*Phys Rev Lett ; 127(17): 171801, 2021 Oct 22.*

##### RESUMO

Using a dataset of 6.32 fb^{-1} of e^{+}e^{-} annihilation data collected with the BESIII detector at center-of-mass energies between 4178 and 4226 MeV, we have measured the absolute branching fraction of the leptonic decay D_{s}^{+}âτ^{+}ν_{τ} via τ^{+}âe^{+}ν_{e}ν[over ¯]_{τ}, and find B_{D_{s}^{+}âτ^{+}ν_{τ}}=(5.27±0.10±0.12)×10^{-2}, where the first uncertainty is statistical and the second is systematic. The precision is improved by a factor of 2 compared to the previous best measurement. Combining with f_{D_{s}^{+}} from lattice quantum chromodynamics calculations or the |V_{cs}| from the CKMfitter group, we extract |V_{cs}|=0.978±0.009±0.012 and f_{D_{s}^{+}}=(251.1±2.4±3.0) MeV, respectively. Combining our result with the world averages of B_{D_{s}^{+}âτ^{+}ν_{τ}} and B_{D_{s}^{+}âµ^{+}ν_{µ}}, we obtain the ratio of the branching fractions B_{D_{s}^{+}âτ^{+}ν_{τ}}/B_{D_{s}^{+}âµ^{+}ν_{µ}}=9.72±0.37, which is consistent with the standard model prediction of lepton flavor universality.

*Phys Rev Lett ; 127(12): 121802, 2021 Sep 17.*

##### RESUMO

The absolute branching fraction of Λâpµ^{-}ν[over ¯]_{µ} is reported for the first time based on an e^{+}e^{-} annihilation sample of 10×10^{9} J/ψ events collected with the BESIII detector at sqrt[s]=3.097 GeV. The branching fraction is determined to be B(Λâpµ^{-}ν[over ¯]_{µ})=[1.48±0.21(stat)±0.08(syst)]×10^{-4}, which is improved by about 30% in precision over the previous indirect measurements. Combining this result with the world average of B(Λâpe^{-}ν[over ¯]_{e}), we obtain the ratio {[Γ(Λâpµ^{-}ν[over ¯]_{µ})]/[Γ(Λâpe^{-}ν[over ¯]_{e})]} to be 0.178±0.028, which agrees with the standard model prediction assuming lepton flavor universality. The asymmetry of the branching fractions of Λâpµ^{-}ν[over ¯]_{µ} and Λ[over ¯]âp[over ¯]µ^{+}ν_{µ} is also determined, and no evidence for CP violation is found.

*Phys Rev Lett ; 127(13): 131801, 2021 Sep 24.*

##### RESUMO

Using 2.93 fb^{-1} of e^{+}e^{-} collision data taken with the BESIII detector at a center-of-mass energy of 3.773 GeV, the observation of the D^{0}âK_{1}(1270)^{-}e^{+}ν_{e} semileptonic decay is presented. The statistical significance of the decay D^{0}âK_{1}(1270)^{-}e^{+}ν_{e} is greater than 10σ. The branching fraction of D^{0}âK_{1}(1270)^{-}e^{+}ν_{e} is measured to be (1.09±0.13_{-0.16}^{+0.09}±0.12)×10^{-3}. Here, the first uncertainty is statistical, the second is systematic, and the third originates from the assumed branching fraction of K_{1}(1270)^{-}âK^{-}π^{+}π^{-}. The fraction of longitudinal polarization in D^{0}âK_{1}(1270)^{-}e^{+}ν_{e} is determined for the first time to be 0.50±0.19_{stat}±0.08_{syst}.

