The recent result that neutrinos appear to travel faster than light could be tested at the Large Hadron Collider (LHC), according to a pair of physicists in the US. Although the European particle accelerator would not be able to fully confirm or refute the result, it would be able to test a mechanism that is thought to occur when neutrinos move faster than light.
The result that neutrinos may travel faster than light came in September, when physicists at the OPERA experiment in Italy reported that neutrinos travelling 730 km underground appeared to arrive 60 ns too early. If the result is correct, it will contradict Einstein’s theory of special relativity, which says the speed of light is the maximum speed possible.
Indeed, many physicists have pointed out that the OPERA result should be incompatible with other reported neutrino behaviour. In 1987, for example, a wave of neutrinos arrived at Earth as a result of a distant supernova explosion three hours before astronomers saw the light from the event. However, if neutrinos were as superluminal as the OPERA result suggests, their arrival would have been early not by three hours, but by more than three years.
Depleted at high energies
At the end of September, theorists Sheldon Glashow and Andrew Cohen at Boston University in the US highlighted another potential problem. They developed a theoretical framework that would allow neutrinos to travel slightly faster than light, in accordance with the OPERA result. However, they found that the framework opened up other processes that particle physics would normally forbid. In particular, say Glashow and Cohen, a superluminal neutrino should be able to decay into an electron–positron pair plus a less energetic neutrino. As a result, the spectrum of neutrinos at OPERA should be depleted at high energies – but this is not what the OPERA collaboration saw.
Now, Hooman Davoudiasl of the Brookhaven National Laboratory in New York and Thomas Rizzo of SLAC National Laboratory in California have re-examined Glashow and Cohen’s theory. True, the framework would open up neutrino decay in a vacuum, Davoudiasl and Rizzo say, but the OPERA neutrinos were travelling mostly through rock. Perhaps the rock stalls the decay for some reason – for example by making the neutrinos transform or “oscillate” into different types – which would mean Glashow and Cohen’s theoretical framework would still be compatible with the OPERA result.
If so, then Glashow and Cohen’s mechanism should turn up in other places – notably at the LHC, say Davoudiasl and Rizzo. Neutrinos are produced in the particle accelerator, for example when energetic top quarks decay, but they are not normally observed because they pass straight through the detectors. But if Glashow and Cohen’s mechanism is at work, then some of the neutrinos should themselves decay, at roughly a metre from where they are produced. To someone studying the particle trails, this decay should manifest as an energetic electron–positron pair appearing suddenly, as if from nowhere. “This is a relatively easy signal to spot at the LHC,” says Rizzo.
“Using a pile-driver to break an egg”
Glashow and Cohen agree with Davoudiasl and Rizzo’s analysis. However, they think it would be too much effort: although a positive result would favour the existence of superluminal neutrinos, a null result would only suggest that the theoretical framework is faulty. On the other hand, other “long baseline” experiments, such as the MINOS experiment at Fermilab in the US, do have the ability to refute the OPERA result. Davoudiasl and Rizzo’s experiment would be “like using a pile-driver to break an egg”, says Glashow.
Rizzo agrees that a long-baseline experiment – that is, another OPERA-like experiment that detects neutrinos sent over many kilometres – is the best way forward. But he points out that it might take more than a year for such an experiment to be conducted with sufficient statistical certainty. “It is interesting to perform as many other, albeit model-dependent, tests using as many techniques as possible while we wait,” he says. Rizzo adds that existing datasets from the LHC’s ATLAS and CMS experiments should reveal the neutrino decays, if they exist. “It may be possible to obtain the results within a matter of a few months,” he says.
The research will be published in Physical Review D and a preprint is available on arXiv.
