| This thesis is mainly constitutive of two analyses at BESIII:searches for lepton number violating decays?-?p(?+)e-e-and measurement of the branching fraction of electromagnetic Dalitz decay ?(3686)?e+e+?c.The standard model of particle physics has achieved great success since the establishment.However,there are many compelling reasons to believe that the standard model is not the ultimate theory.Searches for new physics beyond the standard model have been performed in various experiments with high energy or high luminosity.Lepton number violating decay is one of the most sensitive processes to new physics.All results from experiments will be translated into stringent conditions for theoretical models.The lepton number violating decay B1-?B2+l-l-(B=baryon,l=e,?)is a unique process,where the underlying mechanism at quark level is similar to that of neutrinoless double beta decay.In this decay,two down-type(d or s)quarks are converted into two up-quarks and the lepton number is changed by two units.Using a data sample of(1310.6±7.0)x 106 J/? events taken with the BESIII detector at the center-of-mass energy of 3.097 GeV,we search for the lepton number violating decays ?-?p(?+)e+e-for the first time.A double-tag method is used to suppress background events and reduce systematic uncertainties.The ?-candidates are tagged in J/???(1385)+?-decay.In the ?-??+e-e-channel,due to the limited phase space(M?--M?+(?)8 MeV/c2)and the small Lorentz boost of?-,the leptons have very small momenta and cannot be reconstructed in the detector.Therefore,the processes investigated in this analysis are?-?pe-e-and the rare inclusive decay?-??+X,where X represents any particle or particle combination.No obvious signal candidates beyond standard model are found,and the upper limits on the branching fractions at the 90%confidence level are determined to be:B(?-?pe-e-)<6.7×10-5 B(?-??+X)<1.2×10-4The electromagnetic Dalitz decays,V?Pl+l-,where V and P are vector meson and pseudo-scalar meson,and l denotes lepton,provide an ideal opportunity to probe the structure of hadronic states and to investigate the fundamental mechanisms of the interactions between photons and hadrons.Besides,considering that the dark photon U may couple to the standard model particles,V?PU?Pl+l-,the V?Pl+l-process has a chance to search for a dark photon with low mass.Now,there are few studies of electromagnetic Dalitz transition between heavy vector meson and heavy pseudo-scalar meson.Therefore,we perform a theoretical study on the ?((?))??c(?b)l+l-decays and the partial decay widths are determined under the standard model.The partial decay width of ?(3686)?e+e-?c is estimated to be(8.9±1.3)× 10-3 keV.The polarization of ?(T)has also been investigated.At BES?,the ? produced from e+e-collision is totally polarized in the transverse state,so that the angular distribution of P would have the form of 1+cos2 ?,where ? is the polar angle of P in the rest frame of V.With a data sample of(448.1±.9)× 106 ?(3686)events taken with BESIII detector at the center-of-mass energy of 3.686 GeV,the branching fraction of electromagnetic Dalitz decay ?(3686)?e+e-?c is measured for the first time.To improve the statistical significance,we just reconstruct the lepton pair and investigate the recoiling mass of the lepton pair to obtain the signal yields.The branching fraction is determined to be:B(?(3686)?e+e-?c)=(3.79±0.47±0.25)×10-5 where the uncertainties are statistical and systematic uncertainties,respectively.And the statistical significance is more than 9 standard deviations.The result is consistent with the theoretical prediction.Due to the high background in the distribution of e+einvariant mass,we do not perform a search for the dark photon. |
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