Renormalization group analysis and the top quark mass in the SU(4) × O(4) string model

We perform a two-loop renormalization group analysis for the gauge couplings in the SU(4) × O(4) model. We use the string theory prediction for the unification scale and the experimentally acceptable low energy values for ₃ and sin² θ_w, to determine the magnitudes of the various symmetry breaking scales as well as the value of the common gauge coupling at the unification scale. We solve the coupled differential system for the gauge and top and bottom Yukawa couplings, and determine the top mass as a function of two parameters which could be chosen to be the ratio of the Higgs VEV's that give masses to the up and down quarks and the value of the top coupling at the unification scale. We find a relatively heavy top quark mass which lies in the range 130m_t180 GeV.     

The dimensionally reduced effective theory for quarks in high-temperature QCD

We show that QCD undergoes a partial dimensional reduction at high temperatures also in the quark sector. In the kinematic region relevant to screening physics, where the lowest Matsubara modes are close to their “mass shells”, all static Green functions involving both quarks and gluons are reproducible in the high-T limit by a renormalizable three-dimensional Lagrangian up to order ²(T) 1/1nT. This three-dimensional theory only contains explicitly the lightest bosonic and fermionic Matsubara modes, while the heavier modes correct the tree-level couplings and generate extra local vertices. We also find that the quark degrees of freedom that have been retained in the reduced theory are non-relativistic in the high-T limit. We then improve our result to order ⁴(T) through an explicit non-relativistic expansion, in the spirit of the heavy-quark effective theory. This effective theory is relevant for studying QCD screening phenomena with observables made from quarks, e.g. mesonic and baryonic currents, already at temperatures not much higher than the chiral transition temperature T_c.     

New re1ults on CP violation in B-meson decays

37 years after the discovery of CP violation in π⁺π⁻ decays of neutral K mesons, the experiments BABAR and BELLE have found a second system which violates CP symmetry. J K_S^O decays of neutral B mesons show very large symmetry breaking. The effect has now, with the most recent data, a significance of about 13 standard deviations. The presentation describes the B-meson factories, the experiments, and their analyses. The results are in agreement with the Standard Model explanation of CP violation, i. e. with different couplings of the Higgs boson to quarks and antiquarks.     

Azimuthal angular dependence of decay lepton in e^+ e^- \to t \bar{t}

We discuss top quark production and its subsequent decay as used for searching new physics at lepton colliders. The angular dependence of the decay leptons is calculated including both QCD corrections and anomalous couplings. The off-diagonal spin basis for the top and anti-top quarks is shown to be useful to probe the anomalous couplings. Received: 11 August 2000 / Published online: 27 November 2000     

Flavor-Changing Bottom-Strange Associated in the Littlest Higgs Model with T-parity at the ILC

In the littlest Higgs model with T-parity (LHT) the mirror quarks induce the special flavor structures and some new flavor-changing couplings, which could greatly enhance the production rates of the flavor-changing processes. We in this paper study some bottom and anti-strange production processes in the LHT model at the International Linear Collider (ILC), i.e.,e⁺e^- →b\bar{s} andγγ→b\bar{s}. The results show that the production rates of these processes are sizeable for the favorable values of the parameters. Therefore, it is quite possible to test the LHT model or make some constraints on the relevant parameters of the LHT through the detection of these processes at the ILC.     

The fermion mass matrices near a fixed point

The structure of the fermion mass matrices is investigated in the neighbourhood of an infrared fixed point with special emphasis on the mixing angles which determine the charged current couplings. We find that it is necessary to introduce a fourth generation of quarks whose masses are predicted. The mass of the top quark is close to 35 GeV. The solution for a two family problem is found, which predicts very small charged current couplings of the very heavy to the lighter quarks.     

