Performance study of Cascade Reconstruction at the Glashow Resonance in IceCube

Abstract

IceCube is a one cubic kilometer neutrino telescope at the South Pole. Its primary goal is to discover high energy cosmic neutrinos and anti-neutrinos from astrophysical sources. Observation of the spectrum near the characteristic energy $E_\nu\approx6.3\,\mbox{PeV}$ of the Glashow resonance, the interaction of anti-neutrinos with atomic electrons via $\bar{\nu}_e+e^{-}\rightarrow W^{-}$, is of particular interest. Since the cross section for this process can be calculated from first principles, it is possible to quantify separately the fluxes for neutrinos and anti-neutrinos if the resonance is observed above a continuum. In turn, such a separation will give unique insights into the astrophysics properties of the sources.\\ We conducted the first IceCube performance studies and optimizations for likelihood-based algorithms to reconstruct (anti-)neutrino-induced particle showers (cascades) in the energy range of the Glashow resonance using simulated data from electron (anti-)neutrino Monte Carlo generators and detector response simulations. For hadronic showers in the energy range $1\,{\rm{PeV}}<E_{\nu}<10\,{\rm{PeV}}$ that are well contained within the IceCube instrumented volume, we achieved an energy resolution of $10\%< \sigma \left( \Delta E / E\right)<14\%$ depending on the ice model and the shower position in the detector. The position and direction resolution varied between $1.1\,\mbox{m}<\sigma\left(\Delta x,\Delta y, \Delta z\right)<4.2\,\mbox{m}$ and $8^\circ<\theta_{RMS}<27^\circ$, respectively. We verified and refined the methods on experimental data using an in-situ laser as a pulsed light source with constant brightness and a single wavelength of $\lambda=337\,\mbox{nm}$. The energy resolution for reconstructed laser events was found to be $\sigma\left(\Delta E/E\right)=1.8\%$ from the reconstructed energy of $E\pm\delta E_{stat}=\left(527\pm9\right)\,{\rm{TeV}}$, where the uncertainty is statistical. For $83\%-92\%$ of the laser events, we reconstructed the zenith angle to within $\Delta\theta<2^\circ$ and found a position resolution of $0.3\,\mbox{m}<\sigma\left(\Delta x,y,z\right)<0.4\,\mbox{m}$ from the reconstructed positions. The existence of considerable systematic effects is evidenced by a shift of the reconstructed laser position from the true position by $3.7\,\mbox{m}$. Such effects arise, for example, from differences in photon propagation at different wavelengths. The laser data represent a best case scenario, in view of its illumination of the detector and the monochromatic laser emission. \\ The simulation results confirm IceCube's capability to observe astrophysical neutrino fluxes near the Glashow resonance and form a first demonstration, corroborated by an analysis of laser data, of IceCube's pointing capability with the cascade detection channel in this energy range.