arXiv subjects

B. S. Acharya

Publications and source records attributed to B. S. Acharya.

At least 55 records · Page 3Linked to original sources

arXiv2001

Pachmarhi Array of Čerenkov Telescopes and its Sensitivity

Pachmarhi Array of Čerenkov Telescopes (PACT) has been designed to search for celestial TeV $γ-$rays using the wavefront sampling technique. PACT, located at Pachmarhi, (latitude 22$^\circ$ 28$^\prime$ N, longitude 76$^\circ$ 26$^\prime$ E, altitude 1075 m) consists of 25 telescopes deployed over an area of 80 m $\times$ 100m. Each telescopes consists of 7 parabolic reflectors, each viewed by a fast phototube behind a 3$^\circ$ mask at the focus. The density and the arrival time of the photons at the PMT are recorded for each shower. The energy threshold and collection area of the array are estimated, from Monte Carlo simulations, to be $\sim$ 900 GeV and 10$^5$ m$^2$ respectively. The accuracy in determination of arrival angle of a shower is estimated to be about 0.1$^\circ$ in the near vertical direction. About 99% of the off-axis hadronic events could be rejected from directional information alone. Further, at least 75% of the on-axis hadronic events could be rejected using species sensitive parameters derived from timing and density measurements. These cuts on data to reject background would retain $\sim$ 44% of the $γ-$ray signal. The sensitivity of the array for a 5$σ$ detection of $γ-$ray signal at a threshold energy of 1 TeV has been estimated to be $\sim$ 4.1 $\times$ 10$^{-12}$ photons cm$^{-2}$ s$^{-1}$ for an on source exposure of 50 hours. The PACT set-up has been fully commissioned and is collecting data. The details of the system parameters and sensitivity will be presented.

astro-ph↗

arXiv2001

A Possible High Altitude High Energy Gamma Ray Observatory in India

Recently an Indian Astronomical Observatory has been set up at Hanle (32$^\circ$ 46$^\prime$ 46$^{\prime\prime}$ N, 78$^\circ$ 57$^\prime$ 51$^{\prime\prime}$ E, 4515m amsl) situated in the high altitude cold desert in the Himalayas. The Observatory has 2-m aperture optical-infrared telescope, recently built by the Indian Institute of Astrophysics. We have carried out systematic simulations for this observation level to study the nature of Čerenkov light pool generated by gamma ray and proton primaries incident vertically at the top of the atmosphere. The differences in the shape of the lateral distributions of Čerenkov light with respect to that at lower altitudes is striking. This arises primarily due to the proximity of the shower maximum to the observation site. The limited lateral spread of the Čerenkov light pool and near 90% atmospheric transmission at this high altitude location makes it an ideal site for a gamma ray observatory. This results in a decrease in the gamma ray energy threshold by a factor of 2.9 compared to that at sea-level. Several parameters based on density and timing information of Čerenkov photons, including local and medium range photon density fluctuations as well as photon arrival time jitter could be efficiently used to discriminate gamma rays from more abundant cosmic rays at tens of GeV energies.

astro-ph↗

arXiv2001

Very High Energy Gamma Ray Emission from Crab Nebula with the PACT Array

The Crab nebula has proved to be the nearest to a standard candle in VHE $γ-$ ray astronomy. Results on the gamma ray emission from the nebula at various energies have come in the last decade mostly from imaging telescopes. The aim of the new Pachmarhi Atmospheric Cerenkov Telescope (PACT) array has been to use the temporal and spatial distribution of Cerenkov photons in distinguishing between proton and gamma ray showers. The array, with timing information from 175 mirrors, is ideally suited for precise estimation of the arrival direction. Preliminary results from the recent data taken on Crab has shown a 12$σ$ signal with the flux in good agreement with those of the other experiments.

