Search for Λ[over ¯]-Λ Baryon-Number-Violating Oscillations in the Decay J/ψ→pK

We report on the first search for Λ[over ¯]-Λ oscillations in the decay J/ψ→pK^{-}Λ[over ¯]+c.c. by analyzing 1.31×10^{9} J/ψ events accumulated with the BESIII detector at the BEPCII collider. The J/ψ events are produced using e^{+}e^{-} collisions at a center of mass energy sqrt[s]=3.097 GeV. No evidence for hyperon oscillations is observed. The upper limit for the oscillation rate of Λ[over ¯] to Λ hyperons is determined to be P(Λ)=[B(J/ψ→pK^{-}Λ+c.c.)/B(J/ψ→pK^{-}Λ[over ¯]+c.c.)]<4.4×10^{-6} corresponding to an oscillation parameter δm_{ΛΛ[over ¯]} of less than 3.8×10^{-18} GeV at the 90% confidence level.

Observation of J/ψ Electromagnetic Dalitz Decays to X(1835), X(2120), and X(2370).

Using a sample of about 10^{10} J/ψ events collected at a center-of-mass energy sqrt[s]=3.097 GeV with the BESIII detector, the electromagnetic Dalitz decays J/ψ→e^{+}e^{-}π^{+}π^{-}η^{'}, with η^{'}→γπ^{+}π^{-} and η^{'}→π^{+}π^{-}η, have been studied. The decay J/ψ→e^{+}e^{-}X(1835) is observed with a significance of 15σ, and also an e^{+}e^{-} invariant-mass dependent transition form factor of J/ψ→e^{+}e^{-}X(1835) is presented for the first time. The intermediate states X(2120) and X(2370) are also observed in the π^{+}π^{-}η^{'} invariant-mass spectrum with significances of 5.3σ and 7.3σ. The corresponding product branching fractions for J/ψ→e^{+}e^{-}X, X→π^{+}π^{-}η^{'} [X=X(1835), X(2120), and X(2370)] are reported.

Observation of the Singly Cabibbo Suppressed Decay Λ_{c}

The singly Cabibbo-suppressed decay Λ_{c}^{+}→nπ^{+} is observed for the first time with a statistical significance of 7.3σ by using 3.9 fb^{-1} of e^{+}e^{-} collision data collected at center-of-mass energies between 4.612 and 4.699 GeV with the BESIII detector at BEPCII. The branching fraction of Λ_{c}^{+}→nπ^{+} is measured to be (6.6±1.2_{stat}±0.4_{syst})×10^{-4}. By taking the upper limit of branching fractions of Λ_{c}^{+}→pπ^{0} from the Belle experiment, the ratio of branching fractions between Λ_{c}^{+}→nπ^{+} and Λ_{c}^{+}→pπ^{0} is calculated to be larger than 7.2 at the 90% confidence level, which disagrees with most predictions of the available phenomenological models. In addition, the branching fractions of the Cabibbo-favored decays Λ_{c}^{+}→Λπ^{+} and Λ_{c}^{+}→Σ^{0}π^{+} are measured to be (1.31±0.08_{stat}±0.05_{syst})×10^{-2} and (1.22±0.08_{stat}±0.07_{syst})×10^{-2}, respectively, which are consistent with previous results.

Measurement of the Cross Section for e

Based on electron-positron collision data collected with the BESIII detector operating at the Beijing Electron-Positron Collider II storage rings, the value of R≡σ(e^{+}e^{-}→hadrons)/σ(e^{+}e^{-}→µ^{+}µ^{-}) is measured at 14 center-of-mass energies from 2.2324 to 3.6710 GeV. The resulting uncertainties are less than 3.0% and are dominated by systematic uncertainties.

First Measurement of Polarizations in the Decay D

Using a data sample corresponding to an integrated luminosity of 2.93 fb^{-1} collected at a center-of-mass energy sqrt[s]=3.773 GeV by the BESIII detector, the decay D^{0}→ωϕ is observed for the first time. The branching fraction is measured to be (6.48±0.96±0.40)×10^{-4} with a significance of 6.3σ, where the first and second uncertainties are statistical and systematic, respectively. An angular analysis reveals that the ϕ and ω mesons from the D^{0}→ωϕ decay are transversely polarized. The 95% confidence level upper limit on longitudinal polarization fraction is set to be less than 0.24, which is inconsistent with current theoretical expectations and challenges our understanding of the underlying dynamics in charm meson decays.

