Ultrahigh-energy cosmic neutrinos remain elusive

by RaoOfPhysics | 07 Jun 2015

No signs of tell-tale particles despite years of data collection

We are all bathed in a constant stream of particles knows as cosmic rays¹, which are produced mostly outside the Solar System. These high-energy particles were initially suspected to have Earthly origins, but Victor Hess demonstrated in 1912 that they arrive on our planet from non-terrestrial sources. For decades, the enigmatic nature of cosmic rays has captivated scientists, who have sought explanations for their origins and the energies they possess.

In 1966, Kenneth Greisen as well as Georgiy Zatsepin and Vadim Kuzmin proposed that the energy of cosmic rays originating from very distant sources wouldn’t exceed a certain value. The Greisen–Zatsepin–Kuzmin (GZK) limit comes from energy losses that ultrahigh-energy cosmic rays (UHECRs) experience when they interact with the photons from the cosmic microwave background radiation. Indeed the flux of UHECRs tails off at energies above the GZK limit, as predicted. However, there may be alternative explanations as to why this is the case, which can be tested by probing the composition, and therefore the sources, of the UHECRs.

One such probe involves searching for ultrahigh-energy cosmic neutrinos. Neutrinos are particles that interact with matter extremely weakly, meaning that they can pass through lightyears of lead without interacting in any way with the dense metal. Cosmic neutrinos can have two possible origins: they could be produced through the interactions of cosmic-ray particles such as protons with the cosmic microware background or they could be the result of the decay of ephemeral particles produced at the cosmic-ray source. Observing and studying ultrahigh-energy neutrinos could shed light on the nature of UHECR that originate in very distant cosmic-ray sources. It would also help validate or rule out theories that attempt to explain the UHECR energy spectrum.

The Pierre Auger Observatory in Argentina is designed to detect ultrahigh-energy cosmic rays. It is operated by an international collaboration of more than 500 scientists and is the largest UHECR observatory with a detection area of around 3000 km² — the area of Rhode Island or around thrice the area of Hong Kong. The Surface Detector (SD) array of the observatory can detect neutrinos that have energies of around and greater than 1 exaelectronvolt (10¹⁸ eV). These energies are over 150,000 times the energy we can produce in our most powerful particle accelerator, the Large Hadron Collider. The neutrinos are not observed directly but through cascading showers of particles (mostly electrons or their heavier cousins known as muons) produced when the neutrinos interact with the Earth’s atmosphere.

Three of the stations of the Surface Detectors in the Pierre Auger Observatory in a row towards the Andes, at sunrise. Image by Lorenzo Caccianiga / CC BY-SA 3.0

The Auger team analysed data collected between January 2004 and June 2013, corresponding to around six and a half years of continuous operation. During this period, they didn’t observe a single candidate for neutrino-induced showers. In the absence of an observation, the null result has helped the Auger scientists establish their most stringent limit on the flux of ultrahigh-energy neutrinos in the energy range of 1e17 eV to 2.5e19 eV. The limits thus set will allow theorists to further fine-tune their models and predictions on the nature and behaviour of ultrahigh-energy cosmic rays.

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Citation:

Pierre Auger Collaboration (2015). Improved limit to the diffuse flux of ultrahigh energy neutrinos from the Pierre Auger Observatory. Physical Review D, 91(9). DOI: 10.1103/PhysRevD.91.092008