*Phys Rev Lett ; 127(8): 082002, 2021 Aug 20.*

##### RESUMO

We report a measurement of the observed cross sections of e^{+}e^{-}âJ/ψX based on 3.21 fb^{-1} of data accumulated at energies from 3.645 to 3.891 GeV with the BESIII detector operated at the BEPCII collider. In analysis of the cross sections, we measured the decay branching fractions of B(ψ(3686)âJ/ψX)=(64.4±0.6±1.6)% and B(ψ(3770)âJ/ψX)=(0.5±0.2±0.1)% for the first time. The energy-dependent line shape of these cross sections cannot be well described by two Breit-Wigner (BW) amplitudes of the expected decays ψ(3686)âJ/ψX and ψ(3770)âJ/ψX. Instead, it can be better described with one more BW amplitude of the decay R(3760)âJ/ψX. Under this assumption, we extracted the R(3760) mass M_{R(3760)}=3766.2±3.8±0.4 MeV/c^{2} , total width Γ_{R(3760)}^{tot}=22.2±5.9±1.4 MeV, and product of leptonic width and decay branching fraction Γ_{R(3760)}^{ee}B[R(3760)âJ/ψX]=(79.4±85.5±11.7) eV. The significance of the R(3760) is 5.3σ. The first uncertainties of these measured quantities are from fits to the cross sections and second systematic.

*Phys Rev Lett ; 126(9): 092002, 2021 Mar 05.*

##### RESUMO

We present an analysis of the process ψ(3686)âΩ^{-}Ω[over ¯]^{+} (Ω^{-}âK^{-}Λ, Ω[over ¯]^{+}âK^{+}Λ[over ¯], Λâpπ^{-}, Λ[over ¯]âp[over ¯]π^{+}) based on a dataset of 448×10^{6} ψ(3686) decays collected with the BESIII detector at the BEPCII electron-positron collider. The helicity amplitudes for the process ψ(3686)âΩ^{-}Ω[over ¯]^{+} and the decay parameters of the subsequent decay Ω^{-}âK^{-}Λ (Ω[over ¯]^{+}âK^{+}Λ[over ¯]) are measured for the first time by a fit to the angular distribution of the complete decay chain, and the spin of the Ω^{-} is determined to be 3/2 for the first time since its discovery more than 50 years ago.

*Phys Rev Lett ; 126(10): 102001, 2021 Mar 12.*

##### RESUMO

We report a study of the processes of e^{+}e^{-}âK^{+}D_{s}^{-}D^{*0} and K^{+}D_{s}^{*-}D^{0} based on e^{+}e^{-} annihilation samples collected with the BESIII detector operating at BEPCII at five center-of-mass energies ranging from 4.628 to 4.698 GeV with a total integrated luminosity of 3.7 fb^{-1}. An excess of events over the known contributions of the conventional charmed mesons is observed near the D_{s}^{-}D^{*0} and D_{s}^{*-}D^{0} mass thresholds in the K^{+} recoil-mass spectrum for events collected at sqrt[s]=4.681 GeV. The structure matches a mass-dependent-width Breit-Wigner line shape, whose pole mass and width are determined as (3982.5_{-2.6}^{+1.8}±2.1) MeV/c^{2} and (12.8_{-4.4}^{+5.3}±3.0) MeV, respectively. The first uncertainties are statistical and the second are systematic. The significance of the resonance hypothesis is estimated to be 5.3 σ over the contributions only from the conventional charmed mesons. This is the first candidate for a charged hidden-charm tetraquark with strangeness, decaying into D_{s}^{-}D^{*0} and D_{s}^{*-}D^{0}. However, the properties of the excess need further exploration with more statistics.

