Search For Exotic Hadronic States And Charmed Baryon Decay At BESⅢ

Quantum chromodynamics(QCD)is the basic theory for describing strong interactions.The property of "asymptotic freedom" for QCD theory at high energy has b een confirmed by a large number of experiments and allows systematic calculations in QCD using perturbation theory.In the low-energy region,the QCD theoretical method based on perturbation theory is ineffective due to the strong coupling effect between quarks and gluons.Among them,the mechanism of "quark confinement" is one of the most important research topics in particle physics for a long time.Experimentally,hadron spectroscopy has established a link among experimental observables and the effective freedom of QCD in the non-perturbative region,which is important for the development of phenomenological models and the study of non-perturbative QCD theory.Meanwhile,studying of charmed baryon provide an excellent laboratory for understanding the nonperturbative QCD dynamices and for testing the validity of theoretical modes.Experimentally,progress in the investigations of Ac+ decay has been relatively slow due to the lack of experimental data in recent decades,especially for Cabibbo-suppressed decays whose branching fractions are usually smaller than 10-3.Therefore,further precise measurements of the branching fractions of Ac+hadronic weak decays are eagerly sought in order to confront theory.Moreover,experimental measurements can also be taken as input to constrain these phenomenological models,as they quantify the non-factorizable effects,and thus will help to improve our understanding of the dynamics of charmed baryons.We report a branching fraction measurement of the singly Cabibbo-suppressed decay Λc+→ΛK+ using a data sample collected with the BESⅢ detector at the BEPCII storage ring.The data span center-of-mass energies from 4.599 to 4.950 GeV and correspond to an integrated luminosity of 6.44 fb-1.The branching fraction of Λc+→ΛK+ relative to that of the Cabibbo-favored decay Λc+→Λπ+ is measured to be R=B(Λc+→Λk+)/B(Λc+→Λπ+)=(4.78 ± 0.34 ± 0.20)%.Combining with the world-average value of V(Λc+→Λπ+),we obtain B(Λc+→ΛK+)=(6.21 ± 0.44 ± 0.26 ± 0.34)× 10-4.Here the first uncertainties are statistical,the second systematic,and the third comes from the uncertainty of theΛc+→Λπ+ branching fraction.This result,which is more precise than previous measurements,does not agree with theoretical predictions,and suggests that non-factorizable contributions have been under-estimated in current models.The measured branching fraction of Λc+→ΛK+ is significantly lower(～40%)than the predictions based on the SU(3)flavor symmetry,constituent quark model,or current algebra.As the pure factorizable contribution is reliably calculated for the relative branching fraction(Rfac=(7.43±0.14)%),we determine the contribution from the non-factorizable effect to be Rnon-fac=R-Rfac=-(2.65±0.42)%,which is negative and has a size comparable to the factorizable contribution.This indicates that the non-factorizable contributions in Λc+ decay are important and have been significantly under-estimated in current theoretical models.In the conventional quark model,hadrons are made up of two or three quarks.Whereas,QCD allows the existence of new types of hadrons,such as glueballs,hybrids and multi-quark states.The radiative decays of the J/Ψ meson provide such a gluon-rich environment and are regarded as one of the most promising hunting grounds for glueballs.Glueballs have nonexotic quantum numbers,hence will be mixed into the conventional meson spectrum and difficult to be identified experimentally.It requires systematic studies to identify a glueball by searching for outnumbering of conventional quark model states and comparing a candidates properties to the expected properties of glueballs and conventional mesons.The flavor filter reactions J/Ψ→γX,X→γφ could play an important role in unraveling the quark content of intermediate resonance X.It serves as a prob and helps us to understand the properties of the intermediate resonance states.Based on a sample of(10.09±0.04)× 109 J/Ψ events collected with the BESⅢdetector,a partial wave analysis of J/Ψ→γγφ is performed by using covariant tensor approach for the first time.The η(1475),X(1835),f1(1420),f2(1525),f2(2010),f1(1285),f1(1510),f2(1950),fo(2200)and ηc decaying into γφ have been observed with significance larger than 5σ.The structure with a mass around 1.4 GeV in γφ system are come from the contribution of η(1475)and f1(1420).The mass,width and product of BRs for the η(1475)are measured to be M=1422.0 ± 2.1-7.1+4.3 MeV/c2,Γ=86.3±2.7-17.1+3.7 MeV and B(J/Ψ→γη(1475)→γγφ)=(3.57 ±0.18-0.55+0.51)×10-6,respectively.The existence of the X(1835)is also confirmed with JPC=0-+.The mass,width and product of BRs of X(1835)are determined to be 1849.3±3.0-10.0+5.4 MeV/c2,179.6±8.7-24.9+15.9 MeVO,the B(J/Ψ→γX(1835)→γγφ)=(3.37 ± 0.19-1.00+0.59)×10-6.For the first time,the decay pattern of ηc→γφ is observed in γφ system,yielding a product branching fraction of B(J/Ψ→γηc→γγφ)=2.08±0.26-0.62+0.32×10-7.This measurement provides new insights into the decay characteristics ofηc.No significant signal for η(1295),η(1405),η1(1855)and X(2370)are observed in γφ invariant-mass distribution.The upper limits for product branching fractions at 90%C.L.are determined to be B(J/Ψ→γη(1295)→γγφ)<2.8 × 10-7,B(J/Ψ→γη(1405)→γγφ)<3.5 × 10～-8,B(J/Ψ→γX(2370)→γγφ)<1.5 × 10-7,B(J/Ψ→γη1(1855)→γγφ)<4.7× 10-6.