Subir Sarkar | University of Oxford (original) (raw)

Refereed papers by Subir Sarkar

Research paper thumbnail of Multimessenger Gamma-Ray and Neutrino Coincidence Alerts Using HAWC and

Astrophysical Journal, 2021

The High Altitude Water Cerenkov (HAWC) and IceCube observatories, through the Astrophysical Mult... more The High Altitude Water Cerenkov (HAWC) and IceCube observatories, through the Astrophysical Multimessenger Observatory Network (AMON) framework, have developed a multimessenger joint search for extragalactic astrophysical sources. This analysis looks for sources that emit both cosmic neutrinos and gamma rays that are produced in photohadronic or hadronic interactions. The AMON system is running continuously, receiving subthreshold data (i.e., data that are not suited on their own to do astrophysical searches) from HAWC and IceCube, and combining them in real time. Here we present the analysis algorithm, as well as results from archival data collected between 2015 June and 2018 August, with a total live time of 3.0 yr. During this period we found two coincident events that have a false-alarm rate (FAR) of <1 coincidence yr–1, consistent with the background expectations. The real-time implementation of the analysis in the AMON system began on 2019 November 20 and issues alerts to the community through the Gamma-ray Coordinates Network with an FAR threshold of <4 coincidences yr–1.

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Research paper thumbnail of A Test of the Cosmological Principle with Quasars

Astrophysical Journal Letters, 2021

We study the large-scale anisotropy of the universe by measuring the dipole in the angular distri... more We study the large-scale anisotropy of the universe by measuring the dipole in the angular distribution of a fluxlimited, all-sky sample of 1.36 million quasars observed by the Wide-field Infrared Survey Explorer (WISE). This sample is derived from the new CatWISE2020 catalog, which contains deep photometric measurements at 3.4 and 4.6 μm from the cryogenic, post-cryogenic, and reactivation phases of the WISE mission. While the direction of the dipole in the quasar sky is similar to that of the cosmic microwave background (CMB), its amplitude is over twice as large as expected, rejecting the canonical, exclusively kinematic interpretation of the CMB dipole with a p-value of 5 × 10 −7 (4.9σ for a normal distribution, one-sided), the highest significance achieved to date in such studies. Our results are in conflict with the cosmological principle, a foundational assumption of the concordance ΛCDM model. Unified Astronomy Thesaurus concepts: Large-scale structure of the universe (902); Cosmic microwave background radiation (322); Observational cosmology (1146); Infrared astronomy (786); Quasars (1319); Active galactic nuclei (16); Cosmology (343)

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Research paper thumbnail of Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation

JCAP, 2021

The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for "-ray astron... more The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for "-ray astronomy, provides unique capabilities to address significant open questions in astrophysics, cosmology, and fundamental physics. We study some of the salient areas of "-ray cosmology that can be explored as part of the Key Science Projects of CTA, through simulated observations of active galactic nuclei (AGN) and of their relativistic jets. Observations of AGN with CTA will enable a measurement of "-ray absorption on the extragalactic background light with a statistical uncertainty below 15% up to a redshift z = 2 and to constrain or detect "-ray halos up to intergalactic-magnetic-field strengths of at least 0.3 pG. Extragalactic observations with CTA also show promising potential to probe physics beyond the Standard Model. The best limits on Lorentz invariance violation from "-ray astronomy will be improved by a factor of at least two to three. CTA will also probe the parameter space in which axion-like particles could constitute a significant fraction, if not all, of dark matter. We conclude on the synergies between CTA and other upcoming facilities that will foster the growth of "-ray cosmology.

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Research paper thumbnail of Blast from the past: Constraints on the dark sector from the BEBC WA66 beam dump experiment

SciPost Physics, 2021

We derive limits on millicharged dark states, as well as particles with electric or magnetic dipo... more We derive limits on millicharged dark states, as well as particles with electric or magnetic dipole moments, from the number of observed forward electron scattering events at the Big European Bubble Chamber in the 1982 CERN-WA-066 beam dump experiment. The dark states are produced by the 400 GeV proton beam primarily through the decays of mesons produced in the beam dump, and the lack of excess events places bounds extending up to GeV masses. These improve on bounds from all other experiments, in particular CHARM II.

