LHCb is a single-arm forward spectrometer dedicated to the study of the CP-violation and rare decays in the b-quark sector. An efficient and high precision tracking system is a key requirement of the experiment. The LHCb Silicon Tracker Project consists of two sub-detectors that make use of silicon micro-strip technology: the Tracker Turicensis located upstream of the spectrometer magnet and the Inner Tracker which covers the innermost part of the tracking stations after the magnet. In total an area of 12 m^2 is covered by silicon. In September 2008 and June 2009, injection tests from the SPS to the LHC were performed. Bunches of order 5x10^9 protons were dumped onto a beam stopper (TED) located upstream of LHCb. This produced a spray of ~10 GeV muons in the LHCb detector. Though the occupancy in this environment is relatively large, these TED runs have allowed a first space and time alignment of the tracking system. Results of these studies together and the overall detector performance obtained in the TED running will be discussed

Adeva, B

Anderson, J

Bauer, C

Bay, A

Bettler, M-O

Blanc, F

Britsch, M

Buechler, A

Chiapolini, N

Conti, G

Fave, V

Frei, R

Fungueiri no Pazos, J

Gallas, A

Gauvin, N

Haefeli, G

Hangartner, V

Hofmann, W

Iakovenko, V

Keune, A

Knecht, M

Luisier, J

Maciuc, F

Muresan, R

Nakada, T

Needham, M

Nicolas, L

Okhrimenko, O

Pazos-Alvarez, A

Perrin, A

Pló-Casasus, M

Potterat, C

Pugatch, V

Pérez-Trigo, E

Rodriguez Perez, P

Saborido, J

Salzmann, C

Schmelling, M

Schneider, O

Steiner, S

Steinkamp, O

Tobin, M

Tran, M

Van Tilburg, J

Vollhardt, A

Voss, H

Vázquez, P

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LHCb-PROC-2009-04410/05/2011PoS(RD09)010Commissioning of the LHCb Silicon Tracker usingdata from LHC injection testsMathias Knecht∗†Ecole Polytechnique Fédérale de LausanneE-mail: mathias.knecht@epfl.chLHCb is a single-arm forward spectrometer dedicated to the study of CP violation and rare decaysin the b-quark sector. An efficient and high-precision tracking system is a key requirement of theexperiment. The LHCb Silicon Tracker consists of two sub-detectors that make use of siliconmicro-strip technology: the Tracker Turicensis located upstream of the spectrometer magnet andthe Inner Tracker which covers the innermost part of the tracking stations downstream of themagnet. Together, these two detectors cover an area of 12 m2 with silicon. In September 2008 andJune 2009 injection tests from the SPS to the LHC were performed. Bunches of approximately2–5× 109 protons were dumped onto a beam stopper (TED), producing a cone of secondaryparticles heading towards LHCb. This produced a spray of ≃ 10 GeV/c muons in the LHCbdetector. Although the occupancy in these events is significantly higher than that expected fornormal running conditions, the TED runs have allowed a first space and time alignment of thetracking system. The results of these studies and the overall detector performance obtained in theTED running are discussed.9th International Conference on Large Scale Applications and Radiation Hardness of SemiconductorDetectors30 September - 2 October 2009Florence, Italy∗Speaker.†On behalf of the LHCb Silicon Tracker group. For the complete list of authors, see Ref. [1].c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(RD09)010Commissioning of the LHCb Silicon Tracker using data from LHC injection tests Mathias Knecht1. The LHCb DetectorLHCb is an experiment at the Large Hadron Collider (LHC), dedicated to the study of CPviolation and rare decays in the b-quark sector. It is designed to run with pp collisions at a center-of-mass energy of 14 TeV/c2. Since the b-quark production cross-section is peaked at high pseudo-rapidity, the detector has been designed as a single-arm forward spectrometer. The layout of thedetector is shown in Fig. 1. The acceptance covers an angle of 15–300 (15–250) mrad with respectto the beam axis in the bending plane (non-bending plane).The tracking system includes the Vertex Locator (VELO), which is a silicon detector for pri-mary vertex reconstruction surrounding the interaction