Search For And Study Of Exotic Hadrons At BES?

Since the Quantum Chromodynamics was build,the physics problems at per-turbation region can be solved perfectly,but it do not work well for the physical phenomena outside perturbation region,and more seriously it lacks rules on how the hadrons are formed by quarks.Many physicists have been trying to search for new hadron structure besides meson and baryon for half century to understand the QCD and the physical world better.The BESIII detector,which is working on Beijing electron-positron collider,is build to study the physics in tau-charm sector and this sector is located in transition region between perturbation and non-perturbation regions in QCD.On the other hand,most of the exotic hadron candidates can be produced in this sector,so it is extremely important for under-standing QCD to study the hadrons' decay and produced mechanism and search for new hadron structure in this sector.Up to now,several good candidates for exotic hadrons,such as X(3872),Y(4140),Y(4260),Y(2175)and D*sO(2317),have been observed by abnormal de-cays in some collider experiments.All of them have obvious unconventional char-acters compared to normal meson or baryon,so people believe some of them have new structures and developed may theoretical models to explain their existences,but it is still far from understanding their natures due to lack of experimental information.In this thesis,we used the electron positron annihilation data above 3.7 GeV to search for and study these states at BESIII,including search for their new produced and decay modes,and more importantly their relations,to provide key information to understand these states.Using the data collected with BESIII detector around center-of-mass energy(?)=4.26 GeV,we observed the famous exotic hadron candidate X(3872)via the process of e+e-???+?-(?)J/? with a statistical significance larger than 10?for the first time.The mass of X(3872)is measured as 3872.6±0.5±0.2 MeV/c2,here the uncertainties are statistical and systematic,respectively.The Born cross sections at each energy points are measured and the result indicates that the X(3872)is very likely to come from the radiative transition of Y(4260)with equivalent branching fraction of Y(4260)??+?-J/?.It is the first time that the two exotic hadron candidates,X(3872)and Y(4260),are connected,which means they should have similar structures.Besides B decays and hadron collision,this process is the new produced mode of X(3872),and is the second decay mode of Y(4260)which has been observed.After the observation of X(3872)at BESIII,we tried to use the data samples to search for another exotic hadron candidate Y(4140)through e+e-???J/?process,since the Y(4140)may has same type of structure with X(3872).The three decay modes of ?,K+K-,KS0KD0 and ?+?-?0 are analyzed separately,and no obvious signal can be observed from the combined result.We estimated the upper limits of cross sections at 90%confidence level as 0.35,0.38 and 0.33 pb at 4.23,4.26 and 4.36 GeV,respectively.Using the cross sections of ?X(3872)and the calculation of the partial decay width ?(Y(4140)? ?J/?)in molecule figure,we can get the ratio of cross sections a(e+e-??Y(4140))/?(e+e-??X(3872))around 0.1 or even smaller at 4.23 and 4.26 GeV.The famous exotic hadron candidate in strangeonium sector,Y(2175),has similar property with Y(4260),we observed the process of e+e-? ?Y(2175)using the data above 3.7 GeV for the first time,the joint statistical signifi-cance from all the data is larger than 10?.The mass and width of Y(2175)are measured to be 2135 ± 8 ± 9 MeV/c2 and 104 ± 24 ± 12 MeV,respectively,here the first uncertainties are statistical and second one systematic.From the distribution of cross sections,no sign of any vector charmonium or charmoni-umlike states can be observed,which is consist with the expectation of vector-pseudoscalar processes.The contribution of ?(3686)is searched for with a sample of(447.9 ± 2.9)x 106 ?(3686)events,no obvious signal can be observed and the ratio of branching fractions between ?(3686)and J/? decays is estimated as B*(?(3686)? ?Y(2175))/B*(J/? ??Y(2175))=(0.23 ± 0.29 ± 0.13)%after considering the phase space difference.Huge suppression is observed compared to 12%rule,and it is a new vector-pseudoscalar mode which fails this rule.We searched for the similar process e+e-??'Y(2175)with same data sample,and no obvious signal is observed,the upper limit of ratio between cross sections?(e+e-??'Y(2175))/a(e+e-??Y(2175))is estimated as 0.43 at 90%confi-dence level.Besides the quarkonium,the exotic hadron candidates appear in charm me-son sector too and the most famous one is Ds0*(2317).We observed the signal of Ds0*(2317)in the recoil mass spectrum of Ds*using the data sample at 4.6 GeV with a integrated luminosity of 566.93 pb-1 for the first time.The mass of Ds0*(2317)is measured to be(2318.3 ±1.2 ± 1.2)MeV/c2,and the statistical significance is 5.8?.The absolute branching fract,ion B(Ds0*(2317)±??0Ds± is measured as 1.00-0.14±0.14 for the first t.ime using a partial reconstruction technique of Ds0*(2317)in the recoil side of Ds*,here the first uncertainty is statistical and second one systematic.The result shows that the ?0Ds mode is overwhelming-ly dominated the Ds0*(2317)decays,which is different with the expectation of conventional cs hypothesis while consist with the calculation in molecule figure.Above the open-charm threshold,all the vector charmonium states are expect-ed to decay into a pair of charm mesons dominantly.So with the precise cross sections of open-charm channels,we can hunt every conventional vector charmo-nium states and the exotic hadron candidates can be excluded or confirmed in this region.We measured the cross section of e+e-? Ds+Ds-precisely using all the data above Ds+Ds-threshold at BES?,including more than 100 data samples at different centre-of-mass energies up to 4.6 GeV.The Ds+ and Ds-are reconstructed and analyzed separately,and combined together.From the result of the combined cross sections,we observe strange structure around 4.23 GeV.This structure could be caused by the interference between Y(4260)and other charmonium state.