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Research paper thumbnail of Sensitivity of the Cherenkov Telescope Array to a dark matter signal from the Galactic centre

JCAP, 2021

We provide an updated assessment of the power of the Cherenkov Telescope Array (CTA) to search fo... more We provide an updated assessment of the power of the Cherenkov Telescope Array (CTA) to search for thermally produced dark matter at the TeV scale, via the associated gamma-ray signal from pair-annihilating dark matter particles in the region around the Galactic centre. We find that CTA will open a new window of discovery potential, significantly extending the range of robustly testable models given a standard cuspy profile of the dark matter density distribution. Importantly, even for a cored profile, the projected sensitivity of CTA will be su cient to probe various well-motivated models of thermally produced dark matter at the TeV scale. This is due to CTA's unprecedented sensitivity, angular and energy resolutions, and the planned observational strategy. The survey of the inner Galaxy will cover a much larger region than corresponding previous observational campaigns with imaging atmospheric Cherenkov telescopes. CTA will map with unprecedented precision the large-scale di↵use emission in high-energy gamma rays, constituting a background for dark matter searches for which we adopt state-of-the-art models based on current data. Throughout our analysis, we use up-to-date event reconstruction Monte Carlo tools developed by the CTA consortium, and pay special attention to quantifying the level of instrumental systematic uncertainties, as well as background template systematic errors, required to probe thermally produced dark matter at these energies.

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Research paper thumbnail of Is there really a Hubble tension

Classical & Quantum Gravity, 2021

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Research paper thumbnail of Do supernovae indicate an accelerating universe

European Physical Journal Special Topics, 2021

In the late 1990's, observations of two directionally-skewed samples of, in total, 93 Type Ia sup... more In the late 1990's, observations of two directionally-skewed samples of, in total, 93 Type Ia supernovae were analysed in the framework of the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology. Assuming these to be 'standard(isable) candles' it was inferred that the Hubble expansion rate is accelerating as if driven by a positive Cosmological Constant Λ in Einstein's theory of gravity. This is still the only direct evidence for the 'dark energy' that is the dominant component of today's standard ΛCDM cosmological model. Other data such as baryon acoustic oscillations (BAO) in the large-scale distribution of galaxies, temperature fluctuations in the cosmic microwave background (CMB), measurement of stellar ages, the rate of growth of structure, etc are all 'concordant' with this model but do not provide independent evidence for accelerated expansion. The recent discussions about whether the inferred acceleration is real rests on analysis of a larger sample of 740 SNe Ia which shows that these are not quite standard candles, and more importantly highlights the 'corrections' that are applied to analyse the data in the FLRW framework. The latter holds in the reference frame in which the CMB is isotropic, whereas observations are carried out in our heliocentric frame in which the CMB has a large dipole anisotropy. This is assumed to be of kinematic origin i.e. due to our non-Hubble motion driven by local inhomogeneity in the matter distribution which has grown under gravity from primordial density perturbations traced by the CMB fluctuations. The ΛCDM model predicts how this peculiar velocity should fall off as the averaging scale is raised and the universe becomes sensibly homogeneous. However observations of the local 'bulk flow' are inconsistent with this expectation and convergence to the CMB frame is not seen. Moreover, the kinematic interpretation implies a corresponding dipole in the sky distribution of high redshift quasars, which is rejected by observations at 4.9σ. Hence the peculiar velocity corrections employed in supernova cosmology are inconsistent and discontinuous within the data. The acceleration of the Hubble expansion rate is in fact anisotropic at 3.9σ and aligned with the bulk flow. Thus dark energy could be an artefact of analysing data assuming that we are idealised observers in an FLRW universe, when in fact the real universe is inhomogeneous and anisotropic out to distances large enough to impact on cosmological analyses.

