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OPERA review serves up a feast for physics geeks

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Interpreting the OPERA signal

If you’re in the mood for statistics, Antonio Palazzo of Technische Universitat Munchen has the paper for you.

It’s worth a look, even if only to dispel the pop-science idea that the neutrino observations were as simple as switching on a beam near the LHC and waiting for it to arrive at Gran Sasso. Rather, the researchers looked at a huge number of interactions between neutrinos and their detectors, applying a statistical analysis to derive a “probability density function” of the neutrino emission times.

That density function is designed to identify the point in two somewhat “smeared” waveforms – the proton events that produced the neutrinos, and the neutrinos detected at Gran Sasso – for which the experimenters can state “these neutrinos were definitely produced by the CERN proton beam at time X”.

Palazzo’s question is whether the right statistical techniques were applied: “it seems that the single waveforms are first summed together and then their sum is normalized to the total number of neutrino interactions in OPERA … such a procedure, if effectively adopted in OPERA, is questionable”, the paper states.

To help resolve this, Palazzo has asked that the OPERA researchers publish the timestamps of the experiment’s 16,000 detected neutrino interactions, along with those of the associated proton waveforms.

Walter Winter, however, doesn’t agree with Palazzo, asserting in this paper that “possible smearing effects … do not change the OPERA results”. That’s not, however, his main concern: Winter also questions the popular (among physicists) assumption that the superluminals must be “sterile” neutrinos.

His re-analysis of the reported data suggests that the “sterile superluminal” theory “can probably be ruled out”: some non-sterile types of superluminal neutrinos are, he suggests, necessary to explain the number of superluminals apparently observed.

The superluminal neutrino laser

A personal favourite of mine in the numerous attempts to explain the OPERA results is this paper by Rafael Torrealba at the University Centro Occidental in Venezuela.

In spite of some translational difficulty, Torrealba proposes the neat idea that the CERN experiment has actually invented a kind of “neutrino laser”: “the starting point of neutrinos could be shifted in time or driven by stimulated emission, as happens for an ultrashort pulse LASER traveling through an amplifier plasma with an initial population inversion”.

“In other words”, he writes, “the traveling distance of the stimulated neutrinos is shorter than that of the not stimulated ones.”

The world still turns

GPS relativity isn’t the only “mundane” source of possible error: two authors have independently asked whether the Earth’s rotation was properly accounted for in the OPERA analysis.

Markus Kuhn of the University of Cambridge (here) asks whether the effect is properly accounted for – while admitting that it would only have a 2ns influence on the reported result, insufficient to invalidate the apparently superluminal trip.

Dominique Monderen – whose paper doesn’t give an affiliation, proving that this review process truly is open to all – also queries the impact of the Earth’s rotation, noting that “the distance [of the neutrino trip] and the time of flight are measured in two different inertial time frames.”

Distance, she says, is measured in a static time frame, but the time of flight experiences Earth’s rotation.

Any of the discussions hitting the wires over at Arxiv.org could be the answer, or none of them might be.

However, before seizing on any single item as being a candidate for refuting the OPERA results, science journalists – who really should know better – would do well to keep a couple of things in mind.

First, the publication of the OPERA results at such an early stage was designed to attract exactly this kind of public scrutiny.

Second, nearly all of the explanations, questions and refutations will themselves need to be reviewed before they can be accepted. ®

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