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There has recently been a dramatic renewal of interest in hadron spectroscopy and charm physics. This renaissance has been driven in part by the discovery of a plethora of charmonium-like XYZ states at BESIII and B factories, and the observation of an intriguing proton-antiproton threshold enhancement and the possibly related X(1835) meson state at BESIII, as well as the threshold measurements of charm mesons and charm baryons.
We present a detailed survey of the important topics in tau-charm physics and hadron physics that can be further explored at BESIII during the remaining operation period of BEPCII. This survey will help in the optimization of the data-taking plan over the coming years, and provides physics motivation for the possible upgrade of BEPCII to higher luminosity.
Using a data sample of 448.1 × 106 ψ(3686) events collected with the BESIII detector at the BEPCII collider, we report the first observation of the electromagnetic Dalitz decay ψ(3686) → η e+e−, with significances of 7.0σ and 6.3σ when reconstructing the η meson via its decay modes η → γπ+π− and η → π+π−η (η → γγ ), respectively. The weighted average branching fraction is determined to be B(ψ(3686) → η e+e−) = (1.90 ± 0.25 ± 0.11) × 10−6, where the first uncertainty is statistical and the second systematic.
Using a data sample with an integrated luminosity of 2.93 fb−1 taken at the center-of-mass energy of 3.773 GeV, we search for the Majorana neutrino (𝜈𝑚) in the lepton number violating decays 𝐷→𝐾𝜋𝑒+𝑒+. No significant signal is observed, and the upper limits on the branching fraction at the 90% confidence level are set to be ℬ(𝐷0→𝐾−𝜋−𝑒+𝑒+)<2.8×10−6, ℬ(𝐷+→𝐾0𝑆𝜋−𝑒+𝑒+)<3.3×10−6 and ℬ(𝐷+→𝐾−𝜋0𝑒+𝑒+)<8.5×10−6. The Majorana neutrino is searched for with different mass assumptions ranging from 0.25 to 1.0 GeV/𝑐2 in the decays 𝐷0→𝐾−𝑒+𝜈𝑚,𝜈𝑚→𝜋−𝑒+ and 𝐷+→𝐾0𝑆𝑒+𝜈𝑚,𝜈𝑚→𝜋−𝑒+, and the upper limits on the branching fraction at the 90% confidence level are at the level of 10−7∼10−6, depending on the mass of the Majorana neutrino. The constraints on the mixing matrix element |𝑉𝑒𝜈𝑚|2 are also evaluated.
The cross section of the process e+e−→K+K− is measured at a number of center-of-mass energies s√ from 2.00 to 3.08 GeV with the BESIII detector at the Beijing Electron Positron Collider (BEPCII). The results provide the best precision achieved so far. A resonant structure around 2.2 GeV is observed in the cross section line shape. A Breit-Wigner fit yields a mass of M=2239.2±7.1±11.3~and a width of Γ=139.8±12.3±20.6 MeV, where the first uncertainties are statistical and the second ones are systematic. In addition, the time-like electromagnetic form factor of the kaon is determined at the individual center-of-mass energy points.
The baryonic decay D+s→pn¯ is observed, and the corresponding branching fraction is measured to be (1.21±0.10±0.05)×10−3, where the first uncertainty is statistical and second systematic. The data sample used in this analysis was collected with the BESIII detector operating at the BEPCII e+e− double-ring collider with a center-of-mass energy of 4.178~GeV and an integrated luminosity of 3.19~fb−1. The result confirms the previous measurement by the CLEO Collaboration and is of greatly improved precision, which may deepen our understanding of the dynamical enhancement of the W-annihilation topology in the charmed meson decays.
We search for rare decays of D mesons to hadrons accompany with an electron-positron pair (h(h')e+e−), using an e+e− collision sample corresponding to an integrated luminosity of 2.93 fb−1 collected with the BESIII detector at s√ = 3.773 GeV. No significant signals are observed, and the corresponding upper limits on the branching fractions at the 90% confidence level are determined. The sensitivities of the results are at the level of 10−5∼10−6, providing a large improvement over previous searches.
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 𝐷+𝑠→𝜋+𝜋−𝑒+𝜈𝑒, where the 𝐷+𝑠 is produced via the process 𝑒+𝑒−→𝐷*±𝑠𝐷∓𝑠. We observe the 𝑓0(980) in the 𝜋+𝜋− system and the branching fraction of the decay 𝐷+𝑠→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 𝐷+𝑠→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 𝑓𝑓0+(0) and the 𝑐→𝑠 Cabibbo-Kobayashi-Maskawa matrix element |𝑉𝑐𝑠| is determined for the first time to be 𝑓𝑓0+(0)|𝑉𝑐𝑠|=0.504±0.017stat±0.035syst. Furthermore, the decay 𝐷+
𝑠→𝑓0(500)𝑒+𝜈𝑒 is searched for the first time but no signal is found. The upper limit on the branching fraction of 𝐷+𝑠→𝑓0(500)𝑒+𝜈𝑒, 𝑓0(500)→𝜋+𝜋− decay is set to be 3.3×10−4 at 90% confidence level.
A narrow structure in the pΛ¯ system near the mass threshold, named as X(2085), is observed in the process e+e−→pK−Λ¯ with a statistical significance greater than 20σ. Its spin and parity are determined for the first time to be JP=1+ in an amplitude analysis, with statistical significance greater than 5σ over other quantum numbers. The pole positions of X(2085) are measured to be Mpole=(2086±4±6)~MeV and Γpole=(56±5±16) MeV, where the first uncertainties are statistical and the second ones are systematic. The analysis is based on the study of the process e+e−→pK−Λ¯ and uses the data samples collected with the BESIII detector at the center-of-mass energies s√=4.008, 4.178, 4.226, 4.258, 4.416, and 4.682 GeV with a total integrated luminosity of 8.35 fb−1.
Based on a sample of 448.1×106 ψ(3686) events collected with the BESIII detector, a study of ψ(3686)→ΛΛ¯π0 and ψ(3686)→ΛΛ¯η is performed. Evidence of the isospin-violating decay ψ(3686)→ΛΛ¯π0 is found for the first time with a statistical significance of 3.7σ, the branching fraction B(ψ(3686)→ΛΛ¯π0) is measured to be (1.42±0.39±0.59)×10−6, and its corresponding upper limit is determined to be 2.47×10−6 at 90\% confidence level. A partial wave analysis of ψ(3686)→ΛΛ¯η shows that the peak around Λη invariant mass threshold favors a Λ∗ resonance with mass and width in agreement with the Λ(1670). The branching fraction of the ψ(3686)→ΛΛ¯η is measured to be (2.34±0.18±0.52)×10−5. The first uncertainties are statistical and the second are systematic.
Based on e+e− collision data corresponding to an integrated luminosity of 4.5 fb−1 collected at the center-of-mass energies between 4.600 and 4.699 GeV with the BESIII detector at BEPCII, the absolute branching fraction of the inclusive decay Λ+c→n+X, where X refers to any possible final state particles, is measured. The absolute branching fraction is determined to be B(Λ+c→n+X)=(32.4±0.7±1.5)%, where the first uncertainty is statistical and the second systematic. Assuming CP symmetry, the measurement indicates that about one-fourth of Λ+c (Λ¯−c) decay modes with a neutron (an anti-neutron) in the final state have not been observed.