arXiv subjects

V. Prasad

Publications and source records attributed to V. Prasad.

12 recordsLinked to original sources

arXiv2017

A path integral approach for the colloidal glass transition based on an analogy with the $λ$-transition in liquid helium

We describe a model for the colloidal glass transition that is based on Feynman's theory for the $λ$-transition in liquid helium. Our model essentially counts the number of configurations of dynamic loops, strings or clusters of different sizes, and determines the glass transition volume fraction $ϕ_{g}$ from the distribution of these heterogeneities. Since confinement restricts the available number of configurations for these loops, strings and clusters, its effect on $ϕ_{g}$ can also be calculated in a relatively straightforward manner.

cond-mat.soft↗

arXiv2015

On the second stable homotopy group of the Eilenberg-Maclane space and the Schur Multiplier

We prove that for a finitely generated group $G$, the second stable homotopy group $π_2^S(K(G,1))$ of the Eilenberg-Maclane space $K(G,1)$ is completely determined by the Schur multiplier $H_2(G)$. We also prove that the second stable homotopy group $π_2^S(K(G,1))$ is equal to the Schur multiplier $H_2(G)$ for a torsion group $G$ with no elements of order $2$ and show that for such groups, $π_2^S(K(G,1))$ is a direct factor of $π_{3}(SK(G,1))$, where $S$ denotes suspension and $π_2^S$ the second stable homotopy group. We compute $π_{3}(SK(G,1))$ and $π_2^S(K(G,1))$ for symmetric, alternating, general linear groups over finite fields and some infinite general linear groups $G$. We also obtain a bound for the Schur multiplier of all finite groups $G$ analogous to Green's bound for $p$-groups.

math.GR↗

arXiv2010

Precise Measurements of Direct CP Violation, CPT Symmetry, and Other Parameters in the Neutral Kaon System

We present precise tests of CP and CPT symmetry based on the full dataset of K to pipi decays collected by the KTeV experiment at Fermi National Accelerator Laboratory during 1996, 1997, and 1999. This dataset contains 16 million K to 2pi0 and 69 million K to pi+pi- decays. We measure the direct CP violation parameter Re(epsilon'/epsilon) = (19.2 pm 2.1)x10-4. We find the KL-KS mass difference Deltam = (5270 pm 12)x10^6 hbar/s and the KS lifetime tauS = (89.62 pm 0.05)x10-12 s. We also measure several parameters that test CPT invariance. We find the difference between the phase of the indirect CP violation parameter, epsilon, and the superweak phase, phi_epsilon - phi_SW = (0.40 pm 0.56) degrees. We measure the difference of the relative phases between the CP violating and CP conserving decay amplitudes for K to pi+pi- (phi+-) and for K to 2pi0 (phi00), Delta phi = (0.30 pm 0.35) degrees. From these phase measurements, we place a limit on the mass difference between K0 and K0bar, DeltaM < 4.8 x 10-19 GeV/c^2 at 95% C.L. These results are consistent with those of other experiments, our own earlier measurements, and CPT symmetry.

hep-ex↗

arXiv2009

Flow fields in soap films: relating surface viscosity and film thickness

We follow the diffusive motion of colloidal particles in soap films with varying $h/d$, where $h$ is the thickness of the film and $d$ the diameter of the particles. The hydrodynamics of these films are determined by looking at the correlated motion of pairs of particles as a function of separation $R$. The Trapeznikov approximation [A. A. Trapeznikov, \emph{PICSA} (1957)] is used to model soap films as an effective interface in contact with bulk air phases, that behaves as a 2D fluid. The flow fields determined from correlated particle motions show excellent agreement with what is expected for the theory of 2D fluids for all our films where $0.6 \leq h/d \leq 14.3$, with the surface viscosity matching that predicted by Trapeznikov. However, for thicker films with $h/d > 7 \pm 3$, single particle motion is faster than expected. Additionally, while the flow fields still match those expected for 2D fluids, the parameters of these flow fields change markedly for thick films. Our results indicate a transition from 2D to 3D fluid-like behavior occurs at this value of $h/d$.

cond-mat.soft↗

arXiv2008

2D to 3D transition in soap films demonstrated by microrheology

We follow the diffusive motion of colloidal particles of diameter $d$ in soap films of varying thickness $h$ with fluorescence microscopy. Diffusion constants are obtained both from one- and two-particle microrheological measurements of particle motion in these films. These diffusion constants are related to the surface viscosity of the interfaces comprising the soap films, by means of the Trapeznikov approximation [A. A. Trapeznikov, \emph{PICSA} (1957)] and Saffman's equation for diffusion in a 2D fluid. Unphysical values of the surface viscosity are found for thick soap films ($h/d > 7$), indicating a transition from 2D to 3D behavior.

cond-mat.soft↗

arXiv2008

Observation of the Decay Xi^0 --> Sigma^+ mu^- nu(bar)

The Xi^0 muon semi-leptonic decay has been observed for the first time with nine identified events using the KTeV beam line and detector at Fermilab. The decay is normalized to the Xi^0 beta decay mode and yields a value for the ratio of decay rates of $(1.8^{+0.7}_{-0.5}(stat.)\pm0.2(syst.))\times 10^{-2}$. This is in agreement with the SU(3) flavor symmetric quark model.

