Electron–positron colliders operating in the GeV center-of-mass range, or tau-charm energy region, have been proved to enable competitive frontier research due to several unique features. With the progress of high-energy physics in the last two decades, a new-generation Tau-Charm factory, called the Super Tau-Charm Facility (STCF), has been actively promoted by the particle physics community in China. STCF has the potential to address fundamental questions such as the essence of color confinement and the matter–antimatter asymmetry within the next decades. The main design goals of the STCF are a center-of-mass energy ranging from 2 to 7 GeV and a luminosity surpassing 5 × 1034 cm−2 s−1 that is optimized at a center-of-mass energy of 4 GeV, which is approximately 50 times that of the currently operating Tau-Charm factory—BEPCII. The STCF accelerator has two main parts: a double-ring collider with a crab-waist collision scheme and an injector that provides top-up injections for both electron and positron beams. As a typical third-generation electron–positron circular collider, the STCF accelerator faces many challenges in both accelerator physics and technology. In this paper, the conceptual design of the STCF accelerator complex is presented, including the ongoing efforts and plans for technological research and development, as well as the required infrastructure. The STCF project aims to secure support from the Chinese central government for its construction during the 15th Five-Year Plan (2026–2030).

Using e+e− collision data corresponding to an integrated luminosity of 7.33 fb−1 recorded by the BESIII detector at center-of-mass energies between 4.128 and 4.226 GeV, we present an analysis of the decay Ds+→π+π−e+νe, where the Ds+ is produced via the process e+e−→Ds*±Ds∓. We observe the f0(980) in the π+π− system and the branching fraction of the decay Ds+→f0(980)e+νe with f0(980)→π+π− measured to be (1.72±0.13stat±0.10syst)×10−3, where the uncertainties are statistical and systematic, respectively. The dynamics of the Ds+→f0(980)e+νe decay are studied with the simple pole parametrization of the hadronic form factor and the Flatté formula describing the f0(980) in the differential decay rate, and the product of the form factor f+f0(0) and the c→s Cabibbo-Kobayashi-Maskawa matrix element |Vcs| is determined for the first time to be f+f0(0)|Vcs|=0.504±0.017stat±0.035syst. Furthermore, the decay Ds+→f0(500)e+νe is searched for the first time but no signal is found. The upper limit on the branching fraction of Ds+→f0(500)e+νe, f0(500)→π+π− decay is set to be 3.3×10−4 at 90% confidence level.Received 23 March 2023Revised 29 November 2023Accepted 28 February 2024DOI:https://doi.org/10.1103/PhysRevLett.132.141901Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLeptonic, semileptonic & radiative decaysMultiquark bound statesQuantum chromodynamicsStrong interactionPhysical SystemsCharm quarkMesonsPropertiesForm factorsParticles & Fields

The Circular Electron Positron Collider (CEPC) is a large scientific project initiated and hosted by China, fostered through extensive collaboration with international partners. The complex comprises four accelerators: a 30 GeV Linac, a 1.1 GeV Damping Ring, a Booster capable of achieving energies up to 180 GeV, and a Collider operating at varying energy modes (Z, W, H, and ttbar). The Linac and Damping Ring are situated on the surface, while the Booster and Collider are housed in a 100 km circumference underground tunnel, strategically accommodating future expansion with provisions for a Super Proton Proton Collider (SPPC). The CEPC primarily serves as a Higgs factory. In its baseline design with synchrotron radiation (SR) power of 30 MW per beam, it can achieve a luminosity of 5e34 /cm^2/s^1, resulting in an integrated luminosity of 13 /ab for two interaction points over a decade, producing 2.6 million Higgs bosons. Increasing the SR power to 50 MW per beam expands the CEPC's capability to generate 4.3 million Higgs bosons, facilitating precise measurements of Higgs coupling at sub-percent levels, exceeding the precision expected from the HL-LHC by an order of magnitude. This Technical Design Report (TDR) follows the Preliminary Conceptual Design Report (Pre-CDR, 2015) and the Conceptual Design Report (CDR, 2018), comprehensively detailing the machine's layout and performance, physical design and analysis, technical systems design, R&D and prototyping efforts, and associated civil engineering aspects. Additionally, it includes a cost estimate and a preliminary construction timeline, establishing a framework for forthcoming engineering design phase and site selection procedures. Construction is anticipated to begin around 2027-2028, pending government approval, with an estimated duration of 8 years. The commencement of experiments could potentially initiate in the mid-2030s.

