ICARUS: Neutrinos Travel At Light Speed. Period.

By Tommaso Dorigo

Little less than one year ago the world of fundamental physics was shaken by the bold claim of the OPERA collaboration, which produced a measurement of the time of flight of neutrinos traveling underground from Geneva to the Gran Sasso mine in central Italy. The beam of neutrinos, produced by the CERN SpS proton synchrotron, was observed to produce interactions in the large mass of the OPERA detector with about 60 nanosecond anticipation with respect to what would be expected for a particle traveling at exactly the speed of light (2439096.1+-0.3 nanoseconds, since the flight path is of 731221.95+-0.09 meters).

Following the CERN announcement of the OPERA result physicists around the world busied themselves with pointing out several issues with the measurement, among which the statistical analysis of the data, the precise measurement of several delays in the detection process, and other hardware components, and remained generally quite cold….

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Neutrino watch

… Speed claim baffles CERN theoryfest

Typical event recorded in ICARUS. Evidence for a pair of γ’s from a π_o (tracks 16a and 16b) with a momentum of 912 MeV/c pointing at the primary vertex, showing the typical behavior of γ conversions in the TPCN LAr Imaging chamber.

Even a meeting of elite minds at Europe’s top particle physics lab couldn’t do it: reconciling neutrinos that appear to break the cosmic speed limit with the laws of physics is still beyond us. However, a paper on the speeding neutrinos has been accepted for publication and the first preliminary results from a comparable experiment are out.

“For the moment, there is no explanation that works,” says physicist Ignatios Antoniadis, who helped to organise the meeting at CERN near Geneva, Switzerland, last Friday. It was three weeks to the day after physicists in the OPERA collaboration at Gran Sasso, Italy, announced that neutrinos travelling from CERN had apparently moved faster than light.

Frantic calculation, speculation and debate have followed in the wake of the announcement. The meeting’s goal was to “review the situation and discuss whether it is possible [that neutrinos broke the speed of light]” , says Antoniadis.

The biggest challenge yet to the OPERA result comes from Nobel laureate Sheldon Glashow and his Boston University colleague Andrew Cohen in a paper posted online a few weeks ago.

First publication

Physical Review Letters has agreed to publish the paper, making it the first scientific journal to accept work on the OPERA result.

In the paper, Glashow and Cohen point out that if neutrinos can travel faster than light, then when they do so they should sometimes radiate an electron paired with its antimatter equivalent – a positron – through a process called Cerenkov radiation, which is analogous to a sonic boom. Each electron-positron pair should carry away a large chunk of the neutrinos’ energy: Cohen and Glashow calculated that at the end of the experiment, the neutrinos should have had energies no higher than about 12 gigaelectronvolts. But OPERA saw plenty of neutrinos with energies upwards of 40 GeV.

“It doesn’t correspond to the energies measured at all,” says CERN physicist Christophe Grojean.

Another strike against the speedy neutrinos comes from the fact that neutrinos are linked to certain other particles – electrons, muons and tau particles – via the weak nuclear force. Because of that link, neutrinos can’t travel faster than light unless electrons do too – although electrons needn’t travel as fast as the neutrinos.

Speedy electrons

CERN physicist Gian Giudice, who spoke at the seminar, and colleagues looked into what would happen if electrons travelled faster than light by one part in 100,000,000, a speed consistent with the OPERA neutrino measurement. Such speedy electrons should emit a cone of Cerenkov radiation in empty space – but previous experiments show that they don’t.

The only way out, theorists at the meeting decided, was to break another supposedly fundamental law of nature – the conservation of energy. But that suggestion seems even more ludicrous than breaking the speed of light.

“At the moment, there is no concrete model that really avoids all these theoretical constraints,” Grojean says. “That’s why it’s so interesting. We cannot explain it in terms of known physics.”

