Deformation studies on CMS endcap modules and misalignment studies on the CMS tracker

In this thesis two related topics were investigated: * The deformation of detector modules located in the endcap of the CMS tracker * The effect of misalignment of the tracker support structure on the performance of the CMS tracker 0.1 Deformation Measurements In chapter two the possible deformations of the TEC modules under temperature changes have been investigated. A test setup to measure module deformation was designed and built. The optical measurement principle of the test stand utilises the reflecting wafers of the modules. The polished side of the sensors acts as a mirror, that reflects a regularly patterned grid of white dots towards a CCD camera. Any deformation of the surface leads to a distortion of the reflected pattern. These distortions are used to reconstruct the shape of the surface. The setup is capable of reconstructing sagittas of at least 10 µm over the length of the biggest TEC modules (209 mm). After calibration of the test stand first measurements were conducted on a dummy module. The measurements showed severe deformations of the frame (120 µm) and the hybrid (200 µm), and medium deformation of the wafers (40 µm). In order to reduce the deformation of the module components, several solutions were proposed and tested: * The deformation of the frame was mainly caused by a small heat collecting piece (HCP) glued on one side of the frame. Adding a counter part for each HCP on the other side of the carbon fiber frame to cancel their effects was shown to be the right way to reduce frame deformations. It was also seen that glue films of a homogenous thickness are important to ensure the performance of this compensation method. Finally, changing the material of the HCP from aluminum to compressed graphite removed the frame deformation due to the HCPs almost completely (< 10 µm). This led to the first change in the design of the TEC modules. * In order to reduce the deformation of the wafers, a special silicon glue, capable of absorbing more stresses than the epoxy glue used before, was tested. The maximum deformation of the wafers was reduced to a value below 20 µm. * The deformation in the hybrid area was reduced by a change of the frame material supporting the hybrid from carbon fibre to compressed graphite. Due to the well matching CTEs of the hybrid material and the graphite no deformations could be measured after the change. This led to a second modification in the design of the TEC modules. Due to the partial substitution of graphite for carbon fibre in the module frames the total cost for the modules of the two endcaps could be reduced by about 400.000 SFr. 0.2 Misalignment Studies The simulation results concerning the misalignment of the whole CMS tracker have been presented in chapter three of this thesis. First the deformation scenario was described, consisting of random, gaussian movements of all levels of the tracker support structure. The amplitudes of the random movements were based on the precision that could be achieved during the production and assembly of the CMS tracker. The tracker has a laser alignment system to monitor possible deformations of the support structure. The laser alignment system was included in the simulations in two levels, each leading to its own alignment scenario. The first level aligns the subdetectors of the tracker (TIB/TID,TOB,TEC) to each other with a precision of 100 µm. In addition the second level aligns the discs of the two endcaps to the same precision of 100 µm. In total three different misalignment cases were investigated: * The misaligned tracker without laser alignment * The misaligned tracker with laser alignment on level one * The misaligned tracker with laser alignment on level two For all cases the performance of the CMS tracker (efficiency, fake track rate, momentum resolution) was evaluated. This was done by using two event processes: * pp -> Z+X -> muon(+) muon(-) +X * pp -> b bbar +X -> Jets+X The simulations showed the great impact of tracker deformations on tracker performance. Not only the reconstructed track parameters are affected, but also the overall ability to find the true particle tracks is dramatically reduced for events with high particle densities. The laser alignment system was shown to greatly enhance the performance of a misaligned tracker. It increases the accuracy of the momentum resolution to values about 5 to 7 times better than without alignment. For the endcaps this is only two times worse than for the ideally aligned tracker. The track finding efficiency is reasonably increased for great particle densities. But the most important improvement is the reduction of the fake rate by a factor of 15 to a value that should greatly help the alignment with tracks to be successful.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2003

Author dc:creator

• König, Stefan

Contributors dc:contributor

• Schael, Stefan

• info:eu-repo/classification/ddc/530
• CMS-Detektor
• Deformationsmessung
• Spurdetektor
• Justieren
• Physik
• CMS Tracker alignment

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123547%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:61938