Measurement of the Absolute Branching Fraction of D_{s}

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}^{+}→τ^{+}ν_{τ} via τ^{+}→e^{+}ν_{e}ν[over ¯]_{τ}, and find B_{D_{s}^{+}→τ^{+}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}^{+}→τ^{+}ν_{τ}} and B_{D_{s}^{+}→µ^{+}ν_{µ}}, we obtain the ratio of the branching fractions B_{D_{s}^{+}→τ^{+}ν_{τ}}/B_{D_{s}^{+}→µ^{+}ν_{µ}}=9.72±0.37, which is consistent with the standard model prediction of lepton flavor universality.

First Measurement of the Absolute Branching Fraction of Λ→pµ

The absolute branching fraction of Λ→pµ^{-}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(Λ→pµ^{-}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(Λ→pe^{-}ν[over ¯]_{e}), we obtain the ratio {[Γ(Λ→pµ^{-}ν[over ¯]_{µ})]/[Γ(Λ→pe^{-}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 Λ→pµ^{-}ν[over ¯]_{µ} and Λ[over ¯]→p[over ¯]µ^{+}ν_{µ} is also determined, and no evidence for CP violation is found.

Observation of D

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}→K_{1}(1270)^{-}e^{+}ν_{e} semileptonic decay is presented. The statistical significance of the decay D^{0}→K_{1}(1270)^{-}e^{+}ν_{e} is greater than 10σ. The branching fraction of D^{0}→K_{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)^{-}→K^{-}π^{+}π^{-}. The fraction of longitudinal polarization in D^{0}→K_{1}(1270)^{-}e^{+}ν_{e} is determined for the first time to be 0.50±0.19_{stat}±0.08_{syst}.

Direct Measurement of the Branching Fractions B(ψ(3686)→J/ψX) and B(ψ(3770)→J/ψX), and Observation of the State R(3760) in e

We report a measurement of the observed cross sections of e^{+}e^{-}→J/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)→J/ψX)=(64.4±0.6±1.6)% and B(ψ(3770)→J/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)→J/ψX and ψ(3770)→J/ψX. Instead, it can be better described with one more BW amplitude of the decay R(3760)→J/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)→J/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.

Observation of a Near-Threshold Structure in the K

We report a study of the processes of e^{+}e^{-}→K^{+}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.

Model-Independent Determination of the Spin of the Ω

We present an analysis of the process ψ(3686)→Ω^{-}Ω[over ¯]^{+} (Ω^{-}→K^{-}Λ, Ω[over ¯]^{+}→K^{+}Λ[over ¯], Λ→pπ^{-}, Λ[over ¯]→p[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)→Ω^{-}Ω[over ¯]^{+} and the decay parameters of the subsequent decay Ω^{-}→K^{-}Λ (Ω[over ¯]^{+}→K^{+}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.

Observation of the Doubly Cabibbo-Suppressed Decay D

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^{+}→K^{+}π^{+}π^{-}π^{0} is reported. After removing decays that contain narrow intermediate resonances, including D^{+}→K^{+}η, D^{+}→K^{+}ω, and D^{+}→K^{+}ϕ, the branching fraction of the decay D^{+}→K^{+}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^{+}→K^{+}π^{+}π^{-}π^{0} over D^{+}→K^{-}π^{+}π^{+}π^{0} is found to be (1.81±0.15)%, which corresponds to (6.28±0.52)tan^{4}θ_{C}, where θ_{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^{±}→K^{±}π^{±}π^{∓}π^{0} is also determined, and no evidence for CP violation is found. In addition, the first evidence for the D^{+}→K^{+}ω decay, with a statistical significance of 3.3σ, is presented and the branching fraction is measured to be B(D^{+}→K^{+}ω)=(5.7_{-2.1}^{+2.5}_{stat}±0.2_{syst})×10^{-5}.

Σ

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^{-}→J/ψ[ψ(3686)]→Σ^{+}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^{+}→pπ^{0}(α_{0}=-0.998±0.037±0.009) and Σ[over ¯]^{-}→p[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.