hep-ex↗

arXiv2008

Improved Cosmological Constraints from New, Old and Combined Supernova Datasets

We present a new compilation of Type Ia supernovae (SNe Ia), a new dataset of low-redshift nearby-Hubble-flow SNe and new analysis procedures to work with these heterogeneous compilations. This ``Union'' compilation of 414 SN Ia, which reduces to 307 SNe after selection cuts, includes the recent large samples of SNe Ia from the Supernova Legacy Survey and ESSENCE Survey, the older datasets, as well as the recently extended dataset of distant supernovae observed with HST. A single, consistent and blind analysis procedure is used for all the various SN Ia subsamples, and a new procedure is implemented that consistently weights the heterogeneous data sets and rejects outliers. We present the latest results from this Union compilation and discuss the cosmological constraints from this new compilation and its combination with other cosmological measurements (CMB and BAO). The constraint we obtain from supernovae on the dark energy density is $Ω_Λ= 0.713^{+0.027}_{-0.029} (stat)}^{+0.036}_{-0.039} (sys)}$, for a flat, LCDM Universe. Assuming a constant equation of state parameter, $w$, the combined constraints from SNe, BAO and CMB give $w=-0.969^{+0.059}_{-0.063}(stat)^{+0.063}_{-0.066} (sys)$. While our results are consistent with a cosmological constant, we obtain only relatively weak constraints on a $w$ that varies with redshift. In particular, the current SN data do not yet significantly constrain $w$ at $z>1$. With the addition of our new nearby Hubble-flow SNe Ia, these resulting cosmological constraints are currently the tightest available.

astro-ph↗

arXiv2006

Two-Particle Microrheology of quasi-2D Viscous Systems

We study the correlated motions of colloidal particles in a quasi-2D system (Human Serum Albumin (HSA) protein molecules at an air-water interface) for different surface viscosities $η_{s}$. We observe a transition in the behavior of the correlated motion, from 2-D interface dominated at high $η_{s}$ to bulk fluid-dependent at low $η_{s}$. The correlated motions can be scaled onto a master curve which captures the features of this transition. This master curve also characterizes the spatial dependence of the flow field of a viscous interface in response to a force. From the flow field and the correlated particle motions, we calculate a two-particle MSD (mean square displacement) for direct comparison with rheological measurements.

cond-mat.soft↗

arXiv2004

Measurements of the Branching Fractions and Decay Distributions for KL->πμνγand KL->πeνγ

We present measurements of R_Kl3rad = Gamma(KL->π\ellνγ; Egcm > 10 MeV)/Gamma(KL->π\ellν), where ell = mu or e, and Egcm is the photon energy in the kaon rest frame. These measurements are based on KL decays collected in 1997 by the KTeV (E832) experiment at Fermilab. With samples of 1385 KL->πμνγand 14221 KL->π\eνγcandidates, we find R_Km3rad = (0.530 +- 0.019)% and R_Ke3rad = (4.942 +- 0.062)%. We also examine distributions of photon energy and lepton-photon angle.

hep-ex↗

arXiv2004

Spectroscopic confirmation of high-redshift supernovae with the ESO VLT

We present VLT FORS1 and FORS2 spectra of 39 candidate high-redshift supernovae that were discovered as part of a cosmological study using Type Ia supernovae (SNe Ia) over a wide range of redshifts. From the spectra alone, 20 candidates are spectrally classified as SNe Ia with redshifts ranging from z=0.212 to z=1.181. Of the remaining 19 candidates, 1 might be a Type II supernova and 11 exhibit broad supernova-like spectral features and/or have supernova-like light curves. The candidates were discovered in 8 separate ground-based searches. In those searches in which SNe Ia at z ~ 0.5 were targeted, over 80% of the observed candidates were spectrally classified as SNe Ia. In those searches in which SNe Ia with z > 1 were targeted, 4 candidates with z > 1 were spectrally classified as SNe Ia and later followed with ground and space based observatories. We present the spectra of all candidates, including those that could not be spectrally classified as supernova.

astro-ph↗

arXiv2004

Spectroscopic Observations and Analysis of the Peculiar SN 1999aa

We present an extensive new time-series of spectroscopic data of the peculiar SN 1999aa in NGC 2595. Our data set includes 25 optical spectra between -11 and +58 days with respect to B-band maximum light, providing an unusually complete time history. The early spectra resemble those of a SN 1991T-like object but with a relatively strong Ca H&K absorption feature. The first clear sign of Si II 6355, characteristic of Type Ia supernovae, is found at day -7 and its velocity remains constant up to at least the first month after B-band maximum light. The transition to normal-looking spectra is found to occur earlier than in SN 1991T suggesting SN 1999aa as a possible link between SN 1991T-like and Branch-normal supernovae. Comparing the observations with synthetic spectra, doubly ionized Fe, Si and Ni are identified at early epochs. These are characteristic of SN 1991T-like objects. Furthermore, in the day -11 spectrum, evidence is found for an absorption feature which could be identified as high velocity C II 6580 or H-alpha. At the same epoch C III 4648.8 at photospheric velocity is probably responsible for the absorption feature at 4500 A. High velocity Ca is found around maximum light together with Si II and Fe II confined in a narrow velocity window. Implied constraints on supernovae progenitor systems and explosion hydrodynamical models are briefly discussed.

astro-ph↗

arXiv2000

Enhancement of magnetoresistance in manganite multilayers

Magnanite multilayers have been fabricated using La0.67Ca0.33MnO3 as the ferromagnetic layer and Pr0.7Ca0.3MnO3 and Nd0.5Ca0.5MnO3 as the spacer layers. All the multilayers were grown on LaAlO3 (100) by pulse laser deposition. An enhanced magnetoresistnace (defined (RH- R0)/R0) of more than 98% is observed in these multilayers. Also a low field magnetoresistance of 41% at 5000 Oe is observed in these multilayer films. The enhanced MR is attributed to the induced double exchange in the spacer layer, which is giving rise to more number of conducting carriers. This is compared by replacing the spacer layer with LaMnO3 where Mn exists only in 3+ state and no enhancement is observed in the La0.67Ca0.33MnO3 / LaMnO3 multilayers as double exchange mechanism can not be induced by external magnetic fields.

cond-mat.mtrl-sci↗