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| CMS-TRK-17-001 ; CERN-EP-2018-144 | ||
| Precision measurement of the structure of the CMS inner tracking system using nuclear interactions | ||
| CMS Collaboration | ||
| 9 July 2018 | ||
| JINST 13 (2018) P10034 | ||
| Abstract: The structure of the CMS inner tracking system has been studied using nuclear interactions of hadrons striking its material. Data from proton-proton collisions at a center-of-mass energy of 13 TeV recorded in 2015 at the LHC are used to reconstruct millions of secondary vertices from these nuclear interactions. Precise positions of the beam pipe and the inner tracking system elements, such as the pixel detector support tube, and barrel pixel detector inner shield and support rails, are determined using these vertices. These measurements are important for detector simulations, detector upgrades, and to identify any changes in the positions of inactive elements. | ||
| Links: e-print arXiv:1807.03289 [physics.ins-det] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
(upper) Schematic view of the CMS tracker detector [1], and (lower) closeup view of the region around the original BPIX detector with labels identifying pixel detector support tube, BPIX detector outer and inner shields, three BPIX detector layers, and beam pipe. |
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Figure 1-a:
Schematic view of the CMS tracker detector [1]. |
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Figure 1-b:
Closeup view of the region around the original BPIX detector with labels identifying pixel detector support tube, BPIX detector outer and inner shields, three BPIX detector layers, and beam pipe. |
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Figure 2:
(left) Photograph of one half of the BPIX detector showing longitudinal support, three layers, and inner shield. (right) Photograph showing an end of the BPIX detector while standing on the installation cassette. Optical targets, indicated by the numbers 2001, 2002, and 2003, are used to locate the BPIX detector within the CMS cavern. Photographs by Antje Behrens, CERN. |
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Figure 2-a:
Photograph of one half of the BPIX detector showing longitudinal support, three layers, and inner shield. |
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Figure 2-b:
Photograph showing an end of the BPIX detector while standing on the installation cassette. Optical targets, indicated by the numbers 2001, 2002, and 2003, are used to locate the BPIX detector within the CMS cavern. Photographs by Antje Behrens, CERN. |
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Figure 3:
Schematic view of NI vertex reconstruction: (left) a cluster of $ {P_{\mathrm {C}}} $ positions ($ {P_{\mathrm {C}}} $, $ {P_{\mathrm {C2}}} $, and $ {P_{\mathrm {C3}}} $) with the distance of closest approach $ {d_{\mathrm {m}}} $ (labeled $d_{\mathrm {m1}}$), shown for $ {P_{\mathrm {C}}} $; (center) the algorithm uses the three $ {P_{\mathrm {C}}} $ points to identify an aggregate position $ {P_{\mathrm {G}}} $; (right) after refitting the track helices, the best vertex $ {P_{\mathrm {G}}'} $ is found with indicated incoming direction from the primary vertex position, $ {P_{\mathrm {V}}} $, and outgoing system. Black curves correspond to reconstructed charged particle tracks. |
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Figure 3-a:
Schematic view of NI vertex reconstruction: a cluster of $ {P_{\mathrm {C}}} $ positions ($ {P_{\mathrm {C}}} $, $ {P_{\mathrm {C2}}} $, and $ {P_{\mathrm {C3}}} $) with the distance of closest approach $ {d_{\mathrm {m}}} $ (labeled $d_{\mathrm {m1}}$), shown for $ {P_{\mathrm {C}}} $; Black curves correspond to reconstructed charged particle tracks. |
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Figure 3-b:
Schematic view of NI vertex reconstruction: the algorithm uses the three $ {P_{\mathrm {C}}} $ points to identify an aggregate position $ {P_{\mathrm {G}}} $; Black curves correspond to reconstructed charged particle tracks. |
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Figure 3-c:
Schematic view of NI vertex reconstruction: after refitting the track helices, the best vertex $ {P_{\mathrm {G}}'} $ is found with indicated incoming direction from the primary vertex position, $ {P_{\mathrm {V}}} $, and outgoing system. Black curves correspond to reconstructed charged particle tracks. |
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Figure 4:
Hadrography of the tracker detector in the $x$-$y$ plane in the barrel region ($ | z | < $ 25 cm). The density of NI vertices is indicated by the color scale. The signatures of the beam pipe, the BPIX detector with its support, and the first layer of the TIB detector can be observed above the background of misreconstructed NIs. |
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Figure 5:
