Recent Belle II results on BSM physics

We report on recent results obtained by the Belle II experiment related to searches for beyond-the-standard-model physics. The $B$ meson decay data sample used corresponds to an integrated luminosity of $L = 189$ fb$^{-1}$. In recent years, discrepancies from standard-model predictions, which suggest violation of lepton-flavor universality, have emerged in multiple measurements of $B$-meson decays. In this document two analyses aimed to search for violation of the light-lepton universality are reported: the inclusive $R(X_{e/\mu})$ measurement and an angular analysis of the $B^0 \to D^{*-} l^+ \nu_l$, with $l=e,\mu$, decays. In addition, beyond-the-standard-model physics can manifests itself with the production of new particles. The search for a long-lived spin-0 particle in $b\to s$ transitions, and its interpretation in two dark sector models, is reported.Comment: Presented at DIS2023: XXX International Workshop on Deep-Inelastic Scattering and Related Subjects, Michigan State University, USA, 27-31 March 2023. Updated version includes a reference to a preprint which is just release

Flavor Changing Effects in Family Nonuniversal Z' Models

Flavor-changing and CP-violating interactions of Z' to fermions are generally present in models with extra U(1) gauge symmetry that are string-inspired or related to broken gauged family symmetry. We study the consequences of such couplings in fermion electric dipole moments, muon g-2, and K and B meson mixings. From experimental limits or measured values, we constrain the off-diagonal Z' couplings to fermions. Some of these constraints are comparable or stronger than the existing constraints obtained from other observables.Comment: 17 pages, 2 figure

Charged-Lepton Flavour Physics

This writeup of a talk at the 2011 Lepton-Photon symposium in Mumbai, India, summarises recent results in the charged-lepton flavour sector. I review searches for charged-lepton flavour violation, lepton electric dipole moments and flavour-conserving CP violation. I also discuss recent progress in tau-lepton physics and in the Standard Model prediction of the muon anomalous magnetic moment.Comment: Presented at Lepton-Photon 2011, Mumbai, India; 23 pages, 14 figure

Status of the BELLE II Pixel Detector

The Belle II experiment at the super KEK B-factory (SuperKEKB) in Tsukuba, Japan, has been collecting $e^+e^−$ collision data since March 2019. Operating at a record-breaking luminosity of up to $4.7×10^{34} cm^{−2}s^{−1}$, data corresponding to $424 fb^{−1}$ has since been recorded. The Belle II VerteX Detector (VXD) is central to the Belle II detector and its physics program and plays a crucial role in reconstructing precise primary and decay vertices. It consists of the outer 4-layer Silicon Vertex Detector (SVD) using double sided silicon strips and the inner two-layer PiXel Detector (PXD) based on the Depleted P-channel Field Effect Transistor (DePFET) technology. The PXD DePFET structure combines signal generation and amplification within pixels with a minimum pitch of $(50×55) μm^2$. A high gain and a high signal-to-noise ratio allow thinning the pixels to $75 μm$ while retaining a high pixel hit efficiency of about $99$. As a consequence, also the material budget of the full detector is kept low at $≈0.21$$\frac{X}{X_0}$ per layer in the acceptance region. This also includes contributions from the control, Analog-to-Digital Converter (ADC), and data processing Application Specific Integrated Circuits (ASICs) as well as from cooling and support structures. This article will present the experience gained from four years of operating PXD; the first full scale detector employing the DePFET technology in High Energy Physics. Overall, the PXD has met the expectations. Operating in the intense SuperKEKB environment poses many challenges that will also be discussed. The current PXD system remains incomplete with only 20 out of 40 modules having been installed. A full replacement has been constructed and is currently in its final testing stage before it will be installed into Belle II during the ongoing long shutdown that will last throughout 2023

Search for Axionlike Particles Produced in e⁺ e⁻ Collisions at Belle II

International audienceWe present a search for the direct production of a light pseudoscalar a decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process e+e-→γa, a→γγ in the mass range 0.2

