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The super τ-charm facility (STCF) is an electron−positron collider proposed by the Chinese particle physics community. It is designed to operate in a center-of-mass energy range from 2 to 7 GeV with a peak luminosity of 0.5 × 10 35 cm −2·s −1 or higher. The STCF will produce a data sample about a factor of 100 larger than that of the present τ-charm factory - the BEPCII, providing a unique platform for exploring the asymmetry of matter-antimatter (charge-parity violation), in-depth studies of the internal structure of hadrons and the nature of non-perturbative strong interactions, as well as searching for exotic hadrons and physics beyond the Standard Model. The STCF project in China is under development with an extensive R&D program. This document presents the physics opportunities at the STCF, describes conceptual designs of the STCF detector system, and discusses future plans for detector R&D and physics case studies.

The complicated structure of the neutron cannot be calculated using first-principles calculations due to the large colour charge of quarks and the self-interaction of gluons. Its simplest structure observables are the electromagnetic form factors1, which probe our understanding of the strong interaction. Until now, a small amount of data has been available for the determination of the neutron structure from the time-like kinematical range. Here we present measurements of the Born cross section of electron–positron annihilation reactions into a neutron and anti-neutron pair, and determine the neutron’s effective form factor. The data were recorded with the BESIII experiment at centre-of-mass energies between 2.00 and 3.08 GeV using an integrated luminosity of 647.9 pb−1. Our results improve the statistics on the neutron form factor by more than a factor of 60 over previous measurements, demonstrating that the neutron form factor data from annihilation in the time-like regime is on par with that from electron scattering experiments. The effective form factor of the neutron shows a periodic behaviour, similar to earlier observations of the proton form factor. Future works—both theoretical and experimental—will help illuminate the origin of this oscillation of the electromagnetic structure observables of the nucleon.Form factors encode the structure of nucleons. Measurements from electron–positron annihilation at BESIII reveal an oscillating behaviour of the neutron electromagnetic form factor, and clarify a long-standing photon–nucleon interaction puzzle.

A bstract We report a determination of the CKM angle ϕ 3 , also known as γ , from a combination of measurements using samples of up to 711 fb − 1 from the Belle experiment and up to 362 fb − 1 from the Belle II experiment. We combine results from analyses of B + → DK + , B + → Dπ + , and B + → D * K + decays, where D is an admixture of D 0 and D ¯ 0 mesons, in a likelihood fit to obtain ϕ 3 = (75.2 ± 7.6) ° . We also briefly discuss the interpretation of this result.

A bstract We present the result of a search for the charged-lepton-flavor violating decay τ − → μ − μ + μ − using a 424 fb − 1 sample of data recorded by the Belle II experiment at the SuperKEKB e + e − collider. The selection of e + e − → τ + τ − events is based on an inclusive reconstruction of the non-signal tau decay, and on a boosted decision tree to suppress background. We observe one signal candidate, which is compatible with the expectation from background processes. We set a 90% confidence level upper limit of 1 . 9 × 10 − 8 on the branching fraction of the τ − → μ − μ + μ − decay, which is the most stringent bound to date.

A bstract We present a measurement of the ratio R μ = B τ − → μ − ν ¯ μ ν τ / B τ − → e − ν ¯ e ν τ of branching fractions B of the τ lepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKB e + e − collider. The sample has an integrated luminosity of 362 ± 2 fb − 1 at a centre-of-mass energy of 10.58 GeV. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result, R μ = 0.9675 ± 0.0007 ± 0.0036, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to the W boson in τ decays of 0.9974 ± 0.0019, in agreement with the standard model expectation of unity.

