Tag Archives: OPERA

So has the fat lady sang at OPERA?

Well, it had to happen. The OPERA team has identified two potential problems in their measurement of neutrino velocities. (You remember that story, right? The one where it seemed as if the neutrinos were superluminal…)

It turns out that there’s a problem with an atomic clock that they used to get start/stop times for the measurement. (The error here would tend to increase the measured time-of-flight, and thus reduce the measured speed.) There was also a problem with the optical fibre connection between the main clock and the GPS system. (Surprisingly, the error here would tend to increase the measured speed.)

The identification of these two systematic errors means that the OPERA team can no longer claim to have seen superluminal neutrinos. Further experiments later this year, both at OPERA and elsewhere, will surely put the story to bed once and for all.

What has been fascinating here, though, has been the reaction of the scientific community to the claim. I think we all knew that this result was never going to stand. But that doesn’t mean the OPERA team were wrong to publish. Their initial result caught the public imagination, and their identification of systematic errors in the experiment showed the public how science progresses in the real world.

They showed that science is sometimes messy, sometimes confusing. But they also showed that science is transparent, and eventually it gives us knowledge we can rely on. Well done OPERA.

Light dark matter?

As I’m sure you all know, the best model we have of the universe says that about 80% of its matter content is in some unknown form we call ‘dark matter’. (Most of the total mass-energy content is in some unknown form we call ‘dark energy’, but that’s another story.) Perhaps the best suggestion regarding the nature of dark matter is that it consists of WIMPs – weakly interacting massive particles. But what those WIMPs are, and precisely how heavy they are, remains unclear.

Pie chart showing amounts of dark energy, dark matter, normal matter

Normal matter forms only a small part of the mass-energy inventory of the universe
Credit: NASA

There’s no general acceptance amongst the scientific community that dark matter particles have been directly detected, but there have been tantalising hints of WIMP detection in recent years. The Gran Sasso lab in Italy (which is home to the OPERA experiment, which recently observed the famous superluminal neutrino anomaly) is also home to the CRESST and DAMA experiments. Both experiments have made observations that are consistent with the detection of dark matter particles (it’s a strong claim in the case of DAMA). The Soudan mine in America is home to the COGENT experiment, which also saw events that are consistent with dark matter detection. Furthermore, the PAMELA cosmic-ray mission, which has been in orbit since 2006, has seen an abundance of positrons that some scientists have argued could be the product of dark matter annihilation.

In all the above cases the dark matter particles that are observed would be “light” particles – in other words, of relatively small mass.

A recent paper has added to the list of possible sightings of dark matter. The ARCADE ballon-borne experiment has been observing the sky in the radio spectrum, between 3-90 GHz, and has seen an excess of isotropic radiation. The paper suggests that this excess could be the result of WIMP annihilation: when WIMPs annihilate then many theoretical models suggest that they will generate pairs of electrons and positrons, which in turn will emit synchrotron radiation when they travel through magnetic fi elds. For this mechanism to explain the ARCADE results the WIMPs would need to have a mass in the range 10-20 GeV. This is not particularly massive; the WIMPs would be quite light.

So throwing all this evidence together can we conclude that dark matter particles are light? Well, no. The CRESST, DAMA and COGENT observations can all have other explanations, and in any case it’s difficult to reconcile all the data; whether the PAMELA excess can be attributed to annihilation of light dark matter particles has recently been called into question; and the ARCADE data could be the result of messy, poorly understood galactic astrophysics.

So maybe dark matter is light. But we’re far from knowing for sure.

When will the fat lady sing at OPERA?

Some of the world’s finest physicists and cosmologists have in recent weeks been pouring scorn on the now infamous OPERA result. (If you’ve just been released from one of those Mars simulation missions, such as Mars500, then I guess it’s possible that you might have missed what has the potential to be the biggest physics result in a century: the report by the OPERA collaboration that muon neutrinos produced by CERN travelled ever-so-slightly faster than light while on their way to detectors at Gran Sasso.) I’m sure that those scientists, many of whom I admire tremendously, are right: those neutrinos are surely not travelling faster than light. It wasn’t as if the neutrinos acted like resublimated thiotimoline, somehow arriving at the OPERA detectors before they were produced. The OPERA team were making tremendously difficult measurements, and at this point it’s safer to assume that their finding is the result of some unknown source of error in the experiment. But there’s one point on which I think those eminent critics of OPERA have it wrong.

The criticism is that the OPERA team contacted the media and called a press conference before they published their results in a peer-reviewed paper: irresponsible behaviour, clearly, particularly where such a controversial result is involved. Thing is, the OPERA researchers didn’t announce their results at a press conference: they announced them at a CERN seminar. And they didn’t draft a press release: they submitted a technical preprint to arXiv. Surely they did everything that responsible scientists should do?

Once, not many years ago, you could put a preprint on arXiv and you knew you’d be reaching an audience of physicists. We now live in a world of blogs (well, you’re reading this one aren’t you?) and Twitter. Put a preprint on arXiv that says in effect “Einstein was wrong” and you may as well shout it out loud while standing naked at Speaker’s Corner. Perhaps unfortunately for OPERA, in the modern world of social media there’s no way that the original seminar could go unnoticed; the press conferences that followed were inevitable – and then so was the criticism that the collaboration hadn’t followed proper processes.

Neutrino beam going from CERN to Gran Sasso

CERN sends neutrinos directly through the Earth to the Gran Sasso Laboratory, some 730km away Credit: CERN

If there’s a criticism to be made of OPERA it is, I believe, that they hadn’t ruled out all sources of systematic error before giving that initial CERN seminar. Indeed, that’s probably why ten senior members of the collaboration decided not to sign the arXiv submission. One obvious concern with the experiment, which many physicists voiced immediately, is that CERN was sending long neutrino pulses (about 10 microseconds long) to Gran Sasso; the effect they were observing, though, involved a shift that was a tiny fraction of that pulse length (the shift was about 60 nanoseconds). For their analysis to work, the collaboration needed to know the shape of the neutrino pulse quite precisely; but they were only able to infer the neutrino pulse shape. (The neutrinos come from protons smashing into a target; OPERA infer the neutrino pulse shape from the initial proton pulse shape.) Get that inference just a little bit wrong and they would end up seeing things that just aren’t there.

Fortunately, there’s a really simple way to get round this difficulty: repeat the experiment, but send a series of short neutrino pulses separated by large gaps. That way you don’t need to know the neutrino pulse shape: each pulse from CERN is unambiguously linked to the OPERA detector.

The OPERA collaboration has now run precisely this experiment. They asked CERN to generate proton pulses lasting just 3 nanoseconds, and recorded 20 neutrino events. And the result? Well, again the neutrinos reached Gran Sasso about 60 nanoseconds before light itself could have reached there. The anomaly remains.

So when will the fat lady sing at OPERA? When will we know what systematic error is to blame for this bizarre result? (And for what it’s worth I think it will turn out to be a systematic, probably to do the use of GPS in the experiment.) Well, it’s clear that independent checks are required. The first project to be in a position to do those checks is likely to be MINOS at Fermilab. We might get results from MINOS some time in 2012. If MINOS replicates the OPERA result… well, then we’ll be living in interesting times.