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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.

Using data samples collected by the Belle and Belle II experiments at the Υ(4 S ) resonance with integrated luminosities of 571 fb − 1 and 365 fb − 1 , respectively, we measure the pseudoscalar B -meson mass difference to be m ( B 0 ) − m ( B + ) = (0 . 495 ± 0 . 024 ± 0 . 005) MeV/ c 2 . The results are based on a simultaneous fit to the variable$${\\overline{M} }_{bc}$$, which is related to the B momentum, for B 0 and B + candidates; and to the energy dependence of$$\\mathcal{R}=\\sigma ({e}^{+}{e}^{-}\\to {B}^{0}{\\overline{B} }^{0}) / \\sigma ({e}^{+}{e}^{-}\\to {B}^{+}{B}^{-})$$, which is measured using changes in the average center-of-mass energy over the data taking periods. The phase-space hypothesis$$\\mathcal{R}={({p}_{{B}^{0}} /{p}_{{B}^{+}})}^{3}$$, upon which previous measurements rely, is strongly disfavored by our fit; the measured mass-difference value for the phase-space hypothesis also differs significantly from our measurement. We constrain$$\\mathcal{R}$$in a broader energy range than covered by the direct measurement and extract the energy dependence of$$\\mathcal{R}$$in the range from the$$B\\overline{B }$$threshold up to 10.59 GeV. We interpret the results using a phenomenological model and constrain the parameters of the$$B\\overline{B }$$potential in the isovector channel.

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.

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 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 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.

We present a measurement of the time-dependent CP asymmetry in$${B}^{0}\\to {K}_{\\text{S}}^{0}{\\pi }^{+}{\\pi }^{-}\\gamma $$decays using a data set of 365 fb − 1 recorded by the Belle II experiment and the final data set of 711 fb − 1 recorded by the Belle experiment at the Υ(4S) resonance. The direct and mixing-induced time-dependent CP violation parameters C and S are determined along with two additional quantities, S + and S − , defined in the two halves of the$${m}^{2}\\left({K}_{\\text{S}}^{0}{\\pi }^{+}\\right)-{m}^{2}\\left({K}_{\\text{S}}^{0}{\\pi }^{-}\\right)$$plane. The measured values are C = − 0 . 17 ± 0 . 09 ± 0 . 04, S = − 0 . 29 ± 0 . 11 ± 0 . 05, S + = −0 . 57 ± 0 . 23 ± 0 . 10 and S − = 0 . 31 ± 0 . 24 ± 0 . 05, where the first uncertainty is statistical and the second systematic.

A bstract We study the processes χ bJ ω and χ bJ ( π + π − π 0 ) non −ω ( J = 0, 1, 2) at center-of-mass energies from 10.73 to 11.02 GeV using a 142 . 5 fb − 1 data sample collected with the Belle detector at the KEKB asymmetric-energy e + e − collider; and at GeV using a 19 . 8 fb − 1 sample collected with Belle II at SuperKEKB. We find that the Υ(10753) state decays into χ bJ ω but not into χ bJ ( π + π − π 0 ) non −ω , while the Υ(10860) state, in contrast, decays into χ bJ ( π + π − π 0 ) non −ω but not into χ bJ ω . The mass and width of the Υ(10753) state are measured to be (10756 . 1 ± 3 . 4(stat . ) ± 2 . 7(syst . )) MeV/ c 2 and (32 . 2 ± 11 . 3(stat . ) ± 14 . 9(syst . )) MeV. The products of the partial width to e + e − and branching fractions for Υ(10753) → χ b 1 ω and Υ(10753) → χ b 2 ω are (1 . 57 ± 0 . 27(stat . ) ± 0 . 22(syst . )) eV and (1 . 39 ± 0 . 41(stat . ) ± 0 . 33(syst . )) eV.

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.