Astronomers from the University of Iowa and the Naval Research Laboratory have used the Moon to eliminate a number of causes for ultra-high energy neutrinos.

Neutrinos are chargeless, massless particles found throughout the Universe. Due to their non-interacting nature they are very difficult to detect, with billions passing through you every second. In the past scientists have devised a number of imaginative detection methods, often involving filling large underground caverns with heavy or pure water and waiting for sporadic emissions of radiation. Now a group of scientists are using the Moon to aid in detecting very high energy neutrinos.

Using the Very Large Array (VLA) radio telescope and the Extended Very Large Array (EVLA) Ted Jaeger of the University of Iowa and the Naval Research Laboratory and Robert Mutel and Kenneth Gayley of the University of Iowa searched for neutrinos interacting with the Moon. Although it is not possible to detect neutrinos directly using a radio telescope, it is possible to detect the radio waves produced by the interaction of neutrinos with the Moon. It was these emissions that the team was trying to detect.

After satisfying themselves that the experiment was working correctly by launching a helium balloon transmitter above the VLA, the detectors were then turned towards the lunar edge. Taking measurements for 200 hours the group report in the December issue of Astroparticle Physics that they had not detected any ultra-high energy neutrino signals.

This technique has been used since 1995, but this was the first time that measurements of this sensitivity have been recorded. Theoretical predictions of the source of the high energy neutrinos range between black holes at the centre of distant galaxies to exploding stars and even tears in the fabric of space-time. Due to the absence of any emissions some of these sources can now be ruled out.

“Our observations have set a new upper limit – the lowest yet – for the amount of the type of neutrinos we sought,” Mutel said. “This limit eliminates some models that proposed bursts of these neutrinos coming from the halo of the Milky Way Galaxy”.

In order to test other predictions more sensitive measurements will need to be taken. According to Mutel, “when we develop the ability to detect these particles, we will open a new window for observing the Universe and advancing our understanding of basic astrophysics”.