Particle-flow reconstruction and global event description with the CMS detector

TL;DR

This paper presents the CMS particle-flow reconstruction framework, which jointly interprets signals from the tracker, ECAL, HCAL, and muon systems to identify and reconstruct all final-state particles in each collision.Key innovations include iterative tracking with enhanced efficiency, tracker-based electron seeding with Gaussian-sum filtering, a sophisticated calorimeter clustering and calibration scheme, and a robust link algorithm that assembles PF blocks across subdetectors.Performance studies in simulation show PF jets, MET, electrons, muons, taus, and isolation outpace traditional methods, with substantial gains in resolution and pileup robustness, validated by Run 1 data at 8 TeV with average pileup around 20.The PF approach enables efficient pileup mitigation and is integrated into the CMS high-level trigger, making it the backbone of CMS physics analyses and guiding future detector upgrades for higher pileup.

Abstract

The CMS apparatus was identified, a few years before the start of the LHC operation at CERN, to feature properties well suited to particle-flow (PF) reconstruction: a highly-segmented tracker, a fine-grained electromagnetic calorimeter, a hermetic hadron calorimeter, a strong magnetic field, and an excellent muon spectrometer. A fully-fledged PF reconstruction algorithm tuned to the CMS detector was therefore developed and has been consistently used in physics analyses for the first time at a hadron collider. For each collision, the comprehensive list of final-state particles identified and reconstructed by the algorithm provides a global event description that leads to unprecedented CMS performance for jet and hadronic tau decay reconstruction, missing transverse momentum determination, and electron and muon identification. This approach also allows particles from pileup interactions to be identified and enables efficient pileup mitigation methods. The data collected by CMS at a centre-of-mass energy of 8 TeV show excellent agreement with the simulation and confirm the superior PF performance at least up to an average of 20 pileup interactions.

Figures (33)

• Figure 1: A sketch of the specific particle interactions in a transverse slice of the CMS detector, from the beam interaction region to the muon detector. The muon and the charged pion are positively charged, and the electron is negatively charged.
• Figure 2: Event display of an illustrative jet made of five particles only in the $(x,y)$ view (upper panel), and in the ($\eta,\varphi$) view on the ECAL surface (lower left) and the HCAL surface (lower right). In the top view, these two surfaces are represented as circles centred around the interaction point. The $\mathrm{K}^0_\mathrm{L}$, the $\pi^-$, and the two photons from the $\pi^0$ decay are detected as four well-separated ECAL clusters denoted $\mathrm{E}_{1,2,3,4}$. The $\pi^+$ does not create a cluster in the ECAL. The two charged pions are reconstructed as charged-particle tracks $\mathrm{T}_{1,2}$, appearing as vertical solid lines in the ($\eta,\varphi$) views and circular arcs in the $(x,y)$ view. These tracks point towards two HCAL clusters $\mathrm{H}_{1,2}$. In the bottom views, the ECAL and HCAL cells are represented as squares, with an inner area proportional to the logarithm of the cell energy. Cells with an energy larger than those of the neighbouring cells are shown in dark grey. In all three views, the cluster positions are represented by dots, the simulated particles by dashed lines, and the positions of their impacts on the calorimeter surfaces by various open markers.
• Figure 3: Total thickness $t$ of the inner tracker material expressed in units of interaction lengths $\lambda_l$ (left) and radiation lengths $X_0$ (right), as a function of the pseudorapidity $\eta$. The acronyms TIB, TID, TOB, and TEC stand for "tracker inner barrel", "tracker inner disks", "tracker outer barrel", and "tracker endcaps", respectively. The two figures are taken from Ref. cms_tracking_paper.
• Figure 4: Efficiency (left) and misreconstruction rate (right) of the global combinatorial track finder (black squares); and of the iterative tracking method (green triangles: prompt iterations based on seeds with at least one hit in the pixel detector; red circles: all iterations, including those with displaced seeds), as a function of the track $p_{\mathrm{T}}$, for charged hadrons in multijet events without pileup interactions. Only tracks with $\lvert \eta \rvert < 2.5$ are considered in the efficiency and misreconstruction rate determination. The efficiency is displayed for tracks originating from within 3.5$\text{\,cm}$ of the beam axis and $\pm 30$$\text{\,cm}$ of the nominal centre of CMS along the beam axis.
• Figure 5: Maps of nuclear interaction vertices for data collected by CMS in 2011 at $\sqrt{s} = 7$$\,\text{Te\spaceV}$, corresponding to an integrated luminosity of 1$\,\text{nb}^\text{$-$1}$, in the longitudinal (left) and transverse (right) cross sections of the inner part of the tracker, exhibiting its structure in concentric layers around the beam axis.