The beam pipe region viewed in the $x$-$y$ plane for $ | z | < $ 25 cm before background subtraction. The density of NI vertices is indicated by the color scale. $(0,0)$ is the origin of the CMS offline coordinate system, which is discussed in Section 7.1. The blue point in the center of the distribution corresponds to the average beam spot position of $ {x_{\text {bs}}} = $ 0.8 mm and $ {y_{\text {bs}}} = $ 0.9 mm in 2015. |
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Figure 6:
The density of NI vertices versus ${\rho ({x_{0}}, {y_{0}})}$ for a $\phi $ slice of the beam pipe located near $\phi = $ 0 (black line) for $ | z | < $ 25 cm before background subtraction. The green hatched area corresponds to the signal region, the red hatched area corresponds to the sideband region used to fit the background, and the blue hatched area corresponds to the estimated background in the signal region. |
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Figure 7:
The beam pipe region with the fitted values for a circle of radius {R} and center $({x_{0}}, {y_{0}})$ for $ | z | < $ 25 cm. The $x$-$y$ plane after background subtraction (upper), and the $r$-$\phi $ coordinates before background subtraction (lower), are shown. The density of NI vertices is indicated by the color scale. The red line shows the fitted circle. The blue point in the center of the $x$-$y$ plane corresponds to the average beam spot position of $ {x_{\text {bs}}} =$ 0.8 mm and $ {y_{\text {bs}}} =$ 0.9 mm in 2015. |
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Figure 7-a:
The beam pipe region with the fitted values for a circle of radius {R} and center $({x_{0}}, {y_{0}})$ for $ | z | < $ 25 cm. The $x$-$y$ plane after background subtraction is shown. The density of NI vertices is indicated by the color scale. The red line shows the fitted circle. The blue point in the center of the $x$-$y$ plane corresponds to the average beam spot position of $ {x_{\text {bs}}} =$ 0.8 mm and $ {y_{\text {bs}}} =$ 0.9 mm in 2015. |
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Figure 7-b:
The beam pipe region with the fitted values for a circle of radius {R} and center $({x_{0}}, {y_{0}})$ for $ | z | < $ 25 cm. The $r$-$\phi $ coordinates before background subtraction is shown. The density of NI vertices is indicated by the color scale. The red line shows the fitted circle. The blue point in the center of the $x$-$y$ plane corresponds to the average beam spot position of $ {x_{\text {bs}}} =$ 0.8 mm and $ {y_{\text {bs}}} =$ 0.9 mm in 2015. |
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Figure 8:
The BPIX detector inner shield region viewed in the $x$-$y$ plane for $ | z | < $ 25 cm before background subtraction and removal of the $\phi $ regions with additional structures. The density of NI vertices is indicated by the color scale. The inner shield itself is the visible circle of radius $r = $ 3.8 cm. Modules in the first BPIX detector layer are visible at larger radius. The small bumps that can be seen around the shield correspond to cables connected to the first BPIX detector layer. |
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Figure 9:
The BPIX detector inner shield with the fitted values for two half-circles of common radius R and centers $({{x_{0}} ^{\text {far}}}, {{y_{0}} ^{\text {far}}})$ and $({{x_{0}} ^{\text {near}}}, {{y_{0}} ^{\text {near}}})$ for $ | z | < $ 25 cm. The $x$-$y$ plane after background subtraction (upper), and the $r$-$\phi $ coordinates before background subtraction (lower), are shown. The density of NI vertices is indicated by the color scale. The red and black lines at around $r = $ 3.8 cm show the fitted half-circles on the far and near sides, respectively. The blue point at the center of the $x$-$y$ plane corresponds to the average beam spot position of $ {x_{\text {bs}}} =$ 0.8 mm and $ {y_{\text {bs}}} =$ 0.9 mm in 2015. Modules in the first BPIX detector layer are visible (lower) at larger radius. |
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Figure 9-a:
The BPIX detector inner shield with the fitted values for two half-circles of common radius R and centers $({{x_{0}} ^{\text {far}}}, {{y_{0}} ^{\text {far}}})$ and $({{x_{0}} ^{\text {near}}}, {{y_{0}} ^{\text {near}}})$ for $ | z | < $ 25 cm. The $x$-$y$ plane after background subtraction is shown. The density of NI vertices is indicated by the color scale. The red and black lines at around $r = $ 3.8 cm show the fitted half-circles on the far and near sides, respectively. The blue point at the center of the $x$-$y$ plane corresponds to the average beam spot position of $ {x_{\text {bs}}} =$ 0.8 mm and $ {y_{\text {bs}}} =$ 0.9 mm in 2015. |
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Figure 9-b:
The BPIX detector inner shield with the fitted values for two half-circles of common radius R and centers $({{x_{0}} ^{\text {far}}}, {{y_{0}} ^{\text {far}}})$ and $({{x_{0}} ^{\text {near}}}, {{y_{0}} ^{\text {near}}})$ for $ | z | < $ 25 cm. The $r$-$\phi $ coordinates before background subtraction is shown. The density of NI vertices is indicated by the color scale. The red and black lines at around $r = $ 3.8 cm show the fitted half-circles on the far and near sides, respectively. Modules in the first BPIX detector layer are visible at larger radius. |