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Research paper thumbnail of Measurement of the Atmospheric \nu_e Flux in IceCube

Physical Review Letters, Apr 10, 2013

We report the first measurement of the atmospheric electron neutrino flux in the energy range bet... more We report the first measurement of the atmospheric electron neutrino flux in the energy range between approximately 80 GeV and 6 TeV, using data recorded during the first year of operation of IceCube's DeepCore low-energy extension. Techniques to identify neutrinos interacting within the DeepCore volume and veto muons originating outside the detector are demonstrated. A sample of 1029 events is observed in 281 days of data, of which 496±66(stat)±88(syst) are estimated to be cascade events, including both electron neutrino and neutral current events. The rest of the sample includes residual backgrounds due to atmospheric muons and charged current interactions of atmospheric muon neutrinos. The flux of the atmospheric electron neutrinos is consistent with models of atmospheric neutrinos in this energy range. This constitutes the first observation of electron neutrinos and neutral current interactions in a very large volume neutrino telescope optimized for the TeV energy range.

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Research paper thumbnail of Measurement of the cosmic ray energy spectrum with IceTop-73

Physical Review D, 2013

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Research paper thumbnail of Neutron oscillations and the primordial magnetic field

Astrophysical Letters Communications, 1989

... Reprints available directly from the publisher Printed in the United States of America Photoc... more ... Reprints available directly from the publisher Printed in the United States of America Photocopying permitted by license only Neutron Oscillations and the Primordial Magnetic Field SUBIR SARKAR Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX1 1 OQX, UK ...

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Research paper thumbnail of South Pole glacial climate reconstruction from multi-borehole laser particulate stratigraphy

Journal of Glaciology, 2013

The IceCube Neutrino Observatory and its prototype, AMANDA, were built in South Pole ice, using p... more The IceCube Neutrino Observatory and its prototype, AMANDA, were built in South Pole ice, using powerful hot-water drills to cleanly bore >100 holes to depths up to 2500 m. The construction of these particle physics detectors provided a unique opportunity to examine the deep ice sheet using a variety of novel techniques. We made high-resolution particulate profiles with a laser dust logger in eight of the boreholes during detector commissioning between 2004 and 2010. The South Pole laser logs are among the most clearly resolved measurements of Antarctic dust strata during the last glacial period and can be used to reconstruct paleoclimate records in exceptional detail. Here we use manual and algorithmic matching to synthesize our South Pole measurements with ice-core and logging data from Dome C, East Antarctica. We derive impurity concentration, precision chronology, annual-layer thickness, local spatial variability, and identify several widespread volcanic ash depositions usefu...

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Research paper thumbnail of The integrated Sachs-Wolfe imprint of cosmic superstructures: a problem for ΛCDM

Journal of Cosmology and Astroparticle Physics, 2012

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Research paper thumbnail of On the APM power spectrum and the CMB anisotropy: Evidence for a phase transition during inflation

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Research paper thumbnail of Testing astrophysical models for the PAMELA positron excess with cosmic ray nuclei

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Research paper thumbnail of Cosmic Neutrinos from Unstable Relic Particles

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Research paper thumbnail of Extremely high energy cosmic rays from relic particle decays

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Research paper thumbnail of CMB Anisotropy in the Decaying Neutrino Cosmology

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Research paper thumbnail of Ruling out a critical density baryonic universe

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Research paper thumbnail of Neutrino diagnostics of ultra-high energy cosmic ray protons

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Research paper thumbnail of Big Bang nucleosynthesis and physics beyond the Standard Model

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Research paper thumbnail of Multimessenger Gamma-Ray and Neutrino Coincidence Alerts Using HAWC and

Astrophysical Journal, 2021

The High Altitude Water Cerenkov (HAWC) and IceCube observatories, through the Astrophysical Mult... more The High Altitude Water Cerenkov (HAWC) and IceCube observatories, through the Astrophysical Multimessenger Observatory Network (AMON) framework, have developed a multimessenger joint search for extragalactic astrophysical sources. This analysis looks for sources that emit both cosmic neutrinos and gamma rays that are produced in photohadronic or hadronic interactions. The AMON system is running continuously, receiving subthreshold data (i.e., data that are not suited on their own to do astrophysical searches) from HAWC and IceCube, and combining them in real time. Here we present the analysis algorithm, as well as results from archival data collected between 2015 June and 2018 August, with a total live time of 3.0 yr. During this period we found two coincident events that have a false-alarm rate (FAR) of <1 coincidence yr–1, consistent with the background expectations. The real-time implementation of the analysis in the AMON system began on 2019 November 20 and issues alerts to the community through the Gamma-ray Coordinates Network with an FAR threshold of <4 coincidences yr–1.

