arXiv subjects

N. Braun

Publications and source records attributed to N. Braun.

At least 55 records · Page 3Linked to original sources

arXiv2020

Studies of the semileptonic $\bar B^0\to D^{*+}\ell^-\barν_\ell$ and $B^-\to D^{0}\ell^-\barν_\ell$ decay processes with 34.6 fb$^{-1}$ of Belle II data

We report measurements of the $\bar{B}^0 \to D^{*+} \ell^{-} \barν_l$ and $B^- \to D^{0} \ell^{-} \barν_l$ processes using 34.6 fb$^{-1}$ of collision events recorded by the Belle II experiment at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. For the $B^-\to D^{0}\ell^-\barν_\ell$ channel, we present first studies that isolate this decay from other semileptonic processes and backgrounds. We report a measurement of the $\bar{B}^0 \to D^{*+} \ell^{-} \barν_l$ branching fraction and obtain ${\cal B}(\bar{B}^0 \to D^{*+} \ell^{-} \barν_l) = \left(4.60 \pm 0.05_{\mathrm{stat}}\pm0.17_{\mathrm{syst}} \pm 0.45_{π_s}\right) \%$, in agreement with the world average. Here, the uncertainties are statistical, systematic, and related to slow pion reconstruction, respectively. The systematic uncertainties are limited by the statistics of auxiliary measurements and will improve in the future. We also report differential branching fractions in five bins of the hadronic recoil parameter $w$ for $\bar{B}^0 \to D^{*+} \ell^{-} \barν_l$, unfolded to account for resolution and efficiency effects.

hep-ex↗

arXiv2020

A calibration of the Belle II hadronic tag-side reconstruction algorithm with $B \rightarrow X\ell ν$ decays

Tag-side reconstruction is an important method for reconstructing $B$ meson decays with missing energy. The Belle II tag-side reconstruction algorithm, Full Event Interpretation, relies on a hierarchical reconstruction of $B$ meson decays with multivariate classification employed at each stage of reconstruction. Given the large numbers of classifiers employed and decay chains reconstructed, the performance of the algorithm on data and simulation differs significantly. Here, calibration factors are derived for hadronic tag-side $B$ decays by measuring a signal side decay, $B \rightarrow X\ell ν$, in $34.6$ fb$^{-1}$ of Belle II data. For a very loose selection on the tag-side $B$ multivariate classifier, the calibration factors are $0.65 \pm 0.02$ and $0.83 \pm 0.03$ for tag-side $B^{+}$ and $B^{0}$ mesons, respectively.

hep-ex↗

arXiv2020

Rediscovery of $B \to ϕK^{()}$ decays and measurement of the longitudinal polarization fraction $f_L$ in $B \to ϕK^{}$ decays using the Summer 2020 Belle II dataset

We utilize a sample of 34.6 fb$^{-1}$, collected by the Belle II experiment at the SuperKEKB asymmetric energy $e^+e^-$ collider, to search for the $B^+ \to ϕK^+$, $B^+ \to ϕK^{*+}$, $B^0 \to ϕK^0_S$, and $B^0 \to ϕK^{*0}$ decays. Charmless hadronic $B$ decays represent an important part of the Belle II physics program, and are an ideal benchmark to test the detector capabilities in terms of tracking efficiency, charged particle identification, vertexing, and advanced analysis techniques. Each channel is observed with a significance that exceeds 5 standard deviations, and we obtain measurements of their branching ratios that are in good agreement with the world averages. For the $B \to ϕK^*$ modes, we also perform a measurement of the longitudinal polarization fraction $f_L$.

hep-ex↗

arXiv2020

Measurement of two-particle correlations in hadronic $e^{+}e^{-}$ collisions at Belle

