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Institute
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Using a total of 5.25 fb−1 of e+e− collision data with center-of-mass energies from 4.236 to 4.600 GeV, we report the first observation of the process e+e− → ηψ(2S) with a statistical significance of 4.9 standard deviations. The data sets were collected by the BESIII detector operating at the BEPCII storage ring. We measure the yield of events integrated over center-of-mass energies and also present the energy dependence of the measured cross section.
Search for the lepton number violating decay Σ⁻ → pe⁻e⁻ and the rare inclusive decay Σ⁻ → Σ⁺X
(2021)
Using a data sample of (1310.6±7.0)×106 𝐽/𝜓 events taken with the BESIII detector at the center-of-mass energy of 3.097 GeV, we search for the first time for the lepton number violating decay Σ−→𝑝𝑒−𝑒− and the rare inclusive decay Σ−→Σ+𝑋, where 𝑋 denotes any possible particle combination. The Σ− candidates are tagged in 𝐽/𝜓→¯Σ(1385)+Σ− decays. No signal candidates are found, and the upper limits on the branching fractions at the 90% confidence level are determined to be ℬ(Σ−→𝑝𝑒−𝑒−)<6.7×10−5 and ℬ(Σ−→Σ+𝑋)<1.2×10−4.
Using data samples with a total integrated luminosity of 20.1 fb−1 collected by the BESIII detector operating at the BEPCII collider, the cross section of the process 𝑒+𝑒−→𝜋+𝜋−𝜓(3686) is measured at center-of-mass energies between 4.0076 and 4.6984 GeV. The measured cross section is consistent with previous results, and with much improved precision. A fit to the measured energy-dependent cross section, which includes three Breit-Wigner functions and a nonresonant contribution, confirms the existence of the charmonium-like states 𝑌(4220), 𝑌(4390), and 𝑌(4660). This is the first observation of the 𝑌(4660) at the BESIII experiment.
The process e+e−→ϕη is studied at 22 center-of-mass energy points (√s) between 2.00 and 3.08 GeV using 715 pb−1 of data collected with the BESIII detector. The measured Born cross section of e+e−→ϕη is found to be consistent with BABAR measurements, but with improved precision. A resonant structure around 2.175 GeV is observed with a significance of 6.9σ with mass (2163.5±6.2±3.0) MeV/c2 and width (31.1+21.1−11.6±1.1) MeV, where the first uncertainties are statistical and the second are systematic.
The integrated luminosities of the data samples collected in the BESIII experiment in 2016--2017 at center-of-mass energies between 4.19 and 4.28 GeV are measured with a precision better than 1% by analyzing large-angle Bhabha scattering events. The integrated luminosities of the old data sets collected in 2010--2014 are updated by considering correction related to the detector performance, offsettting the effect of newly discovered readout errors in the electromagnetic calorimeter that happen haphazardly.
Using 2.93 fb−1 of e+e− collision data collected with the BESIII detector at a center-of-mass energy of 3.773 GeV, we measure the absolute branching fractions of the decays D0→K−e+νe and D+→K¯0e+νe to be (3.567±0.031stat±0.025syst)% and (8.68±0.14stat±0.16syst)%, respectively. Starting with the process e+e−→DD¯, a new reconstruction method is employed to select events that contain candidates for both D→K¯e+νe and D¯→Ke−ν¯e decays. The branching fractions reported in this work are consistent within uncertainties with previous BESIII measurements that selected events containing D→K¯e+νe and hadronic D¯ decays. Combining our results with the lifetimes of the D0 and D+ mesons and the previous BESIII measurements leads to a ratio of the two decay partial widths of Γ¯D0→K−e+νeΓ¯D+→K¯0e+νe=1.039±0.021. This ratio supports isospin symmetry in the D0→K−e+νe and D+→K¯0e+νe decays within 1.9σ.