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Research paper thumbnail of A Test of the Cosmological Principle with Quasars

Astrophysical Journal Letters, 2021

We study the large-scale anisotropy of the universe by measuring the dipole in the angular distri... more We study the large-scale anisotropy of the universe by measuring the dipole in the angular distribution of a fluxlimited, all-sky sample of 1.36 million quasars observed by the Wide-field Infrared Survey Explorer (WISE). This sample is derived from the new CatWISE2020 catalog, which contains deep photometric measurements at 3.4 and 4.6 μm from the cryogenic, post-cryogenic, and reactivation phases of the WISE mission. While the direction of the dipole in the quasar sky is similar to that of the cosmic microwave background (CMB), its amplitude is over twice as large as expected, rejecting the canonical, exclusively kinematic interpretation of the CMB dipole with a p-value of 5 × 10 −7 (4.9σ for a normal distribution, one-sided), the highest significance achieved to date in such studies. Our results are in conflict with the cosmological principle, a foundational assumption of the concordance ΛCDM model. Unified Astronomy Thesaurus concepts: Large-scale structure of the universe (902); Cosmic microwave background radiation (322); Observational cosmology (1146); Infrared astronomy (786); Quasars (1319); Active galactic nuclei (16); Cosmology (343)

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Research paper thumbnail of Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation

JCAP, 2021

The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for "-ray astron... more The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for "-ray astronomy, provides unique capabilities to address significant open questions in astrophysics, cosmology, and fundamental physics. We study some of the salient areas of "-ray cosmology that can be explored as part of the Key Science Projects of CTA, through simulated observations of active galactic nuclei (AGN) and of their relativistic jets. Observations of AGN with CTA will enable a measurement of "-ray absorption on the extragalactic background light with a statistical uncertainty below 15% up to a redshift z = 2 and to constrain or detect "-ray halos up to intergalactic-magnetic-field strengths of at least 0.3 pG. Extragalactic observations with CTA also show promising potential to probe physics beyond the Standard Model. The best limits on Lorentz invariance violation from "-ray astronomy will be improved by a factor of at least two to three. CTA will also probe the parameter space in which axion-like particles could constitute a significant fraction, if not all, of dark matter. We conclude on the synergies between CTA and other upcoming facilities that will foster the growth of "-ray cosmology.

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Research paper thumbnail of Blast from the past: Constraints on the dark sector from the BEBC WA66 beam dump experiment

SciPost Physics, 2021

We derive limits on millicharged dark states, as well as particles with electric or magnetic dipo... more We derive limits on millicharged dark states, as well as particles with electric or magnetic dipole moments, from the number of observed forward electron scattering events at the Big European Bubble Chamber in the 1982 CERN-WA-066 beam dump experiment. The dark states are produced by the 400 GeV proton beam primarily through the decays of mesons produced in the beam dump, and the lack of excess events places bounds extending up to GeV masses. These improve on bounds from all other experiments, in particular CHARM II.

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Research paper thumbnail of Sensitivity of the Cherenkov Telescope Array to a dark matter signal from the Galactic centre

