NARC Neutrino Experiment

Abstract

Cosmic rays have historically been the source of much of our information about the extraterrestrial world. It is believed that among the most energetic cosmic rays are those which may be produced by massive black holes which could exist at the centers of some galaxies (aka "Active Galactic Nuclei", or "AGN", or radio-emitting galaxies called 'blazars'). Additional ultra-high energy neutrinos may be produced by the (as-yet-unidentified) process responsible for the ultra-high energy cosmic rays observed in present Extensive Air Shower experiments, such as the Auger Experiment. We describe an on-going experimental effort to detect ultra high energy electron neutrinos through their interactions with ice molecules in the Antarctic icecap, based on the principle of "radio coherence". Experimentally, we measure a long-wavelength (radiofrequency) pulse resulting from this interaction. A prototype experiment ("NARC") presently operating at the South Pole is described.

This experimental effort has been made possible only through the cooperation, assistance, and support of the IceCube collaboration, and the financial support of the National Science Foundation's Office of Polar Programs through grant number 0826747, the State of Kansas, and the Cottrell Research Corporation.

Longer Description

Conventional wisdom tells us that the matter which comprises the known universe was created in a cataclysmic explosion some 10-20 billion years ago. In this explosion, massive particles, such as protons, neutrons, and electrons, condensed out of the fireball of energy that comprised this "Big Bang". Stars formed as these particles became bound to each other; galaxies formed as these stars clumped themselves together under their mutual gravitational attraction. At the centers of many galaxies where the density of stars and gas should be the greatest, many models of galactic evolution tell us that supermassive black holes, some 100 thousand - 10 billion times more massive than our sun, are likely to have formed.

For some black holes, known as Active Galactic Nuclei, or "AGN", the region approximately 10-100 Schwarzschild radii from the black hole (or, about 10-100 times as large as the distance across the black hole) could be a place where extremely violent interactions between particles occur. In this region, the gravitational force sucking particles into the black hole is countered by the intense outward radiation pressure emanating from the black hole itself, resulting in an extremely turbulent region called a "shock front". In this region, violent interactions are capable of imparting ultra high energies ("UHE", corresponding to energies up to PeV-EeV - these are comparable to the kinetic energy of a tennis ball moving at 50 mph concentrated into a single proton) to the particles located around the shock front. Direct experimental detection of these ultra high energy particles would provide strong support for AGN theories and would also probe the Universe in a hitherto unmapped energy domain.

There are primarily three types of highly energetic, stable particles that should form in these violent interactions - protons, neutrinos and gamma rays. Unfortunately, both the protons and the gamma rays emerging from the black hole region are likely to suffer collisions with other stray particles (primarily Cosmic Microwave Background photons left over from the Big Bang which permeate all of the Universe with a density of approximately 100/cc) between their formation point and our observation point on earth. CMB photons are sufficiently copious in the Universe that they should form a relatively thick, opaque barrier between the earth and protons or gamma rays from AGN. Nevertheless, indications of AGN sources have successfully been observed optically all over the sky at a variety of energies. For example, quasars, energetic sources identified by their high red shifts, are candidates for older AGN. A more recently formed, albeit only modestly sized AGN is now believed to be located at the center of the Milky Way. If our understanding of AGN is correct, there should also be a substantial number of neutrinos arriving at earth, pointing back to extragalactic AGN sources. Because neutrinos have such small interaction cross-sections, they should remain unencumbered in their paths outward from the black holes. They therefore offer the possibility for terrestrial detection of AGN's.

In addition to the neutrinos believed to be emerging from the region around black holes, there should be other high energy neutrino sources, including pointlike cosmic powerhouses such as the Crab Nebula in our own galaxy, and the more distant, intensely energetic galaxy, Markarian 421. There is also, quite unexpectedly, recent evidence from extensive air shower experiments that another source of UHE particles may be lurking nearby. Recently, ultra high-energy particles in extensive air showers (EAS) have been observed by the Pierre Auger Experiment in Malargue, Argentina. These events have generated an extraordinary amount of interest within the scientific community, as their observation (if these particles are protons) 'guarantees' an accompanying ultra-high energy neutrino flux due to proton/CMB-photon collisions.

The goal of the NARC experiment is to construct a relatively low-cost experiment capable of detecting neutrinos such as those from AGN. Neutrinos, however, interact only rarely with conventional particle detectors, and therefore cannot be simply measured by using something like a usual laboratory Geiger Counter. Since neutrinos are so inert, we will use a very massive detector - the Antarctic icecap at the South Pole - to probe a very large volume sensitive to the rare neutrino interactions. We are using an array of radiowave receivers in the Antarctic ice and measure the radiation produced when UHE neutrinos interact with the ice.

The detection scheme takes advantage of a known effect called "Coherent Radio Emission", which is ideally matched to this type of experiment. Although we cannot detect neutrinos directly, we are able to detect the spray of particles produced on the very rare occasions when a neutrino (in our case, an electron neutrino) crashes head-on with a particle such as an ice molecule. The detection scheme works as follows: when a high-energy electron neutrino collides with nuclear target material such as polar ice, a charged current interaction can occur - the neutrino essentially converts itself into a more familiar particle, the electron. The electron produced in this interaction can initiate a large "shower" of particles (primarily electrons, positrons [antimatter partners of electrons] and gammas) which moves from the original neutrino-ice molecule collision point at nearly the velocity of light. As the shower moves through the ice, it emits radiation ("Cerenkov radiation") over a wide range of electromagnetic frequencies, ranging from high frequency, low wavelength visible light (wavelengths of approximately 0.000005 meters) and longer wavelength, radiowave frequencies (wavelengths approximately 10 cm - 1m, corresponding to the frequency range that a typical FM receiver might use.) The NARC experiment aims to measure the long-wavelength radiation with standard radiowave receivers.

Current Status

The RICE experiment, which began deployments in 1996 and initiated continuous data-taking in 1999, continues to run at the South Pole. Current (2009) efforts are focused on attempts to correlate data taken by the RICE experimental hardware with data taken by Digital Radio Modules (constructed by KU, U of Wisconsin, U of Maryland, and U of Hawaii) deployed in IceCube holes to depths of 250--1500 meters. In addition, we are attempting to identify the signature of air showers impacting the ice. A parallel experiment, based in TUNKA Valley, Siberia, attempts to correlate the radio air shower signal produced by extensive air showers with signals registered by a ground air Cherenkov array.

Last modified: April 10, 2009 / Contact: Dave Besson