Based on an e+e− collision data sample corresponding to an integrated luminosity of 2.93 fb−1 collected with the BESIII detector at √s=3.773 GeV, the first amplitude analysis of the singly Cabibbo-suppressed decay D+→K+K0Sπ0 is performed. From the amplitude analysis, the K∗(892)+K0S component is found to be dominant with a fraction of (57.1±2.6±4.2)%, where the first uncertainty is statistical and the second systematic. In combination with the absolute branching fraction B(D+→K+K0Sπ0) measured by BESIII, we obtain B(D+→K∗(892)+K0S)=(8.69±0.40±0.64±0.51)×10−3, where the third uncertainty is due to the branching fraction B(D+→K+K0Sπ0). The precision of this result is significantly improved compared to the previous measurement. This result also differs from most of theoretical predictions by about 4σ, which may help to improve the understanding of the dynamics behind.
Though immensely successful, the standard model of particle physics does not offer any explanation as to why our Universe contains so much more matter than antimatter. A key to a dynamically generated matter–antimatter asymmetry is the existence of processes that violate the combined charge conjugation and parity (CP) symmetry1. As such, precision tests of CP symmetry may be used to search for physics beyond the standard model. However, hadrons decay through an interplay of strong and weak processes, quantified in terms of relative phases between the amplitudes. Although previous experiments constructed CP observables that depend on both strong and weak phases, we present an approach where sequential two-body decays of entangled multi-strange baryon–antibaryon pairs provide a separation between these phases. Our method, exploiting spin entanglement between the double-strange Ξ− baryon and its antiparticle2 Ξ¯+
, has enabled a direct determination of the weak-phase difference, (ξP − ξS) = (1.2 ± 3.4 ± 0.8) × 10−2 rad. Furthermore, three independent CP observables can be constructed from our measured parameters. The precision in the estimated parameters for a given data sample size is several orders of magnitude greater than achieved with previous methods3. Finally, we provide an independent measurement of the recently debated Λ decay parameter αΛ (refs. 4,5). The ΛΛ¯
asymmetry is in agreement with and compatible in precision to the most precise previous measurement.
Using e+e− annihilation data corresponding to a total integrated luminosity of 6.32 fb−1 collected at the center-of-mass energies between 4.178 and 4.226 GeV with the BESIII detector, we perform an amplitude analysis of the decay D+s→K−K+π+π+π− and determine the relative fractions and phases of different intermediate processes. Absolute branching fraction of D+s→K−K+π+π+π− decay is measured to be (6.60±0.47stat.±0.35syst.)×10−3. The dominant intermediate process is D+s→a1(1260)+ϕ,ϕ→K−K+,a1(1260)+→ρπ+,ρ→π+π−, with a branching fraction of (5.16±0.41stat.±0.27syst.)×10−3.
The decays D → K−π+π+π− and D → K−π+π 0 are studied in a sample of quantum-correlated DD¯ pairs produced through the process e+e− → ψ(3770) → DD¯, exploiting a data set collected by the BESIII experiment that corresponds to an integrated luminosity of 2.93 fb−1 . Here D indicates a quantum superposition of a D0 and a D¯ 0 meson. By reconstructing one neutral charm meson in a signal decay, and the other in the same or a different final state, observables are measured that contain information on the coherence factors and average strong-phase differences of each of the signal modes. These parameters are critical inputs in the measurement of the angle γ of the Unitarity Triangle in B− → DK− decays at the LHCb and Belle II experiments. The coherence factors are determined to be RK3π = 0.52+0.12−0.10 and RKππ0 = 0.78 ± 0.04, with values for the average strong-phase differences that are δ K3π D = (167+31−19)◦ and δKππ0D = (196+14−15◦ , where the uncertainties include both statistical and systematic contributions. The analysis is re-performed in four bins of the phase-space of the D → K−π+π+π− to yield results that will allow for a more sensitive measurement of γ with this mode, to which the BESIII inputs will contribute an uncertainty of around 6◦.