JCAP, 2021

We provide an updated assessment of the power of the Cherenkov Telescope Array (CTA) to search fo... more We provide an updated assessment of the power of the Cherenkov Telescope Array (CTA) to search for thermally produced dark matter at the TeV scale, via the associated gamma-ray signal from pair-annihilating dark matter particles in the region around the Galactic centre. We find that CTA will open a new window of discovery potential, significantly extending the range of robustly testable models given a standard cuspy profile of the dark matter density distribution. Importantly, even for a cored profile, the projected sensitivity of CTA will be su cient to probe various well-motivated models of thermally produced dark matter at the TeV scale. This is due to CTA's unprecedented sensitivity, angular and energy resolutions, and the planned observational strategy. The survey of the inner Galaxy will cover a much larger region than corresponding previous observational campaigns with imaging atmospheric Cherenkov telescopes. CTA will map with unprecedented precision the large-scale di↵use emission in high-energy gamma rays, constituting a background for dark matter searches for which we adopt state-of-the-art models based on current data. Throughout our analysis, we use up-to-date event reconstruction Monte Carlo tools developed by the CTA consortium, and pay special attention to quantifying the level of instrumental systematic uncertainties, as well as background template systematic errors, required to probe thermally produced dark matter at these energies.

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Research paper thumbnail of Is there really a Hubble tension

Classical & Quantum Gravity, 2021

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Research paper thumbnail of Do supernovae indicate an accelerating universe

European Physical Journal Special Topics, 2021

In the late 1990's, observations of two directionally-skewed samples of, in total, 93 Type Ia sup... more In the late 1990's, observations of two directionally-skewed samples of, in total, 93 Type Ia supernovae were analysed in the framework of the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology. Assuming these to be 'standard(isable) candles' it was inferred that the Hubble expansion rate is accelerating as if driven by a positive Cosmological Constant Λ in Einstein's theory of gravity. This is still the only direct evidence for the 'dark energy' that is the dominant component of today's standard ΛCDM cosmological model. Other data such as baryon acoustic oscillations (BAO) in the large-scale distribution of galaxies, temperature fluctuations in the cosmic microwave background (CMB), measurement of stellar ages, the rate of growth of structure, etc are all 'concordant' with this model but do not provide independent evidence for accelerated expansion. The recent discussions about whether the inferred acceleration is real rests on analysis of a larger sample of 740 SNe Ia which shows that these are not quite standard candles, and more importantly highlights the 'corrections' that are applied to analyse the data in the FLRW framework. The latter holds in the reference frame in which the CMB is isotropic, whereas observations are carried out in our heliocentric frame in which the CMB has a large dipole anisotropy. This is assumed to be of kinematic origin i.e. due to our non-Hubble motion driven by local inhomogeneity in the matter distribution which has grown under gravity from primordial density perturbations traced by the CMB fluctuations. The ΛCDM model predicts how this peculiar velocity should fall off as the averaging scale is raised and the universe becomes sensibly homogeneous. However observations of the local 'bulk flow' are inconsistent with this expectation and convergence to the CMB frame is not seen. Moreover, the kinematic interpretation implies a corresponding dipole in the sky distribution of high redshift quasars, which is rejected by observations at 4.9σ. Hence the peculiar velocity corrections employed in supernova cosmology are inconsistent and discontinuous within the data. The acceleration of the Hubble expansion rate is in fact anisotropic at 3.9σ and aligned with the bulk flow. Thus dark energy could be an artefact of analysing data assuming that we are idealised observers in an FLRW universe, when in fact the real universe is inhomogeneous and anisotropic out to distances large enough to impact on cosmological analyses.

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Research paper thumbnail of Measurement of the Atmospheric \nu_e Flux in IceCube

Physical Review Letters, Apr 10, 2013

We report the first measurement of the atmospheric electron neutrino flux in the energy range bet... more We report the first measurement of the atmospheric electron neutrino flux in the energy range between approximately 80 GeV and 6 TeV, using data recorded during the first year of operation of IceCube's DeepCore low-energy extension. Techniques to identify neutrinos interacting within the DeepCore volume and veto muons originating outside the detector are demonstrated. A sample of 1029 events is observed in 281 days of data, of which 496±66(stat)±88(syst) are estimated to be cascade events, including both electron neutrino and neutral current events. The rest of the sample includes residual backgrounds due to atmospheric muons and charged current interactions of atmospheric muon neutrinos. The flux of the atmospheric electron neutrinos is consistent with models of atmospheric neutrinos in this energy range. This constitutes the first observation of electron neutrinos and neutral current interactions in a very large volume neutrino telescope optimized for the TeV energy range.