The enhancement of charged-particle pairs with large pseudorapidity difference and small azimuthal angle difference, often referred to as the ``ridge signal'', is a phenomenon widely observed in high multiplicity proton-proton, proton-ion and deutron-ion collisions, which is not yet fully understood. In heavy-ion collisions, the hydrodynamic expansion of the Quark-Gluon Plasma is one of the possible explanations of the origin of the ridge signal. Measurements in the $e^+e^-$ collision system, without the complexities introduced by hadron structure in the initial state, can serve as a complementary probe to examine the formation of a ridge signal. The first measurement of two-particle angular correlation functions in high multiplicity $e^+e^-$ collisions at $\sqrt{s}=10.52$ GeV is reported. The hadronic $e^+e^-$ annihilation data collected by the Belle detector at KEKB are used in this study. Two-particle angular correlation functions are measured over the full azimuth and large pseudorapidity intervals which are defined by either the electron beam axis or the event thrust as a function of charged particle multiplicity. The measurement in the event thrust analysis, with mostly outgoing quark pairs determining the reference axis, is sensitive to the region of additional soft gluon emissions. No significant ridge signal is observed with either coordinates analyses. Near side jet correlations appear to be absent in the thrust axis analysis. The measurements are compared to predictions from various event generators and expected to provide new constraints to the phenomenological models in the low energy regime.

hep-ex↗

arXiv2020

First flavor tagging calibration using 2019 Belle II data

We report on the first calibration of the standard Belle II $B$-flavor tagger using the full data set collected at the $Υ(4{\rm S})$ resonance in 2019 with the Belle II detector at the SuperKEKB collider, corresponding to 8.7 fb$^{-1}$ of integrated luminosity. The calibration is performed by reconstructing various hadronic charmed $B$-meson decays with flavor-specific final states. We use simulation to optimize our event selection criteria and to train the flavor tagging algorithm. We determine the tagging efficiency and the fraction of wrongly identified tag-side $B$~candidates from a measurement of the time-integrated $B^0-\overline{B}^0$ mixing probability. The total effective efficiency is measured to be $\varepsilon_{\rm eff} = \big(33.8 \pm 3.6(\text{stat}) \pm 1.6(\text{sys})\big)\%$, which is in good agreement with the predictions from simulation and comparable with the best one obtained by the Belle experiment. The results show a good understanding of the detector performance and offer a basis for future calibrations.

hep-ex↗

arXiv2020

Measurement of the branching fraction ${\cal B}(\bar B^0\to D^{*+}\ell^-\barν_\ell)$ with early Belle II data

We measure the branching fractions for the decays $\bar{B}^0 \to D^{*+} e^{-} \barν_e$ and $\bar{B}^0 \to D^{*+} μ^{-} \barν_μ$ using $8.70 \pm 0.09~{\rm fb}^{-1}$ of data collected by the Belle II experiment at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. Candidate signal decays are reconstructed with the subsequent decays $D^{*+}\to D^0 π^+$ and $D^0\to K^-π^+$. We obtain the results ${\cal B}(\bar{B}^0 \to D^{*+} e^{-} \barν_e)$ = $(4.55\pm0.14(\mathrm{stat})\pm0.35 (\mathrm{syst})) \%$ and ${\cal B}(\bar{B}^0 \to D^{*+} μ^{-} \barν_μ)$ = $(4.84\pm 0.13(\mathrm{stat})\pm0.37(\mathrm{syst})) \%$, in agreement with the world averages. The measurements serve to validate the full chain of detector operation and calibration, data collection and processing, and production of physics results in the case of semileptonic $B$-meson decays.

