# Performance of the LHCb RICH detector at the LHC

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## Abstract

The LHCb experiment has been taking data at the Large Hadron Collider (LHC) at CERN since the end of 2009. One of its key detector components is the Ring-Imaging Cherenkov (RICH) system. This provides charged particle identification over a wide momentum range, from 2-100 GeV/c. The operation and control software, and online monitoring of the RICH system are described. The particle identification performance is presented, as measured using data from the LHC. Excellent separation of hadronic particle types (pion, kaon and proton) is achieved.

## Figures and captions

 Side view of the LHCb spectrometer, with the two RICH detectors indicated lhcb.eps [512 KiB] HiDef png [615 KiB] Thumbnail [332 KiB] Invariant mass distribution for B$\rightarrow h^+h^-$ decays [3] in the LHCb data before the use of the RICH information (left), and after applying RICH particle identification (right). The signal under study is the decay B$^0\rightarrow\pi^+\pi^-$, represented by the turquoise dotted line. The contributions from different $b$-hadron decay modes (B$^0 \rightarrow$ K$\pi$ red dashed-dotted line, B$^0\rightarrow$3-body orange dashed-dashed line, B$_s \rightarrow$ KK yellow line, B$_s \rightarrow$ K$\pi$ brown line, $\Lambda_b \rightarrow$ pK purple line, $\Lambda_b \rightarrow$ p$\pi$ green line), are eliminated by positive identification of pions, kaons and protons and only the signal and two background contributions remain visible in the plot on the right. The grey solid line is the combinatorial background mpipi_[..].eps [1 MiB] HiDef png [397 KiB] Thumbnail [180 KiB] mpipi_[..].eps [1 MiB] HiDef png [359 KiB] Thumbnail [166 KiB] RICH data-flow through the online system. Events selected by the L0 trigger are sent to the High Level Trigger (HLT) farm and, if they pass this trigger requirements, are sent to storage. A fraction of these events (typically 10%) is also sent to the monitoring farm. Online monitoring algorithms examine the data for irregularities and send messages to the slow-control (ECS) that can trigger automatic actions. Special triggers are sent directly to the calibration farm bypassing the High Level Trigger mondat[..].eps [2 MiB] HiDef png [261 KiB] Thumbnail [117 KiB] Distribution of the midpoints in RICH 1 (left) and RICH 2 (right) after time alignment with pp collisions. The RMS deviations of the HPDs are approximately 1 ns R1_mid[..].eps [46 KiB] HiDef png [75 KiB] Thumbnail [84 KiB] R2_mid[..].eps [46 KiB] HiDef png [77 KiB] Thumbnail [86 KiB] Spatial residuals demonstrating the resolution with which the light spots of the test pattern in RICH 1 are identified. The plot shows the distance from the measured light spot centre to the nearest test point. The dotted and solid lines are before and after the calibration respectively, along the $x$ direction (left) and along $y$ (right) of the anode plane, projected on the photocathode plane. The solid line is the Gaussian fit resol_x.eps [10 KiB] HiDef png [120 KiB] Thumbnail [121 KiB] resol_y.eps [10 KiB] HiDef png [122 KiB] Thumbnail [122 KiB] Spatial residuals demonstrating the resolution with which the light spots of the test pattern in RICH 2 are identified. The plot shows the distance from the measured light spot centre to the nearest test point. The dotted and solid lines are before and after the calibration respectively. Most of the photodetectors of RICH 2 are in a region free from magnetic field residual values (region around $x$=0 of the dotted histogram). Where these are different from zero, the distorsions induced are visible in the two satellite peaks of opposite sign (the magnetic field changes sign in the upper and lower part of the photodetector matrix plane). The left plot is the measurement along the $x$, the right plot along $y$ of the anode plane, projected on the photocathode plane. The solid line is the Gaussian fit resX_PC.eps [7 KiB] HiDef png [135 KiB] Thumbnail [139 KiB] resY_PC.eps [7 KiB] HiDef png [134 KiB] Thumbnail [139 KiB] $\Delta\theta_C$ plotted as a function of the azimuthal angle $\phi$ and fitted with $\theta_{x} \cos(\phi) + \theta_{y}\sin(\phi)$, for one side of the RICH 2 detector. The left-hand plot is prior to alignment, and shows a dependency of the angle $\theta_{C}$ on the angle $\phi$. The right-hand plot is after the alignment correction, and $\Delta \theta_{C}$ is uniform in $\phi$ RICH2_[..].eps [60 KiB] HiDef png [325 KiB] Thumbnail [273 KiB] RICH2_[..].eps [60 KiB] HiDef png [304 KiB] Thumbnail [263 KiB] The Cherenkov angular resolution (c.f. Sect. 4.2), after all corrections have been applied, as a function of run number. a) for RICH 1 and b) for RICH 2. The period of time covered on the x-axis corresponds to about 8 months of running rich1-res.eps [197 KiB] HiDef png [260 KiB] Thumbnail [286 KiB] rich2-res.eps [198 KiB] HiDef png [253 KiB] Thumbnail [284 KiB] Single photoelectron resolution for the RICH 