*Phys Rev Lett ; 125(14): 141802, 2020 Oct 02.*

##### RESUMO

Using 2.93 fb^{-1} of e^{+}e^{-} collision data collected at a center-of-mass energy of 3.773 GeV with the BESIII detector, the first observation of the doubly Cabibbo-suppressed decay D^{+}âK^{+}π^{+}π^{-}π^{0} is reported. After removing decays that contain narrow intermediate resonances, including D^{+}âK^{+}Î·, D^{+}âK^{+}ω, and D^{+}âK^{+}Ï, the branching fraction of the decay D^{+}âK^{+}π^{+}π^{-}π^{0} is measured to be (1.13±0.08_{stat}±0.03_{syst})×10^{-3}. The ratio of branching fractions of D^{+}âK^{+}π^{+}π^{-}π^{0} over D^{+}âK^{-}π^{+}π^{+}π^{0} is found to be (1.81±0.15)%, which corresponds to (6.28±0.52)tan^{4}Î¸_{C}, where Î¸_{C} is the Cabibbo mixing angle. This ratio is significantly larger than the corresponding ratios for other doubly Cabibbo-suppressed decays. The asymmetry of the branching fractions of charge-conjugated decays D^{±}âK^{±}π^{±}π^{∓}π^{0} is also determined, and no evidence for CP violation is found. In addition, the first evidence for the D^{+}âK^{+}ω decay, with a statistical significance of 3.3σ, is presented and the branching fraction is measured to be B(D^{+}âK^{+}ω)=(5.7_{-2.1}^{+2.5}_{stat}±0.2_{syst})×10^{-5}.

*Phys Rev Lett ; 125(5): 052004, 2020 Jul 31.*

##### RESUMO

From 1310.6×10^{6} J/ψ and 448.1×10^{6} ψ(3686) events collected with the BESIII experiment, we report the first observation of Σ^{+} and Σ[over ¯]^{-} spin polarization in e^{+}e^{-}âJ/ψ[ψ(3686)]âΣ^{+}Σ[over ¯]^{-} decays. The relative phases of the form factors ΔΦ have been measured to be (-15.5±0.7±0.5)° and (21.7±4.0±0.8)° with J/ψ and ψ(3686) data, respectively. The nonzero value of ΔΦ allows for a direct and simultaneous measurement of the decay asymmetry parameters of Σ^{+}âpπ^{0}(α_{0}=-0.998±0.037±0.009) and Σ[over ¯]^{-}âp[over ¯]π^{0}(α[over ¯]_{0}=0.990±0.037±0.011), the latter value being determined for the first time. The average decay asymmetry, (α_{0}-α[over ¯]_{0})/2, is calculated to be -0.994±0.004±0.002. The CP asymmetry A_{CP,Σ}=(α_{0}+α[over ¯]_{0})/(α_{0}-α[over ¯]_{0})=-0.004±0.037±0.010 is extracted for the first time, and is found to be consistent with CP conservation.

*Phys Rev Lett ; 125(6): 062001, 2020 Aug 07.*

##### RESUMO

The double-polarization observable E and helicity-dependent cross sections σ_{1/2}, σ_{3/2} have been measured for the photoproduction of π^{0} pairs off quasifree protons and neutrons at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost 4π covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the Î³NâNπ^{0}π^{0} reaction. They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. The comparison of the prediction and the refit demonstrates the large impact of the new data.

*Phys Rev Lett ; 124(23): 231801, 2020 Jun 12.*

##### RESUMO

By analyzing a data sample corresponding to an integrated luminosity of 2.93 fb^{-1} collected at a center-of-mass energy of 3.773 GeV with the BESIII detector, we measure for the first time the absolute branching fraction of the D^{+}âÎ·µ^{+}ν_{µ} decay to be B_{D^{+}âÎ·µ^{+}ν_{µ}}=(10.4±1.0_{stat}±0.5_{syst})×10^{-4}. Using the world averaged value of B_{D^{+}âÎ·e^{+}ν_{e}}, the ratio of the two branching fractions is determined to be B_{D^{+}âÎ·µ^{+}ν_{µ}}/B_{D^{+}âÎ·e^{+}ν_{e}}=0.91±0.13_{(stat+syst)}, which agrees with the theoretical expectation of lepton flavor universality within uncertainty. By studying the differential decay rates in five four-momentum transfer intervals, we obtain the product of the hadronic form factor f_{+}^{Î·}(0) and the câd Cabibbo-Kobayashi-Maskawa matrix element |V_{cd}| to be f_{+}^{Î·}(0)|V_{cd}|=0.087±0.008_{stat}±0.002_{syst}. Taking the input of |V_{cd}| from the global fit in the standard model, we determine f_{+}^{Î·}(0)=0.39±0.04_{stat}±0.01_{syst}. On the other hand, using the value of f_{+}^{Î·}(0) calculated in theory, we find |V_{cd}|=0.242±0.022_{stat}±0.006_{syst}±0.033_{theory}.

