arXiv subjects

A. Budano

Publications and source records attributed to A. Budano.

At least 109 records · Page 6Linked to original sources

arXiv2024

Determination of $|V_{ub}|$ from simultaneous measurements of untagged $B^0\toπ^- \ell^+ ν_{\ell}$ and $B^+\toρ^0 \ell^+ν_{\ell}$ decays

We present a measurement of $|V_{ub}|$ from a simultaneous study of the charmless semileptonic decays $B^0\toπ^- \ell^+ ν_{\ell}$ and $B^+\toρ^0 \ell^+ν_{\ell}$, where $\ell = e, μ$. This measurement uses a data sample of 387 million $B\overline{B}$ meson pairs recorded by the Belle~II detector at the SuperKEKB electron-positron collider between 2019 and 2022. The two decays are reconstructed without identifying the partner $B$ mesons. We simultaneously measure the differential branching fractions of $B^0\toπ^- \ell^+ ν_{\ell}$ and $B^+\toρ^0 \ell^+ν_{\ell}$ decays as functions of $q^2$ (momentum transfer squared). From these, we obtain total branching fractions $B(B^0\toπ^- \ell^+ ν_{\ell}) = (1.516 \pm 0.042 (\mathrm{stat}) \pm 0.059 (\mathrm{syst})) \times 10^{-4}$ and $B(B^+\toρ^0 \ell^+ν_{\ell}) = (1.625 \pm 0.079 (\mathrm{stat}) \pm 0.180 (\mathrm{syst})) \times 10^{-4}$. By fitting the measured $B^0\toπ^- \ell^+ ν_{\ell}$ partial branching fractions as functions of $q^2$, together with constraints on the non-perturbative hadronic contribution from lattice QCD calculations, we obtain $|V_{ub}|$ = $(3.93 \pm 0.09 \pm 0.13 \pm 0.19) \times 10^{-3}$. Here, the first uncertainty is statistical, the second is systematic, and the third is theoretical.

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arXiv2024

New graph-neural-network flavor tagger for Belle II and measurement of $\sin2ϕ_1$ in $B^0 \to J/ψK^0_\text{S}$ decays

We present GFlaT, a new algorithm that uses a graph-neural-network to determine the flavor of neutral $B$ mesons produced in $Υ(4S)$ decays. It improves previous algorithms by using the information from all charged final-state particles and the relations between them. We evaluate its performance using $B$ decays to flavor-specific hadronic final states reconstructed in a 362 $\text{fb}^{-1}$ sample of electron-positron collisions collected at the $Υ(4S)$ resonance with the Belle II detector at the SuperKEKB collider. We achieve an effective tagging efficiency of $(37.40 \pm 0.43 \pm 0.36) \%$, where the first uncertainty is statistical and the second systematic, which is $18\%$ better than the previous Belle II algorithm. Demonstrating the algorithm, we use $B^{0}\to J/ψK^0_\text{S}$ decays to measure the mixing-induced and direct $CP$ violation parameters, $S = (0.724 \pm 0.035 \pm 0.009)$ and $C = (-0.035 \pm 0.026 \pm 0.029)$.

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arXiv2024

Measurement of branching fractions, CP asymmetry, and isospin asymmetry for $\boldsymbol{B\rightarrowργ}$ decays using Belle and Belle II data

We present measurements of $B^{+}\rightarrowρ^{+}γ$ and $B^{0}\rightarrowρ^{0}γ$ decays using a combined data sample of $772 \times 10^6$ $B\overline{B}$ pairs collected by the Belle experiment and $387\times 10^6$ $B\overline{B}$ pairs collected by the Belle II experiment in $e^{+}e^{-}$ collisions at the $Υ(4S)$ resonance. After an optimized selection, a simultaneous fit to the Belle and Belle II data sets yields $114\pm 12$ $B^{+}\rightarrowρ^{+}γ$ and $99\pm 12$ $B^{0}\rightarrowρ^{0}γ$ decays. The measured branching fractions are $(13.1^{+2.0 +1.3}_{-1.9 -1.2})\times 10^{-7}$ and $(7.5\pm 1.3^{+1.0}_{-0.8})\times 10^{-7}$ for $B^{+}\rightarrowρ^{+}γ$ and $B^{0}\rightarrowρ^{0}γ$ decays, respectively, where the first uncertainty is statistical and the second is systematic. We also measure the isospin asymmetry $A_{\rm I}(B\rightarrowργ)=(10.9^{+11.2 +7.8}_{-11.7 -7.3})\%$ and the direct CP asymmetry $A_{CP}(B^{+}\rightarrowρ^{+}γ)=(-8.2\pm 15.2^{+1.6}_{-1.2})\%$.

