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Kink Finder at Belle II

Denis Bodrov, Xinping Xu, Dmitrii Gavrilov, Pavel Pakhlov, Valerio Bertacchi, Tadeas Bilka, Arkodip Biswas, Giulia Casarosa, Priyanka Cheema, Luigi Corona, Giacomo De Pietro, Thanh V. Dong, Patrick Ecker, Torben Ferber, Romulus Godang, Alexander Heidelbach, Tommy Lam, Martina Laurenza, Thomas Lueck, Ludovico Massaccesi, Frank Meier, Lea Reuter, Bianca Scavino, Karin Schoenning, Justin Skorupa, Stefano Spataro, Tien Manh Tran, Vidya Vobbilisetti, Christian Wessel, Jaroslaw Wiechczynski

TL;DR

The paper develops a dedicated Kink Finder for Belle II to reconstruct in-flight decays and scatterings that create kinks, addressing limitations of standard track finding and clone suppression. It handles two cases: separate mother/daughter tracks and combined tracks that are split, using a preselection of candidates, vertex fitting, hit reassignment, and a combined-track splitting mechanism. Performance studies on MC samples show the Kink Finder boosting in-flight-decay kink reconstruction efficiency from about $11\%$ to roughly $40\%$, along with substantial improvements in vertex/momentum resolutions, a ~60\% reduction in clones, and measurable PID-fake-rate reductions for low-$p_T$ pions. These enhancements expand Belle II’s physics reach, enabling more precise measurements (e.g., Michel parameters) and expanding the toolkit for flavor and beyond-Standard-Model studies, while outlining concrete avenues for speedups and further accuracy improvements.$

Abstract

We present a track-finding algorithm for the Belle II experiment that specifically targets so-called kinks: signatures of charged particles decaying or scattering in-flight in the detector material, resulting in a sudden and significant change of the particle's flight direction. Our benchmark studies of this Kink Finder show that the reconstruction efficiency for such signatures is about 40%, compared to a value of around 11% for the standard Belle II track-finding algorithm. Our studies also show that the Kink Finder significantly improves the resolution of the secondary track parameters, suppresses the number of cloned tracks, and reduces the PID misidentification rates for kaon and pions.

Kink Finder at Belle II

TL;DR

The paper develops a dedicated Kink Finder for Belle II to reconstruct in-flight decays and scatterings that create kinks, addressing limitations of standard track finding and clone suppression. It handles two cases: separate mother/daughter tracks and combined tracks that are split, using a preselection of candidates, vertex fitting, hit reassignment, and a combined-track splitting mechanism. Performance studies on MC samples show the Kink Finder boosting in-flight-decay kink reconstruction efficiency from about to roughly , along with substantial improvements in vertex/momentum resolutions, a ~60\% reduction in clones, and measurable PID-fake-rate reductions for low- pions. These enhancements expand Belle II’s physics reach, enabling more precise measurements (e.g., Michel parameters) and expanding the toolkit for flavor and beyond-Standard-Model studies, while outlining concrete avenues for speedups and further accuracy improvements.$

Abstract

We present a track-finding algorithm for the Belle II experiment that specifically targets so-called kinks: signatures of charged particles decaying or scattering in-flight in the detector material, resulting in a sudden and significant change of the particle's flight direction. Our benchmark studies of this Kink Finder show that the reconstruction efficiency for such signatures is about 40%, compared to a value of around 11% for the standard Belle II track-finding algorithm. Our studies also show that the Kink Finder significantly improves the resolution of the secondary track parameters, suppresses the number of cloned tracks, and reduces the PID misidentification rates for kaon and pions.

Paper Structure

This paper contains 14 sections, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Belle II experiment
  3. Kinks at Belle II
  4. Kink Finder algorithm
  5. Selection of the kink candidates
  6. Reconstructed track pairs
  7. Vertex fit
  8. Hit reassignment algorithm
  9. Combined tracks
  10. Performance
  11. Reconstructed track pairs
  12. Combined tracks
  13. Problems and further improvement
  14. Conclusion

Figures (6)

  • Figure 1: CDC event display of three MC simulated events with kinks (a)--(c). Blue and red hits represent reconstructed mother and daughter tracks from the kink, respectively. All remaining hits in the CDC are yellow.
  • Figure 2: Resolution of the $r$- and $z$-components of the vertex position. Panel (a, b) are obtained with the $\tau^-\rightarrow\xspace K^-\nu_\tau$ sample and panel (c, d) with the $\tau^-\rightarrow\xspace \pi^-\nu_\tau$ sample.
  • Figure 3: Resolution of the daughter momentum in the laboratory frame for transverse component (a) and $z$-component (b); distribution (c) and resolution (d) of the daughter momentum in the mother rest frame. The plots are obtained with the $\tau^-\rightarrow\xspace K^-\nu_\tau$ sample. The two peaks in (c) correspond to the $K^-\rightarrow\xspace\pi^-\pi^0$ (left) and $K^-\rightarrow\xspace\mu^-\bar{\nu}_\mu$ (right) decays.
  • Figure 4: Resolution of the daughter momentum in the laboratory frame for transverse component (a) and $z$-component (b); distribution (c) and resolution (d) of the daughter momentum in the mother rest frame. The plots are obtained with the $\tau^-\rightarrow\xspace \pi^-\nu_\tau$ sample.
  • Figure 5: Daughter particle momentum in the mother rest frame with muon and pion mass hypotheses (a) and pion and kaon mass hypotheses (b). The various sources of kinks in $B\bar{B}$ sample are shown as stacked histograms.
  • ...and 1 more figures