region, the silicon Tracker Turicensis (TT)located before the dipole magnet, and three tracking stations (T1, T2, T3) located after the mag-net. The tracking stations are are composed of the silicon Inner Tracker (IT) and the straw-tubeOuter Tracker (OT). Other important detectors are the Ring Imaging Cherenkov (RICH) countersfor particle identification, the Electromagnetic and Hadronic Calorimeters (ECAL/HCAL), and theMuon stations M1-M5. LHCb uses a cartesian coordinate system with its origin at the nominalLHC interaction point, the z axis along the beam axis, the x axis horizontal and the y axis vertical.A detailed description of the detector can be found in Ref. [2].M1M3M2M4 M5RICH2HCALECALSPD/PSMagnetz5my5m10m 15m 20mTTT1T2T3VertexLocatorFigure 1: Layout of the LHCb detector in the non-bending (y-z) plane. The beam pipe crosses LHCb alongthe z axis at y = 0 and the interaction point is located at z = 0.2. The Silicon TrackerThe Silicon Tracker consists of the Inner Tracker (IT) located downstream of the spectrometermagnet and the Tracker Turicensis (TT) situated upstream of the magnet. Both make use of siliconmicro-strip technology. A total area of 12 m2 is covered by 16 layers of silicon sensors. Due totheir similarity the two detectors have been developed as a common project.The IT is composed of 336 silicon micro-strip modules arranged in three stations with fourboxes each. The stations are separated by ≃ 70 cm in z. The boxes are located at 7 mm from thebeam pipe, covering the very forward region of the acceptance. Each box contains 28 modulesarranged in four layers. Two of the four layers have vertical readout strips, the other two layers2PoS(RD09)010Commissioning of the LHCb Silicon Tracker using data from LHC injection tests Mathias Knechthave readout strips tilted by a stereo angle of +5◦ and −5◦. Although the IT covers only 1.5%of the geometrical acceptance, it covers about 20% of the pseudo-rapidity range, leading to highparticle densities. The readout pitch is 198 µm, and the total number of channels is 129′000. Thelayout of a layer is shown in Fig. 2 (left). The 336 modules are of two types: “Short” modules(11 cm in length), built with one 320 µm-thick silicon sensor, used in upper and lower boxes and“Long” modules (22 cm in length), built with two 410 µm-thick silicon sensors, used in side boxes.The Signal-over-Noise ratio (S/N) of the IT modules is about 15.Figure 2: Silicon sensors arranged in x–y planes around the beam pipe (each rectangle represents onesensor). Left: one of the 12 IT detection layers. Right: the four TT layers.The TT (Fig. 2, right) located just before the magnet is composed of 128 half-modules arrangedin four layers. The four layers have the same orientation as for the IT: two layers are vertical, and theother two are tilted by +5◦ and −5◦. In contrast to the IT it covers the full LHCb acceptance. Thehalf-modules are made of seven 500 µm-thick CMS-OB2 sensors (each sensor is 9.4 cm long). Thereadout pitch is 183 µm, and the total number of channels is 143′000. The S/N of the TT modulesis about 12.After the installation was completed in early summer 2008, the commissioning of both IT andTT was started using data collected in cosmic runs as well as LHC injection test runs. Analysis ofthe data taken in the 2008 running period allowed to identify several readout channels faults, whichwere fixed for the 2009 run. Currently 99.7% of the channels are operational in each detector.3. Cosmic RunningThe probability for a cosmic particle triggered by the calorimeters to pass through IT or TT isextremely low because of the geometric configuration of the detector. In a sample of 2.6 millioncosmic events triggered by the calorimeters, 1000 cosmics were observed for which track segmentswere reconstructed in at least one IT box. In the same sample three tracks were reconstructedthrough three IT boxes. Though the statistics were limited these events were useful to have a firstcoarse time and space alignment. An example of a “golden” cosmic event, hitting three IT boxes,is shown in Fig. 3.3PoS(RD09)010Commissioning of the LHCb Silicon Tracker using data from