hep-ex↗

arXiv2020

Charmless $B$ decay reconstruction in 2019 Belle II data

We report on the reconstruction of various charmless $B$ decays from electron-positron collisions at the energy corresponding to the $Υ(4S)$ resonance collected with the Belle II detector at the SuperKEKB collider. We use simulation to devise optimized event selections and apply them to the full data set collected in 2019, corresponding to 8.7\,fb$^{-1}$ of integrated luminosity. We fit the difference between half of the collision energy and the $B$ candidate energy (in the $Υ(4S)$ frame) for events restricted to a signal-rich range in beam-energy-constrained mass to search for charmless signals. Signal yields of approximately 80, 15, 20, 30, 90, and 160 decays are reconstructed for the channels $B^0 \to K^+π^-$, $B^0 \to π^+π^-$, $B^+ \to K^0_S(\to π^+π^-)π^+$, $B^+ \to K^+π^0(\to γγ)$, $B^+ \to K^+K^-K^+$, and $B^+ \to K^+π^-π^+$, respectively. Yields and background contaminations are compatible with those expected from simulation and comparable with those obtained by the Belle experiment. The results show a good understanding of the detector performance and offer a reliable basis to assess projections for future reach.

hep-ex↗

arXiv2020

Measurement of the $B^0$ lifetime using fully reconstructed hadronic decays in the 2019 Belle II dataset

This document presents the measurement of $B^0$ meson lifetimes using the 2019 Belle II dataset that corresponds to an integrated luminosity of $8.7 \pm 0.2$ fb$^{-1}$. Each candidate is fully reconstructed with hadronic decay final states on the signal side, while the rest-of-event technique allows to infer the decay vertex position on the other (tag) side. $B^0$ lifetime is extracted from an unbinned maximum likelihood fit to the distribution of the difference between the signal side $B^0$ candidate and the tag side decay times. The measured lifetime is $τ_{B^0} = 1.48 \pm 0.28 \pm 0.06 $ ps, where the first uncertainty is statistical and the second is systematic.

hep-ex↗

arXiv2019

Measurement of the integrated luminosity of the Phase 2 data of the Belle II experiment

From April to July 2018, a data sample at the peak energy of the $Υ(4S)$ resonance was collected with the Belle~II detector at the SuperKEKB electron-positron collider. This is the first data sample of the Belle~II experiment. Using Bhabha and digamma events, we measure the integrated luminosity of the data sample to be ($496.3 \pm 0.3 \pm 3.0$)~pb$^{-1}$, where the first uncertainty is statistical and the second is systematic. This work provides a basis for future luminosity measurements at Belle~II.

hep-ex↗

arXiv2019

Search for $B^+ \to μ^+\, ν_μ$ and $B^+ \to μ^+\, N$ with inclusive tagging

We report the result for a search for the leptonic decay of $B^+ \to μ^+ \, ν_μ$ using the full Belle data set of 711 fb${}^{-1}$ of integrated luminosity at the $Υ(4S)$ resonance. In the Standard Model leptonic $B$-meson decays are helicity and CKM suppressed. To maximize sensitivity an inclusive tagging approach is used to reconstruct the second $B$ meson produced in the collision. The directional information from this second $B$ meson is used to boost the observed $μ$ into the signal $B$ meson rest-frame, in which the $μ$ has a monochromatic momentum spectrum. Though its momentum is smeared by the experimental resolution, this technique improves the analysis sensitivity considerably. Analyzing the $μ$ momentum spectrum in this frame we find $\mathcal{B}(B^+ \to μ^+ \, ν_μ) = \left( 5.3 \pm 2.0 \pm 0.9 \right) \times 10^{-7}$ with a one-sided significance of 2.8 standard deviations over the background-only hypothesis. This translates to a frequentist upper limit of $\mathcal{B}(B^+ \to μ^+ \, ν_μ) < 8.6 \times 10^{-7}$ at 90% CL. The experimental spectrum is then used to search for a massive sterile neutrino, $B^+ \to μ^+ \, N$, but no evidence is observed for a sterile neutrino with a mass in a range of 0 - 1.5 GeV. The determined $B^+ \to μ^+ \, ν_μ$ branching fraction limit is further used to constrain the mass and coupling space of the type II and type III two-Higgs-doublet models.