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arXiv2024

Search for the decay $B_s^0\to J/ψπ^0$ at Belle experiment

We have analyzed 121.4 fb$^{-1}$ of data collected at the $Υ(5S)$ resonance by the Belle experiment using the KEKB asymmetric-energy $e^+e^-$ collider to search for the decay $B_s^0\to J/ψπ^0$. We observe no signal and report an upper limit on the branching fraction $\mathcal{B}(B_s^0\to J/ψπ^0)$ of $1.21\times 10^{-5}$ at 90\% confidence level. This result is the most stringent, improving the previous bound by two orders of magnitude.

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arXiv2024

Search for the baryon number and lepton number violating decays $τ^-\to Λπ^-$ and $τ^-\to \barΛπ^-$ at Belle II

We present a search for the baryon number $B$ and lepton number $L$ violating decays $τ^- \rightarrow Λπ^-$ and $τ^- \rightarrow \barΛ π^-$ produced from the $e^+e^-\to τ^+τ^-$ process, using a 364 fb$^{-1}$ data sample collected by the Belle~II experiment at the SuperKEKB collider. No evidence of signal is found in either decay mode, which have $|Δ(B-L)|$ equal to $2$ and $0$, respectively. Upper limits at 90\% credibility level on the branching fractions of $τ^- \rightarrow Λπ^-$ and $τ^- \rightarrow \barΛπ^-$ are determined to be $4.7 \times 10^{-8}$ and $4.3 \times 10^{-8}$, respectively.

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arXiv2024

The Belle II Detector Upgrades Framework Conceptual Design Report

We describe the planned near-term and potential longer-term upgrades of the Belle II detector at the SuperKEKB electron-positron collider operating at the KEK laboratory in Tsukuba, Japan. These upgrades will allow increasingly sensitive searches for possible new physics beyond the Standard Model in flavor, tau, electroweak and dark sector physics that are both complementary to and competitive with the LHC and other experiments.

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arXiv2024

Evidence of $h_{b}(\text{2P}) \to Υ(\text{1S})η$ decay and search for $h_{b}(\text{1P,2P}) \to Υ(\text{1S})π^0$ with the Belle detector

We report the first evidence for the $h_{b}(\text{2P}) \to Υ(\text{1S})η$ transition with a significance of $3.5$ standard deviations. The decay branching fraction is measured to be $\mathcal{B}[h_{b}(\text{2P}) \to Υ(\text{1S})η]=(7.1 ~^{+3.7} _{-3.2}\pm 0.8)\times10^{-3}$, which is noticeably smaller than expected. We also set upper limits on $π^0$ transitions of $\mathcal{B}[h_{b}(\text{2P}) \to Υ(\text{1S})π^0] < 1.8\times10^{-3}$, and $\mathcal{B}[h_{b}(\text{1P})\to Υ(\text{1S})π^0] < 1.8\times10^{-3}$, at the $90\%$ confidence level. These results are obtained with a $131.4$~fb$^{-1}$ data sample collected near the $Υ(\text{5S})$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider.

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arXiv2024

Measurement of the branching fraction of the decay $B^- \to D^0 ρ(770)^-$ at Belle II

We measure the branching fraction of the decay $B^- \to D^0 ρ(770)^-$ using data collected with the Belle II detector. The data contain 387 million $B\overline{B}$ pairs produced in $e^+e^-$ collisions at the $Υ(4S)$ resonance. We reconstruct $8360\pm 180$ decays from an analysis of the distributions of the $B^-$ energy and the $ρ(770)^-$ helicity angle. We determine the branching fraction to be $(0.939 \pm 0.021\mathrm{(stat)} \pm 0.050\mathrm{(syst)})\%$, in agreement with previous results. Our measurement improves the relative precision of the world average by more than a factor of two.

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arXiv2024

Search for a $μ^+μ^-$ resonance in four-muon final states at Belle II

We report on a search for a resonance $X$ decaying to a pair of muons in $e^{+}e^{-}\rightarrow μ^+ μ^- X$ events in the 0.212-9.000 GeV/$c^{2}$ mass range, using 178 fb$^{-1}$ of data collected by the BelleII experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of $X$ beyond the standard model: a $Z^{\prime}$ vector boson in the $L_μ-L_τ$ model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90$\%$ confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the $L_μ-L_τ$ model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below 6 GeV/$c^{2}$, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the $L_μ-L_τ$ model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search.