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Research paper thumbnail of Measurement of the cosmic ray energy spectrum with IceTop-73

Physical Review D, 2013

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Research paper thumbnail of Neutron oscillations and the primordial magnetic field

Astrophysical Letters Communications, 1989

... Reprints available directly from the publisher Printed in the United States of America Photoc... more ... Reprints available directly from the publisher Printed in the United States of America Photocopying permitted by license only Neutron Oscillations and the Primordial Magnetic Field SUBIR SARKAR Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX1 1 OQX, UK ...

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Research paper thumbnail of South Pole glacial climate reconstruction from multi-borehole laser particulate stratigraphy

Journal of Glaciology, 2013

The IceCube Neutrino Observatory and its prototype, AMANDA, were built in South Pole ice, using p... more The IceCube Neutrino Observatory and its prototype, AMANDA, were built in South Pole ice, using powerful hot-water drills to cleanly bore >100 holes to depths up to 2500 m. The construction of these particle physics detectors provided a unique opportunity to examine the deep ice sheet using a variety of novel techniques. We made high-resolution particulate profiles with a laser dust logger in eight of the boreholes during detector commissioning between 2004 and 2010. The South Pole laser logs are among the most clearly resolved measurements of Antarctic dust strata during the last glacial period and can be used to reconstruct paleoclimate records in exceptional detail. Here we use manual and algorithmic matching to synthesize our South Pole measurements with ice-core and logging data from Dome C, East Antarctica. We derive impurity concentration, precision chronology, annual-layer thickness, local spatial variability, and identify several widespread volcanic ash depositions usefu...

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Research paper thumbnail of The integrated Sachs-Wolfe imprint of cosmic superstructures: a problem for ΛCDM

Journal of Cosmology and Astroparticle Physics, 2012

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Research paper thumbnail of On the APM power spectrum and the CMB anisotropy: Evidence for a phase transition during inflation

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Research paper thumbnail of Testing astrophysical models for the PAMELA positron excess with cosmic ray nuclei

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Research paper thumbnail of Cosmic Neutrinos from Unstable Relic Particles

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Research paper thumbnail of Extremely high energy cosmic rays from relic particle decays

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Research paper thumbnail of CMB Anisotropy in the Decaying Neutrino Cosmology

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Research paper thumbnail of Ruling out a critical density baryonic universe

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Research paper thumbnail of Neutrino diagnostics of ultra-high energy cosmic ray protons

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Research paper thumbnail of Big Bang nucleosynthesis and physics beyond the Standard Model

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Research paper thumbnail of Cosmic rays, neutrinos & multi-messenger astronomy with IceCube

Physics News, 2021

on the IceCube experiment. Subsequently he was a Carlsberg Postdoctoral fellow at the Niels Bohr ... more on the IceCube experiment. Subsequently he was a Carlsberg Postdoctoral fellow at the Niels Bohr Institute, Copenhagen and is now a Reader in the Dept. of High Energy Physics at TIFR. His interests include the phenomenology of astroparticle physics and cosmology, as well as the statistical interpretation of empirical data.

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Research paper thumbnail of Evidence for an inflationary phase transition from the LSS and CMB anisotropy data

Nuclear Physics B - Proceedings Supplements, 2001

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Research paper thumbnail of The solution to Olbers' paradox

Physics World, 2002

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Research paper thumbnail of Acceleration of Electrons in Supernova Remnants

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Research paper thumbnail of Study of Homalite and Pershore ADC (CR-39) Plastic Detector for Cosmic-Ray Composition Studies

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Research paper thumbnail of Cosmological Parameters and the Baryon Density from CMB and Galaxy Fluctuations

Astrophysics and Space Science Library, 2002

Recent observations of the CMB fluctuations (eg. Boomerang and Maxima) show an anomalously low se... more Recent observations of the CMB fluctuations (eg. Boomerang and Maxima) show an anomalously low second acoustic peak, which has been interpreted as an indication of a high primordial Big-Bang nucleosynthesis (BBN) baryon density. We reexamine the prediction by Adams et al. (1997) that this would be the consequence of a &amp;quot;step&amp;quot; in the primordial spectrum induced by a spontaneous symmetry

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Research paper thumbnail of A hadronic explanation of the lepton anomaly

Journal of Physics: Conference Series, 2014

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Research paper thumbnail of Neutrino detectors as probes of massive cosmological relics

Nuclear Physics B - Proceedings Supplements, 1992

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Research paper thumbnail of Introduction to Big Bang Cosmology

Recent Developments in Particle Physics and Cosmology, 2001

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Research paper thumbnail of The Standard Big Bang Cosmology

Astrophysics and Space Science Library, 2000

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Research paper thumbnail of Does dark energy really exist?

Scientific American, 2009

Scientific American is the world's premier magazine of scientific discovery and technologica... more Scientific American is the world's premier magazine of scientific discovery and technological innovation for the general public. Readers turn to it for a deep understanding of how science and technology can influence human affairs and illuminate the natural world.

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Research paper thumbnail of Are Gamma Ray Sources also Cosmic Ray Sources?

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Research paper thumbnail of The evolution of the radio spectra of supernova remnants

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Research paper thumbnail of Lifetime significance

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Research paper thumbnail of Questioning the concordance model

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Research paper thumbnail of Darkness Visible

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Research paper thumbnail of Probing the high energy universe with cosmic rays and neutrinos

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Research paper thumbnail of The'PAMELA anomaly'indicates a nearby cosmic ray accelerator

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Research paper thumbnail of Does Cosmological Structure Formation Require Dark Energy?

Cral-Ipnl: Dark Energy and Dark Matter: Observations, Experiments and Theories, 2009

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Research paper thumbnail of Gamma-Ray Emission from Supernova Remnants

Astrophysics and Space Science Library, 2004

ABSTRACT

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Research paper thumbnail of Palaeogeography of microbial mats in terrigenous clastics–environmental distribution of associated sedimentary features and the role of geologic time

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Research paper thumbnail of Off-platform dolomitization in Proterozoic Chanda Limestone Formation, Adilabad, Andhra Pradesh

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Research paper thumbnail of Tectonics and Sedimentation of Proterozoic Basins of Peninsular India

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Research paper thumbnail of Multi-Mode Generation of Carbonate Tabular Intraclast Deposits: Unnamed Proterozoic Formation, Maharashtra

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Research paper thumbnail of Capacitive Micromachined Ultrasonic Transducer Based Gas Sensor Modeling and Simulation

Applied Mechanics and Materials, 2011

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Research paper thumbnail of 6 (b). Some unusual and/or problematic inferred mat-related

... All photos: Subir Sarkar, Santanu Banerjee, Snehasis Chakraborty, Pradip Samanta. Page 4. In:... more ... All photos: Subir Sarkar, Santanu Banerjee, Snehasis Chakraborty, Pradip Samanta. Page 4. In: Atlas of microbial mat features preserved within the clastic rock record, Schieber, J., Bose, PK, Eriksson, PG, Banerjee, S., Sarkar, S., Altermann, W., and Catuneau, O., (Eds.)J ...

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Research paper thumbnail of Intermittency in the Slow Particle Emission During Hadron–Nucleus Interactions

Czechoslovak journal of …, 2003

The study of the angular distribution of slow particles during high energy hadron-nucleus interac... more The study of the angular distribution of slow particles during high energy hadron-nucleus interaction indicates that emission of slow particles takes place from a thermally non-equilibrated system. This evidence has come out from the presence of intermittency — a phenomenon ...

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Research paper thumbnail of Ordered, stoichiometric and sucrosic dolomitization: A result of prolonged exposure to warm sea water: Proterozoic Chanda Limestone, Adilabad, India

Carbonates and Evaporites, 1993

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Research paper thumbnail of Intermolecular structure of liquid D- tert -butanol by neutron-diffraction data

Applied Physics A: Materials Science & Processing, 2002

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Research paper thumbnail of Dissolution seams: Some observations from the Proterozoic Chanda Limestone, Adilabad, India

Carbonates and Evaporites, 1996

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Research paper thumbnail of C12-enrichment along intraformational unconformities within Proterozoic Bhander Limestone, Son valley, India and its implication

