arXiv subjects

Valerio Bertacchi

Publications and source records attributed to Valerio Bertacchi.

8 recordsLinked to original sources

arXiv2026

Measurements of electroweak penguins and $B$ decays to final states with missing energy at Belle and Belle II

The Belle and Belle II experiments have collected a 1.3 ab$^{-1}$ sample of $e^+e^-\to B\bar B$ collisions at $\Upsilon(4S)$ centre-of-mass energy. This is ideal environment to search for rare electroweak penguin $B$ decays and notably those involving $B$ decays to final states with missing energy. Results on these datasets of $b\to s \ell^+\ell^-$ $(\ell=e,\mu)$, $b\to s\tau^+\tau^-$, and $b\to s\nu\bar \nu$ transitions are presented.

hep-ex↗

arXiv2025

Kink Finder at Belle II

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.

hep-ex↗

arXiv2024

End-to-End Multi-Track Reconstruction using Graph Neural Networks at Belle II

We present the study of an end-to-end multi-track reconstruction algorithm for the central drift chamber of the Belle II experiment at the SuperKEKB collider using Graph Neural Networks for an unknown number of particles. The algorithm uses detector hits as inputs without pre-filtering to simultaneously predict the number of track candidates in an event and their kinematic properties. In a second step, we cluster detector hits for each track candidate to pass to a track fitting algorithm. Using a realistic full detector simulation including beam-induced backgrounds and detector noise taken from actual collision data, we find significant improvements in track finding efficiencies for tracks in a variety of different event topologies compared to the existing baseline algorithm used in Belle II. For events with a hypothetical long-lived massive particle with a mass in the GeV-range decaying uniformly along its flight direction into two charged particles, the GNN achieves a combined track finding and fitting efficiency of 85.4% with a fake rate of 2.5%, compared to 52.2% and 4.1% for the baseline algorithm. This is the first end-to-end multi-track machine learning algorithm for a drift chamber detector that has been utilized in a realistic particle physics environment.

physics.ins-det↗

arXiv2023

Belle II status and prospects for studies of neutral currents

The Belle II experiment at the SuperKEKB energy-asymmetric electron-positron collider is a substantial upgrade of the B factory facility at the Japanese KEK laboratory. Belle II collected a sample of $362~\mathrm{fb}^{-1}$ at the $Υ(4S)$ resonance between 2019 and 2022, with a maximum peak luminosity of $4.7 \times 10^{34} \mathrm{cm}^{-2}s^{-1}$. Belle II is currently facing a long shutdown period, required for several upgrades of the detector and the collider. Data taking will resume at the end of 2023. We report the recent measurements which involve neutral current transitions in $B$ meson decays. In particular, we present the current status and future prospects for the branching fractions measurements of the radiative decays $B\to K^*γ$ and the fully inclusive $B\to X_sγ$, the search for $B^+\to K^+ν\bar ν$ decays, the measurement of the branching fractions of $B\to J/ψ(\to \ell\ell)K$ and $B\to K^*\ell\ell$. Finally, we show the perspectives of the search for $B\to K^{*}ττ$ and the searches of lepton flavor violating channels $B\to K^{(*)}\ell\ell'$, with $\ell=e, μ, τ$.

hep-ex↗

arXiv2023

Time-dependent, hadronic B decays and electroweak penguins at Belle II

We report the recent measurements performed using the data sample collected from 2019 to 2022 by the Belle~II experiment~\cite{Belle2:TDR} at $Υ(4S)$ resonance, corresponding to an integrated luminosity of $362~\mathrm{fb}^{-1}$. We present the measurement of $B$ lifetime and mixing frequency, the time dependent CP-violation analyses using the $B^0\to JψK_S^0$, $B^0\to ϕK_S^0$, $B^0\to K_S^0K_S^0K_S^0$, $B^0\to K_S^0π^0$ decays and the implication of the latter for the isospin sum rule, the observation of $B\to D^{(*)} K^-K_S^0$ decays, the analysis of $B\to X_sγ$. We also present the measurements of the angle $γ$ using $B\to DK$ decays, with the combined dataset collected by Belle and Belle II.

hep-ex↗

arXiv2021

Track Finding at Belle II

This paper describes the track-finding algorithm that is used for event reconstruction in the Belle II experiment operating at the SuperKEKB B-factory in Tsukuba, Japan. The algorithm is designed to balance the requirements of a high efficiency to find charged particles with a good track parameter resolution, a low rate of spurious tracks, and a reasonable demand on CPU resources. The software is implemented in a flexible, modular manner and employs a diverse selection of global and local track-finding algorithms to achieve an optimal performance.

physics.ins-det↗

arXiv2020

DeepCore: Convolutional Neural Network for high $p_T$ jet tracking

Tracking in high-density environments, such as the core of TeV jets, is particularly challenging both because combinatorics quickly diverge and because tracks may not leave anymore individual "hits" but rather large clusters of merged signals in the innermost tracking detectors. In the CMS collaboration, this problem has been addressed in the past with cluster splitting algorithms, working layer by layer, followed by a pattern recognition step where a high number of candidate tracks are tested. Modern Deep Learning techniques can be used to better handle the problem by correlating information on multiple layers and directly providing proto-tracks without the need of an explicit cluster splitting algorithm. Preliminary results will be presented with ideas on how to further improve the algorithms.

physics.ins-det↗

arXiv2020

Impact of the PDFs on the Z and W lineshapes at LHC

The parton distribution functions (PDFs) of the proton play a role in determining the lineshape of $Z$ and $W$ bosons produced at the LHC. In particular, the mode of the gauge boson virtuality is shifted with respect to the pole due to the dependence of the partonic luminosity on the boson virtuality. The knowledge of this shift contributes to the systematic uncertainty for a direct measurement of the boson mass. A detailed study of the shift and of its systematic uncertainty due to the limited knowledge of the PDFs is obtained using a tree-level model of $Z$ and $W$ boson production in proton-proton collisions at $\sqrt{s}=13$ TeV. A Monte Carlo simulation is further used to validate the tree-level model and study the dependence of the shift on the transverse momentum of the gauge bosons. The tree-level calculation is found to provide a good description of the shift. The systematic uncertainty on the lineshape due to the PDFs is estimated to be below one MeV in the phase-space relevant for a future high-precision mass measurement of the gauge boson masses at the LHC.

hep-ex↗