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Figure 10:
The region of the pixel detector support tube viewed in the $x$-$y$ plane for $ | z | < $ 25 cm before background subtraction and removal of the $\phi $ regions with additional structures. The density of NI vertices is indicated by the color scale. Two circular structures are visible. The circle with the smaller radius corresponds to the BPIX detector outer shield, while the one with the larger radius is the pixel detector support tube (also visible in Fig. 4. |
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Figure 11:
The pixel detector support tube with the fitted values for an ellipse with semi-minor axis $ {R_{\mathrm {x}}} $, semi-major axis $ {R_{\mathrm {y}}} $, and center $({x_{0}}, {y_{0}})$ for $ | z | < $ 25 cm. The $x$-$y$ plane after background subtraction (upper), and the $r$-$\phi $ coordinates before background subtraction (lower), are shown. The density of NI vertices is indicated by the color scale. The red line shows the fitted ellipse. The blue point in the center of the $x$-$y$ plane corresponds to the average beam spot position of $ {x_{\text {bs}}} =$ 0.8 mm and $ {y_{\text {bs}}} =$ 0.9 mm in 2015. |
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Figure 11-a:
The pixel detector support tube with the fitted values for an ellipse with semi-minor axis $ {R_{\mathrm {x}}} $, semi-major axis $ {R_{\mathrm {y}}} $, and center $({x_{0}}, {y_{0}})$ for $ | z | < $ 25 cm. The $x$-$y$ plane after background subtraction is shown. The density of NI vertices is indicated by the color scale. The red line shows the fitted ellipse. The blue point in the center of the $x$-$y$ plane corresponds to the average beam spot position of $ {x_{\text {bs}}} =$ 0.8 mm and $ {y_{\text {bs}}} =$ 0.9 mm in 2015. |
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Figure 11-b:
The pixel detector support tube with the fitted values for an ellipse with semi-minor axis $ {R_{\mathrm {x}}} $, semi-major axis $ {R_{\mathrm {y}}} $, and center $({x_{0}}, {y_{0}})$ for $ | z | < $ 25 cm. The $r$-$\phi $ coordinates before background subtraction (lower) is shown. The density of NI vertices is indicated by the color scale. The red line shows the fitted ellipse. |
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Figure 12:
The BPIX detector support rails after background subtraction in the $x$-$y$ plane for the combined tracker detector barrel and endcap regions. Horizontal red lines correspond to the fit of the BPIX detector support rails. The density of NI vertices is indicated by the color scale. |
| Tables | |
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Table 1:
Results of the fit to the beam pipe with a circle, the BPIX detector inner shield with two half-circles, and the pixel detector support tube with an ellipse. Only systematic uncertainties are provided, since the statistical uncertainties are negligible. |
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Table 2:
Results of the fitted $y$ coordinate of the bottom and top BPIX detector support rails with a horizontal line. Only systematic uncertainties are provided, since the statistical uncertainties are negligible. |
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Table 3:
Systematic uncertainties in the position and radius measurements of three passive detector elements. |
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Table 4:
Results from the survey of the CMS central beam pipe positions on January 12, 2015. |
| Summary |
| Nuclear interactions have a reputation of being undesirable events that degrade the quality of the reconstruction of charged and neutral hadrons. In this analysis, it has been demonstrated that they can be used to produce a high-precision map of the material inside the tracker. Using a data set that corresponds to an integrated luminosity of 2.5 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV, a large sample of secondary hadronic interactions was collected. After background subtraction, the positions of the secondary vertices were used to determine the locations of passive material with a precision of the order of 100 $\mu$m. The positions of the beam pipe and the inner tracker structures (pixel detector support tube, and BPIX inner shield and support rails) were determined with a precision that depends on the structure under study. No significant position bias was identified through the technique, and statistical uncertainties were negligible. The positions of the structures under consideration were probed with a precision better than the typical installation tolerances and are found to be compatible with previous survey measurements. |
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