First Observation of D

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^{+}→ηµ^{+}ν_{µ} decay to be B_{D^{+}→ηµ^{+}ν_{µ}}=(10.4±1.0_{stat}±0.5_{syst})×10^{-4}. Using the world averaged value of B_{D^{+}→ηe^{+}ν_{e}}, the ratio of the two branching fractions is determined to be B_{D^{+}→ηµ^{+}ν_{µ}}/B_{D^{+}→ηe^{+}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_{+}^{η}(0) and the c→d Cabibbo-Kobayashi-Maskawa matrix element |V_{cd}| to be f_{+}^{η}(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_{+}^{η}(0)=0.39±0.04_{stat}±0.01_{syst}. On the other hand, using the value of f_{+}^{η}(0) calculated in theory, we find |V_{cd}|=0.242±0.022_{stat}±0.006_{syst}±0.033_{theory}.

Measurements of Absolute Branching Fractions of Fourteen Exclusive Hadronic D Decays to η.

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 η, e.g., D^{0}→K^{-}π^{+}η, K_{S}^{0}π^{0}η, K^{+}K^{-}η, K_{S}^{0}K_{S}^{0}η, K^{-}π^{+}π^{0}η, K_{S}^{0}π^{+}π^{-}η, K_{S}^{0}π^{0}π^{0}η, and π^{+}π^{-}π^{0}η; D^{+}→K_{S}^{0}π^{+}η, K_{S}^{0}K^{+}η, K^{-}π^{+}π^{+}η, K_{S}^{0}π^{+}π^{0}η, π^{+}π^{+}π^{-}η, and π^{+}π^{0}π^{0}η. Among these decays, the D^{0}→K^{-}π^{+}η and D^{+}→K_{S}^{0}π^{+}η decays have the largest branching fractions, which are B(D^{0}→K^{-}π^{+}η)=(1.853±0.025_{stat}±0.031_{syst})% and B(D^{+}→K_{S}^{0}π^{+}η)=(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.

Study of Open-Charm Decays and Radiative Transitions of the X(3872).

The processes X(3872)→D^{*0}D[over ¯]^{0}+c.c.,γJ/ψ,γψ(2S), and γD^{+}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)→D^{*0}D^{0}[over ¯]+c.c. and find evidence for X(3872)→γJ/ψ with statistical significances of 7.4σ and 3.5σ, respectively. No evident signals for X(3872)→γψ(2S) and γD^{+}D^{-} are found, and the upper limit on the relative branching ratio R_{γψ}≡{B[X(3872)→γψ(2S)]}/{B[X(3872)→γJ/ψ]}<0.59 is set at 90% confidence level. Measurements of branching ratios relative to decay X(3872)→π^{+}π^{-}J/ψ are also reported for decays X(3872)→D^{*0}D^{0}[over ¯]+c.c.,γψ(2S),γJ/ψ, and γD^{+}D^{-}, as well as the non-D^{*0}D^{0}[over ¯] three-body decays π^{0}D^{0}D^{0}[over ¯] and γD^{0}D^{0}[over ¯].

Observation of a Resonant Structure in e

A partial-wave analysis is performed for the process e^{+}e^{-}→K^{+}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^{-}→K^{+}K^{-}π^{0}π^{0}, as well as the Born cross sections for the subprocesses e^{+}e^{-}→ϕπ^{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^{-}→K^{+}K^{-}π^{0}π^{0} and ϕπ^{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^{-}→K_{1}^{+}(1270)K^{-} and K^{*+}(892)K^{*-}(892). The resonant parameters of the observed structure suggest it can be identified with the ϕ(2170), thus the results provide valuable input to the internal nature of the ϕ(2170).

Measurement of the Cross Section for e

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^{-}→Ξ^{-}Ξ[over ¯]^{+} by means of a single baryon-tag method. After performing a fit to the dressed cross section of e^{+}e^{-}→Ξ^{-}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)→Ξ^{-}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).

Measurement of Proton Electromagnetic Form Factors in e

The process of e^{+}e^{-}→pp[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^{-}→pp[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.

Observation of the Leptonic Decay D

We report the first observation of D^{+}→τ^{+}ν_{τ} with a significance of 5.1σ. We measure B(D^{+}→τ^{+}ν_{τ})=(1.20±0.24_{stat}±0.12_{syst})×10^{-3}. Taking the world average B(D^{+}→µ^{+}ν_{µ})=(3.74±0.17)×10^{-4}, we obtain R_{τ/µ}=Γ(D^{+}→τ^{+}ν_{τ})/Γ(D^{+}→µ^{+}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.