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D. Biswas

Publications and source records attributed to D. Biswas.

At least 163 records · Page 9Linked to original sources
arXiv2023

First Measurement of the EMC Effect in $^{10}$B and $^{11}$B

The nuclear dependence of the inclusive inelastic electron scattering cross section (the EMC effect) has been measured for the first time in $^{10}$B and $^{11}$B. Previous measurements of the EMC effect in $A \leq 12$ nuclei showed an unexpected nuclear dependence; $^{10}$B and $^{11}$B were measured to explore the EMC effect in this region in more detail. Results are presented for $^9$Be, $^{10}$B, $^{11}$B, and $^{12}$C at an incident beam energy of 10.6~GeV. The EMC effect in the boron isotopes was found to be similar to that for $^9$Be and $^{12}$C, yielding almost no nuclear dependence in the EMC effect in the range $A=4-12$. This represents important, new data supporting the hypothesis that the EMC effect depends primarily on the local nuclear environment due to the cluster structure of these nuclei.

nucl-ex↗
arXiv2023

First measurement of the Q^2 distribution of X(3915) single-tag two-photon production

We report the first measurement of the $Q^2$ distribution of $X(3915)$ produced by single-tag two-photon interactions. The decay mode used is $X(3915) \rightarrow J/ψω$. The covered $Q^2$ region is from 1.5 (GeV/$c$)$^2$ to 10.0 (GeV/$c$)$^2$. We observe $7.9\pm 3.1({\rm stat.})\pm 1.5({\rm syst.})$ events, where we expect $4.1\pm 0.7$ events based on the $Q^2=0$ result from the no-tag two-photon process, extrapolated to higher $Q^2$ region using the $c\bar{c}$ model of Schuler, Berends, and van Gulik. The shape of the distribution is also consistent with this model; we note that statistical uncertainties are large.

hep-ex↗
arXiv2023

Study of the muon decay-in-flight in the $τ^-\toμ^-\barν_μν_τ$ decay to measure the Michel parameter $ξ^\prime$

We present the first measurement of the Michel parameter $ξ^\prime$ in the $τ^-\toμ^-\barν_μν_τ$ decay using the full data sample of $988\,\text{fb}^{-1}$ collected by the Belle detector operating at the KEKB asymmetric energy $e^+ e^-$ collider. The method is based on the reconstruction of the $μ^- \to e^- \barν_eν_μ$ decay-in-flight in the Belle central drift chamber and relies on the correlation between muon spin and its daughter electron momentum. We study the main sources of the background that can imitate the signal decay, such as kaon and pion decays-in-flight and charged particle scattering on the detector material. Highly efficient methods of their suppression are developed and applied to select 165 signal-candidate events. We obtain $ξ^\prime=0.22\pm0.94\pm0.42$ where the first uncertainty is statistical, and the second one is systematic. The result is in agreement with the Standard Model prediction of $ξ^\prime=1$.

hep-ex↗
arXiv2023

First measurement of the Michel parameter $ξ^\prime$ in the $τ^-\toμ^-\barν_μν_τ$ decay at Belle

We report the first measurement of the Michel parameter $ξ^\prime$ in the $τ^-\toμ^-\barν_μν_τ$ decay with a new method proposed just recently. The measurement is based on the reconstruction of the $τ^-\toμ^-\barν_μν_τ$ events with subsequent muon decay-in-flight in the Belle central drift chamber. The analyzed data sample of $988\,\text{fb}^{-1}$ collected by the Belle detector corresponds to approximately $912\times10^6$ $τ^+ τ^-$ pairs. We measure $ξ^\prime=0.22\pm0.94(\text{stat})\pm0.42(\text{syst})$, which is in agreement with the standard model prediction of $ξ^\prime=1$. Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.

hep-ex↗
arXiv2023

Search for an invisible $Z^\prime$ 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 $Z^{\prime}$ boson that couples only to the heavier lepton families. We search for such a $Z^\prime$ through its invisible decay in the process $e^+ e^- \to μ^+ μ^- Z^{\prime}$. We use a sample of electron-positron collisions at a center-of-mass energy of 10.58GeV collected by the Belle II experiment in 2019-2020, corresponding to an integrated luminosity of 79.7fb$^{-1}$. 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 $3 \times 10^{-3}$ at low $Z^{\prime}$ masses to 1 at $Z^{\prime}$ masses of 8$GeV/c^{2}$.