astro-ph↗

arXiv2001

Angular Resolution of Pachmarhi Array of Cerenkov Telescopes

Pachmarhi Array of Čerenkov Telescopes(PACT), consisting of a distributed array of 25 telescopes is used to sample the atmospheric Čerenkov Photon showers. The shower front is fitted to a plane and the direction of arrival of primary particle is obtained. The accuracy in the estimation of the arrival direction of showers has been estimated to be $\sim 0^{\circ}.1~$ using `split' array method. The angular resolution is expected to be even better when a spherical front is used for direction reconstruction or correction for the curvature of the front is applied. This is the best angular resolution among all the currently operating atmospheric Čerenkov telescopes in the world.

astro-ph↗

arXiv2001

Very High Energy Gamma Ray Emission from Crab and Geminga Pulsars

The main inference from the experiments of the '80s that the time-averaged energy spectra of pulsars had to steepen in the GeV-TeV energy region has been reinforced in the '90s from experiments with higher sensitivities. However, results from several experiments from the past and the more sensitive experiments at present can be reconciled by invoking a possibly different component arising in the TeV region. The results of the preliminary analysis of the data being collected with the PACT array will be presented.

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arXiv2001

Observations of Mkn 421 using Pachmarhi Array of Čerenkov Telescopes

Pachmarhi Array of Čerenkov Telescopes (PACT), based on wavefront sampling technique, has been used for detecting TeV gamma rays from galactic and extra-galactic $γ$-ray sources. The Blazar, Mkn 421 was one such extra-galactic source observed during the winter nights of 2000 and 2001. We have carried out a preliminary analysis of the data taken during the nights of January, 2000 and 2001. Results show a significant gamma ray signal from this source during both these periods above a threshold energy of 900 GeV. The source was contemporaneously observed by CAT imaging telescope during the first episode of January 2000 while HEGRA CT1 was observing the source during the second episode. Both these observations have detected variable $γ$-ray emission this source and they reported that it was flaring during both these periods. The light curve in the TeV gamma ray range derived from the first PACT observations during both these episodes is in agreement with that reported by other experiments. The analysis procedure and the preliminary results will be presented and discussed.

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arXiv2001

Gamma Ray and Hadron generated Čerenkov Photon Spectra at Various Observation Altitudes

We study the propagation of Cerenkov photons generated by Very High Energy gamma-rays and hadrons in the atmosphere. The photon production height distributions are estimated from semi-empirical methods and compared with those derived by standard simulation techniques. Incident spectra at various observation altitudes are then derived after applying wavelength dependent corrections due to photon attenuation in the atmosphere during the propagation of photons from the height of production to the height of observation. These are generated both for gamma- and hadron primaries of various energies. The derived production height distributions agree very well with those generated by the simulation package `CORSIKA' at all energies and for both gamma-$ray and proton primaries. The incident photon spectra are found to be both altitude dependent and primary energy dependent. The peak of the incident spectrum shifts towards the shorter wavelength with increasing altitude of observation for a given primary. Also the peak of the photon spectrum shifts towards the shorter wavelength with increasing energy of the primary at given altitude. The fraction of the UV component in the incident Cerenkov spectrum is estimated both for gamma-ray and hadronic primaries at various observation altitudes and energies. Hadron generated Cerenkov spectra are marginally richer in UV light and the difference increases slightly at higher altitudes. The fraction of the UV to the visible light in the Cerenkov spectrum could be a useful parameter to separate gamma-rays from cosmic ray background only if one can measure this fraction very accurately.

