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Institute
By analyzing (448.1Β±2.9)Γ106ββπβ‘(3686) events collected with the BESIII detector operating at the BEPCII collider, the decays of ππβ’π½βΞβ’
Β―Ξβ’π (π½=0, 1, and 2) are observed for the first time with statistical significances of 13.9β’π, 6.7β’π, and 8.2β’π, respectively. The product branching fractions of πβ‘(3686)βπΎβ’ππβ’π½ and ππβ’π½βΞβ’Β―Ξβ’π are measured. Dividing by the world averages of the branching fractions of πβ‘(3686)βπΎβ’ππβ’π½, the branching fractions of ππβ’π½βΞβ’Β―Ξβ’π decays are determined to be (2.31Β±0.30Β±0.21)Γ10β4, (5.86Β±1.38Β±0.68)Γ10β5, and (1.05Β±0.21Β±0.15)Γ10β4 for π½=0, 1 and 2, respectively, where the first uncertainties are statistical and the second systematic.
The radiative hyperon decay ΞβnΞ³ is studied using (10087Β±44)Γ106 J/Ο events collected with the BESIII detector operating at BEPCII. The absolute branching fraction of the decay ΞβnΞ³ is determined with a significance of 5.6Ο to be [0.832Β±0.038(stat.)Β±0.054(syst.)]Γ10β3, which lies significantly below the current PDG value. By analyzing the joint angular distribution of the decay products, the first determination of the decay asymmetry Ξ±Ξ³ is reported with a value of β0.16Β±0.10(stat.)Β±0.05(syst.).
Search for hidden-charm tetraquark with strangeness in eβΊeβ» β KβΊD*β»β D*β° + c.c.
(2023)
We report a search for a heavier partner of the recently observed Zcs(3985)β state, denoted as Zβ²βcs, in the process e+eββK+DββsDβ0+c.c., based on e+eβ collision data collected at the center-of-mass energies of sβ=4.661, 4.682 and 4.699 GeV with the BESIII detector. The Zβ²βcs is of interest as it is expected to be a candidate for a hidden-charm and open-strange tetraquark. A partial-reconstruction technique is used to isolate K+ recoil-mass spectra, which are probed for a potential contribution from Zβ²βcsβDββsDβ0 (c.c.). We find an excess of Zβ²βcsβDββsDβ0 (c.c.) candidates with a significance of 2.1Ο, after considering systematic uncertainties, at a mass of (4123.5Β±0.7stat.Β±4.7syst.) MeV/c2. As the data set is limited in size, the upper limits are evaluated at the 90\% confidence level on the product of the Born cross sections (ΟBorn) and the branching fraction (B) of Zβ²βcsβDββsDβ0, under different assumptions of the Zβ²βcs mass from 4.120 to 4.140 MeV and of the width from 10 to 50 MeV at the three center-of-mass energies. The upper limits of ΟBornβ
B are found to be at the level of O(1) pb at each energy. Larger data samples are needed to confirm the Zβ²βcs state and clarify its nature in the coming years.
Search for hidden-charm tetraquark with strangeness in eβΊeβ» β KβΊD*β»β D*β° + c.c.
(2022)
We report a search for a heavier partner of the recently observed Zcs(3985)β state, denoted as Zβ²βcs, in the process e+eββK+DββsDβ0+c.c., based on e+eβ collision data collected at the center-of-mass energies of sβ=4.661, 4.682 and 4.699 GeV with the BESIII detector. The Zβ²βcs is of interest as it is expected to be a candidate for a hidden-charm and open-strange tetraquark. A partial-reconstruction technique is used to isolate K+ recoil-mass spectra, which are probed for a potential contribution from Zβ²βcsβDββsDβ0 (c.c.). We find an excess of Zβ²βcsβDββsDβ0 (c.c.) candidates with a significance of 2.1Ο, after considering systematic uncertainties, at a mass of (4123.5Β±0.7stat.Β±4.7syst.) MeV/c2. As the data set is limited in size, the upper limits are evaluated at the 90\% confidence level on the product of the Born cross sections (ΟBorn) and the branching fraction (B) of Zβ²βcsβDββsDβ0, under different assumptions of the Zβ²βcs mass from 4.120 to 4.140 MeV and of the width from 10 to 50 MeV at the three center-of-mass energies. The upper limits of ΟBornβ
B are found to be at the level of O(1) pb at each energy. Larger data samples are needed to confirm the Zβ²βcs state and clarify its nature in the coming years.