*Phys Rev Lett ; 124(24): 241802, 2020 Jun 19.*

##### RESUMO

We report the most precise measurements to date of the strong-phase parameters between D^{0} and D[over ¯]^{0} decays to K_{S,L}^{0}π^{+}π^{-} using a sample of 2.93 fb^{-1} of e^{+}e^{-} annihilation data collected at a center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider. Our results provide the key inputs for a binned model-independent determination of the Cabibbo-Kobayashi-Maskawa angle Î³/Ï_{3} with B decays. Using our results, the decay model sensitivity to the Î³/Ï_{3} measurement is expected to be between 0.7° and 1.2°, approximately a factor of three smaller than that achievable with previous measurements, based on the studies of the simulated data. The improved precision of this work ensures that measurements of Î³/Ï_{3} will not be limited by knowledge of strong phases for the next decade. Furthermore, our results provide critical input for other flavor-physics investigations, including charm mixing, other measurements of CP violation, and the measurement of strong-phase parameters for other D-decay modes.

*Phys Rev Lett ; 124(24): 242001, 2020 Jun 19.*

##### RESUMO

The processes X(3872)âD^{*0}D[over ¯]^{0}+c.c.,Î³J/ψ,Î³ψ(2S), and Î³D^{+}D^{-} are searched for in a 9.0 fb^{-1} data sample collected at center-of-mass energies between 4.178 and 4.278 GeV with the BESIII detector. We observe X(3872)âD^{*0}D^{0}[over ¯]+c.c. and find evidence for X(3872)âÎ³J/ψ with statistical significances of 7.4σ and 3.5σ, respectively. No evident signals for X(3872)âÎ³ψ(2S) and Î³D^{+}D^{-} are found, and the upper limit on the relative branching ratio R_{Î³ψ}≡{B[X(3872)âÎ³ψ(2S)]}/{B[X(3872)âÎ³J/ψ]}<0.59 is set at 90% confidence level. Measurements of branching ratios relative to decay X(3872)âπ^{+}π^{-}J/ψ are also reported for decays X(3872)âD^{*0}D^{0}[over ¯]+c.c.,Î³ψ(2S),Î³J/ψ, and Î³D^{+}D^{-}, as well as the non-D^{*0}D^{0}[over ¯] three-body decays π^{0}D^{0}D^{0}[over ¯] and Î³D^{0}D^{0}[over ¯].

*Phys Rev Lett ; 124(24): 241803, 2020 Jun 19.*

##### RESUMO

Using 2.93 fb^{-1} of e^{+}e^{-} collision data taken at a center-of-mass energy of 3.773 GeV with the BESIII detector, we report the first measurements of the absolute branching fractions of 14 hadronic D^{0(+)} decays to exclusive final states with an Î·, e.g., D^{0}âK^{-}π^{+}Î·, K_{S}^{0}π^{0}Î·, K^{+}K^{-}Î·, K_{S}^{0}K_{S}^{0}Î·, K^{-}π^{+}π^{0}Î·, K_{S}^{0}π^{+}π^{-}Î·, K_{S}^{0}π^{0}π^{0}Î·, and π^{+}π^{-}π^{0}Î·; D^{+}âK_{S}^{0}π^{+}Î·, K_{S}^{0}K^{+}Î·, K^{-}π^{+}π^{+}Î·, K_{S}^{0}π^{+}π^{0}Î·, π^{+}π^{+}π^{-}Î·, and π^{+}π^{0}π^{0}Î·. Among these decays, the D^{0}âK^{-}π^{+}Î· and D^{+}âK_{S}^{0}π^{+}Î· decays have the largest branching fractions, which are B(D^{0}âK^{-}π^{+}Î·)=(1.853±0.025_{stat}±0.031_{syst})% and B(D^{+}âK_{S}^{0}π^{+}Î·)=(1.309±0.037_{stat}±0.031_{syst})%, respectively. The charge-parity asymmetries for the six decays with highest event yields are determined, and no statistically significant charge-parity violation is found.