Precise measurement of the Ds+D^+_s lifetime at Belle II

We measure the lifetime of the $D_s^+$ meson using a data sample of 207 fb$^{-1}$ collected by the Belle II experiment running at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. The lifetime is determined by fitting the decay-time distribution of a sample of $116\times 10^3$ $D_s^+\rightarrow\phi\pi^+$ decays. Our result is \tau^{}_{D^+_s} = (498.7\pm 1.7\,^{+1.1}_{-0.8}) fs, where the first uncertainty is statistical and the second is systematic. This result is significantly more precise than previous measurements.Comment: 7 pages, 4 figures, to be submitted to Physical Review Letter

Measurement of the Λc+\Lambda_c^+ lifetime

An absolute measurement of the $\Lambda^{+}_c$ lifetime is reported using $\Lambda_c^+\rightarrow pK^-\pi^+$ decays in events reconstructed from data collected by the Belle II experiment at the SuperKEKB asymmetric-energy electron-positron collider. The total integrated luminosity of the data sample, which was collected at center-of-mass energies at or near the $\Upsilon(4S)$ resonance, is 207.2~\mbox{fb}^{-1}. The result, $\tau(\Lambda^{+}_c) = 203.20 \pm 0.89 \,\mathrm{(stat)} \pm 0.77 \,\mathrm{(syst)}$ fs, is the most precise measurement to date and is consistent with previous determinations.Comment: Accepted for publication in PR

Precise Measurement of the D0^{0} and D+^{+} Lifetimes at Belle II

We report a measurement of the D$^{0}$ and D$^{+}$ lifetimes using D$^{0}$→K$^{-}$π$^{+}$ and D$^{+}$→K$^{-}$π$^{+}$π$^{+}$ decays reconstructed in e$^{+}$e$^{-}$→$\overline{cc}$ data recorded by the Belle II experiment at the SuperKEKB asymmetric-energy e$^{+}$e$^{-}$ collider. The data, collected at center-of-mass energies at or near the Υ(4S) resonance, correspond to an integrated luminosity of 72 fb$^{-1}$. The results, τ(D$^{0}$)=410.5±1.1(stat)±0.8(syst)  fs and τ(D$^{+}$)=1030.4±4.7(stat)±3.1(syst) fs, are the most precise to date and are consistent with previous determinations

First measurement of R(Xτ/ℓ)R(X_{\tau/\ell}) as an inclusive test of the b→cτνb \to c \tau \nu anomaly

We measure the tau-to-light-lepton ratio of inclusive $B$-meson branching fractions $R(X_{\tau/\ell}) \equiv \mathcal{B}(B\to X \tau \nu)/\mathcal{B}(B \to X \ell \nu)$, where $\ell$ indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed $B$ meson and a charged lepton candidate from $189~\mathrm{fb}^{-1}$ of electron-positron collision data collected with the Belle II detector. We find $R(X_{\tau/\ell}) = 0.228 \pm 0.016~(\mathrm{stat}) \pm 0.036~(\mathrm{syst})$, in agreement with standard-model expectations. This is the first direct measurement of $R(X_{\tau/\ell})$

Measurement of branching fractions and direct CPCP asymmetries for B→KπB \to K\pi and B→ππB\to\pi\pi decays at Belle II

We report measurements of the branching fractions and direct $\it{CP}$ asymmetries of the decays $B^0 \to K^+ \pi^-$, $B^+ \to K^+ \pi^0$, $B^+ \to K^0 \pi^+$, and $B^0 \to K^0 \pi^0$, and use these for testing the standard model through an isospin-based sum rule. In addition, we measure the branching fraction and direct $\it{CP}$ asymmetry of the decay $B^+ \to \pi^+\pi^0$ and the branching fraction of the decay $B^0 \to \pi^+\pi^-$. The data are collected with the Belle II detector from $e^+e^-$ collisions at the $\Upsilon(4S)$ resonance produced by the SuperKEKB asymmetric-energy collider and contain $387\times 10^6$ bottom-antibottom meson pairs. Signal yields are determined in two-dimensional fits to background-discriminating variables, and range from 500 to 3900 decays, depending on the channel. We obtain $-0.03 \pm 0.13 \pm 0.04$ for the sum rule, in agreement with the standard model expectation of zero and with a precision comparable to the best existing determinations