Boson Loops Research Articles

B→sνν¯decay in the MSSM: Implication ofb→sγat largetanβ

The decay $b\ensuremath{\rightarrow}s\ensuremath{\nu}\overline{\ensuremath{\nu}}$ is discussed in the minimal supersymmetric standard model with general flavor mixing for squarks, at large $\mathrm{tan}\ensuremath{\beta}$. In this case, in addition to the chargino loop contributions which were analyzed in previous studies, $\mathrm{tan}\ensuremath{\beta}$-enhanced contributions from the gluino and charged Higgs boson loops might become sizable compared with the standard model contribution, at least in principle. However, it is demonstrated that the experimental bounds on the new physics contributions to the radiative decay $b\ensuremath{\rightarrow}s\ensuremath{\gamma}$ should strongly constrain these contributions to $b\ensuremath{\rightarrow}s\ensuremath{\nu}\overline{\ensuremath{\nu}}$, especially on the gluino contribution. We also briefly comment on a possible constraint from the ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ decay.

Effect of charged scalar loops on photonic decays of a fermiophobic Higgs boson

Higgs bosons with very suppressed couplings to fermions (``fermiophobic Higgs bosons,'' ${h}_{f}$) can decay to two photons ($\ensuremath{\gamma}\ensuremath{\gamma}$) with a branching ratio significantly larger than that expected for the standard model Higgs boson for ${m}_{{h}_{f}}<150\text{ }\text{ }\mathrm{GeV}$. Such a particle would give a clear signal at the LHC and can arise in the two Higgs doublet model (type I) in which ${h}_{f}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ is mediated by ${W}^{\ifmmode\pm\else\textpm\fi{}}$ and charged Higgs boson (${H}^{\ifmmode\pm\else\textpm\fi{}}$) loops. We show that the ${H}^{\ifmmode\pm\else\textpm\fi{}}$ loops can cause both constructive and destructive contributions with a magnitude considerably larger than the anticipated precision in the measurement of the photonic decay channel at future hadron and lepton colliders.

Relativistic chiral mean field model for finite nuclei

We study the role of pion for the structure of finite nuclei. We take the chiral sigma model, where the pions are the Nambu–Goldstone bosons of the chiral symmetry breaking. We then take the finite pion mean field in the relativistic mean field approximation. We study first the nuclei in the range of A = 36 to A = 64 with equal number of neutrons and protons. We find that the magic number gap at N = Z = 28 appears due to the finite pion mean field effect. The pion provides a large spin–orbit splitting effect due to a mechanism totally different from the ordinary spin–orbit term of the relativistic origin. On the other hand, we are not able to shift the magic number appearing at A = 36 instead of A = 40 , which is now a motivation to work out the parity and charge projection. The standard projection technique provides an integro-differential equation for the Dirac equation. As an example, we work out 4He in the relativistic chiral mean field model. We find good properties for the ground state energy and the size and the pion energy contribution. The form factor also comes out to be quite satisfactory. We discuss further the renormalization procedure of the linear chiral model by treating both the nucleon loop and the boson loop in the Coleman–Weinberg renormalization scheme with the hope to calculate the negative energy contribution from the nucleon vacuum. We are able to obtain a stable chiral model Lagrangian with the nucleon vacuum polarization effect due to strong cancellation between the nucleon loop and the boson loop.

Pairing in fermion systems with unequal masses: Nonperturbative renormalisation group approach

The application of the nonperturbative renormalisation group approach to a system with two fermion species is studied. Assuming a simple ansatz for the effective action with effective bosons, describing pairing effects we derive a set of approximate flow equations for the effective coupling including boson and fermionic fluctuations. The case of two fermions with different masses but coinciding Fermi surfaces is considered. The phase transition to a phase with broken symmetry is found at a critical value of the running scale. The large mass difference is found to disfavour the formation of pairs. The mean-field results are recovered if the effects of boson loops are omitted. While the boson fluctuation effects were found to be negligible for large values of $p_{F} a$ they become increasingly important with decreasing $p_{F} a$ thus making the mean field description less accurate.

Yukawa induced radiative corrections to the lightest Higgs boson mass in the NMSSM

We compute the leading logarithmic radiative corrections to the lightest Higgs mass in the NMSSM involving the electroweak gauge couplings and the NMSSM specific Yukawa couplings λ and κ (including all mixed combinations), which are induced by chargino, neutralino and Higgs boson loops. The effect of the NMSSM specific Yukawa couplings λ and κ is to increase the upper bound on the lightest Higgs mass by up to ∼1 GeV, but they can also decrease the lightest Higgs mass by up to ∼−20 GeV.

