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Ling-Fong Li

Publications and source records attributed to Ling-Fong Li.

At least 19 recordsLinked to original sources
arXiv2013

Consistency of Loop Regularization Method and Divergence Structure of QFTs Beyond One-Loop Order

We study the problem how to deal with tensor-type two-loop integrals in the Loop Regularization (LORE) scheme. We use the two-loop photon vacuum polarization in the massless Quantum Electrodynamics (QED) as the example to present the general procedure. In the processes, we find a new divergence structure: the regulated result for each two-loop diagram contains a gauge-violating quadratic harmful divergent term even combined with their corresponding counterterm insertion diagrams. Only when we sum up over all the relevant diagrams do these quadratic harmful divergences cancel, recovering the gauge invariance and locality.

hep-ph↗
arXiv2013

Matrix elements of four-quark operators and ΔL=2 hyperon decays

The study of neutrinoless double beta decays of nuclei and hyperons require the calculation of hadronic matrix elements of local four-quark operators that change the total charge by two units ΔQ=2 . Using a low energy effective Lagrangian that induces these transitions, we compute these hadronic matrix elements in the framework of the MIT bag model. As an illustrative example we evaluate the amplitude and transition rate of Σ- -> p e- e-, a decay process that violates lepton number by two units (ΔL=2). The relevant matrix element is evaluated without assuming the usual factorization approximation of the four-quark operators and the results obtained in both approaches are compared.

hep-ph↗
arXiv2007

$ΔL=2$ hyperon semileptonic decays

We compute the rates of semileptonic B_A \to B_Bl^-l^- (l=e or μ) hyperon transitions in a model where intermediate states involve loops of baryons and a Majorana neutrino. These rates turn out to be well below present experimental bounds and other theoretical estimates. From the experimental upper limit on the Ξ^- \to pμ^-μ^- decay, we derive the bound < 22 TeV for the effective Majorana mass of the muon neutrino. Also, an estimate of background contributions for these decays due to the allowed B_A \to B_Bl^-l^-\barν\barν decays are provided.

hep-ph↗
arXiv2007

On Hyperon Decays Involving Two Charged Leptons

The hyperon decays with 2 negative charged leptons in the final states can be of interest as possible tests for the Majorana netrinos. The study of these decays are complementary to the double beta decays of heavy nuclei.

hep-ph↗
arXiv2005

Vector Goldstone Boson and Lorentz Invariance

Spontanous symmetry breaking usually gives spin 0 Goldstone bosons for the case of internal symmetries and spin 1/2 fermions for the supersymmetry. The spontaneous breaking of higher dimensional Lorentz symmetry can give vector Goldstone boson in 4-dimension.

hep-ph↗
arXiv2003

Coupling Constant Unification in Extensions of Standard Model

Unification of electromagnetic, weak, and strong coupling constants is studied in the extension of standard model with additional fermions and scalars. It is remarkable that this unification in the supersymmetric extension of standard model yields a value of Weinberg angle which agrees very well with experiments. We discuss the other possibilities which can also give same result.

hep-ph↗
arXiv2002

Less suppressed mu-e-gamma and tau-mu-gamma loop amplitudes and extra dimension theories

When mu-e-gamma (or tau-mu-gamma) loop involves a vector boson, the amplitude is suppressed by more than two powers of heavy particle masses. However we show that the scalar boson loop diagrams are much less damped. Particularly, the loop amplitude in which the intermediate fermion and scalar boson have comparable masses is as large as possible, as allowed by the decoupling theorem. Such a situation is realized in the "universal extra dimension theory", and can yield a large enough rate to be detectable in current experiments. Our investigation involves precise calculation of the scalar boson loop's dependence on the masses of the intermediate states.

hep-ph↗
arXiv2002

On Vector Goldstone Boson

The possibility that higher dimensional field theories are broken spontaneously, through the usual Nambu-Goldstone mechanism, to 4-dimension is explored. As a consequence, vector Goldstone bosons can arise in this breaking of Lorentzian symmetry from higher dimension to 4-dimension. This can provide a simple mechanism for reduction to 4-dimension in theories with extra dimensions.

hep-ph↗
arXiv2001

Heavy particle electroweak loop effects in extra-dimensional models with bulk neutrinos

