arXiv subjects

A. C. Das

Publications and source records attributed to A. C. Das.

7 recordsLinked to original sources

arXiv2010

Unique Observations of a Geomagnetic SI^+ -- SI^- Pair: Solar Sources and Associated Solar Wind Fluctuations

The paper describes the occurrence of a pair of oppositely directed sudden impulses (SI), in the geomagnetic field ($Δ$X), at ground stations, called SI${^{+}}$ -- SI${^{-}}$ pairs, that occurred between 1835 UT and 2300 UT on 23 April 1998. The SI${^{+}}$ -- SI${^{-}}$ pair, was closely correlated with corresponding variations in the solar wind density, while solar wind velocity and the southward component of the interplanetary magnetic field (Bz) did not show any correspondence. Further, this event had no source on the visible solar disk. However, a rear-side partial halo coronal mass ejection (CME) and an M1.4 class solar flare behind the west limb, took place on 20 April 1998, the date corresponding to the traceback location of the solar wind flows. This event presents empirical evidence, which to our knowledge, is the best convincing evidence for the association of specific solar events to the observations of an SI${^{+}}$ -- SI${^{-}}$ pair. In addition, it shows that it is possible for a rear side solar flare to propagate a shock towards the earth.

physics.space-ph↗

arXiv2003

Near-threshold production of the multi-strange $Ξ^-$ hyperon

The yield for the multi-strange $Ξ^{-}$ hyperon has been measured in 6 AGeV Au+Au collisions via reconstruction of its decay products $π^{-}$ and $Λ$, the latter also being reconstructed from its daughter tracks of $π^{-}$ and p. The measurement is rather close to the threshold for $Ξ^{-}$ production and therefore provides an important test of model predictions. The measured yield for $Ξ^{-}$ and $Λ$ are compared for several centralities. In central collisions the $Ξ^{-}$ yield is found to be in excellent agreement with statistical and transport model predictions, suggesting that multi-strange hadron production approaches chemical equilibrium in high baryon density nuclear matter.

nucl-ex↗

arXiv2003

Comparison of Source Images for protons, $π^-$'s and $Λ$'s in 6 AGeV Au+Au collisions

Source images are extracted from two-particle correlations constructed from strange and non-strange hadrons produced in 6 AGeV Au + Au collisions. Very different source images result from pp vs p$Λ$ vs $π^-π^-$ correlations. These observations suggest important differences in the space-time emission histories for protons, pions and neutral strange baryons produced in the same events.

nucl-ex↗

arXiv2002

Longitudinal Flow of Protons from 2-8 AGeV Central Au+Au Collisions

Rapidity distributions of protons from central $^{197}$Au + $^{197}$Au collisions measured by the E895 Collaboration in the energy range from 2 to 8 AGeV at the Brookhaven AGS are presented. Longitudinal flow parameters derived using a thermal model including collective longitudinal expansion are extracted from these distributions. The results show an approximately linear increase in the longitudinal flow velocity, $<βγ>_{L}$, as a function of the logarithm of beam energy.

nucl-ex↗

arXiv2001

Laying the groundwork at the AGS: Recent results from experiment E895

The E895 Collaboration at the Brookhaven AGS has performed a systematic investigation of Au+Au collisions at 2-8 AGeV, using a large-acceptance Time Projection Chamber. In addition to extensive measurements of particle flow, spectra, two-particle interferometry, and strangeness production, we have performed novel hybrid analyses, including azimuthally-sensitive pion HBT, extraction of the six-dimensional pion phasespace density, and a first measurement of the Lambda-proton correlation function.

nucl-ex↗

arXiv2000

Azimuthal Dependence of Pion Interferometry at the AGS

Two-pion correlation functions, measured as a function of azimuthal emission angle with respect to the reaction plane, provide novel information on the anisotropic shape and orientation of the pion-emitting zone formed in heavy ion collisions. We present the first experimental determination of this information, for semi-central Au+Au collisions at 2-6 AGeV. The source extension perpendicular to the reaction plane is greater than the extension in the plane, and tilt of the pion source in coordinate space is found to be opposite its tilt in momentum space.

nucl-ex↗

arXiv1998

A Preshower Photon Multiplicity Detector for the WA98 Experiment

A high granularity preshower detector has been fabricated and installed in the WA98 Experiment at the CERN SPS for measuring the spatial distribution of photons produced in the forward region in lead ion induced interactions. Photons are counted by detecting the preshower signal in plastic scintillator pads placed behind a 3 radiation length thick lead converter and applying a threshold on the scintillator signal to reject the minimum ionizing particles. Techniques to improve the imaging of the fibre and performance of the detector in the high multiplicity environment of lead-lead collisions are described. Using Monte-Carlo simulation methods and test beam data of pi- and e- at various energies the photon counting efficiency is estimated to be 68% for central and 73% for peripheral Pb+Pb collisions.

hep-ex↗