Carbonates and Evaporites, 1998

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Research paper thumbnail of Orientation correlation of neopentane molecules in liquid state through available diffraction data

Chemical Physics, 2001

ABSTRACT

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Research paper thumbnail of Orientation correlation and intermolecular structure of GeCl4, VCl4 and other tetrachloride liquids

Chemical Physics, 2007

ABSTRACT

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Research paper thumbnail of A low temperature in situ facile technique to enhance ultraviolet emission of zinc oxide nanorods and its mechanistic insights

Chemical Physics Letters, 2011

ABSTRACT A novel approach for enhancing the ultraviolet emission properties of ZnO nanorods (NRs)... more ABSTRACT A novel approach for enhancing the ultraviolet emission properties of ZnO nanorods (NRs) using a in situ facile hydrothermal technique in the presence of excess zinc metal in addition to zinc acetate in the precursor has been demonstrated. The photoluminescence studies show a higher ultra-violet (UV) emission (IUV/IVIS ∼38) in the NRs grown with 15mM excess zinc metal compared to that (IUV/IVIS ∼5.5) of the NRs grown without excess metal. Electron paramagnetic resonance studies reveal a higher intensity signal at g ∼1.95 for NRs grown with 15mM excess zinc indicating the presence of paramagnetic Zni in higher concentration.

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Research paper thumbnail of Multiple level optimization for high frequency ac mobility in GaAs quantum wells under hot-electron condition

Computational Materials Science, 2004

The basic intention of this work is to determine the optimum values of the system parameters of G... more The basic intention of this work is to determine the optimum values of the system parameters of GaAs quantum wells under hot electron condition. The energy loss to LO phonons and the momentum loss through the interaction with LO phonons, deformation acoustic phonons and ionized impurities are incorporated in the calculations. The small-signal ac mobility is calculated on the basis of a heated drifted Fermi–Dirac distribution function for getting the desired ac mobility at a particular dc biasing field. The parameter optimization is done by employing the genetic algorithms to get the desired small-signal ac mobility. This will make feasible the fabrication of a variety of new quantum devices with desired properties.

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Research paper thumbnail of Understanding the applicability and limitations of Wang–Landau method for biomolecules: Met-enkephalin and Trp-cage

Chemical Physics Letters, 2011

Wang–Landau (WL) method is an efficient way to calculate density of states (DOS), thermodynamic a... more Wang–Landau (WL) method is an efficient way to calculate density of states (DOS), thermodynamic and structural properties of a system. In this work, two different flavors of WL method, its original implementation and the t−1 method of Belardinelli et al., are applied to met-enkephalin and Trp-cage to understand their efficacy and limitations. Accurate thermodynamic and structural quantities are calculated for

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Research paper thumbnail of Power Law Tails of Single-Photon States in Parametric Down-Conversion

Europhysics Letters (EPL), 1988

ABSTRACT Photodetection probabilities are calculated far from a nonlinear medium or atom in param... more ABSTRACT Photodetection probabilities are calculated far from a nonlinear medium or atom in parametric down-conversion. These probabilities may be heuristically interpreted as due to single-photon states with photodetection amplitudes having a power law tail of exponent 5/4 in configuration space.

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Research paper thumbnail of Prime controls on Archaean–Palaeoproterozoic sedimentation: Change over time

Gondwana Research, 2007

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Research paper thumbnail of Performance analysis of AODV, AODVUU, AOMDV and RAODV over IEEE 802.15.4 in wireless sensor networks

In this paper the focus is on the performance study of four routing protocols, namely AODV, AODVU... more In this paper the focus is on the performance study of four routing protocols, namely AODV, AODVUU, RAODV and AOMDV. We call these protocols AODV family of protocols as all these protocols consider AODV as the base routing protocol upon which these protocols are improved. Even though AODV and AODVUU are not different protocols, we wanted to see if there

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Research paper thumbnail of Measurement of Radon and Thoron Concentrations in the Indoor Atmosphere and Drinking Water of Eastern Doon Valley, India

Indoor and Built Environment, 2003

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