Based on (2712.4±14.1)×10^6 ψ(3686) events collected with the BESIII detector, we study the decays h_c→3(π^+π^-)π^0, h_c→2(π^+π^-)ω, h_c→2(π^+π^-)π^0η, h_c→2(π^+π^-)η, and h_c→ pp̅ via ψ(3686)→π^0h_c. The decay channel h_c→3(π^+π^-)π^0 is observed for the first time, and its branching fraction is determined to be ( 9.28± 1.14 ± 0.77) ×10^ - 3, where the first uncertainty is statistical and the second is systematic. In addition, first evidence is found for the modes h_c→ 2(π^+π^-)π^0η and h_c→2(π^+π^-)ω with significances of 4.8σ and 4.7σ, and their branching fractions are determined to be (7.55±1.51±0.77)×10^-3 and ( 4.00 ± 0.86 ± 0.35) ×10^ - 3, respectively. No significant signals of h_c→ 2(π^+π^-)η and h_c→ pp̅ are observed, and the upper limits of the branching fractions of these decays are determined to be <6.19×10^-4 and <4.40×10^-5 at the 90 respectively.

With the data samples taken at center-of-mass energies from 2.00 to 3.08 GeV with the BESIII detector at the BEPCII collider, a partial wave analysis on the e+e−→π+π−π0 process is performed. The Born cross sections for e+e−→π+π−π0 and its intermediate processes e+e−→ρπ and ρ(1450)π are measured as functions of s. The results for e+e−→π+π−π0 are consistent with previous results measured with the initial state radiation method within one standard deviation, and improve the uncertainty by a factor of ten. By fitting the line shapes of the Born cross sections for the e+e−→ρπ and e+e−→ρ(1450)π, a structure with mass M=2119±11±15 MeV/c2 and width Γ=69±30±5 MeV is observed with a significance of 5.9σ, where the first uncertainties are statistical and the second ones are systematic. This structure can be interpreted as an excited ω state. Published by the American Physical Society 2024

We present measurements of the Born cross sections for the processes e^+e^-→ωχ_c1 and ωχ_c2 at center-of-mass energies √(s) from 4.308 to 4.951 GeV. The measurements are performed with data samples corresponding to an integrated luminosity of 11.0 fb^-1 collected with the BESIII detector operating at the BEPCII storage ring. Assuming the e^+e^-→ωχ_c2 signals come from a single resonance, the mass and width are determined to be M=(4413.6±9.0±0.8) MeV/c^2 and Γ=(110.5±15.0±2.9) MeV, respectively, which is consistent with the parameters of the well-established resonance ψ(4415). In addition, we also use one single resonance to describe the e^+e^-→ωχ_c1 lineshape, and determine the mass and width to be M=(4544.2±18.7±1.7) MeV/c^2 and Γ=(116.1±33.5±1.7) MeV, respectively. The structure of this lineshape, observed for the first time, requires further understanding.

The energy-dependent cross section for e+e−→ηψ(2S) is measured at 18 center of mass energies from 4.288 to 4.951 GeV using the BESIII detector. Using the same data samples, we also perform the first search for the reaction e+e−→ηX˜(3872), but no evidence is found for the X˜(3872) in the π+π−J/ψ mass distribution. At each of the 18 center of mass energies, upper limits at the 90% confidence level on the cross section for e+e−→ηψ(2S) and on the product of the e+e−→ηX˜(3872) cross section with the branching fraction of X˜(3872)→π+π−J/ψ are reported. Published by the American Physical Society 2024

We report a measurement of decay-time-dependent charge-parity (CP) asymmetries in B0→KS0KS0KS0 decays. We use 387×106 BB¯ pairs collected at the ϒ(4S) resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract the CP-violating parameters S and C from a fit to the distribution of the decay-time difference between the two B mesons. The resulting confidence region is consistent with previous measurements in B0→KS0KS0KS0 and B0→(cc¯)K0 decays and with predictions based on the standard model. Published by the American Physical Society 2024

