19
Oct 2011

Do neutrinos deserve a speeding ticket?

POSTED BY: DIMITRA ATRI
 

Neutrinos are the most unsocial of known subatomic particles. They hardly interact with matter, and most neutrinos can pass through the entire Earth without undergoing any collision! They have a very low mass, no charge and as textbooks say, they travel with the speed of light. But do they? The new experimental results from the OPERA collaboration were posted on Arxiv recently: http://arxiv.org/abs/1109.4897 where the neutrino speed was found to exceed the speed of light by ~0.0025 %.

When you hear something like this, your immediate reaction is that there is something wrong with the calculations or the code has a bug or some experimental mistake. After all, my undergrad students exceed the speed of light all the time! But this wasn’t a result of some undergrads playing in the physics lab but from a highly sophisticated experiment operated by a large collaboration of physicists. In fact, two weeks ago I was at the experiment, holding one of the photographic plates used to detect these very neutrinos in the Gran Sasso underground lab in Italy.

An intense beam of neutrinos is produced at CERN (Geneva, Switzerland) and directed towards Gran Sasso, Italy (~ 730 km), where the OPERA (Oscillation Project with Emulsion-tRacking Apparatus) experiment is based. Most neutrinos never interact with anything and only a very small fraction hits the detectors, producing a signal. This experiment was originally built to study special neutrino properties (neutrino oscillations…saving the explanation for some other post) and the speed measurement was only a side project.

It was 7.30 pm last Friday night and the TIFR theatre was all packed to attend the live webcast of Dario Autiero’s talk from CERN detailing the speed measurements. The presentation was nothing short of a James Bond movie and I was thrilled to learn about the highly sophisticated technologies they used for these super precise measurements. Most of my experiment colleagues were convinced by their methods and believe me, they did an excellent job!

As we all know that speed is the ratio of distance over time and a precise measurement of both needs to be made in order to get an accurate speed. The time measurements were made with nanosecond precision and it is harder to get beyond that with present-day technology. Distance measurements however could not be made as precisely because of logistical reasons. The experiment sits underground inside a 10 km tunnel which experiences heavy traffic. Position measurement uses the triangulation method, which needs a week’s time and no traffic! But unfortunately, the measurements had to be done only with only one lane closed and so they had an uncertainty of 20 cm in their measurement (of ~730 km).

Overall they had a 6 sigma result based on three years of data from 16,000 events, which is non-trivial and needs serious attention. The conclusion of the paper is pretty self explanatory:

  • “Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly.”
  • “We deliberately do not attempt any theoretical or phenomenological interpretation of the results.”

What does this all mean? Well, I would like to quote Carl Sagan here:

“Extraordinary claims require extraordinary evidence.”

Which, in this case translates to better statistics (additional 2-3 years), similar measurements by other collaborations (Fermilab and others) and shutting down the Gran Sasso tunnel traffic for a week!

So…do neutrinos deserve a speeding ticket for breaking the laws of nature? I say, based on their track record, let us leave them this time until someone else catches them!

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Dimitra Atri Posted by
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