Exact Renormalisation Group and pairing in many-fermion systems

We consider pairing phenomena in the framework of the exact renormalisation group. We introduce a boson field to describe pairing effects, and take a simple ansatz for the effective action. We derive a set of approximate flow equations for the effective coupling including boson and fermionic fluctuations. The initial conditions are obtained by renor- malising the interaction to fit the scattering length in vacuum. At some critical value of the running scale, the numerical solutions show a phase transition to a gapped phase. Standard results are recovered if we omit the boson loops.

Pairing in many-fermion systems: an exact renormalisation group treatment

We study the application of the exact renormalisation group to a many-fermion system with a short-range attractive force. We introduce a boson field to describe pairing effects, and take a simple ansatz for the effective action. We derive a set of approximate flow equations for the effective coupling including boson and fermionic fluctuations. The initial conditions are obtained by renormalising the interaction to fit the scattering length in vacuum. At some critical value of the running scale, the numerical solutions show a phase transition to a gapped phase. Standard results are recovered if we omit the boson loops. When boson fluctuations are included, we find that their contributions are significant only in the small-gap regime.

Cutoff schemes in chiral perturbation theory and the quark mass expansion of the nucleon mass

We discuss the use of cutoff methods in chiral perturbation theory. We develop a cutoff scheme based on the operator structure of the effective field theory that allows to suppress high momentum contributions in Goldstone boson loop integrals and by construction is free of the problems traditional cutoff schemes have with gauge invariance or chiral symmetries. As an example, we discuss the chiral expansion of the nucleon mass. We show that the mass of a nucleon in heavy baryon chiral perturbation theory has a well behaved chiral expansion up to effective Goldstone boson masses of 400 MeV when one utilizes standard dimensional regularization techniques. With the help of the here developed cutoff scheme we can demonstrate a well-behaved chiral expansion for the nucleon mass up to 600 MeV of effective Goldstone boson masses. We also discuss in detail the price, in numbers of additional short distance operators involved, that has to be paid for this extended range of applicability of chiral perturbation theory with cutoff regularization. We also compare the fourth-order result for the chiral expansion of the nucleon mass with lattice results and draw some conclusions about chiral extrapolations based on such type of representation.

Little Higgs phenomenology

Recently a new class of models has emerged that addresses the naturalness problem of a light Higgs boson. In these ''little Higgs'' models, the Standard Model Higgs boson is a pseudo-Nambu-Goldstone boson of an approximate global symmetry. The Higgs boson acquires mass radiatively only through ''collective breaking'' of the global symmetry, so that more than one interaction is required to give the Higgs a mass. This protects the Higgs mass from receiving quadratically divergent radiative corrections at one-loop. These models contain new vector bosons, fermions and scalars at the TeV scale that cancel the quadratic divergences in the Higgs mass due to the Standard Model gauge, top quark, and Higgs boson loops. In this talk I review the phenomenology of the little Higgs models, focusing on collider signatures and electroweak precision constraints.

Less suppressed μ→eγ and τ→μγ loop amplitudes and extra dimension theories

When μeγ (or τμγ) 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 for μeγ 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.

Complete two loop bosonic contributions to the muon lifetime in the standard model.

The last missing correction to the muon lifetime in the standard model at O(alpha(2)) coming from gauge and Higgs boson loops is presented. The associated contribution to the parameter Deltar in the on-shell scheme ranges from 6x10(-5) to -4x10(-5) for Higgs boson masses from 100 GeV to 1 TeV. This result translates into a shift of the W boson mass which does not exceed +/-1 MeV in the same range and amounts, in particular, to approximately -0.8 MeV for a 115 GeV Higgs boson.

Quark-quark correlations and baryon electroweak observables

The simple independent quark models have difficulties explaining simultaneously the totality of the known hyperon magnetic moments and hyperon semileptonic decay rates. We show that both the Goldstone boson loop contributions and two-quark effective exchange currents are essential in explaining these observables.

Parity violating bosonic loops at finite temperature

The finite temperature parity-violating contributions to the polarization tensor are computed at one loop in a system without fermions. The system studied is a Maxwell-Chern-Simons-Higgs system in the broken phase, for which the parity-violating terms are well known at zero temperature. At nonzero temperature the static and long-wavelength limits of the parity violating terms have very different structure, and involve nonanalytic log terms depending on the various mass scales. At high temperature the boson loop contribution to the Chern-Simons term goes like T in the static limit and like $T\mathrm{log}T$ in the long-wavelength limit, in contrast with the fermion loop contribution which behaves like $1/T$ in the static limit and like $\mathrm{log}T/T$ in the long wavelength limit.

Dynamical symmetry breaking in the sea of the nucleon.