Using about 23 fbβ1 of data collected with the BESIII detector operating at the BEPCII storage ring, a precise measurement of the e+eββΟ+ΟβJ/Ο Born cross section is performed at center-of-mass energies from 3.7730 to 4.7008 GeV. Two structures, identified as the Y(4220) and the Y(4320) states, are observed in the energy-dependent cross section with a significance larger than 10Ο. The masses and widths of the two structures are determined to be (M,Ξ) = (4221.4Β±1.5Β±2.0 MeV/c2, 41.8Β±2.9Β±2.7 MeV) and (M,Ξ) = (4298Β±12Β±26 MeV/c2, 127Β±17Β±10 MeV), respectively. A small enhancement around 4.5 GeV with a significance about 3Ο, compatible with the Ο(4415), might also indicate the presence of an additional resonance in the spectrum. The inclusion of this additional contribution in the fit to the cross section affects the resonance parameters of the Y(4320) state.
Using about 23 fbβ1 of data collected with the BESIII detector operating at the BEPCII storage ring, a precise measurement of the e+eββΟ+ΟβJ/Ο Born cross section is performed at center-of-mass energies from 3.7730 to 4.7008 GeV. Two structures, identified as the Y(4220) and the Y(4320) states, are observed in the energy-dependent cross section with a significance larger than 10Ο. The masses and widths of the two structures are determined to be (M,Ξ) = (4221.4Β±1.5Β±2.0 MeV/c2, 41.8Β±2.9Β±2.7 MeV) and (M,Ξ) = (4298Β±12Β±26 MeV/c2, 127Β±17Β±10 MeV), respectively. A small enhancement around 4.5 GeV with a significance about 3Ο, compatible with the Ο(4415), might also indicate the presence of an additional resonance in the spectrum. The inclusion of this additional contribution in the fit to the cross section affects the resonance parameters of the Y(4320) state.
By analyzing an electron-positron collision data sample corresponding to an integrated luminosity of 2.93ββfbβ1 taken at the center-of-mass energy of 3.773 GeV with the BESIII detector, we obtain for the first time the absolute branching fractions for seven π·0 and π·+ hadronic decay modes and search for the hadronic decay π·0βπΎ0πβ’πΎ0πβ’π0 with much improved sensitivity. The results are β¬β‘(π·0βπΎ0πβ’π0β’π0β’π0)=(7.64Β±0.30Β±0.29)Γ10β3, β‘(π·0βπΎββ’π+β’π0β’π0β’π0)=9.54Β±0.30Β±0.31)Γ10β3, β¬β‘(π·0βπΎ0πβ’π+β’πββ’π0β’π0)=(12.66Β±0.45Β±0.43)Γ10β3, β¬β‘(π·+βπΎ0πβ’π+β’π0β’π0)=(29.04Β±0.62Β±0.87)Γ10β3, β¬β‘(π·+βπΎ0πβ’π+β’π+β’πββ’π0)=(15.28Β±0.57Β±0.60)Γ10β3, β¬β‘(π·+βπΎ0πβ’π+β’π0β’π0β’π0)=(5.54Β±0.44Β±0.32)Γ10β3, β¬β‘(π·+βπΎββ’π+β’π+β’π0β’π0)=(4.95Β±0.26Β±0.19)Γ10β3, and β¬β‘(π·0βπΎ0πβ’πΎ0πβ’π0)<1.45Γ10β4 at the 90% confidence level. Here, the first uncertainties are statistical, and the second ones are systematic. The newly studied decays greatly enrich the knowledge of the π·βΒ―πΎβ’πβ’πβ’π and π·βΒ―πΎβ’πβ’πβ’πβ’π hadronic decays and open a bridge to access more two-body hadronic π· decays containing scalar, vector, axial, and tensor mesons in the charm sector.