*Phys Rev Lett ; 124(11): 112001, 2020 Mar 20.*

##### RESUMO

A partial-wave analysis is performed for the process e^{+}e^{-}âK^{+}K^{-}π^{0}π^{0} at the center-of-mass energies ranging from 2.000 to 2.644 GeV. The data samples of e^{+}e^{-} collisions, collected by the BESIII detector at the BEPCII collider with a total integrated luminosity of 300 pb^{-1}, are analyzed. The total Born cross sections for the process e^{+}e^{-}âK^{+}K^{-}π^{0}π^{0}, as well as the Born cross sections for the subprocesses e^{+}e^{-}âÏπ^{0}π^{0}, K^{+}(1460)K^{-}, K_{1}^{+}(1400)K^{-}, K_{1}^{+}(1270)K^{-}, and K^{*+}(892)K^{*-}(892), are measured versus the center-of-mass energy. The corresponding results for e^{+}e^{-}âK^{+}K^{-}π^{0}π^{0} and Ïπ^{0}π^{0} are consistent with those of BABAR with better precision. By analyzing the cross sections for the four subprocesses, K^{+}(1460)K^{-}, K_{1}^{+}(1400)K^{-}, K_{1}^{+}(1270)K^{-}, and K^{*+}(892)K^{*-}(892), a structure with mass M=(2126.5±16.8±12.4) MeV/c^{2} and width Γ=(106.9±32.1±28.1) MeV is observed with an overall statistical significance of 6.3σ, although with very limited significance in the subprocesses e^{+}e^{-}âK_{1}^{+}(1270)K^{-} and K^{*+}(892)K^{*-}(892). The resonant parameters of the observed structure suggest it can be identified with the Ï(2170), thus the results provide valuable input to the internal nature of the Ï(2170).

*Phys Rev Lett ; 124(13): 132001, 2020 Apr 03.*

##### RESUMO

We report a measurement of the spin polarization of the recoiling neutron in deuterium photodisintegration, utilizing a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300-700 MeV provides the first measurement of recoil neutron polarization at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarization in a narrow structure centered around E_{Î³}â¼570 MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behavior could be related to the excitation of the d^{*}(2380) hexaquark.

*Phys Rev Lett ; 124(4): 042001, 2020 Jan 31.*

##### RESUMO

The process of e^{+}e^{-}âpp[over ¯] is studied at 22 center-of-mass energy points (sqrt[s]) from 2.00 to 3.08 GeV, exploiting 688.5 pb^{-1} of data collected with the BESIII detector operating at the BEPCII collider. The Born cross section (σ_{pp[over ¯]}) of e^{+}e^{-}âpp[over ¯] is measured with the energy-scan technique and it is found to be consistent with previously published data, but with much improved accuracy. In addition, the electromagnetic form-factor ratio (|G_{E}/G_{M}|) and the value of the effective (|G_{eff}|), electric (|G_{E}|), and magnetic (|G_{M}|) form factors are measured by studying the helicity angle of the proton at 16 center-of-mass energy points. |G_{E}/G_{M}| and |G_{M}| are determined with high accuracy, providing uncertainties comparable to data in the spacelike region, and |G_{E}| is measured for the first time. We reach unprecedented accuracy, and precision results in the timelike region provide information to improve our understanding of the proton inner structure and to test theoretical models which depend on nonperturbative quantum chromodynamics.

