Search for B + → K + ν ν ¯ Decays Using an Inclusive Tagging Method at Belle II
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46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
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Precise Measurement of the D 0 and D + Lifetimes at Belle II
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46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Measurement of the Λ$^+_c$ Lifetime
An absolute measurement of the Λ$^+_c$ lifetime is reported using Λ$^+_c → pK^– π^+$ 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 $\textit{Υ}(4S)$ resonance, is 207.2 fb –1 . The result, $τ(Λ^+_c)$ = 203.20 ± 0.89 ± 0.77 fs , where the first uncertainty is statistical and the second
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for a Dark Photon and an Invisible Dark Higgs Boson in $μ^+ μ^–$ and Missing Energy Final States with the Belle II Experiment
The dark photon $\textit{A'}$ and the dark Higgs boson $\textit{h'}$ are hypothetical particles predicted in many dark sector models. We search for the simultaneous production of A' and h' in the dark Higgsstrahlung process $e^+e^– → A' h'$ with $A' → μ^+μ^–$ and $\textit{h'}$ invisible in electron-positron collisions at a center-of- mass energy of 10.58 GeV in data collected by the Belle II experiment in 2019. With an integrated luminosity of 8.34 fb –1 , we observe no evidence for signal. We obtain exclusion limits at 90% Bayesian credibility in the range of 1.7-5.0 fb on the cross section and in the range of 1.7 × 10 –8 -200 × 10 –8 on the effective coupling $ε^2 × α_D$ for the $\textit{A'}$ mass in the range of 4.0 GeV/$c^2 < M_{A'}$ < 9.7 GeV/$c^2$ and for the $\textit{h'}$ mass $M_{h'} < M_{A'}$, where $\textit{ε}$ is the mixing strength between the standard model and the dark photon and $α_D$ is the coupling of the dark photon to the dark Higgs boson. Our limits are the first in this mass range. Phtysuc
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Observation of e + e – → ω χ b J ( 1 P ) and Search for X b → ω Υ ( 1 S ) at s near 10.75 GeV
We study the processes $e^{+}e^{–} → ωχ_{bJ}$(1P) (J=0, 1, or 2) using samples at center-of-mass energies $\sqrt{s}$ = 10.701, 10.745, and 10.805 GeV, corresponding to 1.6, 9.8, and 4.7 fb –1 of integrated luminosity, respectively. These data were collected with the Belle II detector during special operations of the SuperKEKB collider above the Υ(4S) resonance. We report the first observation of $ωχ_{bJ}$(1P) signals at $\sqrt{s}$ = 10.745 GeV. By combining Belle II data with Belle results at $\sqrt{s}$ = 10.867 GeV, we find energy dependencies of the Born cross sections for $e^{+}e^{–} → ωχ_{b1,b2}$(1P) to be consistent with the shape of the Υ(10753) state. These data indicate that the internal structures of the Υ(10753) and Υ(10860) states may differ. Including data at $\sqrt{s}$ = 10.653 GeV, we also search for the bottomonium equivalent of the X(3872) state decaying into ωΥ(1S). No significant signal is observed for masses between 10.45 and 10.65 GeV/c 2 .
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for Lepton-Flavor-Violating τ Decays to a Lepton and an Invisible Boson at Belle II
We search for lepton-flavor-violating $τ^– \rightarrow e^–α$ $τ^– \rightarrow μ^–α$ decays, where α is an invisible spin-0 boson. The search uses electron-positron collisions at 10.58 GeV center-of-mass energy with an integrated luminosity of 62.8 fb –1 , produced by the SuperKEKB collider and collected with the Belle II detector. We search for an excess in the lepton-energy spectrum of the known $τ^–\rightarrow e^–\bar{v}_{e}v_τ$ decays. We report 95% confidence-level upper limits on the branching-fraction ratio $\mathscr{B}(τ^–\rightarrow e^–α) \ / \mathscr{B}(τ^–\rightarrow e^– \bar{v}_{e}v_τ)$ in the range (1.1-9.7) × 10 –3 and on $\mathscr{B}(τ^–\rightarrow μ^–α) \ / \mathscr{B}(τ^–\rightarrow μ^– \bar{v}_{μ}v_τ)$ in the range (0.7-12.2)×10 –3 for α masses between 0 and 1.6 GeV/c 2 . These results provide the most stringent bounds on invisible boson production from τ decays.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for an Invisible Z ' in a Final State with Two Muons and Missing Energy at Belle II
The $L_μ – L_τ$ extension of the standard model predicts the existence of a lepton-flavor-universality-violating Z' boson that couples only to the heavier lepton families. We search for such a Z' through its invisible decay in the process $e^+e^– → μ^+μ^–$Z'. We use a sample of electron-positron collisions at a center- of-mass energy of 10.58 GeV collected by the Belle II experiment in 2019-2020, corresponding to an integrated luminosity of 79.7 fb –1 . We find no excess over the expected standard-model background. We set 90%-confidence-level upper limits on the cross section for this process as well as on the coupling of the model, which ranges from 3 × 10 –3 at low Z' masses to 1 at Z' masses of 8 GeV=$c^2$.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic B-Meson Decays at Belle II