First observation of ΞcβΊβββΞKβΊΟβ° and evidence of ΞcβΊβββΞKβΊΟβΊΟβ»
(2024)
We present the first observation of the singly Cabibbo-suppressed decay Ξ+cβΞK+Ο0 with a significance of 5.7Ο and the first evidence of Ξ+cβΞK+Ο+Οβ decay with a significance of 3.1Ο, based on e+eβ annihilation data recorded by the BESIII detector at the BEPCII collider. The data correspond to an integrated luminosity of 6.4 fbβ1, in the center-of-mass energy range from 4.600 GeV to 4.950 GeV. We determine the branching fractions of Ξ+cβΞK+Ο0 and Ξ+cβΞK+Ο+Οβ relative to their Cabibbo-favored counterparts to be B(Ξ+cβΞK+Ο0)B(Ξ+cβΞΟ+Ο0)=(2.09Β±0.39stat.Β±0.07syst.)Γ10β2 and B(Ξ+cβΞK+Ο+Οβ)B(Ξ+cβΞΟ+Ο+Οβ)=(1.13Β±0.41stat.Β±0.06syst.)Γ10β2, respectively. Moreover, by combining our measured result with the world average of B(Ξ+cβΞΟ+Ο0), we obtain the branching fraction B(Ξ+cβΞK+Ο0)=(1.49Β±0.27stat.Β±0.05syst.Β±0.08ref.)Γ10β3. This result significantly departs from theoretical predictions based on quark SU(3) flavor symmetry, which is underpinned by the presumption of meson pair S-wave amplitude dominance.
Based on (2712.4Β±14.3)Γ106 Ο(3686) events, we investigate four hadronic decay modes of the P-wave charmonium spin-singlet state hc(1P1)βh+hβΟ0/Ξ· (h=Ο or K) via the process Ο(3686)βΟ0hc at BESIII. The hcβΟ+ΟβΟ0 decay is observed with a significance of 9.6Ο after taking into account systematic uncertainties. Evidences for hcβK+KβΟ0 and hcβK+KβΞ· are found with significances of 3.5Ο and 3.3Ο, respectively, after considering the systematic uncertainties. The branching fractions of these decays are measured to be B(hcβΟ+ΟβΟ0)=(1.36Β±0.16Β±0.14)Γ10β3, B(hcβK+KβΟ0)=(3.26Β±0.84Β±0.36)Γ10β4, and B(hcβK+KβΞ·)=(3.13Β±1.08Β±0.38)Γ10β4, where the first uncertainties are statistical and the second are systematic. No significant signal of hcβΟ+ΟβΞ· is found, and the upper limit of its decay branching fraction is determined to be B(hcβΟ+ΟβΞ·)<4.0Γ10β4 at 90% confidence level.
Evidence for the singly Cabibbo suppressed decay Ξ+cβpΟ0 is reported for the first time with a statistical significance of 3.7Ο based on 6.0 fbβ1 of e+eβ collision data collected at center-of-mass energies between 4.600 and 4.843 GeV with the BESIII detector at the BEPCII collider. The absolute branching fraction of Ξ+cβpΟ0 is measured to be (1.56+0.72β0.58Β±0.20)Γ10β4. Combining with the branching fraction of Ξ+cβnΟ+, (6.6Β±1.2Β±0.4)Γ10β4, the ratio of the branching fractions Ξ+cβnΟ+ and Ξ+cβpΟ0 is calculated to be 4.2+2.2β1.9; this is an important input for the understanding of the decay mechanisms of charmed baryons. In addition, the absolute branching fraction of Ξ+cβpΞ· is measured to be (1.63Β±0.31statΒ±0.11syst)Γ10β3, which is consistent with previous measurements.
Using a sample of (10087Β±44)Γ106ββπ½/π events, which is about 45 times larger than that was previously analyzed, a further investigation on the π½/πβπΎβ’3β’(π+β’πβ) decay is performed. A significant distortion at 1.84ββGeV/π2 in the line shape of the 3β’(π+β’πβ) invariant mass spectrum is observed for the first time, which could be resolved by two overlapping resonant structures, πβ‘(1840) and πβ‘(1880). The new state πβ‘(1880) is observed with a statistical significance larger than 10β’π. The mass and width of πβ‘(1880) are determined to be 1882.1Β±1.7Β±0.7ββMeV/π2 and 30.7Β±5.5Β±2.4ββMeV, respectively, which indicates the existence of a πβ’Β― π bound state.
