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
http://www.newscientist.com

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 ?

http://www.science20.com

Two exercises about neutrino departure times at CERN

Bernd A. Berg, Peter Hoeflich

Two simple exercises are solved, which educators can use to awake interest of their students in subtleties of the CERN Neutrino beam to Grand Sasso (CNGS) experiment. The first one is about the statistical error of the average departure time of neutrinos from CERN. The second one about a hypothetical bias in the departure times…..
Read more: http://arxiv.org/PS_cache/arxiv/pdf/1110/1110.2814v1.pdf

Faster-Than-Light Neutrino Puzzle Claimed Solved by Special Relativity

The relativistic motion of clocks on board GPS satellites exactly accounts for the superluminal effect, says physicist
It’s now been three weeks since the extraordinary news that neutrinos travelling between France and Italy had been clocked moving faster than light. The experiment, known as OPERA, found that the particles produced at CERN near Geneva arrived at the Gran Sasso Laboratory in Italy some 60 nanoseconds earlier than the speed of light allows.

The result has sent a ripple of excitement through the physics community. Since then, more than 80 papers have appeared on the arXiv attempting to debunk or explain the effect. It’s fair to say, however, that the general feeling is that the OPERA team must have overlooked something.

Today, Ronald van Elburg at the University of Groningen in the Netherlands makes a convincing argument that he has found the error.

First, let’s review the experiment, which is simple in concept: a measurement of distance and time.

Results of the OPERA experiment

The distance is straightforward. The location of neutrino production at CERN is fairly easy to measure using GPS. The position of the Gran Sasso Laboratory is harder to pin down because it sits under a kilometre-high mountain. Nevertheless, the OPERA team says it has nailed the distance of 730 km to within 20 cm or so.

The time of neutrino flight is harder to measure. The OPERA team says it can accurately gauge the instant when the neutrinos are created and the instant they are detected using clocks at each end.

But the tricky part is keeping the clocks at either end exactly synchronised. The team does this using GPS satellites, which each broadcast a highly accurate time signal from orbit some 20,000km overhead. That introduces a number of extra complications which the team has to take into account, such as the time of travel of the GPS signals to the ground.

But van Elburg says there is one effect that the OPERA team seems to have overlooked: the relativistic motion of the GPS clocks.

It’s easy to think that the motion of the satellites is irrelevant. After all, the radio waves carrying the time signal must travel at the speed of light, regardless of the satellites’ speed.

But there is an additional subtlety. Although the speed of light is does not depend on the the frame of reference, the time of flight does. In this case, there are two frames of reference: the experiment on the ground and the clocks in orbit. If these are moving relative to each other, then this needs to be factored in.

So what is the satellites’ motion with respect to the OPERA experiment? These probes orbit from West to East in a plane inclined at 55 degrees to the equator. Significantly, that’s roughly in line with the neutrino flight path. Their relative motion is then easy to calculate.

So from the point of view of a clock on board a GPS satellite, the positions of the neutrino source and detector are changing. “From the perspective of the clock, the detector is moving towards the source and consequently the distance travelled by the particles as observed from the clock is shorter,” says van Elburg.

By this he means shorter than the distance measured in the reference frame on the ground.

The OPERA team overlooks this because it thinks of the clocks as on the ground not in orbit.

How big is this effect? Van Elburg calculates that it should cause the neutrinos to arrive 32 nanoseconds early. But this must be doubled because the same error occurs at each end of the experiment. So the total correction is 64 nanoseconds, almost exactly what the OPERA team observes.

That’s impressive but it’s not to say the problem is done and dusted. Peer review is an essential part of the scientific process and this argument must hold its own under scrutiny from the community at large and the OPERA team in particular.

If it stands up, this episode will be laden with irony. Far from breaking Einstein’s theory of relatively, the faster-than-light measurement will turn out to be another confirmation of it.