hep-ex↗

arXiv2019

Experimental determination of the isospin of $Λ_c(2765)^+/Σ_c(2765)^+$

We report an experimental determination of the isospin of $Λ_c(2765)^+/Σ_c(2765)^+$ using 980 fb$^{-1}$ data in the $e^+e^-$ annihilation around $\sqrt{s} = 10.6$ GeV collected by the Belle detector located at the KEKB collider. The isospin partners are searched for in the $Σ_c(2455)^{++/0} π^{0}$ channels, and no evidence was obtained. Thus the isospin is determined to be zero, and the particle is established to be a $Λ_c$.

hep-ex↗

arXiv2019

Measurement of $\mathcal{R}(D)$ and $\mathcal{R}(D^{\ast})$ with a semileptonic tagging method

We report a measurement of the ratios of branching fractions $\mathcal{R}(D) = {\cal B}(\bar{B} \to D τ^- \barν_τ)/{\cal B}(\bar{B} \to D \ell^- \barν_{\ell})$ and $\mathcal{R}(D^{\ast}) = {\cal B}(\bar{B} \to D^* τ^- \barν_τ)/{\cal B}(\bar{B} \to D^* \ell^- \barν_{\ell})$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times10^6$ $B\bar{B}$ events recorded at the $Υ(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. The analysis utilizes a method where the tag-side $B$ meson is reconstructed in a semileptonic decay mode, and the signal-side $τ$ is reconstructed in a purely leptonic decay. The measured values are $\mathcal{R}(D)= 0.307 \pm 0.037 \pm 0.016$ and $\mathcal{R}(D^{\ast})= 0.283 \pm 0.018 \pm 0.014$, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the Standard Model predictions within $0.2$ and $1.1$ standard deviations, respectively, while their combination agrees with the Standard Model predictions within $1.2$ standard deviations.

hep-ex↗

arXiv2019

Measurement of the $D^{\ast-}$ polarization in the decay $B^0 \to D^{\ast -}τ^+ν_τ$

We report the first measurement of the $D^{\ast -}$ meson polarization in the decay $B^0 \to D^{*-} τ^+ν_τ$ using the full data sample of 772$\times 10^6$ $B\bar{B}$ pairs recorded with the Belle detector at the KEKB electron-positron collider. Our result, $F_L^{D^\ast} = 0.60 \pm 0.08 ({\rm stat}) \pm 0.04 ({\rm sys})$, where $F_L^{D^\ast}$ denotes the $D^{\ast-}$ meson longitudinal polarization fraction, agrees within about $1.7$ standard deviations of the standard model prediction.

hep-ex↗

arXiv2018

Search for the rare decay of $B^+ \to \ell^{\,+} ν_{\ell} γ$ with improved hadronic tagging

We present the result of the search for the rare $B$ meson decay of $B^+ \to \ell^{\,+} ν_{\ell} γ$ with $\ell =e,μ$. For the search the full data set recorded by the Belle experiment of $711 \, \mathrm{fb}^{-1}$ integrated luminosity near the $Υ(4S)$ resonance is used. Signal candidates are reconstructed for photon energies $E_γ$ larger than $1 \, \mathrm{GeV}$ using a novel multivariate tagging algorithm. The novel algorithm fully reconstructs the second $B$ meson produced in the collision using hadronic modes and was specifically trained to recognize the signal signature in combination with hadronic tag-side $B$ meson decays. This approach greatly enhances the performance. Background processes that can mimic this signature, mainly charmless semileptonic decays and continuum processes, are suppressed using multivariate methods. The number of signal candidates is determined by analyzing the missing mass squared distribution as inferred from the signal side particles and the kinematic properties of the tag-side $B$ meson. No significant excess over the background-only hypothesis is observed and upper limits on the partial branching fraction $ Δ\mathcal{B} $ with $E_γ> 1 \, \mathrm{GeV}$ individually for electron and muon final states as well as for the average branching fraction of both lepton final states are reported. We find a Bayesian upper limit of $Δ\mathcal{B}( B^{+} \to \ell^{\, +} ν_{\ell} γ) < 3.0 \times 10^{-6}$ at 90% CL and also report an upper limit on the first inverse moment of the light-cone distribution amplitude of the $B$ meson of $λ_B$ at 90% CL.