We report new measurements of the cross sections for the production of Dbar D final states at the Ο(3770) resonance. Our data sample consists of an integrated luminosity of 2.93 fbβ1 of e+eβ annihilation data produced by the BEPCII collider and collected and analyzed with the BESIII detector. We exclusively reconstruct three D0 and six D+ hadronic decay modes and use the ratio of the yield of fully reconstructed Dbar D events ("double tags") to the yield of all reconstructed D or bar D mesons ("single tags") to determine the number of D0bar D0 and D+Dβ events, benefiting from the cancellation of many systematic uncertainties. Combining these yields with an independent determination of the integrated luminosity of the data sample, we find the cross sections to be Ο(e+eβ β D0bar D0) nb and Ο(e+eβ β D+Dβ) = (2.830 Β± 0.011 Β± 0.026) nb, where the uncertainties are statistical and systematic, respectively.
Using a data sample of π+β’πβ collisions corresponding to an integrated luminosity of 567ββpbβ1 collected at a center-of-mass energy of βπ =4.6ββGeV with the BESIII detector, we measure the absolute branching fraction of the inclusive semileptonic Ξ+π decay with a double-tag method. We obtain β¬β‘(Ξ+πβπβ’π+β’ππ)=(3.95Β±0.34Β±0.09)%, where the first uncertainty is statistical and the second systematic. Using the known Ξ+π lifetime and the charge-averaged semileptonic decay width of nonstrange charmed mesons (π·0 and π·+), we obtain the ratio of the inclusive semileptonic decay widths Ξβ‘(Ξ+πβπβ’π+β’ππ)/Β―Ξβ‘(π·βπβ’π+β’ππ)=1.26Β±0.12.
Study of the process eβΊeβ»βββΟβ°Οβ°J/Ο and neutral charmoniumlike state Zc(3900)β°
(2020)
Cross sections of the process π+β’πββπ0β’π0β’π½/π at center-of-mass energies between 3.808 and 4.600 GeV are measured with high precision by using 12.4ββfbβ1 of data samples collected with the BESIII detector operating at the BEPCII collider facility. A fit to the measured energy-dependent cross sections confirms the existence of the charmoniumlike state πβ‘(4220). The mass and width of the πβ‘(4220) are determined to be (4220.4Β±2.4Β±2.3) MeV/π2 and (46.2Β±4.7Β±2.1)ββMeV, respectively, where the first uncertainties are statistical and the second systematic. The mass and width are consistent with those measured in the process π+β’πββπ+β’πββ’π½/π. The neutral charmonium-like state ππβ’(3900)0 is observed prominently in the π0β’π½/π invariant-mass spectrum, and, for the first time, an amplitude analysis is performed to study its properties. The spin-parity of ππβ’(3900)0 is determined to be π½π=1+, and the pole position is (3893.1Β±2.2Β±3.0)βπβ’(22.2Β±2.6Β±7.0)ββMeV/π2, which is consistent with previous studies of electrically charged ππβ’(3900)Β±. In addition, cross sections of π+β’πβ β π0β’ππβ’(3900)0 β π0β’π0β’π½/π are extracted, and the corresponding line shape is found to agree with that of the πβ‘(4220).
The process π+β’πββπβ’πβ² has been studied for the first time in detail using data sample collected with the BESIII detector at the BEPCII collider at center of mass energies from 2.05 to 3.08 GeV. A resonance with quantum numbers π½πβ’πΆ=1ββ is observed with mass π=(2177.5Β±4.8β’(stat)Β±19.5β’(syst))β’MeV/π2 and width Ξ=(149.0Β±15.6β’(stat)Β±8.9β’(syst))ββMeV with a statistical significance larger than 10β’π, including systematic uncertainties. If the observed structure is identified with the πβ‘(2170), then the ratio of partial width between the πβ’πβ² by BESIII and πβ’π by BABAR is (β¬π
πβ’πβ’Ξπ
πβ’π)/(β¬π
πβ’πβ²β’Ξπ
πβ’π)=0.23Β±0.10β’(stat)Β±0.18β’(syst), which is smaller than the prediction of the π β’Β―π β’π hybrid models by several orders of magnitude.