Based on (2.712±0.014)×109 ψ(3686) events collected by the BESIII Collaboration, evidence of the hadronic decay hc→KS0K+π−+c.c. is found with a significance of 4.3σ in the ψ(3686)→π0hc process. The branching fraction of hc→KS0K+π−+c.c. is measured to be (7.3±1.8±0.8)×10−4, where the first and second uncertainties are statistical and systematic, respectively. Combining with the exclusive decay width of ηc→KK¯π, our result indicates inconsistencies with both pQCD and NRQCD predictions. Published by the American Physical Society 2024

AbstractWe present measurements of the branching fractions of eight $$ {\overline{B}}^0 $$ B ¯ 0 → D(*)+K−$$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 , B− → D(*)0K−$$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 decay channels. The results are based on data from SuperKEKB electron-positron collisions at the Υ(4S) resonance collected with the Belle II detector, corresponding to an integrated luminosity of 362 fb−1. The event yields are extracted from fits to the distributions of the difference between expected and observed B meson energy, and are efficiency-corrected as a function of m(K−$$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 ) and m(D(*)$$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 ) in order to avoid dependence on the decay model. These results include the first observation of $$ {\overline{B}}^0 $$ B ¯ 0 → D+K−$$ {K}_S^0 $$ K S 0 , B− → D*0K−$$ {K}_S^0 $$ K S 0 , and $$ {\overline{B}}^0 $$ B ¯ 0 → D*+K−$$ {K}_S^0 $$ K S 0 decays and a significant improvement in the precision of the other channels compared to previous measurements. The helicity-angle distributions and the invariant mass distributions of the K−$$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 systems are compatible with quasi-two-body decays via a resonant transition with spin-parity JP = 1− for the K−$$ {K}_S^0 $$ K S 0 systems and JP = 1+ for the K−K*0 systems. We also present measurements of the branching fractions of four $$ {\overline{B}}^0 $$ B ¯ 0 → D(*)+$$ {D}_s^{-} $$ D s − , B− → D(*)0$$ {D}_s^{-} $$ D s − decay channels with a precision compatible to the current world averages.

Using data samples collected with the BESIII detector operating at the BEPCII storage ring, the cross section of the inclusive process e+e−→η+X, normalized by the total cross section of e+e−→hadrons, is measured at eight center-of-mass energy points from 2.0000 to 3.6710 GeV. These are the first measurements with momentum dependence in this energy region. Our measurement shows a significant discrepancy compared to the existing fragmentation functions. To address this discrepancy, a new QCD analysis is performed at the next-to-next-to-leading order with hadron mass corrections and higher twist effects, which can explain both the established high-energy data and our measurements reasonably well. Published by the American Physical Society 2024

Using a data sample of (10087 +/- 44) x 106 J=psi events collected by the BESIII detector in 2009, 2012, 2018 and 2019, the electromagnetic Dalitz process J=psi -> e+e-eta(1405) is observed via the decay eta(1405) -> pi 0f0(980), f0(980) -> pi+pi-, with a significance of about 9.8 sigma. The branching fraction of this decay is measured to be B(J=psi -> e+e-eta(1405)-> e+e- pi 0f0(980) -> e+e- pi 0 pi+ pi-)=(2.04 +/- 0.20(stat)+/- 0.08(syst))x 10-7. The branching fraction ratio B(J=psi -> e+e- eta(1405))=B(J=psi -> gamma eta(1405)) is determined to be (1.36 +/- 0.17(stat) +/- 0.06(syst)) x 10-2. Furthermore, an e+e- invariant mass dependent transition form factor of J=psi -> e+e-eta(1405) is presented for the first time. The obtained result provides input for different theoretical models and is valuable for the improved understanding the intrinsic structure of the eta(1405) meson.

