Probing HZγ and Hγγ anomalous couplings in the process e~+e~-→ Hγ

We propose to measure the HZγ and Hγγ anomalous couplings in the process e+e-→Hγ with the sequential decay of H→bˉb. The discovery potential of observing the anomalous couplings are explored in detail.Our study shows that future electron–positron colliders have great potential to test the HZγ and Hγγ couplings.Conservative bounds on the two anomalous couplings are also derived when no new physics signal is detected on top of the SM backgrounds.     

Probe R-Parity Violating Supersymmetry in Forward--Backward Asymmetry

We investigate the possibility of probing R-parity violating interactions in forward-backward asymmetries, and conclude that the Tevatron collider is the best place to measure new interactions involving the first generation of quarks. With additional down type squark ~dk contributions, the charge asymmetry in e＋e— events around Z-pole mass region at the Tevatron will be sensitive to R-parity violating couplings λ’11k, which was inaccessible to the LEP Qfb had inclusive measurement. Assuming no apparent deviation from SM prediction is observed, λ’11k〈 0.19 at 95% C.L. with squark mass Mdk=100 GeV can be derived.     

Model-independent Veltman condition,naturalness and the little hierarchy problem

We adopt a bottom-up Effective Field Theory(EFT)approach to derive a model-independent Veltman condition to cancel out the quadratic divergences in the Higgs mass.We show using the equivalence theorem that all the deviations in the Higgs couplings to the W and Z from the SM predictions should vanish.We argue based on tree-level unitarity that any new physics that naturally cancels out the quadratic divergences should be ≤19 TeV.We show that the level of fine-tuning required is unless the O(0:1%-1%) UV sector has a symmetry that forces the satisfaction of the model-independent Veltman condition,in which case all fine-tuning is eliminated.We also conjecture that,if no new physics that couples to the Higgs is observed up to~19 TeV,or if the Higgs couplings to the SM particles conform to the SM predictions,then the Higgs either does not couple to any UV sector or is fine-tuned.     

Production and decay of the 125 GeV Higgs boson in thelittlest Higgs model with T-parity

Motivated by recent search results for the standard model (SM) Higgs boson at the Large Hadron Collider (LHC), we revisit the Higgs phenomenology in the littlest Higgs model with T-parity (LHT). We present the signal strength modifier μ, respectively, for the main search channels qq' → jjh→ jjγγ, qq'→ Vh→ Vγγ, qq'→ Vh → Vbb, gg→ h → γγ, and gg → h → VV in the LHT model. It is found that an enhancement factor of 1.09-1.56 in the qq' → jjh→ jjγγ channel can be obtained for this model in Case B with parameter f in the range 500-1000~GeV. However, the rates for bb, ττ are significantly suppressed relative to the SM predictions, which are still consistent with the current sensitivity. It is hoped that this will be further tested with larger integrated luminosity at the LHC.     

Electroweak vacuum stability and diphoton excess at 750 GeV

Recently,both ATLAS and CMS collaborations at the CERN Large Hadron Collider(LHC) announced their observations of an excess of diphoton events around the invariant mass of 750 GeV with a local significance of 3.6σ and 2.6σ,respectively.In this paper,we interpret the diphoton excess as the on-shell production of a real singlet scalar in the pp →S→γγ channel.To accommodate the observed production rate,we further introduce a vector-like fermion F,which carries both color and electric charges.The viable regions of model parameters are explored for this simple extension of the Standard Model(SM).Moreover,we revisit the problem of electroweak vacuum stability in the same scenario,and find that the requirement for the electroweak vacuum stability up to high energy scales imposes serious constraints on the Yukawa coupling of the vector-like fermion and the quartic couplings of the SM Higgs boson and the new singlet scalar.Consequently,a successful explanation for the diphoton excess and the absolute stability of electroweak vacuum cannot be achieved simultaneously in this economical setup.     

