Impact of CMS Silicon Tracker Misalignment on Track and Vertex Reconstruction

The alignment uncertainties of the CMS Tracker detector, made of a huge amount of independent silicon sensors with an excellent position resolution, affect the performances of the track reconstruction, the track parameters measurement and the vertex reconstruction. In order to study the impact of the mis-alignment of the CMS tracking devices on the previous procedures, realistic estimates for the expected displacements of the tracking systems are supplied in two different scenarios, the first supposed to reproduce the mis-alignment conditions during the first data taking while the second one related to long term data taking condition. Results about the track reconstruction are expressed in terms of the resolution on track parameters, the global efficiency of the track reconstruction and the fake rate in the two scenarios of mis-alignment, by comparing them with the scenario of a perfect alignment of the tracking devices. Primary vertex finding efficiency and position resolution are affected by the tracker mis-alignment, too. Presented at Workshop on Tracking In high Multiplicity Environments, Zurich, October 03-07, 2005 1) Dipartimento Interateneo di Fisica dell’Universitá e del Politecnico di Bari e INFN Sezione di Bari, Bari, Italy 2) CERN, Geneva, Switzerland 3) Physik-Institut, Universitat Zuerich, Zuerich, Switzerland 4) Interuniversity Institute for High Energies (IIHE), Universite Libre de Bruxelles, Brussels, Belgium 1 Tracker misalignment The silicon tracker detector of the CMS detector [1] is made of an inner silicon pixel detector and a silicon microstrip tracker. The pixel detector consists of three cylindrical layers in the barrel at radii 4.4, 7.5 and 10.2 cm (TPB), and two pairs of end-cap disks at |z| = 34.5 cm and 46.5 cm down to a pseudo rapidity η of |2.2| (TPE). The hit position resolution is ∼ 10 μm in the (r-φ) plane and 17 μm in (r,z) plane. The silicon microstrip detector covers radii between 20 and 110 cm. The barrel region is divided into an Inner Barrel (TIB), made of four layers of sensors, and an Outer Barrel (TOB) made of six layers. The TIB is completed on each side by three inner disks (TID). The forward region is equipped with nine end-cap disks (TEC) [3]. The hit position resolution achieved is σr,φ = 40− 60 μm in (r-φ) plane and 500 μm along z. Unavoidable uncertainties on the exact positions of the silicon sensors in the tracker exist due to the mechanical accuracy to position the individual silicon modules within each of the subdetectors (TIB, TOB, and TECs), which is about 50 μm, and to the mechanical accuracy between the subdetectors which will more likely be of the order of mm. The detector positional accuracy, estimated from Monte Carlo Simulation, needed to start the pattern recognition in the CMS silicon tracker has to be about 100 μm. Alignment procedures are implemented with the purpose to determine the absolute position of a sufficient number of mechanical support structure elements with a precision better than 100 μm. This alignment will be performed with the optical laser. A sufficient statistics of reconstructed tracks will be used to determine the positions of the detectors with an accuracy of 10 μm (in order to reconstruct the track parameters with the best resolution). Realistic displacements for the individual detector elements are provided as input to a dedicated software in order to simulate the tracker misalignment and to derive the misalignment effects on the reconstruction. The misalignment of the CMS tracker is introduced: • by displacing the detector modules which host the reconstructed hits while leaving the local hits in place (so no need to generate events with a distorted geometry); • at various hierarchical levels, like for example at the level of misplacing the whole forward end-cap or just one rod or detector module in the outer barrel. Possible displacements implemented are rotations around x, y, z and shifts in x, y, z directions. Two general misalignment scenarios are studied: • the First Data Taking scenario that is supposed to resemble the misalignment conditions during the first data taking, after collecting an integrated data luminosity of < 1 fb−1; the mechanical uncertainties and the laser alignment are expected to reduce the alignment uncertainties at a level of 100 μm for each module. • the Long Term scenario that will address the impact of the alignment uncertainties on the tracking performance, with an integrated data luminosity of few fb−1. A factor 10 of improvement in the alignment uncertainty with respect to the previous scenario is expected to be reached due to the large number of tracks that will allow to carry out a complete track-based alignment down to the sensor level, resulting in an overall alignment uncertainty close to the tracker intrinsic position resolution. Both in the two scenarios all the modules in the tracker are randomly moved according to their mounting precision. Also the ladders in the pixel barrels, the rods in the inner and outer barrels, the blades in the pixel endcaps, the rings in the inner disks and the petals in the endcaps are randomly moved according to their mounting precision. All these random movements are applied in x, y and z direction. For the much complex structures of the barrel layers and the endcap disks a shift in x, y and z direction is also used. Additionally the barrel layers and endcap disks are rotated around the z axis by a fixed amount. The fixed shifts and rotations are evaluated by profiting from the information about the misalignment of these tracker parts given by the laser alignment system, as detailed in Ref. [2]. The resulting values are used as input to a random number generator and the output of the random generator is used as the fixed shift or rotation of the layer or disk. The mounting precisions of modules and substructures like ladders, rods, rings and petals, as expected in the First Data Taking scenario, are detailed in Table 1. Table 2 lists the expected values of the alignment uncertainties for layers (barrel) and disks (endcaps) after the laser alignment is performed, in the First Data Taking scenario.