An intensive work has been done during the last years to develop the ALICE simulation framework AliRoot which is continuously evolving to become also the reconstruction and analysis program. The Dapnia has played an important role and participates actively in the improvement of the response of pad chambers of the Muon arm and in the alignment program. An important contribution was also the evaluation of the sensitivity of the Muon arm to the ratio of ϒʼ/ϒ as a function of pT. This ratio is considered as a signature that should discriminate between the QGP and some other scenarios. By using this ratio it is expected that the effect of the gluon shadowing will cancel out as well as many experimental systematic errors. The value in pp collisions of the ϒʼ/ϒ ratio as a function of pT has been found to be constant by the CDF collaboration at the Tevatron. Therefore, a distortion of this ratio should be an indication of QGP formation. We have also contributed to develop a mixing event technique in view of the combinatorial background subtraction. This technics is particularly useful for the most central events in Pb-Pb collisions.
A spatial resolution of 100 μm is required for the tracking chambers in the bending direction (vertical) in order to achieve the 100 MeV/c2 mass resolution needed to disentangle the resonances of the ϒ family. Then, the position of each tracking detection element has to be known with a precision of a few tens microns. An optimal method to obtain the alignment parameters (the true position and angles) of all detection elements is to perform a least square minimization with respect to track and alignment parameters simultaneously. The limitation of this method comes mainly from the computing time due to the large amount of parameters. To overcome this problem, a special method was developed by V. Blobel, which has been successfully used in previous experiments. The first step was to test the principle of the method in the case of the 156 detection elements of the muon tracking, then a complete implementation of the code was done in the AliRoot framework. We have investigated the effect of several misalignments scenarios, going up to 1 mm in the x and y directions, on the invariant mass spectra of the ϒ and J/ψ resonances.
The simulation shows that the alignment reaches a precision of 20 μm with 105 tracks. The variation in time of the alignment of each half-chamber will be monitored with a dedicated Global Monitoring System (GMS) based on cameras and targets. With the experience collected during test beams, the SPhN group makes an important contribution on the online development. In particular, we did the first implementation of the monitoring program using MOOD (Monitoring Of Online Data & detector debugger), the ALICE official framework. We develop also the needed software for the calibration of the electronics of the tracking chambers (1.1 million channels). The calibration includes also the extraction of the pedestals from dedicated runs that are loaded into the front-end electronics to implement the zero suppression.
We have also started using the DATA GRID, the world-wide data computing system for the LHC experiments.This system will be used to reconstruct and analyze the impressive amount of data (~2 000 Tb per year) produced by ALICE. The group also participates to the Physics WorkingGroup (PWG) on heavy analysis tasks. In particular, we are testing the different analysis steps using the J/ψ channel that will be accessible with the low luminosity available at the beginning of the machine operation. We looked to the different non-prompt contributions to the J/ψ, in particular the radiative decay of χc. The ϒ physics will begin later.
last update : 10-22 00:00:00-2007 (2244)