We present measurements of the branching fractions of eight B^0 → D(*)+K− K_(S)^(∗)0 , B− → D(*)0K− K_(S)^(∗)0 decay channels. The results are based on data from SuperKEKB electron-positron collisions at the Υ(4S) resonance collected with the Belle II detector, corresponding to an integrated luminosity of 362 fb−1. The event yields are extracted from fits to the distributions of the difference between expected and observed B meson energy, and are efficiency-corrected as a function of m(K− K_(S)^(∗)0 ) and m(D(*) K_(S)^(∗)0 ) in order to avoid dependence on the decay model. These results include the first observation of B^0 → D+K− K_S^0 , B− → D*0K− K_S^0 , and B^0 → D*+K− K_S^0 decays and a significant improvement in the precision of the other channels compared to previous measurements. The helicity-angle distributions and the invariant mass distributions of the K− K_(S)^(∗)0 systems are compatible with quasi-two-body decays via a resonant transition with spin-parity JP = 1− for the K− K_S^0 systems and JP = 1+ for the K−K*0 systems. We also present measurements of the branching fractions of four B^0 → D(*)+ D_s^- , B− → D(*)0 D_s^- decay channels with a precision compatible to the current world averages.

We present GFlaT, a new algorithm that uses a graph-neural-network to determine the flavor of neutral B mesons produced in ϒ(4S) decays. It improves previous algorithms by using the information from all charged final-state particles and the relations between them. We evaluate its performance using B decays to flavor-specific hadronic final states reconstructed in a 362 fb−1 sample of electron-positron collisions collected at the ϒ(4S) resonance with the Belle II detector at the SuperKEKB collider. We achieve an effective tagging efficiency of (37.40±0.43±0.36%), where the first uncertainty is statistical and the second systematic, which is 18% better than the previous Belle II algorithm. Demonstrating the algorithm, we use B0→J/ψKS0 decays to measure the mixing-induced and direct CP violation parameters, S=(0.724±0.035±0.009) and C=(−0.035±0.026±0.029). Published by the American Physical Society 2024

Six C-even states, denoted as X, with quantum numbers JPC=0−+, 1±+, or 2±+, are searched for via the e+e−→γDs±Ds*∓ process using (1667.39±8.84) pb−1 of e+e− collision data collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energy of s=(4681.92±0.30) MeV. No statistically significant signal is observed in the mass range from 4.08 GeV/c2 to 4.32 GeV/c2. The upper limits of σ[e+e−→γX]·B[X→Ds±Ds*∓] at a 90% confidence level are determined. Published by the American Physical Society 2024

Momentum measurements for very high momentum charged particles, such as muons from electroweak vector boson decays, are particularly susceptible to charge-dependent curvature biases that arise from misalignments of tracking detectors. Low momentum charged particles used in alignment procedures have limited sensitivity to coherent displacements of such detectors, and therefore are unable to fully constrain these misalignments to the precision necessary for studies of electroweak physics. Additional approaches are therefore required to understand and correct for these effects. In this paper the curvature biases present at the LHCb detector are studied using the pseudomass method in proton-proton collision data recorded at centre of mass energy root s = 13 TeV during 2016, 2017 and 2018. The biases are determined using Z -> mu(+)mu(-) decays in intervals defined by the data-taking period, magnet polarity and muon direction. Correcting for these biases, which are typically at the 10(-4) GeV-1 level, improves the Z -> mu(+)mu(-) mass resolution by roughly 18% and eliminates several pathological trends in the kinematic-dependence of the mean dimuon invariant mass.