LHC injection tests Mathias Knechtz [mm]7600 7800 8000 8200 8400 8600 8800 9000 9200x [mm]-600-400-2000200400600Figure 3: Position of the hits in an event with a cosmic particle traversing three IT boxes, shown in the x-zview (z: coordinate of the sensor, x: coordinate of the mid-point of the sensor). There are three groups offour aligned hits, corresponding to the four layers of one box in each of the three stations. Triangles are hitsassociated to the track while circles are other hits in the event.4. LHC injection testsIn September 2008 and June 2009, injection tests were performed with the LHC machine.These tests consisted of injecting 450 GeV/c2 protons in the transfer line from the SPS to the LHC.Bunches of 2–5× 109 protons were dumped onto a beam stopper (TED), located in the transferline. Since LHCb is located 350 m downstream of the cone of secondary particles produced by thebeam dump, it was possible to observe these particles, about 1000 to 2000 per event in the LHCbacceptance. Monte Carlo simulations indicated that the particles reaching the detector are mainly≃ 10 GeV/c muons. The data were collected without magnetic field. The analysis of the TEDevents was challenging, since particle densities in the Silicon Tracker were about ten times largerthan expected in a typical b¯b event, leading to strip occupancies up to 8%.5. Time AlignmentThe goal of the time alignment was to synchronize the maximum of the signal amplitudefrom the output of the front-end electronics, with the sampling time given by the LHC clock at 40MHz. Different timings had to be set for each specific part of the IT or TT. The timings depend onthree parameters: the time of flight between the tracking stations, the signal cable lengths whichare different for various parts of the detector, and the different capacitances between the detectormodules due to the different readout strip lengths.The method used to find the time of the maximum signal was to reconstruct the pulse shapeas a function of time, by taking several data points with different time delays between the LHCclock and the signal sampling time. The signal amplitude for each delay was determined from themeasured charge distribution of all the clusters (not only the ones associated to a track) as describedbelow. Although we will show the figures for IT, the TT was time-aligned in the same way. Detailscan be found in [3].4PoS(RD09)010Commissioning of the LHCb Silicon Tracker using data from LHC injection tests Mathias KnechtThe charge distribution of ≃ 10 GeV/c muons passing through a given thickness of siliconcan be described by the convolution of a Landau with a Gaussian. The Gaussian width takes intoaccount the effect of detector noise and atomic binding. A typical distribution is shown in Fig. 4(left). The region around the maximum of the distribution is well described by the fitting function.The small peak just above ≃ 10 ADC counts is the tail of the noise distribution. There is a kneein the tail of the curve around 80 ADC, which is due to merged clusters coming from two distinctparticles1 . The value used to measure the signal from the charge distribution is the most probablevalue (MPV).[ADC]0 20 40 60 80 100 120 140 160 180 200entries050100150200250300350MPV=28 ADCtime [ns]0 5 10 15 20 25 30 35 40MPV [ADC]0510152025303540tmax:		 18 nsMPV: 	 30 ADCFigure 4: Left: Charge distribution fitted with the convolution of a Landau and a Gaussian. The MostProbable Value (MPV) is given by the fit, which was performed between 15 and 50 ADC. Right: Pulse-shapescan. The MPV of the charge distribution is plotted versus delay time, and fitted by a function describingwell the expected signal pulse shape.Several sets of data were taken with different delays between trigger and signal sampling time,in steps of 6.25 ns2. For each delay, the Landau was fitted to the charge distribution and the MPVwas extracted. The MPV was then plotted against the different delays, allowing to reconstruct theshape of the pulse in steps of 6.25 ns over more than 50 ns (Fig. 4, right).The measured pulse shape was then fitted with a function which describes the expected