Optimizing Higgs factories by modifying the recoil mass

It is difficult to measure the WW-fusion Higgs production process(e~+e~-→νh)at a lepton collider with a center of mass energy of 240-250 Ge V due to its small rate and the large background from the Higgsstrahlung process with an invisible Z(e~+e~-→h Z,Z→ν).We construct a modified recoil mass variable,m~p_(recoil),defined using only the 3-momentum of the reconstructed Higgs particle,and show that it can separate the WW-fusion and Higgsstrahlung events better than the original recoil mass variable m_(recoil).Consequently,the m~p_(recoil)variable can be used to improve the overall precisions of the extracted Higgs couplings,in both the conventional framework and the effective-field-theory framework.We also explore the application of the m~p_(recoil)variable in the inclusive cross section measurements of the Higgsstrahlung process,while a quantitive analysis is left for future studies.     

Oblique parameters in gauged baryon and lepton numbers with a 125 GeV Higgs

In an extension of the standard model,where baryon number and lepton number are local gauge symmetries,we analyze the effect of corrections from exotic fermions and scalars on the oblique parameters S,T,U.Because a light neutral Higgs ho with mass around 124-126 GeV strongly constrains the corresponding parameter space of this model,we also investigate the gluon fusion process gg→h_0 and two photon decay of the lightest neutral Higgs ho→γγ at the Large Hadron Collider.     

New physical effects on the decay Bs →γγ in the technicolor model

In this paper we calculate the contributions to the branching ratio of Bs →γγ from charged pseudo-Goldstone bosons appearing in the one generation Technicolor model. We find that the theoretical value of the branching ratio, BR（Bs →γγ）, including the contributions of PGBs, P^± and P8^±, is very different from the standard model （SM） prediction. The new physical effects can provide a one to two order of magnitude enhancement of the SM results. It is shown that the decay Bs →γγ can test new physical signals from the technicolor model.     

Note on Higgs Decay into Two Photons H → γγ

The Higgs decay H →γγ due to the virtual W-loop effect is revisited in the unitary gauge by using the symmetry-preserving and divergent-behavior-preserving loop regularization method,which is realized in the fourdimensional space-time without changing original theory.Though the one-loop amplitude of H →γγ is finite as the Higgs boson in the standard model has no direct interaction with the massless photons at tree level,it involves both tensor-type and scalar-type divergent integrals which can in general destroy the gauge invariance without imposing a proper regularization scheme to make them well-defined.As the loop regularization scheme can ensure the consistency conditions between the regularized tensor-type and scalar-type divergent irreducible loop integrals to preserve gauge invariance,we explicitly show the absence of decoupling in the limit M W /M H → 0 and obtain a result agreeing exactly with the earlier one in the literature.We then clarify the discrepancy of the earlier result from the recent one obtained by R.Gastmans,S.L.Wu and T.T.Wu.The advantage of calculation in the unitary gauge becomes manifest in that the non-decoupling arises from the longitudinal contribution of the W gauge boson.     

Charged Higgs Boson Pair Production via Gluon-Gluon Collisions in MSSM with CP Violation

The CP-violating effects to the subprocess gg → H H- are studied in the mSUGRA scenario at the CERN large hadron collider, by taking into account the experimental bounds of electron and neutron electric dipole moments.The CP-violating effects in this process are related to the complex phases ofμ and Af in the mSUGRA scenario. In our calculation we consider smallCP phases ofμ and Af and neglect the effects of neutral Higgs boson mixing. In this case the CP effects to the process mainly come from the complex couplings of Higgs-squark-squark. We find a strong dependence of charged Higgs boson pair production rate on the complex couplings in the parameter space of minimal supersymmetric standard model``     

Two loop electroweak corrections from heavy fermions to b→s＋γ

Applying an effective Lagrangian method and an on-shell scheme, we analyze the electroweak corrections to the rare decay b→, s＋γ from some special two loop diagrams in which a closed heavy fermion loop is attached to the virtual charged gauge bosons or Higgs. At the decoupling limit where the virtual fermions in the inner loop are much heavier than the electroweak scale, we verify the final results satisfying the decoupling theorem explicitly when the interactions among Higgs and heavy fermions do not contain the nondecoupling couplings. Adopting the universal assumptions on the relevant couplings and mass spectrum of new physics, we find that the relative corrections from those two loop diagrams to the SM theoretical prediction on the branching ratio of B → Xsγ can reach 5% as the energy scale of new physics ANp=200 GeV.     