One way to detect the presence of new particles in theories beyond the standard model is through their contribution to electroweak loop effects. We comment on the importance of a consistent inclusion of their mixing angles to ensure that the physical requirement of heavy particle decoupling is fulfilled. We illustrate our points by a detailed discussion of the lepton flavor changing effect mu-e-gamma, investigated recently by Kitano, in the Randall-Sundrum model. Our remarks are equally applicable to models with large compactified dimensions where bulk neutrinos are introduced to account for the observed neutrino oscillations.

hep-ph↗
arXiv2000

Spontaneous Symmetry Breaking and Chiral Symmetry

In this introductory lecture, some basic features of the spontaneous symmetry breaking are discussed. More specifically, $σ$-model, non-linear realization, and some examples of spontaneous symmetry breaking in the non-relativistic system are discussed in details. The approach here is more pedagogical than rigorous and the purpose is to get some simple explanation of some useful topics in this rather wide area. .

hep-ph↗
arXiv1998

Instantons and the singlet-coupling in the chiral quark model

Chiral quark model with a broken-U(3) flavor symmetry can be interpreted as the effective theory of the instanton-dominated non-perturbative QCD. This naturally suggests the possibility of a negative singlet/octet coupling ratio, which has been found, in a previous publication, to be compatible with the phenomenological description of the nucleon spin-flavor structure.

hep-ph↗
arXiv1998

Non-perturbative QCD spin studies

A whole class of non-perturbative QCD studies, e.g. the instanton models, chiral quark models, etc. indicates that the effective degrees of freedom for the physics in the low Q^(2) < 1 GeV^(2) region could be the constituent quarks (CQs) and internal Goldstone bosons (IGBs). This leads to a nucleon structure with the spin being carried by three constituent quark systems, each composed of a massive compact CQ surrounded by a q\bar{q} sea perturbatively generated by the valence quark's IGB emissions. Such a CQ-system has a total angular momentum of 1/2 and a small anomalous magnetic moment, built up from a quark-spin polarization and a significant orbital motion in the quark sea. The distinctive phenomenological signal for such a non-perturbative structure is that the sea-quarks are polarized negatively, while the antiquarks are not polarized. This picture also suggests a negligibly small gluon polarization. All such features can be tested by experiments in the near future.

hep-ph↗
arXiv1997

The Proton Spin and Flavor Structure in the Chiral Quark Model

After a pedagogical review of the simple constituent quark model and deep inelastic sum rules, we describe how a quark sea as produced by the emission of internal Goldstone bosons by the valence quarks can account for the observed features of proton spin and flavor structures. Some issues concerning the strange quark content of the nucleon are also discussed.

hep-ph↗
arXiv1997

Why Naive Quark Model Can Yield a Good Account of the Baryon Magnetic Moments

The chiral quark model suggests that the baryon quark-sea is negatively polarized. This modifies the spin structure as given by the naive quark model and agrees with experimental data. However, for the magnetic moments, there is significant cancellation between the contritutions from this sea spin-polarization and the orbital angular momentum so that effectively the moments are given by the valence constituent quarks alone, as in the NQM.

hep-ph↗
arXiv1997

Chiral Quark Model of Nucleon Spin-Flavor Structure with SU(3) and Axial-U(1) Breakings

The chiral quark model with a nonet of Goldstone bosons can yield an adequate description of the observed proton flavor and spin structure. In a previous publication we have compared the results of a SU(3) symmetric calculation with the phenomenological findings based on experimental measurements and SU(3) symmetry relations. In this paper we discuss their SU(3) and axial-U(1) breaking corrections. With two parameters, we obtain a very satisfactory fit to the F/D ratios for the octet baryon masses and for their axial vector couplings, as well as the different quark flavor contributions to the proton spin. The result also can account for, not only the light u and d quark asymmetry, but also the strange quark content, of the proton sea. SU(3) breaking is the key in reconciling the strange quark fraction as measured in the neutrino charm production and that as deduced from the pion nuleon sigma term.

hep-ph↗
arXiv1995

Antiquark Polarization inside the Proton is Small

Quark contributions to the proton spin as deduced from polarized DIS of leptons off a nucleon target, and the octet baryon magnetic moments, can be used to deduce the antiquark polarizations $Δ_{\overline{q}}$ inside the proton. In this way, the 1992 analysis by Karl is shown to imply $Δ_{\overline{q}}\simeq 0.$ Such a spin structure fits nicely into the chiral quark interpretation of the proton spin and flavor puzzles.

hep-ph↗