A Mean-Field Treatment in Studying Nuclear Matter Through a Thermodynamic Consistent Resummation Scheme

We used the Cornwall, Jackiw and Tomboulis （CJT） resummation scheme to study nuclear matter. In the CJT formalism the meson propagators are treated as the bare propagators and the the higher order loop corrections of the thermodynamic potential are evaluated at the Hartree approximation, while the vacuum fluctuations are ignored. Under these treatments in the CJT formalism we derived exact mean-field theory （MFT） results for the nuclear matter. The results are thermodynamically consistent, and our study indicates that the MFT result is the lowest order resummation result in the CJT resummation scheme. The relation between CJT formalism and MFT is clearly presented through the calculations.

Improved calculation of effective potential and symmetry-restored transition in Higgs model

Early in the 1970s, some authors suggested that the electroweak gauge symmetry should be restored at high enough temperature and density. Recently the electroweak phase transition has attracted much attention, because it is of crucial importance for the baryon asymmetry of the universe. So it is required to discuss the electroweak phase transition in detail. The basical tool for studying the phase transition is effective potential, and naive calculation of the effective potential has two difficulties. Here we will propose a self-consistent calculation of the effecitve potential by the resummation of hard thermal loops, and use this improved method to discuss the symmetry-restored transition in the Higgs model.

Hard-thermal-loop QED thermodynamics

The weak-coupling expansion for thermodynamic quantities in thermal field theories is poorly convergent unless the coupling constant is tiny.We discuss the calculation of the free energy for a hot gas of electrons and photons to three-loop order using hard-thermal-loop perturbation theory （HTLpt）.We show that the hard-thermal-loop perturbation reorganization improves the convergence of the successive approximations to the QED free energy at large coupling,e ～ 2.The reorganization is gauge invariant by construction,and due to the cancellations among various contributions,we obtain a completely analytic result for the resummed thermodynamic potential at three loops.

A Brief Overview of Hard-Thermal-Loop Perturbation Theory

The poor convergence of quantum field theory at finite temperature has been one of the main obstacles in the practical applications of thermal QCD for decades.Here we briefly review the progress of hard-thermal-loop perturbation theory (HTLpt) in reorganizing the perturbative expansion in order to improve the convergence.The quantum mechanical anharmonic oscillator is used as a simple example to show the breakdown of weak-coupling expansion,and variational perturbation theory is introduced as an effective resummation scheme for divergent weak-coupling expansions.We discuss HTLpt thermodynamic calculations for QED,pure-glue QCD,and QCD with N f=3 up to three-loop order.The results suggest that HTLpt provides a systematic framework that can be used to calculate both static and dynamic quantities for temperatures relevant at LHC.

Invariant-mass distribution of top-quark pairs and top-quark mass determination

In this study,we investigate the invariant-mass distribution of top-quark pairs near the 2mt threshold,which strongly influences the determination of the top-quark mass mt.Higher-order non-relativistic corrections lead to large contributions,which are not included in the state-of-the-art theoretical predictions.A factorization formula is derived to resum such corrections to all orders in the strong-coupling,and necessary ingredients are calculated to perform the resummation at next-to-leading power.We combine the resummation with fixed-order results and present phenomenologically relevant numerical results.The resummation effect significantly increases the differential cross-section in the threshold region and makes the theoretical prediction more compatible with experimental data.We estimate that using our prediction in the determination of mt will lead to a value closer to the direct measurement result.

Scalar-pseudoscalar pair production at the Large Hadron Collider at NLO+NLL accuracy in QCD

We thoroughly investigate both transverse momentum and threshold resummation effects on scalar-pseudoscalar pair production via quark-antiquark annihilation at the 13 TeV Large Hadron Collider at QCD NLO+NLL accuracy.A factorization method is introduced to properly supplement the soft-gluon(threshold)resummation contribution from parton distribution functions to the resummed results obtained by the Collins-Soper-Ster-man resummation approach.We find that the impact of the threshold-resummation improved PDFs is comparable to the resummation effect of the partonic matrix element and can even predominate in high invariant mass regions.Moreover,the loop-induced gluon-gluon fusion channel in the type-I two-Higgs-doublet model is considered in our calculations.The numerical results show that the electroweak production via quark-antiquark annihilation dominates over the gluon-initiated QCD production by 1-2 orders of magnitude.