We derive the nonanalytic chiral behavior of the flavor asymmetry d - u. Such behavior is a unique characteristic of Goldstone boson loops in chiral theories, including QCD, and establishes the unambiguous role played by the Goldstone boson cloud in the sea of the proton. Generalizing the results to the SU(3) sector, we show that strange chiral loops require that the s - s distribution be nonzero.

Leading Electroweak Corrections to the Neutral Higgs Boson Production at the Fermilab Tevatron**The project supported in part by National Natural Science Foundation of China, Doctoral Program Foundation of Higher Education, the Post Doctoral Foundation of China, and a grant from the State Commission of Science and Technology of China. ZHU ShouHua gratefully acknowledges the support of K.C. WONG Education Foundation,

We calculate the leading electroweak corrections to the light neutral Higgs boson production via at the Fermilab Tevatron in both the standard model and the minimal supersymmetric model, which arise from the top-quark and Higgs boson loop diagrams. We found that the leading electroweak corrections can exceed the QCD corrections for favorable values of the parameters in the MSSM, but such corrections are only about in the SM, which are much smaller than the QCD corrections. For the mass region of , the leading electroweak corrections can reach for large tan , and these corrections may be observable at a high luminosity Tevatron; at the least, new constraints on the tan can be established.

The longitudinal and transverse responses in the inclusive electron scattering A functional approach

The splitting between the charge-longitudinal and spin-transverse responses is explained in a model whose inputs are the effective interactions in the particle-hole channels in the frame of the first order boson loop expansion. It is shown that the interplay between ω-meson exchange and box diagrams (two-meson exchange with simultaneous excitation of one or two nucleons to Δ's) mainly rules the longitudinal response, while in the transverse one the direct Δ excitations almost cancel the one-loop correction and the response is mainly governed by the ϱ-meson rescattering inside the nucleus. It is also shown that a small variation in the nuclear densities may explain the observed discrepancies between different nuclei.

The Decay of Z → 3γ via Charged Higgs Boson Loops in Some Nonstandard Models11The project partly supported by the National Natural Science Foundation of China and the Grant LWTZ-920411 of the Chinese Academy of Sciences

The fourth rank polarization tensor and the helicity amplitudes of Z → 3γ via charged Higgs boson loops in some nonstandard models with more than one Higgs doublet are calculated by using a nonlinear Rξ gauge. The results are presented in terms of three transcendental functions of B(x), T(x) and I(x, y). We have checked the analytic results in Ref. [6] in comparison with ours.

Loop-induced Higgs boson pair production at e+e- colliders.

We analyze the loop-induced production of Higgs boson pairs at future high-energy ${e}^{+}{e}^{\ensuremath{-}}$ colliders, both in the standard model and in its minimal supersymmetric extension. The cross sections for standard model Higgs pair production through $\frac{W}{Z}$ boson loops, ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{H}^{0}{H}^{0}$, are rather small but the process could be visible for high enough luminosities, especially if longitudinal polarization is made available. In the minimal supersymmetric standard model, the corresponding processes of $\mathrm{CP}$-even or $\mathrm{CP}$-odd Higgs boson pair production, ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}hh, HH, \mathrm{Hh}$, and ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\mathrm{AA}$, have smaller cross sections, in general. The additional contributions from chargino or neutralino and slepton loops are at the level of a few percent in most of the supersymmetric parameter space.

Higgs decay into two photons, dispersion relations and trace anomaly

We examine the contribution of W boson loops to the amplitude of the process H → γγ within the dispersion relation approach, taking up an issue raised in this context recently. We show that the non-vanishing limit of the relevant form factor for m w → 0 is due to a finite subtraction induced by the value of the corresponding trace anomaly. The argument can be turned around and one thus arrives at a dispersive (“infrared”) derivation of the trace anomaly.

Inflation at low scales: General analysis and a detailed model.

Models of inflationary cosmology based on spontaneous symmetry breaking typically suffer from the shortcoming that the symmetry breaking scale is driven to nearly the Planck scale by observational constraints. In this paper we investigate inflationary potentials in a general context, and show that this difficulty is characteristic only of potentials $V(\phi)$ dominated near their maxima by terms of order $\phi^2$. We find that potentials dominated by terms of order $\phi^m$ with \hbox{$m > 2$} can satisfy observational constraints at an arbitrary symmetry breaking scale. Of particular interest, the spectral index of density fluctuations is shown to depend only on the order of the lowest non-vanishing derivative of $V(\phi)$ near the maximum. This result is illustrated in the context of a specific model, with a broken ${\rm SO(3)}$ symmetry, in which the potential is generated by gauge boson loops.