Using 20.3 fb(-1) of e(+)e(-) collision data collected at a center-of-mass energy of 3.773 GeV with the BESIII detector, we present the measurements of the absolute branching fractions of the doubly Cabibbo-suppressed decays D+ -> K+ pi(0), D+ -> K+ eta and D+ -> K+ eta' with the double-tag method, with significantly improved precision compared to the previous measurements. The statistical significance of each signal decay exceeds 10s. The branching fractions are determined to be B(D+ -> K+pi 0) = (1.45 +/- 0.06 +/- 0.08) x 10(-4), B(D+ -> K+eta) = (1.17 +/- 0.10 +/- 0.03) x10(-4) and B(D+ -> K+eta') = (1.88 +/- 0.15 +/- 0.11) x10(-4), where the first uncertainties are statistical and the second systematic. The branching fractions of D+ -> K+ eta and D+ -> K+ eta' are consistent with the world average values. The reported branching fraction of D+ -> K+ pi(0) deviates with the world average value by 3 sigma.

A bstract The first direct measurement of the relative phase between the strong and electromagnetic amplitudes for a J/ψ decaying into a vector-pseudoscalar final state is performed using 26 energy points of e + e − annihilation data between 3 . 00 GeV and 3.12 GeV. The data sets were collected by the BESIII detector with a total integrated luminosity of 452 pb − 1 . By investigating the interference pattern in the cross section lineshape of e + e − → ϕη , the relative phase between the strong and electromagnetic amplitudes of J/ψ decay is determined to be within [133 °, 228 ° ] at 68% confidence level.

A bstract We report results from a study of B ± → DK ± decays followed by D decaying to the CP -even final state K + K − and CP-odd final state K S 0 π 0 , where D is an admixture of D 0 and D ¯ 0 states. These decays are sensitive to the Cabibbo-Kobayashi-Maskawa unitarity-triangle angle ϕ 3 . The results are based on a combined analysis of the final data set of 772 × 10 6 B B ¯ pairs collected by the Belle experiment and a data set of 198 × 10 6 B B ¯ pairs collected by the Belle II experiment, both in electron-positron collisions at the Υ(4 S ) resonance. We measure the CP asymmetries to be A CP + = (+12.5 ± 5.8 ± 1.4)% and A CP− = ( − 16.7 ± 5.7 ± 0.6)%, and the ratios of branching fractions to be R CP + = 1.164 ± 0.081 ± 0.036 and R CP− = 1.151 ± 0.074 ± 0.019. The first contribution to the uncertainties is statistical, and the second is systematic. The asymmetries A CP + and A CP− have similar magnitudes and opposite signs; their difference corresponds to 3.5 standard deviations. From these values we calculate 68.3% confidence intervals of (8.5 ° < ϕ 3 < 16.5 ° ) or (84.5 ° < ϕ 3 < 95.5 ° ) or (163.3 ° < ϕ 3 < 171.5 ° ) and 0.321 < r B < 0.465.

A bstract The Born cross sections of the e + e − → D *+ D * − and e + e − → D *+ D − processes are measured using e + e − collision data collected with the BESIII experiment at center-of-mass energies from 4.085 to 4.600 GeV, corresponding to an integrated luminosity of 15 . 7 fb − 1 . The results are consistent with and more precise than the previous measurements by the Belle, Babar and CLEO collaborations. The measurements are essential for understanding the nature of vector charmonium and charmonium-like states.

A bstract We present measurements of B → K * (892) γ decays using 365 fb − 1 of data collected from 2019 to 2022 by the Belle II experiment at the SuperKEKB asymmetric-energy e + e − collider. The data sample contains (387 ± 6) × 10 6 Υ(4 S ) events. We measure branching fractions ( ) and CP asymmetries ( ) for both B 0 → K *0 γ and B + → K *+ γ decays. The difference in CP asymmetries ( ) and the isospin asymmetry (∆ 0+ ) between these neutral and charged channels are also measured. We obtain the following branching fractions and CP asymmetries: , , , and . The measured difference in CP asymmetries is , and the measured isospin asymmetry is ∆ 0+ = (+4.8 ± 2.0 ± 1.8)%. The first uncertainties listed are statistical and the second are systematic. These results are consistent with world-average values and theory predictions.