We present the first measurement of the ratio of branching fractions of inclusive semileptonic B -meson decays, R ( X e / μ ) = B ( B → X e ν ) / B ( B → X μ ν ) , a precision test of electron-muon universality, using data corresponding to 189 fb - 1 from electron-positron collisions collected with the Belle II detector. In events where the partner B meson is fully reconstructed, we use fits to the lepton momentum spectra above 1.3 GeV / c to obtain R ( X e / μ ) = 1.007 ± 0.009 ( stat ) ± 0.019 ( syst ) , which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurement of C P Violation in B 0 → K S 0 π 0 Decays at Belle II
We report a measurement of the C P -violating parameters C and S in B 0 → K S 0 π 0 decays at Belle II using a sample of 387 × 10 6 B B ¯ events recorded in e + e − collisions at a center-of-mass energy corresponding to the ϒ ( 4 S ) resonance. These parameters are determined by fitting the proper decay-time distribution of a sample of 415 signal events. We obtain C = − 0.0 4 − 0.15 + 0.14 ± 0.05 and S = 0.7 5 − 0.23 + 0.20 ± 0.04 , where the first uncertainties are statistical and the second are systematic. Published by the American Physical Society 2023
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for a τ + τ - Resonance in e + e - → μ + μ - τ + τ - Events with the Belle II Experiment
We report the first search for a nonstandard-model resonance decaying into $τ$ pairs in $e^+e^-$ → $μ^+μ^-τ^+τ^-$ events in the 3.6–10 GeV/c 2 mass range. We use a 62.8 fb -1 sample of $e^+e^-$ collisions collected at a center-of-mass energy of 10.58 GeV by the Belle II experiment at the SuperKEKB collider. The analysis probes three different models predicting a spin-1 particle coupling only to the heavier lepton families, a Higgs-like spin-0 particle that couples preferentially to charged leptons (leptophilic scalar), and an axionlike particle, respectively. We observe no evidence for a signal and set exclusion limits at 90% confidence level on the product of cross section and branching fraction into $τ$ pairs, ranging from 0.7 to 24 fb, and on the couplings of these processes. We obtain world-leading constraints on the couplings for the leptophilic scalar model for masses above 6.5 GeV/c 2 and for the axionlike particle model over the entire mass range.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Precise Measurement of the 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 × 10 3 $D^+_s$ → φπ + decays. Our result is τ$D^+_s$ = (499.5 ± 1.7 ± 0.9) fs, where the first uncertainty is statistical and the second is systematic. This result is significantly more precise than previous measurements.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
First Measurement of R ( X τ / ℓ ) as an Inclusive Test of the b → c τ ν Anomaly
We measure the tau-to-light-lepton ratio of inclusive B -meson branching fractions R ( X τ / ℓ ) ≡ B ( B → X τ ν ) / B ( B → X ℓ ν ) , where ℓ 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 fb − 1 of electron-positron collision data collected with the Belle II detector. We find R ( X τ / ℓ ) = 0.228 ± 0.016 ( stat ) ± 0.036 ( syst ) , in agreement with standard-model expectations. This is the first direct measurement of R ( X τ / ℓ ) . Published by the American Physical Society 2024
Physics↗
Search for Rare b → d ℓ + ℓ − Transitions at Belle
We present the results of a search for the b → d ℓ + ℓ − flavor-changing neutral-current rare decays B + , 0 → ( η , ω , π + , 0 , ρ + , 0 ) e + e − and B + , 0 → ( η , ω , π 0 , ρ + ) μ + μ − using a 711 fb − 1 data sample that contains 772 × 10 6 B B ¯ events. The data were collected at the ϒ ( 4 S ) resonance with the Belle detector at the KEKB asymmetric-energy e + e − collider. We find no evidence for signal and set upper limits on branching fractions at the 90% confidence level in the range ( 3.8 – 47 ) × 10 − 8 depending on the decay channel. The obtained limits are the world’s best results. This is the first search for the channels B + , 0 → ( ω , ρ + , 0 ) e + e − and B + , 0 → ( ω , ρ + ) μ + μ − . Published by the American Physical Society 2024
Physics↗
Bayesian estimations of orientation distribution functions from small-angle scattering enable direct prediction of mechanical stress in anisotropic materials
