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S. Gupta

Publications and source records attributed to S. Gupta.

138 records · Page 8Linked to original sources
arXiv2002

Non-Gaussian Signatures in the Cosmic Background Radiation from Warm Inflation

We calculate the bispectrum of the gravitational field fluctuations generated during warm inflation, where dissipation of the vacuum potential during inflation is the mechanism for structure formation. The bispectrum is non--zero because of the self--interaction of the scalar field. We compare the predictions with those of standard, or `supercooled', inflationary models, and consider the detectability of these levels of non--Gaussianity in the bispectrum of the cosmic microwave background. We find that the levels of non--Gaussianity for warm and supercooled inflation are comparable, and over--ridden by the contribution to the bispectrum due to other physical effects. We also conclude that the resulting bispectrum values will be undetectable in the cosmic microwave background for both the MAP and Planck Surveyor satellites.

astro-ph↗
arXiv2001

Two-species mixture of quantum degenerate Bose and Fermi gases

We have produced a macroscopic quantum system in which a Li-6 Fermi sea coexists with a large and stable Na-23 Bose-Einstein condensate. This was accomplished using inter-species sympathetic cooling of fermionic Li-6 in a thermal bath of bosonic Na-23.

cond-mat.soft↗
arXiv2001

Transport of Bose-Einstein Condensates with Optical Tweezers

We have transported gaseous Bose-Einstein condensates over distances up to 44 cm. This was accomplished by trapping the condensate in the focus of an infrared laser and translating the location of the laser focus with controlled acceleration. Condensates of order 1 million atoms were moved into an auxiliary chamber and loaded into a magnetic trap formed by a Z-shaped wire. This transport technique avoids the optical and mechanical access constraints of conventional condensate experiments and creates many new scientific opportunities.

cond-mat.soft↗
arXiv2001

Realization of Bose-Einstein condensates in lower dimensions

Bose-Einstein condensates of sodium atoms have been prepared in optical and magnetic traps in which the energy-level spacing in one or two dimensions exceeds the interaction energy between atoms, realizing condensates of lower dimensionality. The cross-over into two-dimensional and one-dimensional condensates was observed by a change in aspect ratio and saturation of the release energy when the number of trapped atoms was reduced.

cond-mat.soft↗
arXiv2001

Observation of vortex phase singularities in Bose-Einstein condensates

We have observed phase singularities due to vortex excitation in Bose-Einstein condensates. Vortices were created by moving a laser beam through a condensate. They were observed as dislocations in the interference fringes formed by the stirred condensate and a second unperturbed condensate. The velocity dependence for vortex excitation and the time scale for re-establishing a uniform phase across the condensate were determined.

cond-mat.soft↗
arXiv2000

Amplification of light and atoms in a Bose--Einstein condensate

A Bose-Einstein condensate illuminated by a single off-resonant laser beam (``dressed condensate'') shows a high gain for matter waves and light. We have characterized the optical and atom-optical properties of the dressed condensate by injecting light or atoms, illuminating the key role of long-lived matter wave gratings produced by the condensate at rest and recoiling atoms. The narrow bandwidth for optical gain gave rise to an extremely slow group velocity of an amplified light pulse (1 m/s).

cond-mat.soft↗
arXiv2000

Suppression and enhancement of impurity scattering in a Bose-Einstein condensate

Impurity atoms propagating at variable velocities through a trapped Bose-Einstein condensate were produced using a stimulated Raman transition. The redistribution of momentum by collisions between the impurity atoms and the stationary condensate was observed in a time-of-flight analysis. The collisional cross section was dramatically reduced when the velocity of the impurities was reduced below the speed of sound of the condensate, in agreement with the Landau criterion for superfluidity. For large numbers of impurity atoms, we observed an enhancement of atomic collisions due to bosonic stimulation. This enhancement is analogous to optical superradiance.

cond-mat.soft↗
arXiv1999

Excitation of phonons in a Bose-Einstein condensate by light scattering

Stimulated small-angle light scattering was used to measure the structure factor of a Bose-Einstein condensate in the phonon regime. The excitation strength for phonons was found to be significantly reduced from that of free particles, revealing the presence of correlated pair excitations and quantum depletion in the condensate. The Bragg resonance line strength and line shift agreed with predictions of a local density approximation.

cond-mat↗
arXiv1995

PRODUCTION OF DRELL--YAN PAIRS IN HIGH ENERGY NUCLEON--NUCLEON COLLISIONS

We compute cross sections for the Drell-Yan process in N--N collisions at next-to-leading order in $α_s$. The mass, rapidity, transverse momentum, and angular dependence of these cross sections are presented. An estimate of higher order corrections is obtained from next-to-next-to-leading order calculation of the mass distribution. We compare the results with some of the existing data to show the quality of the agreement between calculations and data. We present predictions for energies which will become available at the RHIC and LHC colliders. Uncertainties in these predictions due to choices of scale, scheme and parton distribution are discussed.

hep-ph↗
arXiv1994

Heavy Quark Production in pp Collisions

A systematic study of the inclusive single heavy quark and heavy-quark pair production cross sections in pp collisions is presented for RHIC and LHC energies. We compare with existing data when possible. The dependence of the rates on the renormalization and factorization scales is discussed. Predictions of the cross sections are given for two different sets of parton distribution functions.

hep-ph↗
arXiv1992

Finite-Size Scaling on the Ising Coexistence Line

We report tests of finite-size scaling ansatzes in the low temperature phase of the two-dimensional Ising model. For moments of the magnetisation density, we find good agreement with the new ansatz of Borgs and Kotecký, and clear evi consequences of the convexity of the free energy are not adequately treated in either of these approaches.\lb {\it Keywords}\/: Finite-size scaling, 2-d Ising, pure-phase susceptibility.

hep-lat↗