hep-ex↗

arXiv2018

The Belle II Core Software

Modern high-energy physics (HEP) enterprises, such as the Belle II experiment at the KEK laboratory in Japan, create huge amounts of data. Sophisticated algorithms for simulation, reconstruction, visualization, and analysis are required to fully exploit the potential of these data. We describe the core components of the Belle II software that provide the foundation for the development of complex algorithms and their efficient application on large data sets.

physics.comp-ph↗

arXiv2018

Measurements of branching fraction and $CP$ asymmetry of the $\bar{B}^{0}(B^{0})\to K^{0}_{S}K^{\mp}π^{\pm}$ decay at Belle

We report the measurement of the branching fraction and $CP$ asymmetry for the $\bar{B}^{0}(B^{0})\to K^{0}_{S}K^{\mp}π^{\pm}$ decay. The analysis is performed on a data sample of 711 $\rm{fb}^{-1}$ collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We obtain a branching fraction of $(3.60\pm0.33\pm0.15)\times10^{-6}$ and an $\mathcal{A}_{CP}$ of $(-8.5\pm8.9\pm0.2)\%$, where the first uncertainties are statistical and the second are systematic. Hints of peaking structures are also observed in the differential branching fraction as functions of Dalitz variables.

hep-ex↗

arXiv2018

Study of charmless decays $B^{\pm} \to K^{0}_{S} K^{0}_{S} h^{\pm}$ ($h=K,π$) at Belle

We report a search for charmless hadronic decays of charged $B$ mesons to the final states $K^{0}_{S} K^{0}_{S} K^{\pm}$ and $K^{0}_{S} K^{0}_{S} π^{\pm}$ . The results are based on a $711 {fb}^{-1}$ data sample that contains $772 \times 10^6$ $B \bar{B}$ pairs, and was collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. For $B^{\pm} \to K^{0}_{S} K^{0}_{S} K^{\pm}$ decays, the measured branching fraction and direct $CP$ asymmetry are $[10.64\pm0.49(stat)\pm 0.44(syst)]\times10^{-6}$ and [$-0.6\pm3.9(stat)\pm 3.4(syst)$] %, respectively. In the absence of a statistically significant signal for $B^{\pm}\to K^{0}_{S} K^{0}_{S} π^{\pm}$, we set the 90 % confidence-level upper limit on its branching fraction at $1.14 \times 10^{-6}$.

hep-ex↗

arXiv2017

Measurement of the tau Michel parameters $\barη$ and $ξκ$ in the radiative leptonic decay $τ^- \rightarrow \ell^- ν_τ \barν_{\ell}γ$

We present a measurement of the Michel parameters of the $τ$ lepton, $\barη$ and $ξκ$, in the radiative leptonic decay $τ^- \rightarrow \ell^- ν_τ \barν_{\ell} γ$ using 711~f$\mathrm{b}^{-1}$ of collision data collected with the Belle detector at the KEKB $e^+e^-$ collider. The Michel parameters are measured in an unbinned maximum likelihood fit to the kinematic distribution of $e^+e^-\rightarrowτ^+τ^-\rightarrow (π^+π^0 \barν_τ)(\ell^-ν_τ\barν_{\ell}γ)$ $(\ell=e$ or $μ)$. The measured values of the Michel parameters are $\barη = -1.3 \pm 1.5 \pm 0.8$ and $ξκ= 0.5 \pm 0.4 \pm 0.2$, where the first error is statistical and the second is systematic. This is the first measurement of these parameters. These results are consistent with the Standard Model predictions within their uncertainties and constrain the coupling constants of the generalized weak interaction.

hep-ex↗