*Phys Rev Lett ; 124(3): 032002, 2020 Jan 24.*

##### RESUMO

Using a total of 11.0 fb^{-1} of e^{+}e^{-} collision data with center-of-mass energies between 4.009 and 4.6 GeV and collected with the BESIII detector at BEPCII, we measure fifteen exclusive cross sections and effective form factors for the process e^{+}e^{-}âΞ^{-}Ξ[over ¯]^{+} by means of a single baryon-tag method. After performing a fit to the dressed cross section of e^{+}e^{-}âΞ^{-}Ξ[over ¯]^{+}, no significant ψ(4230) or ψ(4260) resonance is observed in the Ξ^{-}Ξ[over ¯]^{+} final states, and upper limits at the 90% confidence level on Γ_{ee}B for the processes ψ(4230)/ψ(4260)âΞ^{-}Ξ[over ¯]^{+} are determined. In addition, an excited Ξ baryon at 1820 MeV/c^{2} is observed with a statistical significance of 6.2-6.5σ by including the systematic uncertainty, and the mass and width are measured to be M=(1825.5±4.7±4.7) MeV/c^{2} and Γ=(17.0±15.0±7.9) MeV, which confirms the existence of the J^{P}=3/2^{-} state Ξ(1820).

*Phys Rev Lett ; 123(21): 211802, 2019 Nov 22.*

##### RESUMO

We report the first observation of D^{+}âτ^{+}ν_{τ} with a significance of 5.1σ. We measure B(D^{+}âτ^{+}ν_{τ})=(1.20±0.24_{stat}±0.12_{syst})×10^{-3}. Taking the world average B(D^{+}âµ^{+}ν_{µ})=(3.74±0.17)×10^{-4}, we obtain R_{τ/µ}=Γ(D^{+}âτ^{+}ν_{τ})/Γ(D^{+}âµ^{+}ν_{µ})=3.21±0.64_{stat}±0.43_{syst}., which is consistent with the standard model expectation of lepton flavor universality. Using external inputs, our results give values for the D^{+} decay constant f_{D^{+}} and the Cabibbo-Kobayashi-Maskawa matrix element |V_{cd}| that are consistent with, but less precise than, other determinations.

*Phys Rev Lett ; 123(23): 231801, 2019 Dec 06.*

##### RESUMO

By analyzing a 2.93 fb^{-1} data sample of e^{+}e^{-} collisions, recorded at a center-of-mass energy of 3.773 GeV with the BESIII detector operated at the BEPCII collider, we report the first observation of the semileptonic D^{+} transition into the axial-vector meson D^{+}âK[over ¯]_{1}(1270)^{0}e^{+}ν_{e} with a statistical significance greater than 10σ. Its decay branching fraction is determined to be B[D^{+}âK[over ¯]_{1}(1270)^{0}e^{+}ν_{e}]=(2.30±0.26_{-0.21}^{+0.18}±0.25)×10^{-3}, where the first and second uncertainties are statistical and systematic, respectively, and the third originates from the input branching fraction of K[over ¯]_{1}(1270)^{0}âK^{-}π^{+}π^{0}.

*Phys Rev Lett ; 123(12): 122003, 2019 Sep 20.*

##### RESUMO

The exclusive process e^{+}e^{-}âΛΛ[over ¯], with Λâpπ^{-} and Λ[over ¯]âp[over ¯]π^{+}, has been studied at sqrt[s]=2.396 GeV for measurement of the timelike Λ electric and magnetic form factors, G_{E} and G_{M}. A data sample, corresponding to an integrated luminosity of 66.9 pb^{-1}, was collected with the BESIII detector for this purpose. A multidimensional analysis with a complete decomposition of the spin structure of the reaction enables a determination of the modulus of the ratio R=|G_{E}/G_{M}| and, for the first time for any baryon, the relative phase ΔΦ=Φ_{E}-Φ_{M}. The resulting values are R=0.96±0.14(stat)±0.02(syst) and ΔΦ=37°±12°(stat)±6°(syst), respectively. These are obtained using the recently established and most precise value of the asymmetry parameter α_{Λ}=0.750±0.010 measured by BESIII. In addition, the cross section is measured with unprecedented precision to be σ=118.7±5.3(stat)±5.1(syst) pb, which corresponds to an effective form factor of |G|=0.123±0.003(stat)±0.003(syst). The contribution from two-photon exchange is found to be negligible. Our result enables the first complete determination of baryon timelike electromagnetic form factors.