XXXV Reunión Bienal de la Real Sociedad Española de Física nºpag Título Simposio
Alignment of IBL detector with 2014-2015 ATLAS cosmic Ray Data
J.Jiménez Peña1,*
1Instituto de Física Corpuscular (IFIC), Centro mixto Universitat de València-CSIC,Valencia.
Introduction
Between the Run-I and the future Run-II of the LHC a long technical stop, known as the Long Shutdown 1 (LS1), took place. During the LS1 several maintenance works were performed in the ATLAS Inner Detector (ID) and new detectors as the IBL were installed.
The IBL [1, 2] is a new barrel layer of pixel detector situated closer to the interaction point than the current pixel detector, that allows a better determination of the longitudinal and transversal impact parameters of the tracks. The IBL is mechanically independent from the rest of the pixels, as it is attached to the beam-pipe. Because of that, it is very important to study IBL stability under operational changes. 2014-2015 cosmic ray data taking periods have been the first ones after the LS1 and the first time that data in ATLAS has been recorded using the IBL. This data has been used to perform a first alignment of the IBL and to study its stability.
IBL is situated at a radius of 33.25 mm, while the inner layer of the rest of the pixels is at 50.5 mm from the interaction point. It uses two different technologies of pixel detectors, planar sensors and 3D sensors. It is the first time that the 3D technology has been used in a High Energy Physics experiment. IBL is composed of 14 staves, having each one 12 planar modules in the center of the stave and 8 3D modules, 4 at each end of the stave. A drawing of the IBL design around the beam- pipe and the module layout on the staves can be seen in figure 1.
Figure 1. Drawing of the IBL pixel layer in the ATLAS experiment.
Alignment Basics
Track based alignment algorithms rely on residual minimization [3, 4]. Residuals are defined as the distance between a hit and its extrapolated position in the reconstructed track. The residuals chi2 and its minimization condition are built as follows
where r(a,τ) is the vector of track-to-hit-distance, τ are the track parameters, a are the alignment parameters and V is the covariance matrix of the detector measurements.
χ2= [r(a,τ)]T
tracks
∑
V−1[r(a,τ)], ddaχ2 =0XXXV Reunión Bienal de la Real Sociedad Española de Física nºpag Título Simposio
The alignment procedure is split into three levels in order to cope with a large number of alignable degrees of freedom (d.o.f.) and to mimic the detector assembly structures. IBL has been introduced into the alignment framework as an independent structure separated from the pixel detector.
First alignment of the IBL
Data collected during the ATLAS cosmic ray campaigns has been used to test the performance of the new IBL and to align it for the first time. Initially, as there was no previous experience with the IBL, nominal position and shape was supposed for it. After analysing the first data, large corrections were found to the nominal position. As an example, it was found that a closing flange of the IBL fixing it to the beam-pipe was 2mm wider in reality than in the geometric model of ATLAS, thus resulting in a shift of 2 mm of the IBL along the beam direction. These corrections resulted in an improved performance of the IBL, whose hits were barely being associated to the tracks due to the large initial misalignments.
As enough statistics were collected during the cosmic campaign, IBL was aligned up to module level, in which every module is corrected independently. A tangential bowing of the staves and a new overall length scale of the new IBL detector was detected. The performed module level alignment was able to efficiently account for these deformations. The width of the residual distribution of the IBL was reduced in a factor of 5 with the module alignment.
IBL stability
The cosmic data recorded with ATLAS during 2014 and 2015 were recorded with different setups in order to study the IBL stability under operational changes. The alignment of three samples recorded with different values of the B-Field (0, 1, 2 T) showed that IBL is stable under B-Field variations.
It was found that the shape of the IBL varies with the temperature, causing a tangential bowing of the staves when it is cooled down. The cause of the bowing seems to come from the different thermal expansion coefficients of the cooper flexible bus, glued at one side of the stave, and the carbon fiber stave, what cause an asymmetric torque in the stave.
To study the dependence of this deformation with the temperature data have been recorded at several temperatures [20ºC, -18ºC]. Variations of 300 µm in the mean local-x residual were found between the maximum and minimum temperatures for the central modules of each stave. Once the dependency with the temperature has been parametrized, further studies can be done around the operational temperature of the IBL (-12ºC) to predict what would be the stability of the IBL during Run-II and what would be the impact of it in other actions as the beam spot determination, the b- tagging efficiency, etc.
References
[1] ATLAS Collaboration, Insertable B-Layer: Technical design report, http://cds.cern.ch/record/1291633/files/ATLAS-TDR-019.pdf
[2] ATLAS IBL Collaboration, Prototype ATLAS IBL Modules using the FE-I4A Front-End Readout Chip, J.Instrum. 7 (2012) P11010
[3] ATLAS Collaboration, Alignment of the ATLAS Inner Detector Tracking System with 2010 LHC proton-proton collisions at √s = 7 TeV, http://cds.cern.ch/record/1334582/files/ATLAS-CONF-2011-012.pdf
[4] ATLAS Collaboration, Alignment of the ATLAS Inner Detector and its Performance in 2012, http://cds.cern.ch/record/1741021/files/ATLAS-CONF-2014-047.pdf