Ref: arxiv.org/abs/1110.2685: Times Of Flight Between A Source And A Detector Observed From A GPS Satellite

http://www.technologyreview.com/blog/arxiv/27260/

New theories emerge to disprove OPERA faster-than-light neutrinos claim

— It’s been just two weeks since the Oscillation Project with Emulsion-tRacking Apparatus (OPERA) team released its announcement claiming that they have been measuring muon neutrinos moving faster than the speed of light, causing an uproar in the physics community. Since that time, many papers (perhaps as many as 30 to the preprint server arXiv alone) have been published seeking ways to discredit the findings. Thus far though, only two seem credible.

The first is by Carlo Contaldi of Imperial College London. He says that it’s likely the OPERA team failed to take gravity into their math equations and its effect on the clocks used to time the experiment. This because the degree of gravity at the two stations involved in the experiment (Gran Sasso National Laboratory in Italy and the CERN facility in Geneva) were different, thus one of the clocks would have been running slightly faster than the other, resulting in faulty timing. If this turns out to be the case, the OPERA team will most certainly be embarrassed to have overlooked such a basic problem with their study.

The second is by Andrew Cohen and Sheldon Glashow, who together point out that if the neutrinos in the study were in fact traveling as fast as claimed, they should have been radiating particles as they went, leaving behind a measurable trail; this due to the energy transfer that would occur between particles moving at different speeds. And since the OPERA team didn’t observe any such trail (or at least didn’t report it) it follows that the neutrinos weren’t in fact traveling as fast as were claimed and the resultant speed measurements would have to be attributed to something else.

New Constraints on Neutrino Velocities
Andrew G. Cohen, Sheldon L. Glashow
The OPERA collaboration has claimed that muon neutrinos with mean energy of 17.5 GeV travel 730 km from CERN to the Gran Sasso at a speed exceeding that of light by about 7.5 km/s or 25 ppm. However, we show that such superluminal neutrinos would lose energy rapidly via the bremsstrahlung of electron-positron pairs (νμ→νμ+e++e). For the claimed superluminal neutrino velocity and at the stated mean neutrino energy, we find that most of the neutrinos would have suffered several pair emissions en route, causing the beam to be depleted of higher energy neutrinos. Thus we refute the superluminal interpretation of the OPERA result. Furthermore, we appeal to Super-Kamiokande and IceCube data to establish strong new limits on the superluminal propagation of high-energy neutrinos.

Neither of these papers actually disproves the results found by the OPERA team of course, the first merely suggests there may be a problem with the way the measurements were taken, the second takes more of a “it can’t be true because of…” approach which only highlight the general disbelief in the physics community regarding the very possibility of anything, much less the speed of neutrinos traveling faster than the speed of light, messing with Einstein’s most basic theories. The first can be addressed rather easily by the OPERA team if it so desires, and the second, well, if the neutrinos did in fact travel faster than the speed of light and did so without leaving a trail, a lot of physics theory will have to be rethought. Though that may not necessarily be a bad thing, physics is supposed to be about finding answers to explain the natural world around us after all, even if it means going back to the drawing board now and then.
© 2011 PhysOrg.com

Read also: “Is the OPERA Speedy Neutrino Experiment Self-Contradictory?

Live Chat: Have Neutrinos Broken the Speed Limit of Light?

Nothing can go faster than light, right? Einstein said so. But last week a group of researchers in Italy announced that they’d measured the speed of thousands of neutrinos (tiny, almost massless particles that were fired at their detector from the CERN particle physics lab 730 kilometers away) and found they were traveling slightly faster than light. Is this the beginning of the end for Einstein’s theory of relativity? Have the researchers simply made a mistake in their measurements? Or are the neutrinos, as some versions of string theory allow, taking a shortcut through a higher dimension and arriving in Italy in double-quick time?
Join us for a live chat on this page at 3 p.m. EDT on Thursday, 29 September, to discuss these and other questions with two experts in the field. You can leave your questions in the comments section below before the chat starts.
Upcoming Event:  Have Neutrinos Broken the Speed Limit of Light?