Can Higgs Field Have a Cosmological Origin？

A proposal of the cosmological origin of Higgs particles is given.We show that the Higgs field could be created from the vacuum quantum conformal fluctuation(VQCF) of Anti-de Sitter space-time,the spontaneous breaking of vacuum symmetry and the mass of Higgs particle are related to the cosmological constant of our universe,especially the theoretical estimated mass mφ of Higgs particles is mφ=√-2μ^2=√|Λ|/π.It seems that the Higgs particles with masses in GeV range may be the relics of the VQCF Of Anti-de sitter space in inflational phase of our very early universe.     

New physics searches with Higgs-photon associated production at the Higgs factory

A future Higgs factory is being designed for precise measurement of Higgs characteristics and to search for new physics. In this paper we propose that the Higgs-photon associated production process, e+e-→γh could be a useful channel for new physics. We express new physics model-independently in the effective Lagrangian approach,and find that the new physics effects of γh have only two degrees of freedom, much fewer than the Higgsstrahlung process. This point could be used to reduce the degeneracies of Wilson coefficients. We also calculate for the first time the 95% confidence level(CL) bounds of γh at the Higgs factory, and prove that γh is more sensitive to some dimension-6 operators than the current experimental data. In the optimistic scenario new physics effects may be observed at the CEPC or FCC-ee after the first couple of years of their run.     

Hidden analytic relations for two-loop Higgs amplitudes in QCD

We compute the Higgs plus two-quark and one-gluon amplitudes(H→qqg) and Higgs plus three-gluon amplitudes(H→3 g) in the Higgs effective theory with a general class of operators.By changing the quadratic Casimir CF to CA,the maximally transcendental parts of the H→ qqg amplitudes turn out to be equivalent to that of the H→3 g amplitudes,which also coincide with the counterparts in N=4 SYM.This generalizes the so-called maximal transcendentality principle to the Higgs amplitudes with external quark states,thus the full QCD theory.We further verify that the correspondence applies also to two-loop form factors of more general operators,in both QCD and scalar-YM theory.Another interesting relation is also observed between the planar H→qqg amplitudes and the minimal density form factors in N=4 SYM.     

Probing magnetic moment operators in H_γ production and H→τ~+τ~-γ rare decay

The magnetic moment(a_γ) and weak magnetic moment(a_W) of charged leptons and quarks are sensitive to quantum effects of new physics heavy resonances.In effective field theory,a_γ and a_W are induced by two independent operators.Therefore,one has to measure both ay and aw to shed light on new physics.The aw's of the SM fermions are measured at the LEP.In this work,we analyze the contributions from magnetic and weak magnetic moment operators in the processes of pp→H_γ and gg→H→τ~+τ~-γ at the High-Luminosity Large Hadron Collider.We demonstrate that the two processes can cover most of the parameter space that cannot be probed at the LEP.     

Discriminating the HTM and MLRSM models in collider studies via doubly charged Higgs boson pair production and the subsequent leptonic decays

We present a case study for the doubly charged Higgs boson H~(±±) pair production in e~+e~-and pp colliders with their subsequent decays to four charged leptons.We consider the Higgs Triplet Model(HTM),which is not restricted by the custodial symmetry,and the Minimal Left-Right Symmetric Model(MLRSM).These models include scalar triplets with different complexities of scalar potentials and,because of experimental restrictions,completely different scales of non-standard triplet vacuum expectation values.In both models,a doubly charged Higgs boson H~(±±) can acquire a mass of hundreds of gigaelectronvolts,which can be probed at the HL-LHC,future e~+e~-,and hadron colliders.We take into account a comprehensive set of constraints on the parameters of both models coming from neutrino oscillations,LHC,e~+e~-,and low-energy lepton flavor violating data and assume the same mass of H~(±±).Our finding is that the H~(±±) pair production in lepton and hadron colliders is comparable in both models,though more pronounced in the MLRSM.We show that the decay branching ratios can be different within both models,leading to distinguishable four-lepton signals,and that the strongest are 4μ events yielded by the MLRSM.Typically,we find that the MLRSM signals are one order of magnitude larger those in the HTM.For example,the pp→ 4μMLRSM signal for 1 TeV H~(±±) mass results in a clearly detectable significance of S■11 for the HL-LHC and S■290 for the FCC-hh.Finally,we provide quantitative predictions for the dilepton invariant mass distributions and lepton separations,which help to identify non-standard signals.     

