Prof Subir Sarkar

Abstract:

The Precision IceCube Next Generation Upgrade (PINGU) is a proposed low-energy in-fill array of the IceCube Neutrino Observatory. Leveraging technology proven with IceCube, PINGU will feature the world's largest effective volume for neutrinos at an energy threshold of a few GeV, improving the sensitivity to several aspects of neutrino oscillation physics at modest cost. With its unprecedented statistical sample of low-energy atmospheric neutrinos, PINGU will have highly competitive sensitivity to $\nu_{\mu}$ disappearance, the $\theta_{23}$ octant, and maximal mixing, will make the world's best $\nu_{\tau}$ appearance measurement, allowing a unique probe of the unitarity of the PMNS mixing matrix, and will be able to distinguish the neutrino mass ordering at $3\sigma$ significance with less than 4 years of data. PINGU can also extend the indirect search for solar WIMP dark matter complimentary to the on-going and planned direct dark matter experiments. At the lower end of the energy range, PINGU may use neutrino tomography to directly probe the composition of the Earth's core. With its increased module density, PINGU will improve IceCube's sensitivity to galactic supernova neutrino bursts and enable it to extract the neutrino energy spectral shape.

Abstract:

A critical review is given of the current status of cosmological nucleosynthesis. In the framework of the Standard Model with 3 types of relativistic neutrinos, the baryon-to-photon ratio, $\eta$, corresponding to the inferred primordial abundances of deuterium and helium-4 is consistent with the independent determination of $\eta$ from observations of anisotropies in the cosmic microwave background. However the primordial abundance of lithium-7 inferred from observations is significantly below its expected value. Taking systematic uncertainties in the abundance estimates into account, there is overall concordance in the range $\eta = (5.7-6.7)\times 10^{-10}$ at 95% CL (corresponding to a cosmological baryon density $\Omega_B h^2 = 0.021 - 0.025$). The D and He-4 abundances, when combined with the CMB determination of $\eta$, provide the bound $N_\nu=3.28 \pm 0.28$ on the effective number of neutrino species. Other constraints on new physics are discussed briefly.

Abstract:

We present the results of a first search for self-annihilating dark matter in nearby galaxies and galaxy clusters using a sample of high-energy neutrinos acquired in 339.8 days of live time during 2009/10 with the IceCube neutrino observatory in its 59-string configuration. The targets of interest include the Virgo and Coma galaxy clusters, the Andromeda galaxy, and several dwarf galaxies. We obtain upper limits on the cross section as a function of the weakly interacting massive particle mass between 300 GeV and 100 TeV for the annihilation into bb̄, W+W-, τ+τ -, μ+μ-, and νν̄. A limit derived for the Virgo cluster, when assuming a large effect from subhalos, challenges the weakly interacting massive particle interpretation of a recently observed GeV positron excess in cosmic rays. © 2013 American Physical Society.

Abstract:

We report on the search for neutrino-induced particle-showers, so-called cascades, in the IceCube-40 detector. The data for this search was collected between April 2008 and May 2009 when the first 40 IceCube strings were deployed and operational. Three complementary searches were performed, each optimized for different energy regimes. The analysis with the lowest energy threshold (2 TeV) targeted atmospheric neutrinos. A total of 67 events were found, consistent with the expectation of 41 atmospheric muons and 30 atmospheric neutrino events. The two other analyses targeted a harder, astrophysical neutrino flux. The analysis with an intermediate threshold of 25 TeV lead to the observation of 14 cascade-like events, again consistent with the prediction of 3.0 atmospheric neutrino and 7.7 atmospheric muon events. We hence set an upper limit of $E^2 \Phi_{lim} \leq 7.46\times10^{-8}\,\mathrm{GeV sr^{-1} s^{-1} cm^{-2}}$ (90% C.L.) on the diffuse flux from astrophysical neutrinos of all neutrino flavors, applicable to the energy range 25 TeV to 5 PeV, assuming an $E_{\nu}^{-2}$ spectrum and a neutrino flavor ratio of 1:1:1 at the Earth. The third analysis utilized a larger and optimized sample of atmospheric muon background simulation, leading to a higher energy threshold of 100 TeV. Three events were found over a background prediction of 0.04 atmospheric muon events and 0.21 events from the flux of conventional and prompt atmospheric neutrinos. Including systematic errors this corresponds to a $2.7\sigma$ excess with respect to the background-only hypothesis. Our observation of neutrino event candidates above 100 TeV complements IceCube's recently observed evidence for high-energy astrophysical neutrinos.