be-haviour of the front-end amplifier, in order to extract parameters such as the time of the maximumand the signal amplitude in the maximum of the pulse. This analysis was performed per detectormodule. Distributions of the fitted parameters are shown in Fig. 5. The distribution of the time ofthe maximum shows several peaks corresponding to different data cable lengths. The distributionof the error on the time of the maximum shows that the average precision is about 1 ns. The errorsare bigger for short-sensor modules due to their lower signal amplitude, as seen on the third plot ofthe figure. A few of the short-sensor modules have a higher signal than the rest (the cluster around37 ADC), due to the fact that these special modules have been built with 410 µm sensors, insteadof 320 µm for the other short modules. The higher signal observed on these modules compared tothe long modules of the same thickness is consistent with the expected higher gain of the front-endamplifier for the lower capacitance in this case.1This structure will be smaller during normal data taking due to the lower occupancy.2This corresponds to a quarter of the time between two bunch crossings at 40 MHz.5PoS(RD09)010Commissioning of the LHCb Silicon Tracker using data from LHC injection tests Mathias Knecht[ns]5 10 15 20 25 30051015202530Time of the Maximum (t_max)[ns]0 0.5 1 1.5 2 2.501020304050Absolute error on t_max[ADC]20 22 24 26 28 30 32 34 36 38 4001020304050Signal at t_maxFigure 5: Parameters extracted by the pulse-shape fits per IT-detector module, for the short IT modules(blue/dark grey) and the long IT modules (red/light grey). Left: Time of the maximum. Middle: Error onthe time of the maximum. Right: Signal amplitude in the maximum.6. IT Space AlignmentThe alignment performed in the TED runs which is decribed here is an internal alignment:no alignment with other detectors was performed. The space alignment procedure consists of twosteps: pre-alignment and alignment. In the pre-alignment step, two boxes are fixed and the thirdone is aligned with respect to these two. This step is needed to ensure that the box position in xis known to a sufficient precision to allow a reliable pattern recognition to be performed. In thealignment step, two layers are fixed, and the others are aligned with respect to these. Details canbe found in Refs. [4, 5, 6].6.1 Pre-alignmentThe pre-alignment consisted of estimating the position of the T2-station boxes with respect tothe boxes in station T1 and T3. Firstly, a line was defined with a hit in the last layer of the last box(station T3) and a hit in the first layer of the first box (station T1). This line was required to beconsistent with a particle originating from the TED. A second layer in one of the boxes was used toconfirm the track candidate. Secondly, the distance between the line and the hits in station T2 wascalculated and histogrammed. Fitting a Gaussian plus a flat background to this histogram allowedto extract the position of the middle box in T2 relatively to the fixed T1 and T3 stations (see Fig.6). The observed offsets (700 µm, average over all four boxes) were consistent with the precisionof the survey (≃ 1 mm).6.2 AlignmentThe full alignment procedure was run after the pre-alignment. A sub-set of 16′000 isolated3tracks were selected in the TED events where the occupancy was lowest. Tracks were reconstructedin the IT in a standalone way, and then passed to the full LHCb alignment framework4. The boxeswere aligned for x, y translations and z rotations, the layers for x translations and z rotations, andthe detector modules for x translations. An iterative procedure was used in which tracks were3An isolated track is defined as having less than four hits in a 1 mm window around it.4Based on Kalman Filter, see Refs. [5, 6]6PoS(RD09)010Commissioning of the LHCb Silicon Tracker using data from LHC injection tests Mathias Knechtdx [mm]-4 -3 -2 -1 0 1 2 3 4 020040060080010001200IT Top - Station 2mean =  0.879 +/- 0.007sigma =  0.153 +/- 0.008 Figure 6: Pre-alignment: residuals in the Top box of IT-station T2.selected with low χ2. In order to give a reliable