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arXiv2024

Study of $Υ(10753)$ decays to $π^{+}π^{-}Υ(nS)$ final states at Belle II

We present an analysis of the process $e^{+}e^{-}\toπ^{+}π^{-}Υ(nS)$ (where $n$ = 1, 2, or 3) reconstructed in $19.6\rm$ $\rm fb^{-1}$ of Belle II data during a special run of the SuperKEKB collider at four energy points near the peak of the $Υ(10753)$ resonance. By analyzing the mass distribution of the $π^+π^-Υ(nS)$ system and the Born cross sections of the $e^{+}e^{-}\toπ^{+}π^{-}Υ(nS)$ process, we report the first observation of $Υ(10753)$ decays to the $π^{+}π^{-}Υ(1S)$ and $π^{+}π^{-}Υ(2S)$ final states, and find no evidence for decays to $π^{+}π^{-}Υ(3S)$. Possible intermediate states in the $π^+π^-Υ(1S,2S)$ transitions are also investigated, and no evidence for decays proceeding via the $π^\mp Z_b^\pm$ or $f_0(980)Υ(nS)$ intermediate states is found. We measure Born cross sections for the $e^{+}e^{-}\toπ^{+}π^{-}Υ(nS)$ process that, combined with results from Belle, improve the precision of measurements of the $Υ(10753)$ mass and width by nearly a factor of two to $(10756.3\pm2.7\pm0.6)$ MeV/$c^2$ and $(29.7\pm8.5\pm1.1)$ MeV, respectively. The relative ratios of the Born cross sections at the $Υ(10753)$ resonance peak are also reported for the first time.

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arXiv2024

Search for a heavy neutral lepton that mixes predominantly with the tau neutrino

We report a search for a heavy neutral lepton (HNL) that mixes predominantly with $ν_τ$. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy $e^+ e^-$ collider. The data sample was collected at and just below the center-of-mass energies of the $Υ(4S)$ and $Υ(5S)$ resonances and has an integrated luminosity of $915~\textrm{fb}^{-1}$, corresponding to $(836\pm 12)\times 10^6$ $e^+e^\toτ^+τ^-$ events. We search for production of the HNL (denoted $N$) in the decay $τ^-\to π^- N$ followed by its decay via $N \to μ^+μ^- ν_τ$. The search focuses on the parameter-space region in which the HNL is long lived, so that the $μ^+μ^-$ originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the $τ$ neutrino as a function of the HNL mass.

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arXiv2024

Evidence for $B^{+}\to K^{+}ν\barν$ decays

We search for the rare decay $B^{+}\rightarrow K^{+}ν\barν$ in a $362\ \rm{fb}^{-1}$ sample of electron-positron collisions at the $Υ(4S)$ resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying $B$ meson in $Υ(4S) \to B\kern 0.18em\overline{\kern -0.18em B}{}$ events to suppress background from other decays of the signal $B$ candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying $B$ meson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for the $B^{+}\rightarrow K^{+}ν\barν$ branching fraction of $\left[2.7\pm 0.5(\mathrm{stat})\pm 0.5(\mathrm{syst})\right] \times 10^{-5}$ and $\left[1.1^{+0.9}_{-0.8}(\mathrm{stat}){}^{+0.8}_{-0.5}(\mathrm{syst})\right] \times 10^{-5}$, respectively. Combining the results, we determine the branching fraction of the decay $B^{+}\rightarrow K^{+}ν\barν$ to be $\left[2.3 \pm 0.5(\mathrm{stat})^{+0.5}_{-0.4}(\mathrm{syst})\right]\times 10^{-5}$, providing the first evidence for this decay at $3.5$ standard deviations. The combined result is $2.7$ standard deviations above the standard model expectation.