hep-ex↗
arXiv2023

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 $A^\prime$ and the dark Higgs boson $h^\prime$ are hypothetical particles predicted in many dark sector models. We search for the simultaneous production of $A^\prime$ and $h^\prime$ in the dark Higgsstrahlung process $e^{+}e^{-}\rightarrow A^\prime \, h^\prime$ with $A^\prime \rightarrow μ^+μ^-$ and $ h^\prime$ invisible in electron-positron collisions at a center-of-mass energy of 10.58 GeV 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 $1.7 \times10^{-8}$--$200 \times10^{-8}$ on the effective coupling $\varepsilon^2 \times α_D$ for the $A^\prime$ mass in the range of 4.0 GeV/$c^2$ $< M_{A^\prime}< 9.7$ GeV/$c^2$ and for the $h^\prime$ mass $M_{h^\prime} < M_{A^\prime}$, where $\varepsilon$ 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.

hep-ex↗
arXiv2023

Study of the lineshape of $X(3872)$ using $B$ decays to $D^0\overline{D}{}^{*0}K$

We present a study of the $X(3872)$ lineshape in the decay $B \to X(3872)K\to D^0\overline{D}{}^{*0}K$ using a data sample of $772\times 10^6$ $B\overline{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The peak near the threshold in the $D^0\overline{D}{}^{*0}$ invariant mass spectrum is fitted using a relativistic Breit-Wigner lineshape. We determine the mass and width parameters to be $m = 3873.71 ^{+0.56}_{-0.50} ({\rm stat}) \pm0.13 ({\rm syst}) ~{\rm MeV}/c^2$ and $Γ_0 = 5.2 ^{+2.2}_{-1.5} ({\rm stat}) \pm 0.4 ({\rm syst})~{\rm MeV}$, respectively. The branching fraction is found to be ${\cal B} (B^+\to X(3872)K^+) \times {\cal{B}}(X(3872) \to D^0\overline{D}{}^{*0}) = (0.97 ^{+0.21}_{-0.18} ({\rm stat}) \pm 0.10 ({\rm syst})) \times 10^{-4}$. The signal from $B^0$ decays is observed for the first time with $5.2σ$ significance, and the ratio of branching fractions between charged and neutral $B$ decays is measured to be ${\cal B}(B^0\to X(3872)K^0)/{\cal B}(B^+ \to X(3872)K^+) = 1.34^{+0.47}_{-0.40} ({\rm stat}) ^{+0.10}_{-0.12} ({\rm syst})$. The peak is also studied using a Flatté lineshape. We determine the lower limit on the $D\overline{D}{}^{*}$ coupling constant $g$ to be $0.075$ at 95% credibility in the parameter region where the ratio of $g$ to the mass difference from the $D^0\overline{D}{}^{*0}$ threshold is equal to $-15.11~{\rm GeV}^{-1}$, as measured by LHCb.

hep-ex↗
arXiv2023

Measurement of the $τ$-lepton mass with the Belle~II experiment

We present a measurement of the $τ$-lepton mass using a sample of about 175 million $e^+e^- \to τ^+τ^-$ events collected with the Belle II detector at the SuperKEKB $e^+e^-$ collider at a center-of-mass energy of $10.579\,\mathrm{Ge\kern -0.1em V}$. This sample corresponds to an integrated luminosity of $190\,\mathrm{fb^{-1}}$. We use the kinematic edge of the $τ$ pseudomass distribution in the decay ${τ^-\toπ^-π^+π^-ν_τ}$ and measure the $τ$ mass to be $1777.09 \pm 0.08 \pm 0.11 \,\mathrm{Me\kern -0.1em V\!/c^2}$, where the first uncertainty is statistical and the second systematic. This result is the most precise to date.

