Motivated by models proposed to explain Standard Model anomalies, and the unprecedented τ+τ− data to be collected by the Belle II experiment during the next years, we study the kinematics of tau pair ...decays and propose a new method to search for lepton flavor violating processes in tau lepton decays to invisible beyond Standard Model particles, such as τ → ℓα, where ℓ is either an electron or a muon, and α is a massive particle that escapes undetected. The new method improves by one order of magnitude the expected upper limit on the τ → ℓα production in 3 × 1 prong tau decays and establishes the possibility of performing this search in 1 × 1 prong tau decays which has not been previously considered.
A search for the flavor-changing neutral-current decay B+→ K+νν is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The data sample corresponds to ...an integrated luminosity of 63 fb−1 collected at the Υ(4S) resonance and a sample of 9 fb−1 collected at an energy 60 MeV below the resonance. Because the measurable decay signature involves only a single charged kaon, a novel measurement approach is used that exploits not only the properties of the B+ → K+νν decay, but also the inclusive properties of the other B meson in the Υ(4S) → BB event, to suppress the background from other B meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of B+ → K+νν of 4.1 × 10−5 is set at the 90% confidence level.
We report the first search for a non-standard-model resonance decaying into $\tau$ pairs in $e^{+}e^{-}\rightarrow \mu^{+}\mu^{-} \tau^+\tau^-$ 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 axion-like 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 $\tau$ pairs, ranging from 0.7 fb 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 axion-like particle model over the entire mass range.
The $L_{\mu}-L_{\tau}$ 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 $e^+ e^- \to \mu^+ \mu^- Z^{\prime}$. 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$GeV/c^{2}$.
We study the processes $e^+e^-\to\omega\chi_{bJ}(1P)$ ($J$ = 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 $\Upsilon(4S)$ resonance. We report the first observation of $\omega\chi_{bJ}(1P)$ 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 $e^+e^-\to \omega\chi_{b1,b2}(1P)$ to be consistent with the shape of the $\Upsilon(10753)$ state. These data indicate that the internal structures of the $\Upsilon(10753)$ and $\Upsilon(10860)$ 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 $\omega\Upsilon(1S)$. No significant signal is observed for masses between 10.45 and 10.65 GeV/$c^2$.
A
bstract
We present a measurement of the Cabibbo-Kobayashi-Maskawa unitarity triangle angle
ϕ
3
(also known as
γ
) using a model-independent Dalitz plot analysis of
B
+
→
D
(
K
S
0
h
+
h
−
)
h
+
, ...where
D
is either a
D
0
or
D
¯
0
meson and
h
is either a
π
or
K
. This is the first measurement that simultaneously uses Belle and Belle II data, combining samples corresponding to integrated luminosities of 711 fb
−
1
and 128 fb
−
1
, respectively. All data were accumulated from energy-asymmetric
e
+
e
−
collisions at a centre-of-mass energy corresponding to the mass of the Υ(4
S
) resonance. We measure
ϕ
3
= (78
.
4 ± 11
.
4 ± 0
.
5 ± 1
.
0)°, where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is from the uncertainties on external measurements of the
D
-decay strong-phase parameters.
Motivated by models proposed to explain the Standard Model anomalies, and the unprecedented \(\tau^{+}\tau^{-}\) data to be collected by the Belle~II experiment during the next years, we study the ...kinematics of tau pair decays and propose a new method to search for lepton flavor violating processes in tau lepton decays to invisible beyond Standard Model particles, such as \(\tau \to \ell \alpha\), where \(\ell\) is either an electron or a muon, and \(\alpha\) is a massive particle that escapes undetected. The new method improves by one order of magnitude the expected upper limit on the \(\tau \to \ell \alpha\) production in 3x1 prong tau decays and establishes the possibility of performing this search in 1x1 prong tau decays which has not been previously considered.
PDF