1 (left) and RICH 2 (right) gases, as measured in data for high momentum charged particles. The red line describes the background as determined from the fit using a polynomial function together with the Gaussian for the signal CKThet[..].eps [23 KiB] HiDef png [165 KiB] Thumbnail [148 KiB] CKThet[..].eps [21 KiB] HiDef png [152 KiB] Thumbnail [129 KiB] Single photoelectron resolution for the aerogel as measured in 2011 data with the pp$\rightarrow$pp$\mu^+\mu^-$ events. The red line describes the background as determined from the fit using a polynomial function together with two Gaussians for the signal aero-d[..].eps [25 KiB] HiDef png [177 KiB] Thumbnail [150 KiB] Distribution of $\Delta \theta_C$ for C$_4$F$_{10}$. This plot is produced from kaons and pions from tagged D$^0 \rightarrow$ K$^- \pi^+$ decays in data selected with the criteria described in the text gas1-d[..].eps [25 KiB] HiDef png [175 KiB] Thumbnail [150 KiB] Individual track photon yield distributions for the C$_4$F$_{10}$ (left) and CF$_4$ (right) radiators. The plot is produced from kaons and pions from tagged D$^0 \rightarrow$ K$^- \pi^+$ decays in data selected with the criteria described in the text gas1yield.eps [7 KiB] HiDef png [115 KiB] Thumbnail [119 KiB] gas2yield.eps [7 KiB] HiDef png [122 KiB] Thumbnail [127 KiB] Distribution of the number of pixel hits per event in (a) RICH 1 and (b) RICH 2. An example of a typical LHCb event as seen by the RICH detectors, is shown below the distributions. The upper/lower HPD panels in RICH 1 and the left/right panels in RICH 2 are shown separately nHits-[..].eps [9 KiB] HiDef png [105 KiB] Thumbnail [101 KiB] nHits-[..].eps [9 KiB] HiDef png [105 KiB] Thumbnail [102 KiB] rich1-[..].eps [36 KiB] HiDef png [231 KiB] Thumbnail [182 KiB] rich2-[..].eps [59 KiB] HiDef png [428 KiB] Thumbnail [335 KiB] Reconstructed Cherenkov angle as a function of track momentum in the $\rm C_{4}F_{10}$ radiator CKAngl[..].eps [551 KiB] HiDef png [911 KiB] Thumbnail [333 KiB] Invariant mass distributions of the (a) K$^{0}_{S}$, (b) $\Lambda$ and (c) D$^{0}$ calibration samples. The best fit probability-density-function (pdf), describing both background and signal, is superimposed in blue K0S_Mass.eps [25 KiB] HiDef png [165 KiB] Thumbnail [151 KiB] Lambda[..].eps [26 KiB] HiDef png [176 KiB] Thumbnail [167 KiB] D0_Mass.eps [25 KiB] HiDef png [170 KiB] Thumbnail [158 KiB] Distribution of $\rm\Delta log \mathcal{L}(K - \pi)$ against $\rm\Delta log \mathcal{L}(p - \pi)$ for (a) pions, (b) kaons and (c) protons extracted from the control samples DLL2D_[..].eps [39 KiB] HiDef png [252 KiB] Thumbnail [236 KiB] DLL2D_[..].eps [40 KiB] HiDef png [266 KiB] Thumbnail [246 KiB] DLL2D_[..].eps [30 KiB] HiDef png [213 KiB] Thumbnail [201 KiB] Kaon identification efficiency and pion misidentification rate measured on data as a function of track momentum. Two different $\rm\Delta log \mathcal{L}(K-\pi)$ requirements have been imposed on the samples, resulting in the open and filled marker distributions, respectively KandPi_2_K.eps [20 KiB] HiDef png [189 KiB] Thumbnail [178 KiB] Kaon identification efficiency and pion misidentification rate measured using simulated events as a function of track momentum. Two different $\rm\Delta log \mathcal{L}(K-\pi)$ requirements have been imposed on the samples, resulting in the open and filled marker distributions, respectively MC10_K[..].eps [23 KiB] HiDef png [214 KiB] Thumbnail [194 KiB] Proton identification efficiency and pion misidentification rate measured on data as a function of track momentum. Two different $\rm\Delta log \mathcal{L}(p-\pi)$ requirements have been imposed on the samples, resulting in the open and filled marker distributions, respectively PandPi_2_P.eps [22 KiB] HiDef png [206 KiB] Thumbnail [182 KiB] Proton identification efficiency and kaon misidentification rate measured on data as a function of track momentum. Two different $\rm\Delta log \mathcal{L}(p-K)$ requirements have been imposed on the samples, resulting in the open and filled marker distributions, respectively PandK_2_P.eps [22 KiB] HiDef png [208 KiB] Thumbnail [185 KiB] Pion misidentification fraction versus kaon identification efficiency as measured in 7 TeV LHCb collisions: (a) as a function of track multiplicity, and (b) as a function of the number of reconstructed primary vertices. The efficiencies are averaged over all particle momenta nTracks_v2.eps [8 KiB] HiDef png [219 KiB] Thumbnail [169 KiB] nPV_v2.eps [9 KiB] HiDef png [217 KiB] Thumbnail [164 KiB] Animated gif made out of all figures. DP-2012-003.gif Thumbnail

## Tables and captions

 Comparison of photoelectron yields (N$_{\rm pe}$) determined from D$^* \rightarrow$D$^0\pi^+$ decays in simulation and data, and pp $\rightarrow$ pp $\mu^+ \mu^-$ events in data, using the selections and methods described in the text Table_1.pdf [46 KiB] HiDef png [53 KiB] Thumbnail [29 KiB] tex code

Created on 14 September 2019.Citation count from INSPIRE on 14 September 2019.