It's cosmic show(er) time for CMS
Particles from space are raining on CMS. The collaboration is ready to use these cosmic rays, a natural particle source, to test a 'slice' of their detectors.
The primary goal of the two-month long CMS Magnet Test and Cosmic Challenge (MTCC) is to check the functionality and performance of the superconducting solenoid, which provides the magnetic field of 4 T used to bend charged particles in the inner detectors. This includes its cooling, power supply and control systems. A useful 'by-product' of this is the possibility of performing a 'slice test' of the detector with the field on: detecting and reconstructing cosmic muons through pieces of the final barrel detectors and some end-cap chambers.
Over the past year one set of end-cap disks has been completely equipped with its muon detection system, the end-cap hadron calorimeters have been put in place and most of the barrel muon detectors have been installed and commissioned.
There have been many steps leading to the MTCC. The first step involved cooling the solenoid down to 4.5 K (which has remained at this temperature ever since), after achieving a vacuum inside the coil. (See Bulletin No. 11/2006).
In the next step, the two halves of the complete barrel hadron calorimeter (HCAL) were tested using a radioactive source and then carefully slid inside the solenoid onto rails welded to the interior section. The HCAL is made of layers of brass interwoven with plastic scintillators embedded with wavelength shifting fibres. Only one of the 36 HCAL sections will be tested during the MTCC.
The third step involved the installation of two of the 36 supermodules comprising the electromagnetic calorimeter (ECAL) using a rotatable insertion device known as the 'squirrel cage'. Since each of the supermodules weighs more than three tonnes and comprises 1700 fragile lead-tungstate crystals, as well as support structures, electronics and cooling, this was a delicate task (See Bulletin No. 17/2006).
The prototype tracker was then inserted into the centre of the experiment (See Bulletin 27/2006). The prototype consists of 2 m2 of silicon sensors, a figure larger than any LEP experiment, but small with respect to the real CMS tracker, which will incorporate 200 m2 of sensors. A challenging task was to align carefully the tracker using the two installed ECAL supermodules as guides. A centimeter of clearance is all that will remain between the fully installed ECAL and the tracker.
The sectors of all of the CMS sub-detectors, except for the inner tracking pixel detector and end-cap pre-shower, are now in place and are being commissioned together with the trigger and global data acquisition systems.
In order to operate the solenoid at full power the barrel rings and end-cap disks had to be pushed together, an operation that was completed on 25 July. The CMS Magnet and Infrastructure group will then raise the current slowly to reach the nominal magnetic field at 19500 Amps. Additional tests will be performed on all support systems, including cryogenics and electronic power supplies.
The MTCC provides a unique opportunity for the Collaboration to understand the installed parts of the inner detector operating in the magnetic field before the final commissioning phase of CMS. In August the inner detectors will be linked together through software and the complete trajectories of particles will be tracked. The MTCC will feature parts of the final trigger and data acquisition systems (TriDAS), as well as the event builder (which combines information from the various sub-detectors), control hardware and software and the detector alignment systems.
After completion of the first phase of the MTCC, CMS will be opened once more and the inner detectors removed in order to allow the installation of a 'field mapper'. The yoke will be re-closed and the magnetic field mapped accurately.
The lowering of the large elements (six end-cap yoke disks equipped with muon chambers and the end-cap HCAL; five barrel yoke wheels equipped with muon chambers and the central wheel consisting of the solenoid) into the underground cavern is scheduled to start before the end of the year.