hep-ex↗
arXiv2023

Measurement of the branching fraction and $\it CP$ asymmetry of $B^{0} \rightarrow π^{0} π^{0}$ decays using $198 \times 10^6$ $B\overline{B}$ pairs in Belle II data

We report measurements of the branching fraction and $\it CP$ asymmetry in $B^{0} \to π^{0} π^{0}$ decays reconstructed at Belle II in an electron-positron collision sample containing $198 \times 10^{6}$ $B\overline{B}$ pairs. We measure a branching fraction $\mathcal{B}(\Bpipi) = (1.38 \pm 0.27 \pm 0.22) \times 10^{-6}$ and a $\it CP$ asymmetry $\Acp(\Bpipi) = 0.14 \pm 0.46 \pm 0.07$, where the first uncertainty is statistical and the second is systematic.

hep-ex↗
arXiv2023

A 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, $R(X_{e/μ}) = \mathcal{B}(B\to X \, e \, ν) / \mathcal{B}(B\to X \, μ\, ν)$, a precision test of electron-muon universality, using data corresponding to $189\,\mathrm{fb}^{-1}$ 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 $1.3\,\mathrm{GeV}/c$ to obtain $R(X_{e/μ}) = 1.007 \pm 0.009~(\mathrm{stat}) \pm 0.019~(\mathrm{syst})$, which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.

hep-ex↗
arXiv2023

Search for $C\!P$ violation using $T$-odd correlations in $D_{(s)}^{+}\to K^{+} K^{-}π^{+}π^{0}$, $D_{(s)}^{+}\to K^{+} π^{-}π^{+}π^{0}$, and $D^{+}\to K^{-}π^{+}π^{+}π^{0}$ decays

We search for $C\!P$ violation using $T$-odd correlations in five $D_{(s)}^{+}$ and $D_{(s)}^{-}$ four-body decays. Our analysis is based on 980 $\rm fb^{-1}$ of data collected by the Belle detector at the KEKB energy-asymmetric $e^+e^-$ collider. Our results for the $T$-odd $C\!P$-violating parameter $a^{T\text{-odd}}_{C\!P}$ are: $a^{T\text{-odd}}_{C\!P}({D^{+}\to K^{-}K^{+}π^{+}π^{0}}) = (+2.6\pm 6.6\pm 1.3 )\times10^{-3}$, $a^{T\text{-odd}}_{C\!P}({D^{+}\to K^{+}π^{-}π^{+}π^{0}}) = (-1.3\pm 4.2\pm 0.1 )\times10^{-2}$, $a^{T\text{-odd}}_{C\!P}({D^{+}\to K^{-}π^{+}π^{+}π^{0}}) = (+0.2\pm 1.5\pm 0.8 )\times10^{-3}$, $a^{T\text{-odd}}_{C\!P}({D_s^{+}\to K^{+}π^{-}π^{+}π^{0}}) = (-1.1\pm 2.2\pm 0.1 )\times10^{-2}$, and $a^{T\text{-odd}}_{C\!P}({D_s^{+}\to K^{-}K^{+}π^{+}π^{0}}) = (+2.2\pm 3.3\pm 4.3 )\times10^{-3}$, where the uncertainties are statistical and systematic, respectively. These results are the first such measurements and are all consistent with zero. They include the first measurement for a $D^+_s$ singly Cabibbo-suppressed decay, and the first measurement for a $D$ meson doubly Cabibbo-suppressed decay. We also measure $a^{T\text{-odd}}_{C\!P}$ in different subregions of phase space, where the decays are dominated by different intermediate resonance states such as $D^+\toϕρ^+$, $\bar{K}^{*0}K^{*+}$, and $\bar{K}^{*0}ρ^+$; and $D_s^+\to K^{*+}ρ^{0}$, $K^{*0}ρ^{+}$, $ϕρ^+$, and $\bar{K}^{*0}K^{*+}$. No evidence for $C\!P$ violation is found.

hep-ex↗
arXiv2023

Search for lepton-flavor-violating $τ^- \to \ell^-ϕ$ decays in 2019-2021 Belle II data

