Tracker Performance And The B_c→τν_τ Analysis At The CEPC

The proposed Circular Electron Positron Collider(CEPC)is a next generation particle collider intended to perform high precision measurements of the Standard Model(SM)and explore rare processes and new physics that are beyond the capabilities of existing facilities.The physics objectives put high requirements for the detector’s design and optimization.We study the performances of the CEPC baseline detector on the track reconstructions and the flavor-physics.We begin with the effectiveness of the Silicon External Tracker(SET)in the single muon track reconstructions and the dimuon Higgs mass reconstructions,and conclude the SET significantly improves various resolution at high energies and is essential in meeting the momentum resolution requirement.We then reconstruct the KS0 and Λ with their fully-charged decay products.The KS0 and A are important physics signatures for many flavor-physics measurements and are abundantly produced in the CEPC Z-pole program.They are also long lived particles which need secondary vertex reconstructions.Therefore,they are suitable physics processes to demonstrate the track reconstruction performances.The results show the CEPC baseline detector reaches inclusive efficiency/purity of 40.6%/92.4%and 27.3%/86.4%,and mass resolution of 1.20/0.40(0.24%/0.036%)MeV for KS0 and A,respectively.We also quantify the potential improvements,achieved by improving the low transverse momentum track reconstructions and implementing particle identification capabilities in ideal conditions.Finally we conduct the first dedicated study of flavor-physics performance potentials of CEPC with Bc→τντ,where excellent track reconstructions are essential.The precise determination of the Bc→τντ branching ratio provides an advantageous opportunity for understanding the electroweak structure of the SM,measuring the CKM(Cabibbo-Kobayashi-Maskawa)matrix element |Vcb|and probing new physics models.We discuss the potential of measuring the processes of Bc→τντ with τ decaying leptonically at the CEPC.We conclude that during the Z-pole operation,the channel signal can achieve five σ significance with～109 Z decays,and the signal strength accuracies for Bc→τντ can reach around 1%level at the nominal CEPC Z-pole statistics of one trillion Z decays assuming the total Bc→τντyield is 3.6×106.Our theoretical analysis indicates the accuracy could provide a strong constraint on the general effective Hamiltonian for the b→cτν transition.If the total Bc yield can be determined to O(1%)level of accuracy in the future,these results also imply |Vcb| could be measured up to O(1%)level of accuracy.