OPERA goes on camera

OPERA, the experiment which uses the neutrino beam of CERN’s CNGS facility, has delivered its first neutrino "photos". The core of the detector has been commissioned and has produced images of events resulting from neutrino collisions.

The reconstruction of the core (a few cubic millimetres!) of a neutrino interaction at OPERA. The neutrino arriving from the left of the image has interacted with the lead of a brick, producing various particles identifiable by their tracks visible in the emulsion.

The snapshot is tiny but it was greeted with enthusiasm by the physicists of OPERA. On 2 October, for the first time, the experiment at the Gran Sasso Laboratory in Italy "photographed" an event produced by the beam of neutrinos sent from CERN, 732 kilometres away. One of the 60,000 photosensitive bricks already installed at the heart of the experiment had produced its first particle track.

The commissioning of the OPERA experiment began last year with the detector’s electronic components. 300 neutrino events were detected during this first phase (see Bulletin No. 33-34/2006) but the collaboration, which comprises some 170 physicists from 35 institutes around the world, has been eagerly awaiting the commissioning of the core of the experiment, a multi-layered sandwich of lead plates and nuclear emulsion sheets. This central part will eventually comprise 150,000 bricks, each measuring 10 cm x 13 cm x 7.5 cm and weighing 8.3 kilos. The lead acts as a target mass with which the neutrinos interact, while the emulsion sheets serve as high-precision trackers, producing images of the tracks of charged particles with a precision of just a few microns and an angular resolution of about 2 milliradians. Physicists will thus be able to identify the specific topology of a tau decay occurring when a tau neutrino interacts with the lead.

This is the flavour of neutrino the physicists of OPERA are specifically looking for. There are three types or "flavours" of neutrinos. Recent experiments have shown that neutrinos are capable of changing or "oscillating" between one flavour and another, and the goal of OPERA is to provide direct evidence of this change. The neutrino beam sent by the CNGS installation consists exclusively of muon neutrinos, so the detection of tau neutrinos among the 100 million billion neutrinos sent from CERN every day would constitute the much sought-after direct proof of neutrino oscillation.

The event on 2 October was followed by several similar events over the next few days. The bricks in which they occur are located and removed using a robot, then shared out among the collaboration’s various analysis centres, where they are analysed using a special microscope capable of "reading" the images and measuring the physical characteristics.

Did you know?

The nuclear emulsion technology used by OPERA represents a return to the basics of particle physics but with the precision of the 21st century. During the 1930s, several European teams tried to develop this technique to record high-energy particles. Photographic emulsions had been used in physics for many years (Henri Becquerel discovered radioactivity in 1896 using a photographic technique), but they were not sensitive to isolated charged particles travelling at high speed. In 1946, the British physicist Cecil Powell and his team improved the procedure by significantly increasing the sensitivity of the emulsion. The following year, several plates covered with the new emulsion were exposed to cosmic rays at high altitude, including in a Royal Air Force plane and at the Pic du Midi, which provided evidence of the existence of the pion. Cecil Powell was awarded the Nobel Prize for Physics in 1950 for the development of the method that led to the discovery of the pion.

If you want to know more, you can read a letter by Donald Perkins, a pioneer of the technique, which was published in the CERN Courier: