Pijushpani Bhattacharjee

Improved dark matter search results from PICO-2L Run 2

New data are reported from a second run of the 2-liter PICO-2L C3F8 bubble chamber with a total exposure of 129 kg-days at a thermodynamic threshold energy of 3.3 keV. These data show that measures taken to control particulate contamination in the superheated fluid resulted in the absence of the anomalous background events observed in the first run of this bubble chamber. One single nuclear-recoil event was observed in the data, consistent both with the predicted background rate from neutrons and with the observed rate of unambiguous multiple-bubble neutron scattering events. The chamber exhibits the same excellent electron-recoil and alpha decay rejection as was previously reported. These data provide the most stringent direct detection constraints on weakly interacting massive particle (WIMP)-proton spin-dependent scattering to date for WIMP masses <50 GeV/c2.

Dark Matter Search Results from the PICO−60C3F8 Bubble Chamber

New results are reported from the operation of the PICO-60 dark matter detector, a bubble chamber filled with 52xa0kg of C_{3}F_{8} located in the SNOLAB underground laboratory. As in previous PICO bubble chambers, PICO-60u2009C_{3}F_{8} exhibits excellent electron recoil and alpha decay rejection, and the observed multiple-scattering neutron rate indicates a single-scatter neutron background of less than one event per month. A blind analysis of an efficiency-corrected 1167-kgu2009day exposure at a 3.3-keV thermodynamic threshold reveals no single-scattering nuclear recoil candidates, consistent with the predicted background. These results set the most stringent direct-detection constraint to date on the weakly interacting massive particle (WIMP)-proton spin-dependent cross section at 3.4×10^{-41}u2009u2009cm^{2} for a 30-GeVu2009c^{-2} WIMP, more than 1 order of magnitude improvement from previous PICO results.

Two component dark matter: a possible explanation of 130 GeV γ-ray line from the galactic centre

Recently there has been a hint of a gamma-ray line at 130 GeV originated from the galactic centre after the analysis of the Fermi-LAT satellite data. Being monochromatic in nature, it rules out the possibility of having its astrophysical origin and there has been a speculation that this line could be originated from dark matter annihilation. In this work, we propose a two component dark matter scenario where an extension of the Standard Model by an inert Higgs doublet and a gauge singlet scalar concocted with Z{sub 2} × Z{sub 2} symmetry, is considered. We find that our scenario can not only explain the 130 GeV gamma-ray line through dark matter annihilation but also produce the correct dark matter relic density. We have used the Standard Model Higgs mass around 125 GeV as intimated by the LHC data.

Direct detection of WIMPs: implications of a self-consistent truncated isothermal model of the Milky Way's dark matter halo

Direct detection of Weakly Interacting Massive Particle (WIMP) candidates of Dark Matter (DM) is studied within the context of a self-consistent truncated isothermal model of the finite-size dark halo of the Galaxy. The halo model, based on the ``King model of the phase space distribution function of collisionless DM particles, takes into account the modifications of the phase-space structure of the halo due to the gravitational influence of the observed visible matter in a self-consistent manner. The parameters of the halo model are determined by a fit to a recently determined circular rotation curve of the Galaxy that extends up to ~ 60 kpc. Unlike in the Standard Halo Model (SHM) customarily used in the analysis of the results of WIMP direct detection experiments, the velocity distribution of the WIMPs in our model is non-Maxwellian with a cut-off at a maximum velocity that is self-consistently determined by the model itself. For our halo model that provides the best fit to the rotation curve data, the 90% C.L. upper limit on the WIMP-nucleon spin-independent cross section from the recent results of the CDMS-II experiment, for example, is ~ 5.3 × 10−8 pb at a WIMP mass of ~ 71 GeV. We also find, using the original 2-bin annual modulation amplitude data on the nuclear recoil event rate seen in the DAMA experiment, that there exists a range of small WIMP masses, typically ~ 2–16 GeV, within which DAMA collaborations claimed annual modulation signal purportedly due to WIMPs is compatible with the null results of other experiments. These results, based as they are on a self-consistent model of the dark matter halo of the Galaxy, strengthen the possibility of low-mass (10 GeV) WIMPs as a candidate for dark matter as indicated by several earlier studies performed within the context of the SHM. A more rigorous analysis using DAMA bins over smaller intervals should be able to better constrain the ``DAMA regions in the WIMP parameter space within the context of our model.

Study of low frequency acoustic signals from superheated droplet detector

Abstract The bubble nucleation process in superheated droplet detector (SDD) is associated with the emission of an acoustic pulse that can be detected by an acoustic sensor. We have studied the neutron and gamma-ray induced nucleation events in a SDD with the active liquid R-12 (CCl 2 F 2 , b.p. −29.8xa0°C) using a condenser microphone sensor. A comparative study in the low frequency region (∼0–10xa0kHz) for the neutron and gamma-ray induced nucleation is presented here. From the analysis of the waveforms we observe a significant difference between the neutron and gamma-ray induced acoustic events.

