Toward Hybrid Optical/Radio/Acoustic Detection of EeV Neutrinos

Astrophysical neutrinos at EeV energies promise to be an interesting source for astrophysics and particle physics. Detecting the predicted cosmogenic (Greisen-ZatsepinKusmin, “GZK”) neutrinos at 10 10 eV would test models of cosmic ray production at these energies and probe particle physics at ∼100 TeV center-of-mass energy. IceCube may be able to detect ∼10 GZK events per year with an extension including optical, radio, and acoustic receivers sparsely arrayed surrounding the optical core. Such a detector would feature crosscalibration with coincident events and would allow superior background rejection capability, energy and direction resolution, and confidence in discovered signals compared to single-method detectors. We present estimates of the neutrino effective volume for such a hybrid array both with the single-method sub-arrays independently and requiring combinations of sub-arrays to detect the same events. We also present ideas on hybrid event reconstruction and results from a proof-of-principle Monte Carlo test of a hybrid reconstruction algorithm. 1. Sub-Array and Coincident Effective Volumes Less than one GZK event per year is expected to be detected by km optical neutrino telescopes. To increase this rate to greater than 10 per year in order to do physics and astronomy with angular, temporal, and spectral distributions, alternative techniques such as radio and acoustic are necessary. However, while both methods have been verified with proton bunches in accelerator tests, neither has detected a neutrino. Although both the optical and radio methods may be near the threshold of discovering GZK neutrinos, either method will require careful separation from backgrounds and may require verification with an independent method. It may be possible to build a hybrid detector that can detect a large number of radio and acoustic events, a large fraction of which are in coincidence with one another and a small fraction of which are also detected by an optical detector. A signal seen in coincidence between two of the three methods would uniquely confirm the signal and constrain its parameters better than any single method alone. A large hybrid detector could be realized by expanding the IceCube observatory currently under construction at the South Pole. We estimated the sensitivity of such a detector by exposing all three components to a common Monte Carlo event set and identifying events detected by each method alone and by each combination of multiple methods. We used a configuration consisting of a “small” optical array overlapped by a “large” acoustic/radio array with a similar number of −5 −4 −3 −2 −1 0 1 2 3 4 5 −5 −4 −3 −2 −1 0 1 2 3 4 5 x (km) y (k m ) IceCube optical radio/acoustic Figure 1. Geometry of the simulated hybrid array. 17.5 18 18.5 19 19.5 20 10 0 10 1 10 2 10 3 Log 10 [Eν/eV] V ef f ( km 3 ) A (16.0)