Dynamics of scalar fields in an expanding/contracting cosmos at finite temperature

This study extends the investigation of quantum dissipative effects of a cosmological scalar field by taking into account cosmic expansion and contraction.Cheung,Drewes,Kang,and Kim calculated the effective action and quantum dissipative effects of a cosmological scalar field in a recent work,where analytical expressions for the effective potential and damping coefficient were presented using a simple scalar model with quartic interactions,and the work was conducted using Minkowski-space propagators in loop diagrams.In this work,we incorporate the Hubble expansion and contraction of the cosmic background and focus on the thermal dynamics of a scalar field in a regime where the effective potential changes slowly.Given that the Hubble parameter,H,attains a small but non-zero value,we carry out calculations to the first order in H.If we set H=0,all results match those in flat spacetime.Interestingly,we must integrate over the resonances,which in turn leads to an amplification of the effects of a non-zero H.This is an intriguing phenomenon,which cannot be uncovered in flat spacetime.The implications on particle creations in the early universe will be studied in a forthcoming study.

Analysis of B_(c)→τν_(τ) at CEPC

Precise determination of the B_(c)→τν_(τ) branching ratio provides an advantageous opportunity for under-standing the electroweak structure of the Standard Model,measuring the CKM matrix element |V_(cb)|,and probing new physics models.In this paper,we discuss the potential of measuring the process B_(c)→ τν_(τ)withτdecaying leptonically at the proposed Circular Electron Positron Collider(CEPC).We conclude that during the Z pole opera-tion,the channel signal can achieve five-σsignificance with~10^9 Z decays,and the signal strength accuracies for B_(c)→τν_(τ) can reach around 1%level at the nominal CEPC Z pole statistics of one tillion Z decays,assuming the total B_(c)→τν_(τ) yield is 3.6x 10^(6).Our theoretical analysis indicates the accuracy could provide a strong constraint on the general effective Hamiltonian for the b→cτν transition.If the total Be yield can be determined to 0(1%)level of accuracy in the future,these results also imply|V_(cb)| could be measured up to 0(1%)level of accuracy.