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IOP : Muon identification for LHCb Run 3

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Abstract

Muon identification is of paramount importance for the physics programme of LHCb. In the upgrade phase, starting from Run 3 of the LHC, the trigger of the experiment will be solely based on software. The luminosity increase to $2\times10^{33}$ cm$^{-2}$s$^{-1}$ will require an improvement of the muon identification criteria, aiming at performances equal or better than those of Run 2, but in a much more challenging environment. In this paper, two new muon identification algorithms developed in view of the LHCb upgrade are presented, and their performance in terms of signal efficiency versus background reduction is shown.

Figures and captions

Side view of the muon system in the $y-z$ plane [6].

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Front view of one quadrant of M2 showing the four regions. The intersection of a horizontal and a vertical strip defines a logical pad. The region and channel dimensions scale by a factor of two from one region to the following.

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The LHCb trigger scheme in Run 2 [2].

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Two different combination of muon hits having a similar value of \texttt{MuonDLL}: a combinatorial background event (left) and a clear muon pattern (right).

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Typical invariant mass distributions for $J/\psi\rightarrow \mu^+\mu^-$ (left) and $\Lambda\rightarrow p\pi^-$ (right) calibration samples. The superimposed fit (red line) is composed of a signal (dashed blue) and background (dotted dash green) component [13].

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Spectrum of the $\chi^2_{\mathrm{CORR}}$ , normalised to the degrees of freedom, for muons and protons as evaluated on 2016 calibration samples.

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Proton rejection as a function of muon efficiency for tracks satisfying \texttt{IsMuon} obtained with the $\chi^2_{\mathrm{CORR}}$ (blue) and \texttt{MuonDLL} (black) variables on 2016 calibration data. Low momentum bins, which are not covered by the calibration samples, are not shown. The bands represent statistical uncertainties.

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Classic versus oblivious decision trees. Reproduced from [19].

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Proton rejection as a function of muon efficiency for tracks satisfying \texttt{IsMuon} obtained with the CatBoost algorithm (magenta) and $\chi^2_{\mathrm{CORR}}$ (blue) on 2016 calibration data. Low momentum bins, which are not covered by the calibration samples, are not shown. The bands represent statistical uncertainties.

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Animated gif made out of all figures.

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Tables and captions

Position along the beam axis and thickness in units of radiation length for the main scattering media contributing to the multiple scattering experienced by particles reaching the muon detector.

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Rejection factors of the $\chi^2_{\mathrm{CORR}}$ variable on trigger unbiased events, for a muon efficiency of $\sim 98\%$. The rejection is evaluated on top of the $p_T>800 {\mathrm{ Me V /}c} $, $\text{IP}\chi^2>35$, \texttt{IsMuon} and nPVs $\geq3$ requirements.

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Created on 28 November 2023.