Despite the care the OPERA researchers took to rule out errors in the measurement, that possibility remains. Another unpublished paper on the arxiv.org physics preprint server has attracted attention with its explanation. Ronald van Elburg at the University of Groningen in the Netherlands has calculated that special relativity could have messed up the synchronisation of the clocks at CERN and Gran Sasso. This would make neutrinos appear to arrive 64 nanoseconds early – almost exactly what the OPERA experiment observed.

Icarus test

If this argument holds up, rather than breaking Einstein’s theory of special relativity, the faster-than-light neutrinos would actually end up reaffirming it. But it’s unclear whether the result has legs. “In general, the feeling of theorists is that one should repeat the experiment,” Antoniadis says.

CERN plans to provide a new neutrino beam to do this. Meanwhile, the first glimpses from another detector at the Gran Sasso laboratory don’t look good for the faster-than-light hypothesis. An experiment there called ICARUS (Imaging Cosmic And Rare Underground Signals) has been catching neutrinos travelling from CERN since last year. The 100 or so it has seen do not seem to travel faster than light. ICARUS also doesn’t see any evidence of the Cerenkov-like radiation Glashow and Cohen predicted.

The case is far from closed, however. “For the moment, we don’t have an answer,” Antoniadis says. “That doesn’t mean an answer doesn’t exist.”

References: Glashow and Cohen: arxiv.org/abs/1109.6562; van Elburg: arxiv.org/abs/1110.2685; ICARUS:arxiv.org/abs/1110.3763
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ICARUS Refutes Opera’s Superluminal Neutrinos

a picture of ICARUS in the LNGS cavern

The saga of the superluminal neutrinos took a dramatic turn today, with the publication of a very simple yet definitive study by ICARUS, another neutrino experiment at the Gran Sasso Laboratories, who has looked at the neutrinos shot from CERN since 2010.

The ICARUS team jumped on the chance to test the Opera result based on the article recently published by Cohen and Glashow. The latter argue that superluminal neutrinos should lose energy through  neutral-current weak-interaction radiation -the analogue of Cherenkov radiation for a neutral particle. Given a neutrino moving at a speed v>c as the one measured by Opera, and given the distance traveled to the Gran Sasso cavern, one can relatively easily compute the energy spectrum of observable neutrinos at the cavern, given the production energy spectrum.

The physics is a bit more complicated than I summarized it in the paragraph above, but really, you need not squeeze your brains: there is nothing much to know. What is important is that there is a clean and simple relationship between the superluminal speed and the rate of decrease of the neutrino energy. Neutrinos at CERN are produced with an average energy of 28.2 GeV, and neutrinos at the receiving end – the LNGS where Opera and ICARUS both sit – should have an average energy of only 12.1 GeV for neutrinos detected via charged-current interaction.

Incidentally, a charged-current neutrino interaction occurs when the neutrino “exchanges” a unit of electric charge, along with weak quantum numbers, with a nucleus. The neutrino thus turns into a muon, while the nucleus breaks apart in a shower of light hadrons. The muon is then very easy to detect and measure.

I can imagine the ICARUS team brainstorming all together at a meeting. Everybody brings about their favourite objections to the timing measurement of Opera. Some argue whether they can redo the Opera measurement. Others pass along the tray of donuts. Then somebody brings up the Cohen-Glashow paper: “Look, it is quite easy: we take neutrino interactions, measure their energy, and compare with various hypotheses for the superluminal speed. All based on known physics and hard facts. Can we do it ? Can we ? OMG wait… We have already those neutrino interactions!”

So off they go, and do their homework. And a very good homework it is: in less than three weeks from the appearance of the Cohen-Glashow paper -yesterday evening-, they publish a preprint. Kudos to them for their speed and focus. True, ICARUS is not flooded with neutrino statistics these days -I could not help chuckling at their honest but a bit vintage description of why they lost this or that event, ending up with a statistics of less than 100 interactions (OPERA has 16000, although they’ve run for much longer so far). But those less-than-100 neutrinos do kick ass.

In fact, what do they find ?

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