By analyzing a data sample corresponding to an integrated luminosity of 2.93ββfbβ1 collected at a center-of-mass energy of 3.773 GeV with By analyzing a data sample corresponding to an integrated luminosity of 2.93ββfbβ1 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 π·+βπβ’π+β’ππ decay to be β¬π·+βπβ’π+β’ππ=(10.4Β±1.β’0statΒ±0.β’5syst)Γ10β4. Using the world averaged value of β¬π·+βπβ’π+β’ππ, the ratio of the two branching fractions is determined to be β¬π·+βπβ’π+β’ππ/β¬π·+βπβ’π+β’ππ=0.91Β±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 ππ+β‘(0) and the πβπ Cabibbo-Kobayashi-Maskawa matrix element |ππβ’π| to be ππ+β‘(0)β’|ππβ’π|=0.087Β±0.00β’8statΒ±0.00β’2syst. Taking the input of |ππβ’π| from the global fit in the standard model, we determine ππ+β‘(0)=0.39Β±0.0β’4statΒ±0.0β’1syst. On the other hand, using the value of ππ+β‘(0) calculated in theory, we find |ππβ’π| = 0.242Β±0.02β’2statΒ±0.00β’6systΒ±0.03β’3theory.
We report the first observation of the semimuonic decay π·+βπβ’π+β’ππ using an π+β’πβ collision data sample corresponding to an integrated luminosity of 2.93ββfbβ1 collected with the BESIII detector at a center-of-mass energy of 3.773 GeV. The absolute branching fraction of the π·+βπβ’π+β’ππ decay is measured to be β¬π·+βπβ’π+β’ππ=(17.7Β±1.β’8statΒ±1.β’1syst)Γ10β4. Its ratio with the world average value of the branching fraction of the π·+βπβ’π+β’ππ decay probes lepton flavor universality and it is determined to be β¬π·+βπβ’π+β’ππ/β¬PDG π·+βπβ’π+β’ππ=1.05Β±0.14, in agreement with the standard model expectation within one standard deviation.
We study the electromagnetic Dalitz decay π½/πβπ+β’πββ’π and search for dielectron decays of a dark gauge boson (πΎβ²) in π½/πβπΎβ²β’π with the two π decay modes πβπΎβ’πΎ and πβπ+β’πββ’π0 using (1310.6Β±7.0)Γ106ββπ½/π events collected with the BESIII detector. The branching fraction of π½/πβπ+β’πββ’π is measured to be (1.43Β±0.04β’(stat)Β±0.06β’(syst))Γ10β5, with a precision that is improved by a factor of 1.5 over the previous BESIII measurement. The corresponding dielectron invariant mass dependent modulus square of the transition form factor is explored for the first time, and the pole mass is determined to be Ξ=2.84Β±0.11β’(stat)Β±0.08β’(syst)ββGeV/π2. We find no evidence of πΎβ² production and set 90% confidence level upper limits on the product branching fraction β¬β‘(π½/πβπΎβ²β’π)Γβ¬β‘(πΎβ²βπ+β’πβ) as well as the kinetic mixing strength between the standard model photon and πΎβ² in the mass range of 0.01β€ππΎβ²β€2.4ββGeV/π2.
Search for heavy Majorana neutrino in lepton number violating decays of DβββKΟeβΊeβΊ
(2019)
Using a data sample with an integrated luminosity of 2.93ββfbβ1 taken at the center-of-mass energy of 3.773 GeV, we search for the Majorana neutrino (ππ) in the lepton number violating decays π·βπΎβ’πβ’π+β’π+. No significant signal is observed, and the upper limits on the branching fraction at the 90% confidence level are set to be β¬β‘(π·0βπΎββ’πββ’π+β’π+)<2.8Γ10β6, β¬β‘(π·+βπΎ0πβ’πββ’π+β’π+)<3.3Γ10β6 and β¬β‘(π·+βπΎββ’π0β’π+β’π+)<8.5Γ10β6. The Majorana neutrino is searched for with different mass assumptions ranging from 0.25 to 1.0ββGeV/π2 in the decays π·0βπΎββ’π+β’ππ,ππβπββ’π+ and π·+βπΎ0πβ’π+β’ππ,ππβπββ’π+, and the upper limits on the branching fraction at the 90% confidence level are at the level of 10β7βΌ10β6, depending on the mass of the Majorana neutrino. The constraints on the mixing matrix element |ππβ’ππ|2 are also evaluated.