Supersymmetry for Fermion Masses

It is proposed that supersymmetry （SUSY） may be used to understand fermion mass hierarchies. A family symmetry ZSL is introduced, which is the cyclic symmetry among the three generation SU（2） doublets. SUSY breaks at a high energy scale - 10^11 GeV. The electroweak energy scale- 100 GeV is unnaturally small No additional global symmetry, like the R-parlty, is imposed. The Yukawa couplings and R-parity violating couplings all take their natural values, which are О（10^0 -10^-2）. Under the family symmetry, only the third generation charged ferrnions get their masses. This family symmetry is broken in the soft SUSY breaking terms, which result in a hierarchical pattern of the fermion masses. It turns out that for the charged leptons, the r mass is from the Higgs vacuum expectation value （VEV） and the sneutrino VEVs, the muon mass is due to the sneutrino VEVs, and the electron gains its mass due to both ZZL and SUSY hreaking. The large neutrino mixing are produced with neutralinos playing the partial role of right-handed neutrinos. │Ve3│, which is for Ve-Vr mixing, is expected to be about 0.1. For the quarks, the third generation masses are from the Higgs VEVs, the second generation masses are from quantum corrections, and the down quark mass due to the sneutrino VEVs. It explains me/ms, ms/me, md 〉 mu and so on. Other aspects of the model are discussed.     

CP-odd WWZ couplings induced by vector-like quarks

A minimal extension of the standard model includes extra quarks with charges 2/3 and/or , whose left-handed and right-handed components are both SU(2) singlets. This model predicts new interactions of the flavor-changing neutral current at the tree level, which also violate CP invariance. We study CP-odd anomalous couplings for the W, W, and Z gauge bosons induced by the new interactions at the one-loop level. These couplings become non-negligible only if both up-type and down-type extra quarks are incorporated. Their form factors are estimated to be maximally of order . Such magnitudes are larger than those predicted in the standard model, though smaller than those in certain other models. Received: 26 February 1999 / Published oinline: 12 August 1999     

Electroweak symmetry breaking with hierarchical feature

Electroweak symmetry breaking is attributed to dynamical generation of quark masses. Our study is made in those models in which preons are confined by hypercolor gauge interactions to form massless quarks and leptons as well as heavy hyperhadrons or postons. The hierarchical nature of the breaking can be understood by assuming the necessity of infinite-range color interactions in addition to short-range postonic ones. Self-consistent mass equations forN families of quarks are given in an ideal limit in which left-right asymmetric effects and quark-lepton couplings are neglected.     

Hermitian flavor violation

The fundamental constraint on two Higgs doublet models comes from the requirement of sufficiently suppressing flavor-changing neutral currents. There are various standard approaches for dealing with this problem, but they all tend to share a common feature; all of the Higgs doublets couple very weakly to the first generation quarks. Here we consider a simple two Higgs doublet model which is able to have large couplings to the first generation, while also being safe from flavor constraints. We assume only that there is an SUf(3) flavor symmetry which is respected by the couplings of one of the Higgs doublets, and which is broken by Hermitian Yukawa couplings of the second doublet. As a result of the large permitted couplings to the first generation quarks, this scenario may be used to address the excess in W+dijet events recently observed by CDF at the Tevatron. Moreover, Hermitian Yukawa coupling matrices arise naturally in a broad class of solutions to the strong CP problem, providing a compelling context for the model.     

Heavy Charged Gauge Bosons with General CP Violating Couplings

Heavy gauge bosons such as W＇ are expected to exist in many extensions of the Standard Model. In this paper, the most general Lagrangian for the interaction of W＇ with top and bottom quarks is considered. This Lagrangian consists of V- A and V ＋ A structure with in general complex couplings. Such interactions produce an Electric Dipole Moment （EDM） for the top quark at one loop level. We predict the allowed ranges for the mass and couplings of W＇ by using the upper limit on the top quark EDM.     

Observability at LEP2 hadronic channels of a Z’ with small lepton couplings

We consider the effects of a number of models with one extra Z, with enhanced couplings to quarks, in the final hadronic channels at LEP2. We show that, for a number of representative cases, visible effects could be produced even for very low values of the lepton couplings, much smaller than the existing LEP1/SLC and the future LEP2 (lepton channel) bounds.     