New physical effects on the decay Bs→γγ in the technicolor model

In this paper we calculate the contributions to the branching ratio of Bs→γγ from charged pseudo-Goldstone bosons appearing in the one generation Technicolor model. We find that the theoretical value of the branching ratio, BR(Bs→γγ), including the contributions of PGBs, P± and P±8, is very different from the standard model (SM) prediction. The new physical effects can provide a one to two order of magnitude enhancement of the SM results. It is shown that the decay Bs→γγ can test new physical signals from the technicolor model.     

Neutrino masses and lepton-flavor-violating τ decays in the supersymmetric left-right model

In the supersymmetric left-right model, the light neutrino masses are given by the Type-Ⅱ seesaw mechanism. A duality property of this mechanism indicates that there exist eight possible Higgs triplet Yukawa couplings which result in the same neutrino mass matrix. In this paper, we work out the one-loop renormalization group equations for the effective neutrino mass matrix in the supersymmetric left-right model. The stability of the Type-Ⅱ seesaw scenario is briefly discussed. We also study the lepton-flavor-violating processes (τ→ μγ and τ→eγ) by using the reconstructed Higgs triplet Yukawa couplings.     

Top quark forward-backward asymmetry in the little Higgs model

A sizable difference in top quark pair forward backward asymmetry (AFB) is observed at Tevatron. The discrepancy triggers many new physics beyond the standard model (SM) and then constrains the parameter spaces in them. In this article we calculate the AFB of the top-pair production at Tevatron up to next to leading order (NLO) in the little Higgs model (LHM). We find that the contribution of Z_H can be large enough to make up the gap between SM prediction and experimental data. Then, the parameter space for the couplings between Z_H and quarks are constrained. Thus, this model can fulfill the experimental data, both in AFB and in cross section.     

A general analysis of Wtb anomalous couplings

We investigate new physics effects on the Wtb effective couplings in a model-independent framework.The new physics effects can be parametrized by four independent couplings, f_1~L, f_1~R, f_2~L and f_2~R. We further introduce a set of parameters x_0, x_m, x_p and x_5 which exhibit a linear relation to the single top production cross sections.Using recent data for the t-channel single top production cross section σ_t, t W associated production cross sectionσt W, s-channel single top production cross section σs, and W-helicity fractions F_0, F_L and F_R collected at the 8 Te V LHC and Tevatron, we perform a global fit to impose constraints on the top quark effective couplings. Our global fitting results show that the top quark effective couplings are strongly correlated. We show that(i) improving the measurements of σt and σt W is important in constraining the correlation of(f_1~R, f_2~R) and(f_2~L, f_2~R);(ii) f_1~L and f_2~R are anti-correlated, and are sensitive to all the four experiments;(iii) f_1~R and f_2~Lare also anti-correlated, and are sensitive to the F_0 and FL measurements;(iv) the correlation between f_2~L and f_2~R is sensitive to the precision of the σ_t, σ_t W and F_0 measurements. The effective Wtb couplings are studied in three kinds of new physics models:the G(221) = SU(2)_1?SU(2)_2?U(1)_X models, the vector-like quark models and the Littlest Higgs model with and without T-parity. We show that the Wtb couplings in the left-right model and the un-unified model are sensitive to the ratio of gauge couplings when the new heavy gauge boson's mass(MW) is less than several hundred Ge V, but the constraint is loose if M_W 1 TeV. Furthermore, the Wtb couplings in vector-like quark models and the Littlest Higgs models are sensitive to the mixing angles of new heavy particles and SM particles.