Angular distribution coefficients of Z(W) boson produced in e^+e^- collisions at s^(1/2) = 240 GeV

At the designed circular electron-positron collider(CEPC), similar to the hadron collider, the angular distribution coefficients of the decay lepton pair from the produced Z(W) boson in e^+e^- collisions are predicted. Their dependence on cosθZ(cosθW) are presented in four different polarization frames. Furthermore, the value of the angular distribution coefficients in different bins of cosθZ are presented in the C-S frame. In comparison with the case at the hadron collider, better accurate measurement for Z(W) is expected because there is less background, and W could be reconstructed from its leptonic decay channel. This works provides a method to precisely test the electroweak production mechanism or some effect induced from new physics in the future measurements at the CEPC.

Perturbative QCD for J/ψ inclusive production via initial state radiation in e^+e^- collisions

The process e+e-→J/ψ+X with the center-of-mass(CM)energy in the range from 3.7 to 10.6 GeV is calculated up to the next-to-leading order(NLO)in quantum chromodynamics(QCD).At 10.6 GeV,the result is consistent with the experimental result from Belle.However,the predictions are much smaller than the background in the measurements at BESIII in the low CM energy range from 3.7 to 4.6 GeV.This indicates that the convergence of the QCD perturbative expansion becomes worse as the CM energy is closer to the inclusive J/ψ production threshold.For a further study of the QCD mechanism of J/ψ production in e^+e^-collisions with different CM energies,the initial state radiation effect of e^+e^-→J/ψ +gg and e^+e^-→J/ψ+cc are calculated in QCD NLO.The results are plotted and the number of events for different CM energy bins are provided for SuperKEKB.This provides a method to precisely test the validity of perturbative predictions for J/ψ production in future measurements.

New approach for amplitudes with multiple fermion lines

A new approach for tree-level amplitudes with multiple fermion lines is presented.It primarily focuses on the simplification of fermion lines.By calculating two vectors recursively without any matrix multiplications,the result of a fermion line is reduced to a very compact form depending only on the two vectors.Comparisons with other packages are presented,and the results show that our package FDC provides a very good performance in the processes of multiple fermion lines with this new approach and some other improvements.A further comparison with WHIZARD shows that this new approach has a competitive efficiency in computing pure amplitude squares without phase space integration.

Inclusive γ(1S,2S,3S) photoproduction at the CEPC

Inclusive γ(1S,2S,3S) photoproduction at the future Circular-Electron-Positron-Collider(CEPC)is studied,using the non-relativistic quantum chromodynamics(NRQCD)factorization formalism.Including the contributions from both direct and resolved photons,we present different distributions for the γ(1S,2S,3S) production.Our results suggest that there will be considerable events,implying that well measurements of the T photoproduction can be performed to further study heavy quarkonium physics at electron-positron colliders,in addition to hadron colliders.This supplemental study is very important for clarifying the current situation regarding the heavy quarkonium production mechanism.

Study of the rare decay J/ψ→2γ+hadrons at the BESIII

The two-photon radiative decay process J/ψ→2γ+hadrons was studied,and the main contribution processes,J/ψ→2γ+ggg and J/ψ→2γ+qˉq,were calculated.With the specific conditions at the BESIII,this rare decay process and the main background process e+e?→γγ+hadrons(qˉq)were investigated.The results show that the ratio of signal to background can reach 1.24 with optimized selection criteria at the BESIII.In addition,distributions of the signal and background are presented.All the results show that the signal is large enough to be experimentally measured.

Complete study on polarization of γ(nS) hadroproduction at QCD next-to-leading order

By applying the nonrelativistic quantum chromodynamics factorization formalism toγ(1S.2S,3S)hadroproduction,a complete analysis of the polarization parametersλθ,λθФ、andλФfor the production is presented at QCD next-to-leading order.With the long-distance matrix elements extracted from experimental data for the production rate and polarization parameterλθofγhadroproduction,our results provide a good description of the measured parametersλθФandλФin both the helicity and Collins-Soper frames.In our calculations,the frame invariant parameterλ^-is consistent in the two frames.Finally,we mention that there are discrepancies between the available experimental data and corresponding theoretical predictions forλ^-.