Neutron-gamma discrimination by pulse analysis with superheated drop detector

Abstract Superheated drop detector (SDD) consisting of drops of superheated liquid of halocarbon is irradiated to neutrons and gamma-rays from 252Cf fission neutron source and 137Cs gamma source, respectively, separately. Analysis of pulse height of signals at the neutron and gamma-ray sensitive temperature provides significant information on the identification of neutron and gamma-ray induced events.

Neutrinos from WIMP annihilation in the Sun : Implications of a self-consistent model of the Milky Way's dark matter halo

Upper limits on the spin-independent (SI) as well as spin-dependent (SD) elastic scattering cross sections of low mass (� 2 – 20 GeV) WIMPs (Weakly Interacting Massive Particles) with protons, imposed by the upper limit on the neutrino flux from WIMP annihilation in the Sun given by the Super-Kamiokande (S-K) experiment, and their compatibility with the “DAMA-compatible” regions of the WIMP parameter space — the regions of the WIMP mass versus cross section parameter space within which the annual modulation signal observed by the DAMA/LIBRA experiment is compatible with the null results of other direct detection experiments — are studied within the frame work of a self-consistent model of the finite-size dark matter (DM) halo of the Galaxy. The halo model includes the gravitational influence of the observed visible matter of the Galaxy on the phase space distribution function of the WIMPs constituting the Galaxy’s DM halo in a self-consistent manner. Unlike in the “Standard Halo Model” (SHM) used in earlier analyses, the velocity distribution of the WIMPs in our model is non-Maxwellian, with a high-velocity cutoff determined self-consistently by the model itself. The parameters of the model are determined from a fit to the rotation curve data of the Galaxy. We find that, for our best fit halo model, for SI interaction, while the S-K upper limits do not place additional restrictions on the DAMA-compatible region of the WIMP parameter space

Upper limit on the cosmic gamma-ray burst rate from high energy diffuse neutrino background

We derive upper limits on the ratio f{sub GRB/CCSN}(z){identical_to}R{sub GRB}(z)/R{sub CCSN}(z){identical_to}f{sub GRB/CCSN}(0)(1+z){sup {alpha}}, the ratio of the rate, R{sub GRB}, of long-duration gamma-ray bursts (GRBs) to the rate, R{sub CCSN}, of core-collapse supernovae (CCSNe) in the Universe (z being the cosmological redshift and {alpha}{>=}0), by using the upper limit on the diffuse TeV-PeV neutrino background given by the AMANDA-II experiment in the South Pole, under the assumption that GRBs are sources of TeV-PeV neutrinos produced from decay of charged pions produced in p{gamma} interaction of protons accelerated to ultrahigh energies at internal shocks within GRB jets. For the assumed concordance model of cosmic star formation rate, R{sub SF}, with R{sub CCSN}(z){proportional_to}R{sub SF}(z), our conservative upper limits are f{sub GRB/CCSN}(0){ =}1, already more restrictive than) the current upper limit on this ratio inferred from other astronomical considerations, thus providing a useful independent probe of and constraint on the CCSN-GRB connection. Nondetection of a diffuse TeV-PeV neutrino background by the upcoming IceCube detector in the South Pole after three years of operation, for example, will bring down the upper limit on f{sub GRB/CCSN}(0)morexa0» to below a few x10{sup -5} level, while a detection will confirm the hypothesis of proton acceleration to ultrahigh energies in GRBs and will potentially also yield the true rate of occurrence of these events in the Universe.«xa0less

Constraints on the synchrotron self-Compton mechanism of TeV gamma ray emission from the Milagro TeV source MGRO J2019+37 within the pulsar wind nebula scenario

Origin of the TeV gamma ray emission from MGRO J2019+37 discovered by the Milagro experiment is investigated within the pulsar wind nebula (PWN) scenario using multiwavelength information on sources suggested to be associated with this object. We find that the synchrotron self-Compton (SSC) mechanism of origin of the observed TeV gamma rays within the PWN scenario is severely constrained by the upper limit on the radio flux from the region around MGRO J2019+37 given by the Giant Metrewave Radio Telescope (GMRT) as well as by the x-ray flux upper limit from SWIFT/XRT. Specifically, for the SSC mechanism to explain the observed TeV flux from MGRO J2019+37 without violating the GMRT and/or Swift/XRT flux upper limits in the radio and x-ray regions, respectively, the emission region must be extremely compact with the characteristic size of the emission region restricted to < ∼ O(10 −4 pc) for an assumed distance of ∼ few kpc to the source. This is at least four orders of magnitude less than the characteristic size of the emission region typically invoked in explaining the TeV emission through the SSC mechanism within the PWN scenario. On the other hand, inverse Compton (IC) scattering of the nebular high energy electrons on the cosmic microwave background (CMB) photons can, for reasonable ranges of values of various parameters, explain the observed TeV flux without violating the GMRT and/or SWIFT/XRT flux bounds.

Detecting supernova neutrinos with iron and lead detectors

Supernova (SN) neutrinos can excite the nuclei of various detector materials beyond their neutron emission thresholds through charged current (CC) and neutral current (NC) interactions. The emitted neutrons, if detected, can be a signal for the supernova event. Here we present the results of our study of SN neutrino detection through the neutron channel in lead (