We report a search for lepton-flavor-violating decays $τ^- \to \ell^- ϕ$ ($\ell^- =e^-,μ^-$) at the Belle II experiment, using a sample of electron-positron data produced at the SuperKEKB collider in 2019-2021 and corresponding to an integrated luminosity of 190 fb$^{-1}$. We use a new untagged selection for $e^+e^- \to τ^+τ^-$ events, where the signal $τ$ is searched for as a neutrinoless final state of a single charged lepton and a $ϕ$ meson and the other $τ$ is not reconstructed in any specific decay mode, in contrast to previous measurements by the BaBar and Belle experiments. We find no evidence for $τ^- \to \ell^- ϕ$ decays and obtain upper limits on the branching fractions at 90% confidence level of 23 $\times 10^{-8}$ and 9.7$\times 10^{-8}$ for $τ^- \rightarrow e^-ϕ$ and $τ^- \rightarrow μ^-ϕ$, respectively

hep-ex↗
arXiv2023

Search for Lepton-Flavor-Violating $τ$ Decays to a Lepton and an Invisible Boson at Belle II

We search for lepton-flavor-violating $τ^-\to e^-α$ and $τ^-\toμ^-α$ 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 $τ^-\to e^-\barν_e ν_τ$ and $τ^-\to μ^-\barν_μν_τ$ decays. We report 95\% confidence-level upper limits on the branching-fraction ratio ${\mathcal B}(τ^-\to e^- α) / {\mathcal B}(τ^- \to e^- \barν_e ν_τ)$ in the range $(1.1-9.7) \times 10^{-3}$ and on ${\mathcal B}(τ^-\to μ^- α) / {\mathcal B}(τ^- \to μ^- \barν_μν_τ)$ in the range $(0.7-12.2) \times 10^{-3}$ 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.

hep-ex↗
arXiv2023

Observation of ${B\to D^{(*)} K^- K^{0}_S}$ decays using the 2019-2022 Belle II data sample

We present a measurement of the branching fractions of four $B^{0,-}\to D^{(*)+,0} K^- K^{0}_S$ decay modes. The measurement is based on data from SuperKEKB electron-positron collisions at the $Υ(4S)$ resonance collected with the Belle II detector and corresponding to an integrated luminosity of ${362~\text{fb}^{-1}}$. The event yields are extracted from fits to the distributions of the difference between expected and observed $B$ meson energy to separate signal and background, and are efficiency-corrected as a function of the invariant mass of the $K^-K_S^0$ system. We find the branching fractions to be: \[ \text{B}(B^-\to D^0K^-K_S^0)=(1.89\pm 0.16\pm 0.10)\times 10^{-4}, \] \[ \text{B}(\overline B{}^0\to D^+K^-K_S^0)=(0.85\pm 0.11\pm 0.05)\times 10^{-4},\] \[ \text{B}(B^-\to D^{*0}K^-K_S^0)=(1.57\pm 0.27\pm 0.12)\times 10^{-4}, \] \[ \text{B}(\overline B{}^0\to D^{*+}K^-K_S^0)=(0.96\pm 0.18\pm 0.06)\times 10^{-4},\] where the first uncertainty is statistical and the second systematic. These results include the first observation of $\overline B{}^0\to D^+K^-K_S^0$, $B^-\to D^{*0}K^-K_S^0$, and $\overline B{}^0\to D^{*+}K^-K_S^0$ decays and a significant improvement in the precision of $\text{B}(B^-\to D^0K^-K_S^0)$ compared to previous measurements.

hep-ex↗
arXiv2023

Search for lepton-flavor-violating $τ$ decays into a lepton and a vector meson using the full Belle data sample

Charged-lepton-flavor-violation is predicted in several new physics scenarios. We update the analysis of $τ$ lepton decays into a light charged lepton ($\ell$ = $e^{\pm}$ or $μ^{\pm}$) and a vector meson ($V^0$ = $ρ^0$, $ϕ$, $ω$, $K^{\ast0}$, or $\overline{K}{}^{\ast0}$) using 980 fb$^{-1}$ of data collected with the Belle detector at the KEKB collider. No significant excess of such signal events is observed, and thus 90% credibility level upper limits are set on the $τ\rightarrow \ell V^0$ branching fractions in the range of (1.7--$4.3) \times 10^{-8}$. These limits are improved by 30% on average from the previous results.

