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Performance of the LHCb Outer Tracker

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Abstract

The LHCb Outer Tracker is a gaseous detector covering an area of 5x6 m2 with 12 double layers of straw tubes. The detector with its services are described together with the commissioning and calibration procedures. Based on data of the first LHC running period from 2010 to 2012, the performance of the readout electronics and the single hit resolution and efficiency are presented. The efficiency to detect a hit in the central half of the straw is estimated to be 99.2%, and the position resolution is determined to be approximately 200 um. The Outer Tracker received a dose in the hottest region corresponding to 0.12 C/cm, and no signs of gain deterioration or other ageing effects are observed.

Figures and captions

(a) Module cross section. (b) Arrangement of OT straw-tube modules in layers and stations.

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(a) Design and (b) photograph of the FE electronics mounted in a FE box. Only the boards that read out one monolayer of 64 straws are visible. In addition, the HV boards are not visible in the photograph as they are hidden by the ASDBLR boards.

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Pressure calibration curve of the $^{55}$Fe spectrum, obtained from the dependence of the pulse height $P$ as a function of atmospheric pressure $p$.

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The 2d-hitmap histogram showing the noise occupancy, for each channel, and varying amplifier threshold (1 ADC count $\approx$ 10 mV) [17] for (a) a typical FE-box with good channels and (b) a FE-box with two groups of noisy channels.

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(a) Example of hit-efficiency as function of threshold for a fixed input charge ("high test-pulse") [17]. (b) Stability of the half-efficiency point for channels in one FE-box (1 ADC count $\approx$ 10 mV).

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(a) Example of a linear fit of the measured drift-time as a function of the test-pulse delay [17]. The slope corresponds to unity, if both axis are converted to ns (1 DAC count $\approx$ 0.1 ns, while 1 TDC count $\approx$ 0.39 ns). (b) The slope from the linear fit of the timing measurement for all 128 channels in one FE-box.

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(a) Sketch of the various contributions to the measured TDC time [19], as explained in the text. (b) Picture of a charged particle that traverses a straw.

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The (a) TR-relation distribution follows the shape of a second order polynomial distribution, which leads to a (b) falling drift-time spectrum (black), which, smeared with the time resolution (blue), leads to the shape of the (c) measured drift-time distribution.

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Distribution of differences between $ t_{0}$ constants per FE box, for two different calibrations. The mean shift originates from a change of the overall $ t_{clock}$ time, whereas the spread shows the stability of the delay $t_\mathrm{FE}$ induced by the FE electronics.

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(a) Displacement of modules relative to the survey and (b) hit residuals in the first X-layer of station T2 before (dashed line) and after (continuous line) offline module alignment.

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Average hit residual as function of $y$ coordinate in one particular module (labelled T3L3Q1M7). The four curves show residuals for the four groups of 32 channels within one FE-module. The round markers correspond to one monolayer of 64 straws, whereas the square markers show the residuals of the second monolayer. The vertical dashed lines indicate the position of the wire locators, at every 80 cm along the wire [19].

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(a) Drift-time distribution in module 8, close to the beam, for $75 \mathrm{ns}, 50 \mathrm{ns}, 25 \mathrm{ns}$ bunch-crossing spacing in red, black and blue, respectively. The vertical lines at 64 and 128 TDC counts correspond to 25 and 50 ns, respectively. The distributions correspond to all hits in 3000 events for each bunch-crossing spacing, recorded with an average number of overlapping events of $\mu = 1.2, 1.4$ and 1.2, for $75 \mathrm{ns}, 50 \mathrm{ns}$ and $25 \mathrm{ns}$ conditions, respectively. (b) The drift-time distribution for empty events illustrates the contribution from spillover hits from "busy" previous bunch-crossings (red). The naive expectation of the spillover distribution is shown in black, and is obtained by shifting the nominal drift-time spectrum by $-50 \mathrm{ns}$.

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Straw occupancy for $75 \mathrm{ns}, 50 \mathrm{ns}, 25 \mathrm{ns}$ bunch-crossing spacing in red, black and blue, respectively, for typical run conditions with on average 1.2, 1.4 and 1.2 overlapping events per bunch crossing, respectively. One module contains in total 256 straws, whereas the width of one module is 340 mm. The steps in occupancy at the center of the detector correspond to the location of the shorter S-modules, positioned further from the beam in the $y$-coordinate. The data corresponding to $25 \mathrm{ns}$ bunch-crossing spacing, was recorded with opposite LHCb-dipole polarity, as compared to the other two data sets shown here.

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Coordinate of the origin of charged particles that produce a hit in the OT detector. (a) The blue histogram peaks at $z=0$ and corresponds to hits from particles produced at the $pp$ interaction point and their daughters, while the hits from particles produced in secondary interactions (red) predominantly originate from $z>0$. (b) The longitudinal and transverse position of the origin of charged particles produced in secondary interactions, showing the structure corresponding to the material in the detector.

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(a) Efficiency profile as a function of the distance between the predicted track position and the center of the straw, for straws in the long F-modules closest to the beampipe (module 7). The vertical lines represent the straw tube edge at $|r|=2.45$ mm. (b) Histogram of the average efficiencies per half module (128 channels), at the center of the straw, $|r|<1.25$ mm, for runs 96753, 96763 and 96768 on 22 July 2011.

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(a) Drift-time residual distribution and (b) hit distance residual distribution [19]. The core of the distributions (within $\pm1 \sigma$) are fitted with a Gaussian function and the result is indicated in the figures.

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Improvement in (a) drift-time residual distribution and (b) hit distance residual distribution, (red) before and (blue) after allowing for a different horizontal displacement per half monolayer, corresponding to 64 straws [19].

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The evolution of number of dead and noisy channels as function of run number in the 2011 and 2012 running periods. The definition of dead and noisy channels is given in the text. The three periods with larger number of dead channels, correspond to periods with a problem affecting one entire front-end box.

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Hit efficiency as a function of amplifier threshold in (a) August 2010 and (b) December 2012 for the inner region, defined as $\pm 60 \mathrm{cm}$ in $x$ and $\pm 60 \mathrm{cm}$ in $y$ from the central beam pipe, summed over all OT layers. Note that the threshold value of 1350 mV, where the efficiency is 50%, is much higher than the operational threshold of 800 mV, and is equivalent to multiple times the corresponding average hit charge.

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Animated gif made out of all figures.

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Tables and captions

Main parameters of the OT gas system.

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Average single-hit efficiencies $\varepsilon_{hit}$ near the center of the straws, $|r|<1.25$ mm, for different module positions of the OT detector.

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Created on 12 October 2019.