental yields of the fusion-fission component have been found to be fairly well exlained in the framework of standard statistical model. It is found that there is strong competition between the fusion-fission and the deep-inelastic processes at these energies. The time scale of the deep-inelastic process was estimated to be typically in the range of $\sim$ 10$^{-21}$ - 10$^{-22}$ sec., and it was found to decrease with increasing fragment mass. The angular momentum dissipations in fully energy damped deep-inelastic process have been estimated from the average energies of the deep-inelastic components of the fragment energy spectra. It has been found that, the estimated angular momentum dissipations, for lighter fragments in particular, are more than those predicted by the empirical sticking limit.

nucl-ex↗

arXiv2006

Evidence of large nuclear deformation of $^{32}$S$^{*}$ formed in $^{20}$Ne + $^{12}$C reaction

Deformations of hot composite $^{32}$S$^{*}$ formed in the reaction $^{20}$Ne ($\sim$ 7 -- 10 MeV/nucleon) + $^{12}$C have been estimated from the respective inclusive $α$-particle evaporation spectra. The estimated deformations for $^{32}$S$^{*}$ have been found to be much larger than the `normal' deformations of hot, rotating composites at similar excitations. This further confirms the formation of highly deformed long-lived configuration of $^{20}$Ne + $^{12}$C at high excitations ($\sim$ 70 -- 100 MeV) -- which was recently indicated from the analysis of the complex fragment emission data for the same system. Exclusive $α$-particle evaporation spectra from the decay of hot composite $^{32}$S$^{*}$ also show similar behaviour.

nucl-ex↗

arXiv2005

Survival of orbiting in $^{20}$Ne (7 - 10 MeV/nucleon) + $^{12}$C reactions

The inclusive energy distributions of fragments with Z $\geq$ 3 emitted from the bombardment of $^{12}$C by $^{20}$Ne beams with incident energies between 145 and 200 MeV have been measured in the angular range $θ_{lab} \sim$ 10$^\circ$ - 50$^\circ$. Damped fragment yields in all cases have been found to be characteristic of emission from fully energy equilibrated composites; for B, C fragments, average Q-values, $ $, were independent of the centre of mass emission angle ($θ_{c.m}$), and the angular distributions followed $\sim$1/sin$θ_{c.m}$ like variation, signifying long life times of the emitting di-nuclear systems. Total yields of these fragments have been found to be much larger compared to the standard statistical model predictions of the same. This may be indicative of the survival of orbiting like process in $^{12}$C + $^{20}$Ne system at these energies.

nucl-ex↗