Backstage at OPERA

In the latest in our series of articles on the neutrino beam to Gran Sasso we turn the spotlight this week onto the two experiments under construction at the Gran Sasso Laboratory - OPERA and ICARUS.

In March 2003, installation of the OPERA (Oscillation Project with Emulsion tRacking Apparatus) detector began in Hall C of the INFN's Gran Sasso Laboratory in Italy. The OPERA-CNGS1 Collaboration comprises 170 physicists from 35 research institutes and universities worldwide. It is expected that the complete detector will be ready to receive the CNGS neutrino beam and start data acquisition in August 2006. This article gives an overview of the OPERA experiment.

The OPERA experiment aims to clear up the mystery of neutrino oscillation. But what are more precisely its objectives ?
When cosmic rays interact in the atmosphere, two kinds of neutrino - muon-neutrinos and electron-neutrinos - are produced. In theory there should be twice as many muon-neutrinos as electron-neutrinos, but experiments find too few of the muon type. The OPERA experiment aims to show that neutrino 'oscillations' - where the muon-neutrinos change into a third type, the tau-neutrino - are responsible for this 'atmospheric neutrino deficit' (see the Bulletin 29/2003).
If neutrinos oscillate, the muon-neutrino beam that leaves CERN will contain tau-neutrinos on arrival at the underground Gran Sasso laboratory. The aim is to search for the appearance of these tau-neutrinos. When they interact with the OPERA detector, tau-neutrinos will produce highly unstable tau particles, which decay within one millimetre of the neutrino interaction point. The OPERA detector will recognize these tiny decay kinks by observing the tracks of tau particles using nuclear emulsion films as high-precision trackers.
Of the billions of muon-neutrinos in the CNGS beam only a few will interact with the detector. OPERA expects to observe about 40,000 muon-neutrino events in five years, and about 20 tau-neutrino events in case of oscillation.
The experiment will use muon spectrometers to unambiguously measure muons produced in the tau-decay interactions.

How does it work?
The OPERA detector is composed of two supermodules. Each supermodule is approximately 7 metres by 10 metres by 10 metres and is made up of a target section followed by a muon spectrometer. In front of the upstream supermodule is a plane of Resistive Plate Chambers (RPCs), whose role is to detect particles entering the detector that are not neutrinos, so as to 'veto' these interactions.
OPERA will detect the tau particles using the Emulsion Cloud Chamber (ECC) technique, in which many layers of a special type of photographic emulsion provide precision images of charged-particle tracks, rather like in a cloud chamber, but with much higher spatial resolution (a fraction of a micron).

What can we expect?
The combined analysis of the nuclear emulsions and electronic data will give an excellent signal-to-background ratio. Thanks also to the anticipated 50% improvement of the CNGS neutrino flux with respect to the design value, in five years of data-taking OPERA is expected to detect about 20 tau decays with a background of the order of 0.7 events, using the best-fit value of the data from Super-Kamiokande, the experiment in Japan that established the atmospheric neutrino deficit.
Given the very good efficiency of the ECC technique in identifying and discriminating electrons, it is expected that OPERA will also substantially improve the present limit on electron-neutrino to muon-neutrino oscillations that was set by an experiment using electron-(anti)neutrinos from the CHOOZ nuclear reactor in France.

More details on the web : http://opera.cern.ch


OPERA's components

The target section in each supermodule consists of 31 ‘walls' interleaved with 31 ‘target trackers'. Each wall is made up of 3328 ‘bricks', and each brick consists of a pile of 57 nuclear emulsion sheets interleaved with 56 lead sheets - giving about 900 tonnes of lead in each supermodule. The lead will act as a target mass with which the neutrinos interact, while the emulsion sheets will act as very-high-precision trackers to detect the charged particles created in the decays of the tau particles that are produced in the neutrino interactions, as well as the short path of the tau particle before its decay.
Each target tracker consists of four vertical and four horizontal modules of scintillator strips, which measure the positions of charged tracks. Once the target trackers have located the brick where an interaction has occurred the ‘triggered' brick will be removed from the side, so that the emulsion sheets it contains can be processed and then automatically analysed to validate the neutrino interactions and to search for other interesting events. This will be done in two special scanning stations located in Japan and Italy, and also in precision measurement facilities available in both Japan and Europe.
The muon spectrometer in each supermodule will measure the sign (+ or -) and the momentum of the muons produced by the neutrino interactions in the target section. The spectrometer consists of a 100-square-metre dipole magnet made up of 11+11 iron plates interleaved with 11+11 RPC planes. In front, in the gap and behind the magnet there are three pairs of precision trackers. Their role is to measure the angle at which the muons pass through.