Borexino Collaboration Detects pep Neutrinos

View from inside the Borexino neutrino detector. Image Credit: Borexino Collaboration
View from inside the Borexino neutrino detector. Image Credit: Borexino Collaboration


Solar neutrino physics has quieted down over the past decade. In the past, it had been a source of major excitement and puzzlement for scientists as they struggled to detect these elusive particles emitted from the fusion reactions in the center of the Sun. Although difficult to detect, they provide the most direct probe of the Solar core. Once astronomers learned to detect them and solved the Solar neutrino problem, they were able to confirm their understanding of the main nuclear reaction that powers the sun, the proton-proton (pp) reaction. But now, astronomers have for the first time, detected the neutrinos of another, far rarer nuclear reaction, the proton-electron-proton (pep) reaction.

At any given time, several separate fusion processes are converting the Sun’s hydrogen into helium, creating energy as a byproduct. The main reaction requires the formation of deuterium (hydrogen with an extra neutron in the nucleus) as the first step in a series of events that leads to the creation of stable helium. This typically takes place by the fusion of two protons which ejects a positron, a neutrino, and a photon. However, nuclear physicists predicted an alternative method of creating the necessary deuterium. In it, a proton and electron fuse first, forming a neutron and a neutrino, and then they join with a second proton. Based on solar models, they predicted that only 0.23% of all Deuterium would be created by this process. Given the already elusive nature of neutrinos, the diminished production rate has made these pep neutrinos even more difficult to detect.

While they may be hard to detect, pep neutrinos are readily distinguishable from ones created by the pp reaction. The key difference is the energy they carry. Neutrinos from the pp reaction have a range of energy up to a maximum of 0.42 MeV, while pep neutrinos carry a very select 1.44 MeV.

However, to pick out these neutrinos, the team had to carefully clean the data of signals from cosmic ray strikes which create muons that could then interact with carbon inside the detector to generate a neutrino with similar energy that might create a false positive. In addition, this process would also create a free neutron. To eliminate these, the team rejected all signals of neutrinos that occurred within a short amount of time from a detection of a free neutron. Overall, this indicated that the detector received 4,300 muons passing through it per day, which would generate 27 neutrons per 100tons of detector liquid, and similarly, 27 false positives.

Removing these detections, the team still found a signal of neutrinos with the appropriate energy and used this to estimate the total amount of pep neutrinos flowing through every square centimeter to be about 1.6 billion, per second, which they note is in agreement with predictions made by the standard model used to describe the interior workings of the Sun.

Aside from further confirming astronomers understanding of the processes that power the Sun, this finding also places constraints on another fusion process, the CNO Cycle. While this process is expected to be minor in the Sun (making only ~2% of all helium produced), it is expected to be more efficient in hotter, more massive stars and dominate in stars with 50% more mass than the Sun. Better understanding the limits of this process would help astronomers to clarify how those stars work as well.

Astronomy Without A Telescope – Astronomy On Ice

Well, here’s a bit of a first for AWAT, because this is a story about a telescope. But it’s not your average telescope, being composed of a huge chunk of Antarctic ice with a very large cosmic ray muon filter attached to the back of it, which is called the Earth.

Commenced in 2005, the IceCube Neutrino Observatory is now approaching completion with recent installation of a key component DeepCore. With DeepCore, the Antarctic observatory is now able to observe the southern sky, as well as the northern sky.

Neutrinos have no charge and are weakly interactive with other kinds of matter, making them difficult to detect. The method employed by IceCube and by many other neutrino detectors is to look for Cherenkov radiation which, in the context of IceCube, is emitted when a neutrino interacts with an ice atom creating a highly energized charged particle, such as an electron or a muon – which shoots off at a speed greater than the speed of light, at least greater than the speed of light in ice.

The advantage of using Antarctic ice as a neutrino detector is that it is available in large volumes and thousands of years of sedimentary compression has squeezed most impurities out of it, making it a very dense, consistent and transparent medium. So, not only can you see the little flashes of Cherenkov radiation, but you can also make reliable predictions about the trajectory and energy level of the neutrino which caused each little flash.

The structure of IceCube incorporates strings of evenly spaced basketball-sized Cherenkov detectors lowered into the ice through drill holes to depths of nearly 2.5 kilometers. The DeepCore component is a more compact array of detectors, positioned in the clearest ice deep within IceCube, designed to enhance the sensitivity of IceCube for neutrino energies less than 1 TeV.

Prior to DeepCore being finished, it was only feasible to accurately measure the effects of upwardly moving neutrinos – that is, neutrinos that had already passed through the Earth and, if of a cosmic origin, had actually come from the northern sky. Any downwardly moving neutrinos from the southern sky were lost in noise created by cosmic ray muons which are able to penetrate IceCube, creating their own Cherenkov radiation without neutrinos being involved.

However, with the greater sensitivity offered by DeepCore, coupled with IceTop, which is a set of surface level Cherenkov detectors able to differentiate external muons entering from the surface, it is now possible for IceCube to make neutrino observations of the southern sky as well.

Adapted from Halzen (2009, arXiv:0911.2676)

IceCube’s key scientific goal is to identify neutrino point sources in the sky, which may include supernova and gamma ray bursts. Neutrinos are speculated to account for 99% of the energy release of a Type 2 supernova – suggesting that we may be missing a lot of information when we just focus on the electromagnetic radiation that is emitted.

It is also speculated that IceCube might provide indirect evidence of dark matter. The thinking is that if some dark matter was caught in the centre of the Sun, it would be annihilated by the extreme gravitational compression present there. Such an event should produce a sudden burst of high energy neutrinos, independent of the normal neutrino output resulting from solar fusion reactions. That’s a long chain of suppositions to gain indirect evidence of something, but we’ll see.