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The OPERA experiment and tau neutrino detection October 5, 2006

Posted by dorigo in computers, physics, science.
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OPERA is a neutrino detector built in a cavern under the Gran Sasso mount in central Italy. It is a so-called “long baseline” experiment, which receives neutrinos of the muon type produced by the interactions of 400 GeV protons with the CNGS neutrino beam facility at the CERN laboratory, some 750 km away.

Neutrinos travel below the earth surface and end up traversing the 1800 metric tons of material of which the detector is composed. Given the expected rate of protons delivered by the SpS accelerator (4.5E19 per year), simulations allow to estimate that 31000 of them will produce a weak interaction with the detector mass.

The detector is sketched below. Its heart are 200,000 small bricks made of 56 lead sheets wafered between 57 emulsion plates. Whenever the outer spectrometers detect that an interaction has taken place in the detector volume, the corresponding brick is extracted by a robot and its emulsions are analyzed. The funny thing is that the brick is not replaced: the detector is taken apart piece by piece as running continues, pretty much as HAL 9000 in the famous sequence of “2001: the Space Odyssey” by Stanley Kubrick (boy do I love that movie).

The hope of the experiment is to detect tau-neutrino interactions, which give rise to charged-current interactions yielding a tau lepton. The tau lepton decays after traveling about a millimeter on average, and its charged decay product – an electron or a muon – will produce a track in the emulsion which does not point back to the original neutrino collision: that is the smoking gun for the production of the tau lepton. 

About a hundred such events are expected in 5 years of data taking. A small number of events! But these few events would tell us a lot about the extremely fancy process of neutrino oscillations.

It has been known for a few years now, since the direct evidence obtained by the SuperKamiokande experiment in Japan, that neutrinos have mass, and they change flavor. Muon-neutrinos – those produced by the decay of a muon in the CERN facility – may transmutate into tau-neutrinos before arriving at the OPERA detector. If they do, the number of tau decays observed can be translated into the unknown mass difference of the two neutrino species.

Amazingly, we cannot measure the mass of the neutrino. But we can measure the differences of their masses. It is a whole new field of research the one SuperKamiokande opened ten years ago, and several experiments around the world are trying to determine the parameters of the so-called “neutrino mixing matrix”, a set of numbers that determine the behavior of these super-elusive particles.

Below is a sketch of the OPERA experiment. I have studied it a little today because of my charge to review a graduation thesis by a student which worked on the experiment. It will be fun to learn the details of the first events they have started to collect!

 

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