You shall not pass!

With neutrinos back where they belong - behind the massless photons in vacuum - a lot of the excitement generated by the strange incidents reported last year has died down. After the OPERA experiment announcement on September 23 last year, many observatories, labs and accelerators set about trying to recreate the experiments as well as assisting the Italian Gran Sasso National Lab, the home of OPERA, check for errors with the detection system. Following at their heels were theoretical physicists and mathematicians with different hypotheses each aimed at refuting the results.

On October 18, 2011, another experiment at Gran Sasso, the ominously named ICARUS, published a preprint paper completely contradicting the OPERA results. The ICARUS physicists' conclusion was underpinned by a simple concept: whenever a particle moves, it loses some energy. The rate of energy lost is dependent in a fixed way on the speed at which the particle is moving, and when the particle is moving at a speed greater than that of light's in vacuum, its energy loss must be a specific fraction of its overall energy.

The CERN produced neutrinos at 28.2 GeV, and by the time they reached the OPERA and the ICARUS, they should have had an energy of 12.1 GeV. Unlike ICARUS, OPERA had used a clocking mechanism to determine that the neutrinos were moving faster than light. The ICARUS result, however, showed that there were no neutrinos that had 12.1 GeV of energy, nor that neutrinos possessed any energy in the neighbourhood of that value. Instead, the plot it obtained - of neutrino energy versus number of events - conformed perfectly to the hypothesis that the neutrinos were travelling at the speed of light, no more.

[caption id="attachment_21656" align="aligncenter" width="640" caption="The ICARUS energy-event plot"][/caption]

An important theoretical result inspired the experimental ICARUS result: a paper by Andrew Cohen and Sheldon Glashow contested that superluminal neutrinos could decay into sub-luminal speeds by losing energy in the form of fermions, usually electron-positron pairs. The rate of pair formation, they calculated, was proportional to the sixth power of the neutrinos' energy and the rate of energy decay, proportional to the fifth power. With this, they arrived at a value of around 12.5 GeV as the terminal energy of the neutrinos. OPERA, however, had measured something much higher than this, which meant the energy decay was slow, which meant that they couldn't have been travelling as fast as had been claimed.

At the same time, it wasn't as if the announcement was without supporters: a host of papers were published whose authors seemed determined to validate superluminal travel. Some interesting ones among them are available here, here and here.

Such experiments and solutions, those that sought to prove as well as those that sought to refute the OPERA announcement, are indicative of the spirit of science: that even when an anomalous or conclusively contradictory finding is made, the scientific community utilizes the impetus of the discovery to learn more, to create more knowledge. Even in the case of the the multi-billion dollar hunt for the Higgs boson, all set to be taken to another level in 2012, evidence of the particle's nonexistence will matter as much as its existence. And just was the case when news emerged that neutrinos were well-behaved, after all.