hep-ex↗
arXiv2023

First Observation of $Λπ^+$ and $Λπ^-$ Signals near the $\bar{K}N (I=1)$ Mass Threshold in $Λ_c^+\rightarrowΛπ^+π^+π^-$ Decay

Using the data sample of 980 fb$^{-1}$ collected with the Belle detector operating at the KEKB asymmetric-energy $e^+e^-$ collider, we present the results of an investigation of the $Λπ^+$ and $Λπ^-$ invariant mass distributions looking for substructure in the decay $Λ_c^+\rightarrowΛπ^+π^+π^-$. We find a significant signal in each mass dis\ tribution. When interpreted as resonances, we find for the $Λπ^+$ ($Λπ^-$) combination a mass of $1434.3 \pm 0.6 (\mathrm{stat}) \pm 0.9(\mathrm{syst})$ MeV/$c^2$ [$1438.5 \pm 0.9 (\mathrm{stat}) \pm 2.5(\mathrm{syst})$ MeV/$c^2$], an intrinsic width of $11.5 \pm 2.8 (\mathrm{stat}) \pm 5.3(\mathrm{syst})$ MeV/$c^2$ [$33.0 \pm 7.5 (\mathrm{stat}) \pm 23.6(\mathrm{syst})$ MeV/$c^2$] with a significance of 7.5$σ$ (6.2$σ$). As these two signals are very close to the $\bar{K}N$ threshold, we also investigate the possibility of a $\bar{K}N$ cusp, and find that \ we cannot discriminate between these two interpretations due to the limited size of the data sample.

hep-ex↗
arXiv2023

Novel method for the identification of the production flavor of neutral charmed mesons

We propose a new algorithm for the identification of the production flavor of neutral $D$ mesons in the Belle II experiment. The algorithm exploits the correlation between the flavor of a reconstructed neutral $D$ meson (signal $D$ meson) and the electric charges of particles reconstructed in the rest of the $e^+e^-\to c \bar{c}$ event. These include those originating from the decay of the other charm hadron produced in the event, as well as those possibly produced in association with the signal $D$ meson. We develop the algorithm using simulation and calibrate it in data using decay modes that identify the flavor of the decaying neutral $D$ meson. We use a data sample of $e^+e^-$ collisions, corresponding to $362\,\mathrm{fb}^{-1}$ of integrated luminosity, collected by Belle II at center-of-mass energies near the $Υ(4S)$ mass. The effective tagging efficiency in data is $(47.91 \pm 0.07 \mathrm{(stat)} \pm 0.51 \mathrm{(syst)})\%$, independent of the neutral-$D$-meson decay mode. This charm flavor tagger will approximately double the effective sample size of many $CP$-violation and charm-mixing measurements that so far have exclusively relied on neutral $D$ mesons originating from $D^{*\pm}$ decays. While developed for Belle II, the basic principles underlying the charm flavor tagger can be used in other experiments, including those at hadron colliders.

hep-ex↗
arXiv2023

Study of $e^+e^- \rightarrow Σ^0 \overlineΣ{}^0$ and $Σ^+\overlineΣ{}^- $ by Initial State Radiation Method at Belle

The processes $ e^+e^-\rightarrow Σ^0\overlineΣ{}^0 $ and $ e^+e^-\rightarrowΣ^+\overlineΣ{}^-$ are studied using initial-state-radiation events in a sample of 980 $\,\mbox{fb}^{-1}$ collected with the Belle detector at the KEKB asymmetric-energy $ e^+e^- $ collider. The cross sections from the mass threshold to $ 3{\mathrm{\,Ge\kern -0.1em V\!/}c^2} $ and the effective form factors of $ Σ^0 $ and $ Σ^+ $ are measured. In the charmonium region, we observe the decays $J/ψ\rightarrowΣ^0\overlineΣ{}^0$ and $J/ψ\rightarrowΣ^+\overlineΣ{}^-$ and determine the respective branching fractions.

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