# Mapping the material in the LHCb vertex locator using secondary hadronic interactions

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## Abstract

Precise knowledge of the location of the material in the LHCb vertex locator (VELO) is essential to reducing background in searches for long-lived exotic particles, and in identifying jets that originate from beauty and charm quarks. Secondary interactions of hadrons produced in beam-gas collisions are used to map the location of material in the VELO. Using this material map, along with properties of a reconstructed secondary vertex and its constituent tracks, a $p$-value can be assigned to the hypothesis that the secondary vertex originates from a material interaction. A validation of this procedure is presented using photon conversions to dimuons.

## Figures and captions

 From Ref. [1]: (top left) a photograph of one side of the VELO, taken during assembly, showing the silicon sensors and readout hybrids; (top right) a schematic of both an $r$ and $\phi$ sensor, showing the sensor strips and routing lines; and (bottom) schematics showing the cross section of the $xz$ plane at $y=0$, where the $r(\phi)$ sensors are shown with solid blue (dashed red) lines, and an $xy$ view of overlapping sensors in the closed position. { N.b.}, the modules at positive (negative) $x$ are known as the left or A-side (right or C-side). VeloAs[..].pdf [248 KiB] HiDef png [6 MiB] Thumbnail [334 KiB] randph[..].pdf [165 KiB] HiDef png [3 MiB] Thumbnail [317 KiB] hidef_[..].png [60 KiB] HiDef png [60 KiB] Thumbnail [8 KiB] Reconstructed SVs in the Run 2 data sample showing the $xy$ plane integrated over $z$ within the region of the VELO that contains sensor modules. The left (right) panel shows the central (forward) VELO region. The bins are $0.1\,mm \times0.1\,mm$ in size. sv_xyc.pdf [311 KiB] HiDef png [2 MiB] Thumbnail [190 KiB] sv_xyf.pdf [140 KiB] HiDef png [1 MiB] Thumbnail [125 KiB] Reconstructed SVs in the Run 1 data sample showing the $zr$ plane integrated over $\phi$, where a positive (negative) $r$ value denotes that the SV is closest to material in the right (left) half of the VELO. The bins are $0.1\,mm \times1\,mm$ in size. { N.b.}, the inner-most RF-foil region is nearly semi-circular in the $xy$ plane, which results in sharp edges at smaller $r$ values; however, at large $|y|$ values, the RF-foil is flat producing SVs at larger values of $r$ which can easily be mistaken as background in the $zr$ projection shown here. sv_zr.pdf [351 KiB] HiDef png [1 MiB] Thumbnail [111 KiB] Differences in the sensor locations relative to the survey specifications. The observed deviations are known from VELO alignment studies and accounted for during reconstruction. mod_shifts.pdf [27 KiB] HiDef png [161 KiB] Thumbnail [41 KiB] Comparison between the RF-foil map at $y=0\,mm$ and the description in LHCb simulation. zx_foil.pdf [97 KiB] HiDef png [331 KiB] Thumbnail [68 KiB] Example comparisons between the (filled bins) reconstructed SV locations and (red lines) the material map for: (top) $x$ versus $z$ near $y=0\,mm$; (middle) same as the top but zoomed in on the ${0 < z < 100\,mm }$ region; (bottom left) $x$ versus $y$ near the left-half $r$ sensor at $z=34.4\,mm$; and (bottom right) $x$ versus $y$ near the right-half $\phi$ sensor at $z=49.3\,mm$. For visual clarity: in the top panel, the red vertical lines denote the center of each sensor, while in the bottom panels only the edges of the sensors are shown; and in all panels, only the nominal RF-foil position is shown; { i.e.} the red RF-foil curves do not display its thickness. zx_y0.pdf [219 KiB] HiDef png [886 KiB] Thumbnail [90 KiB] zx_y0_zoom.pdf [131 KiB] HiDef png [644 KiB] Thumbnail [67 KiB] xy_z34.pdf [205 KiB] HiDef png [2 MiB] Thumbnail [164 KiB] xy_z49.pdf [240 KiB] HiDef png [2 MiB] Thumbnail [188 KiB] The normalized photon conversion $p$-value distribution obtained using a subsample of data from an LHCb long-lived dark-photon search [8]. The data are consistent with the photon-conversion hypothesis. Some example dark-photon distributions are also shown for lifetimes of 1 and 10 ps, showing good separation between potential exotic signals and photon conversions. materi[..].pdf [15 KiB] HiDef png [149 KiB] Thumbnail [60 KiB] Animated gif made out of all figures. DP-2018-002.gif Thumbnail

Created on 17 February 2019.Citation count from INSPIRE on 17 February 2019.