estimate of the χ2 in the absence of misalignment,a momentum must be assigned in the track fit. Therefore all tracks were assumed to be ≃ 10 GeV/cmuons. To avoid biases, the χ2 cut was initially kept loose, and gradually tightened after eachiteration. After each iteration the new alignment constants were taken as an input for the nextiteration. The procedure converged after three iterations.The quality of the alignment was validated with an independent data sample from the TEDrun. Reconstructed tracks were used to calculate residuals in every module, without using the hitsin this particular module (unbiased residuals). A Gaussian was then fitted to the unbiased residuals,and the mean (module bias) and the width (resolution) were extracted from the distribution. In Fig.7, the distributions of the mean (left) and width (right) of the unbiased residuals are shown for alllong-sensor modules. The distribution of the mean is centered around zero and has an rms of 9 µm,which is an indication of the remaining misalignment. The distribution of the width is centeredaround 80 µm for x-measuring layers, and 120 µm for stereo layers. The wider distribution of thewidth for stereo-layers is expected since there are only six measurements of this type compared totwelve x measurements. Both numbers are consistent with reconstructed tracks being ≃ 10 GeV/cmuons. The intrinsic resolution of the detector modules was determined in test beams to be around50 µm.7. TT Space AlignmentThe TT has only four layers, therefore no standalone tracking can be done. In order to estimatethe misalignment, residuals in TT with respect to extrapolated tracks from IT and VELO werecalculated. Since the TED is located 350 m away from LHCb, the tracks were almost parallel, anddue to the large acceptance of TT with respect to VELO and IT, the region of TT illuminated withVELO or IT tracks was small. This prevented a complete alignment of TT. The residuals obtainedfrom IT tracks extrapolated to the TT are shown in Fig. 8.8. SummaryThe LHC injection tests have provided a wealth of data for the Silicon Tracker. The timealignment could be done up to 1 ns, and an initial space alignment of IT modules was performed in7PoS(RD09)010Commissioning of the LHCb Silicon Tracker using data from LHC injection tests Mathias KnechtMean   4.762e-05RMS    0.009532bias [mm]-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.10510152025resolution [mm]0 0.05 0.1 0.15 0.2 0.250246810Figure 7: Left: Module bias (distribution of the mean of the unbiased residuals per detector module). Right:Resolution in x (distribution of the width of the unbiased residuals) for x hits (open histogram) and stereohits (hatched histogram).dx [mm]-10 -8 -6 -4 -2 0 2 4 6 8 10 020040060080010001200Res. of IT tracks in TTmean = -0.435 +/- 0.05sigma =  1.13 +/- 0.05 Figure 8: Residuals from IT tracks in TT.which x translations could be determined to a precision of 13 µm. Tracks were observed in all threesilicon detectors of LHCb (VELO, TT, IT). The Silicon Tracker is operational, and the outcome ofthese tests have provided a good starting point for data taking with the first LHC collisions.References[1] M. Knecht et al., Commissioning of the LHCb Silicon Tracker using data from the LHC injectiontests, CERN-LHCb-CONF-2009-044.[2] The LHCb Collaboration, The LHCb detector at the LHC, Journal of Instrumentation, 2008 JINST 3S08005.[3] M. Knecht and M. Needham, Fine time alignment of the LHCb Inner Tracker with the LHC injectiontests, in preparation, CERN-LHCb-PUB-2009-020.[4] M. Needham, Track reconstruction in the LHCb Inner Tracker, CERN-LHCb-PUB-2009-005.[5] L. Nicolas and M. Needham, Alignment of the Inner Tracker stations using first data,CERN-LHCb-PUB-009-012.[6] L. Nicolas et al., Alignment of LHCb tracking stations with tracks fitted with a Kalman filter, 2008IEEE Nuclear Science Symposium Dresden, conference record N20-3, pp. 1714–17198

Commissioning of the LHCb Silicon Tracker using data from the LHC injection tests

Commissioning of the LHCb Silicon Tracker using data from the LHC injection tests

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