Much-debated results suggesting the existence of a fourth kind of neutrino, described as sterile, are to be put to the test in a new experiment under Italy's Gran Sasso mountain. The physicists who have devised the experiment say that by using an existing solar-neutrino detector they can carry out an inexpensive yet thorough search for the hypothetical sterile neutrino.

Neutrinos are chargeless, almost massless subatomic particles that interact with ordinary matter only via the weak nuclear force. As a result they can pass through vast amounts of material undisturbed. To study them, physicists build huge detectors – the idea being that a large number of target nuclei will result in a few neutrino collisions that can be detected.

If they exist, then sterile neutrinos would be even more difficult to detect because they probably would not interact with ordinary matter at all – only with other neutrinos. They would do so via "oscillation", a well-established phenomenon in which ordinary neutrinos transform and re-transform continually from one of three flavours – electron, muon and tau – to another as they travel. Likewise, ordinary neutrinos would oscillate into sterile neutrinos and back again but probably over much shorter distances than those typical of normal neutrino oscillation.

Lines of evidence

The existence of sterile neutrinos is suggested by a number of lines of evidence. These include results from experiments studying the oscillation of ordinary neutrinos and from recent calculations showing that the numbers of neutrinos captured by detectors placed close to nuclear reactors are lower than expected, given all of the different ways that neutrinos can be produced inside those reactors.

If these hints turned out to be real, then the implications would be "enormous", according Marco Pallavicini of the University of Genoa, in Italy, who points out that sterile neutrinos would be the first fundamental particles discovered to lie outside the Standard Model of particle physics. They may also have played a significant role in the evolution of the universe, he adds. Like many physicists, however, he remains sceptical of the particles' existence.

Pallavicini is leading an international collaboration that will search for sterile neutrinos using the Borexino detector at Italy's Gran Sasso National Laboratory, which is used mainly to measure neutrinos emitted by the Sun. The detector contains an array of photomultiplier tubes that record the light emitted when neutrinos interact with electrons inside a 300-tonne sphere of a hydrocarbon scintillator. The new experiment is known as Short Distance Neutrino Oscillations with BoreXino (SOX) and will intercept neutrinos from an intense radioactive source placed several metres away.

Quasi-sinusoidal variation

SOX will establish exactly where each of the source-induced neutrino interactions takes place within the hydrocarbon by recording the precise time that the associated light emission reaches several of the photomultiplier tubes. If sterile neutrinos exist, then the number of interactions taking place as a function of distance from the source would show a small but distinct quasi-sinusoidal variation, with a wavelength on the scale of metres – far too short to be caused by normal neutrino oscillation.

SOX will use one of two radioactive sources. One is chromium-51, which emits electron neutrinos and will be placed in a pit just below the detector; the other is cerium-144, an electron–antineutrino emitter that will be positioned inside Borexino's water shield. The researchers would prefer to use cerium because it would allow the search for sterile neutrinos over a slightly wider range of masses and "mixing angles" (a parameter that determines the strength of oscillations) but they will probably use chromium because it has been used successfully in two previous experiments, albeit it at lower intensities.

Having recently secured the bulk of the funding needed for SOX from the European Research Council – €3.5m – the researchers' next task is to find a supplier of the necessary radioactive material and then obtain the relevant licences to deliver that material to the lab. Pallavicini estimates that the experiment will start taking data in late 2015 and that the first results will appear in 2016. Further funding permitting, the collaboration will carry out a second – and possibly third – round of tests. The latter being the most challenging because it would put a cerium-144 source at the centre of the detector.

Telltale oscillations

The researchers have calculated that even using chromium-51 SOX would almost certainly be sensitive enough to rule the existing reactor anomalies either in or out. However, the experiment will not see any sterile neutrinos if they happen to be particularly light or mix very weakly with standard neutrinos. Pallavicini also admits that other physicists would be sceptical if his collaboration were simply to record a lower-than-expected overall number of neutrinos, which could be caused by sterile oscillations but which might raise suspicions about poor intensity calibrations or low detector efficiency. However, he says that if the data reveal the telltale oscillatory signal, then he would "challenge anyone to come up with a different explanation" to that of sterile neutrinos.

Petr Vogel at the California Institute of Technology in the US, describes the experiment as "challenging, but doable" and believes that it would "give a very strong indication of whether sterile neutrinos exist or not" if the researchers can collect data using both the chromium and cerium.

Also enthusiastic is William Louis of the Los Alamos National Laboratory in New Mexico, US. "[SOX] will help test and resolve the present evidence for sterile neutrinos," he says, "assuming that the proper radioactive sources are delivered." However, he believes that the experiment on its own will not be able to completely prove or disprove the existence of sterile neutrinos, arguing that definitive proof will require the results of several experiments, recording different kinds of oscillation and spanning different energy scales.

SOX is described in a preprint on arXiv.