New formulas and predictions for running fermion masses at higher scales in SM, 2HDM, and MSSM

Including contributions of scale-dependent vacuum expectation values, we derive new analytic formulas and obtain substantially different numerical predictions for the running masses of quarks and charged leptons at higher scales in the SM, 2HDM and MSSM. These formulas exhibit significantly different behaviours with respect to their dependence on gauge and Yukawa couplings from those derived earlier. At one-loop level, the masses of the first two generations are found to be independent of the Yukawa couplings of the third generation in all three effective theories in the small mixing limit. Analytic formulas are also obtained for the running of in 2HDM and MSSM. Other numerical analyses include a study of the third generation masses at high scales as functions of the low-energy values of and the SUSY scale GeV. Received: 1 October 2000 / Revised version: 11 January 2001 / Published online: 12 April 2001     

Relations between the masses of the superpartners of leptons and quarks,the goldstino coupling and the neutral currents

The mass² splittings between leptons and quarks and their spin-0 partners under supersymmetry are related to the goldstino couplings. The bosonic partner of the goldstino cannot be the photon itself. But it should be, in part, a linear combination of the various neutral gauge bosons. As a result, mass relations constrain the neutral current structure of the theory. They require the existence of at least two neutral gauge bosons in addition to the photon and suggest the possibility of a universal mass² splitting between leptons and quarks and their spin-0 partners.     

Four-dimensional heterotic string models with SU(5) and SO(10) grand unification

The gauge group, quark and lepton generations, Higgs scalar structure, couplings of quarks and leptons to Higgs scalars and to colour triplet scalars, and the gauge symmetry breaking are studied for a class of four-dimensional heterotic string models whose boundary conditions include third-integral twists.     

New formulas and predictions for running masses at higher scales in MSSM

The effects of the scale dependent vacuum expectation values (VEVs) on the running masses of quarks and leptons in non-SUSY gauge theories have been considered by a number of authors. Here we use RGEs of the VEVs, and the gauge and Yukawa couplings in the MSSM to analytically derive new one loop formulas for the running masses above the SUSY breaking scale. Some of the masses exhibit a substantially different behaviour with respect to their dependence on the gauge and Yukawa couplings when compared with earlier formulas in the MSSM derived ignoring RGEs of VEVs. In particular, the masses of the first two generations are found to be independent of the Yukawa couplings of the third generation in the small mixing limit. New numerical estimates at the two loop level are also presented. Received: 30 July 1999 / Published online: 6 April 2000     

Study of Top Quark FCNC

We study the one-loop contribution of the effective flavor changing neutral couplings (FCNC) tcZ on the charm quark electric dipole moment. Using the known limits on the top and charm quarks electric dipole moments, we place limits on these FCNC anomalous couplings.     

Octet and decuplet magnetic and axial couplings from one-loop self-consistent perturbation theory

Octet and decuplet wave functions, obtained in a recent precision fit to the spectrum using one-loop self-consistent perturbation theory of mesons and gluons interacting with bagged relativistic quarks, are used to evaluate axial couplings and magnetic moments. Results are fully competitive with other models and provide further support to one-loop self-consistent perturbation theory as a useful model of baryon structure.     

Search for W' boson resonances decaying to a top quark and a bottom quark

We search for the production of a heavy W' gauge boson that decays to third generation quarks in 0.9 fb-1 of pp collisions at square root(s)=1.96 TeV, collected with the D0 detector at the Fermilab Tevatron collider. We find no significant excess in the final-state invariant mass distribution and set upper limits on the production cross section times branching fraction. For a left-handed W' boson with SM couplings, we set a lower mass limit of 731 GeV. For right-handed W' bosons, we set lower mass limits of 739 GeV if the W' boson decays to both leptons and quarks and 768 GeV if the W' boson decays only to quarks. We also set limits on the coupling of the W' boson to fermions as a function of its mass.     

On the corrections of pole dominance in B→π/v

In the framework combining the heavy quark expansion and chiral symmetry for light quarks, we identify a class of corrections to the dominant vector meson pole diagram for B→π/v, in the vicinity of the zero pion recoil point. We also give a qualitative estimate of these effects, using higher states exchanges with couplings constrained by current algebra and PCAC.