Particle theory

Abstract:

A search for an excess of muon neutrinos from dark matter annihilations in the Sun has been performed with the AMANDA-II neutrino telescope using data collected in 812 days of live time between 2001 and 2006 and 149 days of live time collected with the AMANDA-II and the 40-string configuration of IceCube during 2008 and early 2009. No excess over the expected atmospheric neutrino background has been observed. We combine these results with the previously published IceCube limits obtained with data taken during 2007 to obtain a total live time of 1065 days. We provide an upper limit at 90% confidence level on the annihilation rate of captured neutralinos in the Sun, as well as the corresponding muon flux limit at the Earth, both as functions of the neutralino mass in the range 50-5000GeV. We also derive a limit on the neutralino-proton spin-dependent and spin-independent cross section. The limits presented here improve the previous results obtained by the collaboration between a factor of 2 and 5, as well as extending the neutralino masses probed down to 50GeV. The spin-dependent cross section limits are the most stringent so far for neutralino masses above 200GeV, and well below direct search results in the mass range from 50GeV to 5TeV. © 2012 American Physical Society.

Abstract:

The first dedicated search for ultra-high energy (UHE) tau neutrinos of astrophysical origin was performed using the IceCube detector in its 22-string configuration with an instrumented volume of roughly 0.25 km^3. The search also had sensitivity to UHE electron and muon neutrinos. After application of all selection criteria to approximately 200 live-days of data, we expect a background of 0.60 +/- 0.19 (stat.) $^{+0.56}_{-0.58}$ (syst.) events and observe three events, which after inspection emerge as being compatible with background but are kept in the final sample. Therefore, we set an upper limit on neutrinos of all-flavors from UHE astrophysical sources at 90% CL of $E^{2} \Phi(\nu_{x}) < 16.3 * 10^-8 GeV cm^-2 sr^-1 s^-1 over an estimated primary neutrino energy range of 340 TeV to 200 PeV.

Abstract:

We report on a measurement of the cosmic ray energy spectrum with the IceTop air shower array, the surface component of the IceCube Neutrino Observatory at the South Pole. The data used in this analysis were taken between June and October, 2007, with 26 surface stations operational at that time, corresponding to about one third of the final array. The fiducial area used in this analysis was 0.122 km^2. The analysis investigated the energy spectrum from 1 to 100 PeV measured for three different zenith angle ranges between 0{\deg} and 46{\deg}. Because of the isotropy of cosmic rays in this energy range the spectra from all zenith angle intervals have to agree. The cosmic-ray energy spectrum was determined under different assumptions on the primary mass composition. Good agreement of spectra in the three zenith angle ranges was found for the assumption of pure proton and a simple two-component model. For zenith angles {\theta} < 30{\deg}, where the mass dependence is smallest, the knee in the cosmic ray energy spectrum was observed between 3.5 and 4.32 PeV, depending on composition assumption. Spectral indices above the knee range from -3.08 to -3.11 depending on primary mass composition assumption. Moreover, an indication of a flattening of the spectrum above 22 PeV were observed.

Abstract:

In recent years several approaches to quantum gravity have found evidence for a scale dependent spectral dimension of space-time varying from four at large scales to two at small scales of order of the Planck length. The first evidence came from numerical results on four-dimensional causal dynamical triangulations (CDT) [Ambjorn et al., Phys. Rev. Lett. 95 (2005) 171]. Since then little progress has been made in analytically understanding the numerical results coming from the CDT approach and showing that they remain valid when taking the continuum limit. Here we argue that the spectral dimension can be determined from a model with fewer degrees of freedom obtained from the CDTs by "radial reduction". In the resulting "toy" model we can take the continuum limit analytically and obtain a scale dependent spectral dimension varying from four to two with scale and having functional behaviour exactly of the form which was conjectured on the basis of the numerical results.