Abstract We present measurements of the branching fractions of eight B ¯ 0 $$ {\overline{B}}^0 $$ → D (*)+ K − K S ∗ 0 $$ {K}_{(S)}^{\left(\ast \right)0} $$ , B − → D (*)0 K − K S ∗ 0 $$ {K}_{(S)}^{\left(\ast \right)0} $$ decay channels. The results are based on data from SuperKEKB electron-positron collisions at the Υ(4S) resonance collected with the Belle II detector, corresponding to an integrated luminosity of 362 fb −1. The event yields are extracted from fits to the distributions of the difference between expected and observed B meson energy, and are efficiency-corrected as a function of m(K − K S ∗ 0 $$ {K}_{(S)}^{\left(\ast \right)0} $$ ) and m(D (*) K S ∗ 0 $$ {K}_{(S)}^{\left(\ast \right)0} $$ ) in order to avoid dependence on the decay model. These results include the first observation of B ¯ 0 $$ {\overline{B}}^0 $$ → D + K − K S 0 $$ {K}_S^0 $$ , B − → D* 0 K − K S 0 $$ {K}_S^0 $$ , and B ¯ 0 $$ {\overline{B}}^0 $$ → D* + K − K S 0 $$ {K}_S^0 $$ decays and a significant improvement in the precision of the other channels compared to previous measurements. The helicity-angle distributions and the invariant mass distributions of the K − K S ∗ 0 $$ {K}_{(S)}^{\left(\ast \right)0} $$ systems are compatible with quasi-two-body decays via a resonant transition with spin-parity J P = 1− for the K − K S 0 $$ {K}_S^0 $$ systems and J P = 1+ for the K − K* 0 systems. We also present measurements of the branching fractions of four B ¯ 0 $$ {\overline{B}}^0 $$ → D (*)+ D s − $$ {D}_s^{-} $$ , B − → D (*)0 D s − $$ {D}_s^{-} $$ decay channels with a precision compatible to the current world averages.

The ratio of production cross-sections of ψ(2S) over J/ψ mesons as a function of charged-particle multiplicity in proton-proton collisions at a centre-of-mass energy √(s) = 13 TeV is measured with a data sample collected by the LHCb detector, corresponding to an integrated luminosity of 658 pb−1. The ratio is measured for both prompt and non-prompt ψ(2S) and J/ψ mesons. When there is an overlap between the rapidity ranges over which multiplicity and charmonia production are measured, a multiplicity-dependent modification of the ratio is observed for prompt mesons. No significant multiplicity dependence is found when the ranges do not overlap. For non-prompt production, the ψ(2S)-to-J/ψ production ratio is roughly independent of multiplicity, irrespective of the rapidity range over which the multiplicity is measured. The results are compared to predictions of the co-mover model and agree well except in the low multiplicity region. The ratio of production cross-sections of ψ(2S) over J/ψ mesons are cross-checked with other measurements in di-lepton channels and found to be compatible.

A measurement of D ^0 meson production by the LHCb experiment in its fixed-target configuration is presented. The production of D ^0 mesons is studied with a beam of 2.5 TeV protons colliding on a gaseous neon target at rest, corresponding to a nucleon–nucleon centre-of-mass energy of √(s_NN) = 68.5 GeV . The sum of the D ^0 and D^0 production cross-section in pNe collisions in the centre-of-mass rapidity range y^⋆∈ [-2.29, 0] is found to be σ _D^0^y^⋆∈ [-2.29, 0] = 48.2 ± 0.3 ± 4.5 b/nucleon where the first uncertainty is statistical and the second is systematic. The D ^0-D^0 production asymmetry is also evaluated and suggests a trend towards negative values at large negative y^⋆ . The considered models do not account precisely for all the features observed in the LHCb data, but theoretical predictions including 1 % intrinsic charm and 10 % recombination contributions better describe the data than the other models considered.

Abstract The first measurement of $${{J}/\psi }$$ J / ψ and $${{D}} ^0$$ D 0 production in PbNe collisions by the LHCb experiment in its fixed-target configuration is reported. The production of $${{J}/\psi }$$ J / ψ and $${{D}} ^0$$ D 0 mesons is studied with a beam of lead ions with an energy of 2.5 $$\,\text {TeV}$$ TeV per nucleon colliding on gaseous neon targets at rest, corresponding to a nucleon-nucleon centre-of-mass energy of $$\sqrt{s_{\scriptscriptstyle \text {NN}}} =68.5\,\text {GeV} $$ s NN = 68.5 GeV . The $${{J}/\psi }$$ J / ψ / $${{D}} ^0$$ D 0 production cross-section ratio is studied as a function of rapidity, transverse momentum and collision centrality. These data are compared with measurements from $$p\text {Ne}$$ p Ne collisions at the same energy and show no difference in the observed $${{J}/\psi }$$ J / ψ suppression trend when comparing $$p\text {Ne}$$ p Ne and PbNe peripheral collisions with PbNe central collisions.