A new magnet at CERN is going to allow COMPASS (Common Muon Proton Apparatus for Structure and Spectroscopy) maximum acceptance. Thanks to the 5 tonne, 2.5 m long magnet, which arrived last December, many more events are expected compared to the previous data-taking.

COMPASS' new magnet is placed inside the experiment, which will allow for maximum acceptance.

The aim of COMPASS is to study how quarks and gluons form hadrons and, in particular, what contributes to the spin of the nucleon (protons and neutrons). The experiment, which started to take data in 2002, uses a polarized target in which nucleon spins are aligned and hit with a muon beam from the SPS. The muon beam probes inside the particles. A magnetic field of 2.5 tesla is applied, along with a cold temperature of 50mK sustained for the polarized target, making it the coldest place at CERN.

It will now be easier to detect particles produced at large angles from collisions in the target, mainly as a result of the unique characteristics of this new magnet, which has an inner opening of 63 cm. Upcoming improvements to the tracking system will also aid physicists in finding particles that would have been previously absorbed into the former magnet, which had a 27 cm opening. A dramatic increase is expected for those events. 'We will have a larger angle to track and detect,' said Alain Magnon, co-Spokesperson for COMPASS.

The British company, Oxford Danfysik, reworked the magnet in 2003 and 2004 after Oxford Instruments experienced problems completing the project. An International Review Committee involving experts from CEA-Dapnia, CERN and KEK oversaw the redesign of the main magnet coil. Once finished, it spent one year at CEA, where it was tested and instrumented. COMPASS specified to Oxford Instruments that they wanted to obtain a magnetic field uniform to one part of 10,000, or ±10^-4. A perfect uniformity of the field is needed in order to obtain a uniform polarization of the spin inside the target. During testing, the team at CEA found that they were able to establish a magnetic field map with a homogeneity of ±3x10^-5, three times better than required.

To create the uniform field needed, a rather sophisticated magnet, much more complex than a simple solenoid, had to be designed and built. The number and variety of the different coils that form the magnet are the key to the high uniformity. There are two large compensator coils at either end of the magnet and sixteen correction coils placed throughout the volume. Finally, two 'saddle' coils, one on top and one on the bottom, are used to change the orientation of the magnetic field rapidly in order to rotate the particles' spin into another direction.

Although the project for this new magnet had to pass through a number of difficulties, 'it was a very good example of collaboration with a lot of effort by all parties in order to solve the problems,' said Alain Magnon.

Currently, the magnet is fully installed in its intended location in Building 888 and testing is underway to see if it has the same properties as previously demonstrated at CEA. This is important because there are some minor concerns of interference with the magnetic field from large magnets nearby.

In order to take advantage of the new possibilities achievable by the new magnet, COMPASS will upgrade some detectors in its spectrometer before the next run, scheduled to start in June 2006. The tracker, which comprises a total of 320,000 detection channels, will be reinforced particularly to cover the new angles. The Ring Imaging Cherenkov (RICH) will get faster readout and better photon detection in the centre. The experimental programme will span the coming five years. 'We are working hard to make improvements in 2006 for better performance of particle identification and tracking to maximize the acceptance and efficiency of the spectrometer,' explained COMPASS co-Spokesman Gerhard Mallot.

A glimpse inside the sophisticated solenoid magnet.

COMPASS is a collaboration of 250 people from Europe, India, Japan and Russia. The experiment started to take data in 2002 and physicists have already analysed data from 2002 and 2003. Among the first results, one of the most interesting is related to the role played by gluons, the particles that glue the quarks together inside nucleons. COMPASS has already shown that the participation of gluons in the spin of nucleons is not as large as many theorists expected.