Sibaji Raha

Electromagnetic probes of quark gluon plasma

Abstract In very energetic collisions of heavy ions, a novel state of matter — quark gluon plasma — is expected to be transiently formed. We review the suitability of electromagnetically interacting particles, dilepton pairs, single photons, diphotons, etc., as diagnostic tools for quark gluon plasma.

Aerosol Chemistry over a High Altitude Station at Northeastern Himalayas, India

Background There is an urgent need for an improved understanding of the sources, distributions and properties of atmospheric aerosol in order to control the atmospheric pollution over northeastern Himalayas where rising anthropogenic interferences from rapid urbanization and development is becoming an increasing concern. Methodology/Principal Findings An extensive aerosol sampling program was conducted in Darjeeling (altitude ∼2200 meter above sea level (masl), latitude 27°01′N and longitude 88°15′E), a high altitude station in northeastern Himalayas, during January–December 2005. Samples were collected using a respirable dust sampler and a fine dust sampler simultaneously. Ion chromatograph was used to analyze the water soluble ionic species of aerosol. The average concentrations of fine and coarse mode aerosol were found to be 29.5±20.8 µg m−3 and 19.6±11.1 µg m−3 respectively. Fine mode aerosol dominated during dry seasons and coarse mode aerosol dominated during monsoon. Nitrate existed as NH4NO3 in fine mode aerosol during winter and as NaNO3 in coarse mode aerosol during monsoon. Gas phase photochemical oxidation of SO2 during premonsoon and aqueous phase oxidation during winter and postmonsoon were the major pathways for the formation of SO4 2− in the atmosphere. Long range transport of dust aerosol from arid regions of western India was observed during premonsoon. The acidity of fine mode aerosol was higher in dry seasons compared to monsoon whereas the coarse mode acidity was higher in monsoon compared to dry seasons. Biomass burning, vehicular emissions and dust particles were the major types of aerosol from local and continental regions whereas sea salt particles were the major types of aerosol from marine source regions. Conclusions/Significance The year-long data presented in this paper provide substantial improvements to the heretofore poor knowledge regarding aerosol chemistry over northeastern Himalayas, and should be useful to policy makers in making control strategies.

Confinement and phase transitions

Using a density dependent quark mass to express non-perturbative interaction effects, we calculate the critical density for a phase transition between nuclear and quark matter atT=0.

Strange Quark Matter and the Mechanism of Confinement

Abstract It has been suggested in the literature that strange quark matter may be the true ground state of QCD. We investigate the model dependence of such speculations and show that the conclusions depend on the nature of the confinement mechanism.

Can cosmic strangelets reach the earth

The mechanism for the propagation of strangelets with low baryon number through the atmosphere of the Earth has been explored. It has been shown that, under suitable initial conditions, such strangelets may indeed reach depths near mountain altitudes with mass numbers and charges close to the observed values in cosmic ray experiments.

Relics of the cosmological QCD phase transition

The abundance and size distribution of quark nuggets (QN), formed a few microseconds after the big bang due to first order QCD phase transition in the early universe, has been estimated. It appears that stable QNs could be a viable candidate for cosmological dark matter. The evolution of baryon inhomogeneity due to evaporated (unstable) QNs are also examined.

Effect of dust and anthropogenic aerosols on columnar aerosol optical properties over Darjeeling (2200 m asl), eastern Himalayas, India.

Background The loading of atmospheric particulate matter (aerosol) in the eastern Himalaya is mainly regulated by the locally generated anthropogenic aerosols from the biomass burning and by the aerosols transported from the distance sources. These different types of aerosol loading not only affect the aerosol chemistry but also produce consequent signature on the radiative properties of aerosol. Methodology/Principal Findings An extensive study has been made to study the seasonal variations in aerosol components of fine and coarse mode aerosols and black carbon along with the simultaneous measurements of aerosol optical depth on clear sky days over Darjeeling, a high altitude station (2200 masl) at eastern Himalayas during the year 2008. We observed a heavy loading of fine mode dust component (Ca2+) during pre-monsoon (Apr – May) which was higher by 162% than its annual mean whereas during winter (Dec – Feb), the loading of anthropogenic aerosol components mainly from biomass burning (fine mode SO4 2− and black carbon) were higher (76% for black carbon and 96% for fine mode SO4 2−) from their annual means. These high increases in dust aerosols during pre-monsoon and anthropogenic aerosols during winter enhanced the aerosol optical depth by 25 and 40%, respectively. We observed that for every 1% increase in anthropogenic aerosols, AOD increased by 0.55% during winter whereas for every 1% increase in dust aerosols, AOD increased by 0.46% during pre-monsoon. Conclusion/Significance The natural dust transport process (during pre-monsoon) plays as important a role in the radiation effects as the anthropogenic biomass burning (during winter) and their differential effects (rate of increase of the AOD with that of the aerosol concentration) are also very similar. This should be taken into account in proper modeling of the atmospheric environment over eastern Himalayas.

Effect of confinement on the sound velocity in a quark-gluon plasma

Abstract The effect of confinement in a quark-gluon plasma is described through a density dependent quark mass. It is shown that the equation of state resulting from such a description is characterized by a sound velocity which agrees remarkably well with the phenomenologically extracted values from hadronic collisions over a large range of temperatures.

Quark Nuggets as Baryonic Dark Matter

The cosmic first-order phase transition from quarks to hadrons, occurring a few microseconds after the big bang, would lead to the formation of quark nuggets which would be stable on a cosmological timescale if the associated baryon number was larger than a critical value. We examine the possibility that these surviving quark nuggets may not only be viable candidates for cold dark matter but could even close the universe.

Calibration of a solid state nuclear track detector (SSNTD) with high detection threshold to search for rare events in cosmic rays

Abstract We have investigated a commercially available polymer for its suitability as a solid state nuclear track detector (SSNTD). We identified that polymer to be polyethylene terephthalate (PET) and found that it has a higher detection threshold compared to many other widely used SSNTDs which makes this detector particularly suitable for rare event search in cosmic rays as it eliminates the dominant low Z background. Systematic studies were carried out to determine its charge response which is essential before any new material can be used as an SSNTD. In this paper we describe the charge response of PET to 129 Xe, 78 Kr and 49 Ti ions from the REX-ISOLDE facility at CERN, present the calibration curve for PET and characterize it as a nuclear track detector.