Properties of soft materials are influenced by their anisotropic structuring under nonequilibrium fields. Although anisotropic structure-property relationships have been extensively explored theoretically, comparison to experiments requires determination of the microstructural orientation probability distribution function (OPDF) of microstructural elements. Small angle scattering (SAS) measurements encode information about the OPDF, but tools to navigate this connection are incomplete. Here, we develop and validate an explicit framework to link arbitrary OPDFs to SAS measurements. Specifically, we propose, validate, and apply a method, maximum a posteriori scattering inference (MAPSI), whereby the OPDF may be obtained from SAS measurements using a Bayesian estimation method. Using this method, we obtain estimates of the full 3D OPDF for two model semidilute fd-virus (rodlike) dispersions at concentrations that are approximately equal to and twice the overlap concentration. From the OPDF, we calculate its second and fourth moments and compare these to predictions for a dilute suspension of rigid rods and to a recent theory for semidilute suspensions. Finally, we use both the theoretical and measured moments to calculate the stress, both for dilute and semidilute suspensions. These predictions are not only compared to each other, but also to measured values of the shear stress, and point to new insights into the behavior of suspensions of highly elongated particles in the transition between dilute and semidilute behavior. We also use this new framework to provide perspective on the connection between scalar parameterizations of scattering and the OPDF that have frequently been used in the past. Here, the new tools developed in this work provide an unprecedented path toward experimental validation of dynamical theories of rodlike colloids and polymers, and for measurement of nonequilibrium structures and stresses of other complex fluids and soft materials with SAS.
36 MATERIALS SCIENCE↗
Model-agnostic likelihood for the reinterpretation of the 𝐵 + → 𝐾 + $𝑣\bar{𝑣}$ measurement at Belle II
We recently measured the branching fraction of the 𝐵 + → 𝐾 + $𝑣\bar{𝑣}$ decay using 362 fb −1 of on-resonance 𝑒 + 𝑒 − collision data under the assumption of Standard Model kinematics, providing the first evidence for this decay. To facilitate future reinterpretations and maximize the scientific impact of this measurement, we publicly release the full analysis likelihood along with all necessary material required for reinterpretation under arbitrary theoretical models sensitive to this measurement. In this work, we demonstrate how the measurement can be reinterpreted within the framework of the weak effective theory. Using a kinematic reweighting technique in combination with the published likelihood, we derive marginal posterior distributions for the Wilson coefficients, construct credible intervals, and assess the goodness of fit to the Belle II data. For the weak effective theory Wilson coefficients, the posterior mode of the magnitudes |𝐶 VL +𝐶 VR |, |𝐶 SL +𝐶 SR |, and |𝐶 TL | corresponds to the point (11.3, 0.0, 8.2). The respective 95% credible intervals are [1.9, 16.2], [0.0, 15.4], and [0.0, 11.2].
bottom quark↗
First plasma processing trial of a quarter-wave resonator cryomodule at the Facility for Rare Isotope Beams
Studies of plasma processing for reduction of field emission in quarter-wave resonator (QWR) cryomodules are underway at the Facility for Rare Isotope Beams (FRIB), where a total of 104 QWRs are presently in operation. Driving the plasma with a higher-order mode allows for less mismatch at the fundamental power coupler and higher plasma density. The first plasma processing trial for FRIB QWRs in a cryomodule was conducted in January 2024. In-bunker cold tests of the cryomodule showed a significant reduction in field emission x-rays after plasma processing.
Glow & corona plasma discharges↗
Search for B 0 → K *0 𝜏 + 𝜏 − Decays at the Belle II Experiment
We present a search for the rare flavor-changing neutral-current decay 𝐵 0 →𝐾 *0 𝜏 + 𝜏 − with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb −1 data sample recorded at the center-of-mass energy of the ϒ(4𝑆) resonance. One of the 𝐵 mesons produced in the ϒ(4𝑆)→𝐵 0 $\overline{𝐵}$ 0 process is fully reconstructed in a hadronic decay mode, while its companion 𝐵 meson is required to decay into a 𝐾 *0 and two 𝜏 leptons of opposite charge. The 𝜏 leptons are reconstructed in final states with a single electron, muon, charged pion or charged 𝜌 meson, and additional neutrinos. We set an upper limit on the branching fraction of ℬ(𝐵 0 →𝐾 *0 𝜏 + 𝜏 − )<1.8×10 −3 at the 90% confidence level, which is the most stringent constraint reported to date.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