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arXiv2001

Confining Strings from $G_2$-holonomy spacetimes

The low energy physics of M theory near certain singularities of $G_2$-holonomy spacetimes can be described by pure N=1 super Yang-Mills theory in four dimensions. In this note we consider the cases when the gauge group is SO(2n), E6, E7 or E8. Confining strings with precisely the expected charges are naturally identified in proposed ``gravity duals'' of these singular M theory spacetimes.

hep-th↗

arXiv2000

On Realising N=1 Super Yang-Mills in M theory

Pure N=1 super Yang-Mills theory can be realised as a certain low energy limit of M theory near certain singularities in $G_2$-holonomy spaces. For SU(n) and SO(2n) gauge groups these $M$ theory backgrounds can be regarded as strong coupling limits of wrapped D6-brane configurations in Type IIA theory on certain non-compact Calabi-Yau spaces such as the deformed conifold. Various aspects of such realisations are studied including the generation of the superpotential, domain walls, QCD strings and the relation to recent work of Vafa. In the spirit of this recent work we propose a `gravity dual' of M theory near these singularities.

hep-th↗

arXiv2000

Flux, Supersymmetry and M theory on 7-manifolds

Various aspects of low energy M theory compactified to four dimensions are considered. If the supersymmetry parameter is parallel in the unwarped metric, then supersymmetry requires that the warp factor is trivial, the background four-form field strength is zero and that the internal 7-manifold has $G_2$ holonomy (we assume the absence of boundaries and other impurities). A proposal of Gukov - extended here to include M2-brane domain walls - for the superpotential of the compactified theory is shown to yield the same result. Finally, we make some speculative remarks concerning higher derivative corrections and supersymmetry breaking.

hep-th↗

arXiv2000

Studies of the Response of the Prototype CMS Hadron Calorimeter, Including Magnetic Field Effects, to Pion, Electron, and Muon Beams

We report on the response of a prototype CMS hadron calorimeter module to charged particle beams of pions, muons, and electrons with momenta up to 375 GeV/c. The data were taken at the H2 and H4 beamlines at CERN in 1995 and 1996. The prototype sampling calorimeter used copper absorber plates and scintillator tiles with wavelength shifting fibers for readout. The effects of a magnetic field of up to 3 Tesla on the response of the calorimeter to muons, electrons, and pions are presented, and the effects of an upstream lead tungstate crystal electromagnetic calorimeter on the linearity and energy resolution of the combined calorimetric system to hadrons are evaluated. The results are compared with Monte Carlo simulations and are used to optimize the choice of total absorber depth, sampling frequency, and longitudinal readout segmentation.

hep-ex↗

arXiv1999

M theory, Joyce Orbifolds and Super Yang-Mills

We geometrically engineer d=4 N=1 supersymmetric Yang-Mills theories by considering M theory on various Joyce orbifolds. We argue that the superpotential of these models is generated by fractional membrane instantons. The relation of this superpotential to membrane anomalies is also discussed.

hep-th↗

arXiv1997

A Measurement of the W Boson Mass

We present a measurement of the W boson mass using data collected by the D0 experiment at the Fermilab Tevatron during 1994--1995. We identify W bosons by their decays to $eν$ final states. We extract the W mass, M(W), by fitting the transverse mass and transverse electron momentum spectra from a sample of 28,323 W to $eν$ decay candidates. We use a sample of 3,563 dielectron events, mostly due to Z to ee decays, to constrain our model of the detector response. From the transverse mass fit we measure M(W) = $80.44\pm0.10(stat)\pm0.07(syst)$ GeV. Combining this with our previously published result from data taken in 1992--1993, we obtain M(W) $= 80.43\pm0.11$ GeV.

hep-ex↗

arXiv1997

A Measurement of the W Boson Mass

We report a measurement of the W boson mass based on an integrated luminosity of 82 pb$^{-1}$ from \ppbar collisions at $\sqrt{s}=1.8$ TeV recorded in 1994--1995 by the \Dzero detector at the Fermilab Tevatron. We identify W bosons by their decays to $eν$ and extract the mass by fitting the transverse mass spectrum from 28,323 W boson candidates. A sample of 3,563 dielectron events, mostly due to Z to ee decays, constrains models of W boson production and the detector. We measure $\mw=80.44\pm0.10(stat)\pm0.07(syst)$~GeV. By combining this measurement with our result from the 1992--1993 data set, we obtain $\mw=80.43\pm0.11$ GeV.