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arXiv2024

Distillation and Stripping purification plants for JUNO liquid scintillator

The optical and radiochemical purification of the scintillating liquid, which will fill the central detector of the JUNO experiment, plays a crucial role in achieving its scientific goals. Given its gigantic mass and dimensions and an unprecedented target value of about 3% @ 1 MeV in energy resolution, JUNO has set severe requirements on the parameters of its scintillator, such as attenuation length (Lat>20 m at 430 nm), transparency, light yield, and content of radioactive contaminants (238U,232Th<10-15 g/g). To accomplish these needs, the scintillator will be processed using several purification methods, including distillation in partial vacuum and gas stripping, which are performed in two large scale plants installed at the JUNO site. In this paper, layout, operating principles, and technical aspects which have driven the design and construction of the distil- lation and gas stripping plants are reviewed. The distillation is effective in enhancing the optical properties and removing heavy radio-impurities (238U,232Th, 40K), while the stripping process exploits pure water steam and high-purity nitrogen to extract gaseous contaminants (222Rn, 39Ar, 85Kr, O2) from the scintillator. The plant operating parameters have been tuned during the recent com- missioning phase at the JUNO site and several QA/QC measurements and tests have been performed to evaluate the performances of the plants. Some preliminary results on the efficiency of these purification processes will be shown.

physics.ins-det↗

arXiv2024

Refractive index in the JUNO liquid scintillator

In the field of rare event physics, it is common to have huge masses of organic liquid scintillator as detection medium. In particular, they are widely used to study neutrino properties or astrophysical neutrinos. Thanks to its safety properties (such as low toxicity and high flash point) and easy scalability, linear alkyl benzene is the most common solvent used to produce liquid scintillators for large mass experiments. The knowledge of the refractive index is a pivotal point to understand the detector response, as this quantity (and its wavelength dependence) affects the Cherenkov radiation and photon propagation in the medium. In this paper, we report the measurement of the refractive index of the JUNO liquid scintillator between 260-1064 nm performed with two different methods (an ellipsometer and a refractometer), with a sub percent level precision. In addition, we used an interferometer to measure the group velocity in the JUNO liquid scintillator and verify the expected value derived from the refractive index measurements.

physics.ins-det↗

arXiv2024

First Measurement of $R(X_{τ/\ell})$ as an Inclusive Test of the $b \to c τν$ Anomaly

We measure the tau-to-light-lepton ratio of inclusive $B$-meson branching fractions $R(X_{τ/\ell}) \equiv \mathcal{B}(B\to X τν)/\mathcal{B}(B \to X \ell ν)$, 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 $R(X_{τ/\ell}) = 0.228 \pm 0.016~(\mathrm{stat}) \pm 0.036~(\mathrm{syst})$, in agreement with standard-model expectations. This is the first direct measurement of $R(X_{τ/\ell})$.

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arXiv2024

Search for Two-Body $B$ Meson Decays to $Λ^{0}$ and $Ω^{(*)0}_{c}$

We report the results of the first search for Standard Model and baryon-number-violating two-body decays of the neutral $B$ mesons to $Λ^{0}$ and $Ω^{(*)0}_c$ using 711~${\rm fb^{-1}}$ of data collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We observe no evidence of signal from any such decays and set 95\% confidence-level upper limits on the products of $B^0$ and $\bar{B}^0$ branching fractions for these two-body decays with $\mathcal{B}(Ω_{c}^{0} \to π^+ Ω^-)$ in the range between 9.5~$\times 10^{-8}$ and 31.2~$\times 10^{-8}$.

hep-ex↗

arXiv2024

Search for baryon and lepton number violating decays $\boldsymbol{D \rightarrow p\ell}$

We search for the baryon and lepton number violating charm decays, $D \rightarrow p\ell$, where $D$ is either a $D^0$ or a $\overline{D}^0$ and $\ell$ is a muon or an electron, using a data sample of $921\,\mathrm{fb}^{-1}$ collected by the Belle detector at the KEKB asymmetric energy $e^{+}e^{-}$ collider. In the absence of significant signals, we set upper limits on the branching fractions in the range $(5 - 8) \times 10^{-7}$ at a 90\% confidence level, depending on the decay mode.

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arXiv2024

Measurement of $CP$ asymmetries in $B^0 \rightarrow K^0_S K^0_S K^0_S$ decays at Belle II

We report a measurement of decay-time dependent charge-parity ($CP$) asymmetries in $B^0 \rightarrow K^0_S K^0_S K^0_S$ decays. We use $387 \times 10^6 B\bar{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract the $CP$-violating parameters $S$ and $C$ from a fit to the distribution of the decay-time difference between the two $B$ mesons. The resulting confidence region is consistent with previous measurements in $B^0 \rightarrow K^0_S K^0_S K^0_S$ and $B^0 \rightarrow (c\bar{c})K^0$ decays, and with predictions based on the standard model.

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