hep-ex↗

arXiv1997

On Mirror Symmetry for Manifolds of Exceptional Holonomy

We consider Type II string theories on ${\bf T^n}/{{\bf Z_2}^m}$ Joyce orbifolds. This class contains orbifolds which can be desingularised to give manifolds of $G_2$ $({\bf n}$$=$$7)$ and $Spin(7)$ holonomy $({\bf n}$$=$$8)$. In the $G_2$ holonomy case we present two types of $T$-duality transformation which are clearly generalisations of the $T$-duality/mirror transformation in Calabi-Yau spaces. The first maps Type IIA theory on one such space from this class to Type IIB theory on another such space. The second maps Type IIA (IIB) to Type IIA (IIB). In the case of $Spin(7)$ holonomy we present a $T$-duality transformation which maps Type IIA (IIB) theory on one such space to Type IIA (IIB) on another such space. As orbifold conformal field theories these $T$-dual target spaces are related via the inclusion/exclusion of discrete torsion and the $T$-duality is proven to genus $g$ in string perturbation theory. We then apply a Strominger, Yau, Zaslow type argument which suggests that manifolds of $G_2$ holonomy which have a ``mirror'' of the first (second) type admit supersymmetric ${\bf T^3}$ (${\bf T^4}$) fibrations and that manifolds of $Spin(7)$ holonomy for which a mirror exists admit fibration by supersymmetric $4$-tori. Further evidence for this suggestion is given by examining the moduli space structure of wrapped D-branes.

hep-th↗

arXiv1997

A Mirror Pair of Calabi-Yau Fourfolds in Type II String Theory

We give some simple examples of mirror Calabi-Yau fourfolds in Type II string theory. These are realised as toroidal orbifolds. Motivated by the Strominger, Yau, Zaslow argument we give explicitly the mirror transformation which maps Type IIA/IIB on such a fourfold to Type IIA/IIB on the mirror. The mirrors are related by the inclusion/exclusion of discrete torsion. Implicit in the result is a confirmation of mirror symmetry to genus $g$ in the string path integral. Finally, by considering the relationship between M-theory and Type IIA theory, we show how in M-theory on mirror Calabi-Yau fourfolds, mirror symmetry exchanges the Coulomb branch with (one of the) Higgs branches of the theory. This result is relevant to duality in N=2 supersymmetric gauge theories in three dimensions.

hep-th↗

arXiv1997

Higher Dimensional Analogues of Donaldson-Witten Theory

We present a Donaldson-Witten type field theory in eight dimensions on manifolds with $Spin(7)$ holonomy. We prove that the stress tensor is BRST exact for metric variations preserving the holonomy and we give the invariants for this class of variations. In six and seven dimensions we propose similar theories on Calabi-Yau threefolds and manifolds of $G_2$ holonomy respectively. We point out that these theories arise by considering supersymmetric Yang-Mills theory defined on such manifolds. The theories are invariant under metric variations preserving the holonomy structure without the need for twisting. This statement is a higher dimensional analogue of the fact that Donaldson-Witten field theory on hyper-Kähler 4-manifolds is topological without twisting. Higher dimensional analogues of Floer cohomology are briefly outlined. All of these theories arise naturally within the context of string theory.

hep-th↗

arXiv1997

Pachmarhi Array of Cerenkov Telescopes

Very High Energy (VHE) gamma -ray Astronomy observations are planned to be carried out at Pachmarhi in the central Indian state of Madhya Pradesh using the well known atmospheric Cerenkov technique. An array of 25 Cerenkov telescopes is currently under construction. Using this array it is proposed to sample the Cerenkov light pool at various distances from the shower core in order to estimate the lateral distribution parameters of the shower. In this paper we discuss the scientific motivation of this concept of enriching the gamma -ray signal as compared to the standard imaging technique. The current status of the detector development and the expected results will be presented.

astro-ph↗