Littlest Higgs Research Articles

Naturalness sum rules and their collider tests

We present the most general sum rules reflecting the cancellation of ultraviolet divergences in the Higgs potential, which provide necessary conditions for any weakly-coupled extension of the Standard Model to be natural. There is a separate sum rule for the cancellation of the quadratic and logarithmic divergences, and their forms depend on whether the divergences are canceled by same-spin or opposite-spin partners. These sum rules can be applied to mass eigenstates and conveniently used for direct collider tests of naturalness. We study in detail the feasibility of testing these sum rules in the top sector at a future 100 TeV proton collider within two benchmark models, the Little Higgs (LH) and the Maximally Symmetric Composite Higgs (MSCH). We show how the two ingredients of the sum rules, the top partner masses and their Yukawa couplings to the Higgs, can be measured with sufficient accuracy to provide a highly non-trivial quantitative test of the sum rules. In particular, we study observables sensitive to the sign of the top partner Yukawa, which is crucial for verifying the sum rules but is notoriously difficult to measure. We demonstrate that in the benchmark models under study, a statistically significant discrimination between the two possible signs of each Yukawa will be feasible with a 30 ab−1 data set at 100 TeV.

Production of heavy w boson and heavy top quark in the littlest higgs model at high energy lineare-e+ collider

Production of heavy w boson and heavy top quark in the littlest higgs model at high energy lineare-e+ collider

Production of Charged Higgs Boson Associated with Top Partner at the Large Hadron Collider

In the context of the littlest Higgs (LH) model and the left-right twin Higgs (LRTH) model, we study the production of charged Higgs boson associated with top partner at the LHC. We find that, in the LH model, its cross section can be significantly larger for the scale parameter f = 500 GeV, while sharply decreases as f increases. In the LRTH model, this production process mainly transfers to the t̄tb̄bb final state at the Large Hadron Collider and its production rate can reach 167.2 fb.

Effects of the littlest Higgs model on the process e + e − → µ+µ− at a Z 0-factory

We investigate the possibilities of finding the effects of the littlest Higgs (LH) model on the process e + e − → µ+µ− at the leading-order at a possible Z 0-factory. We study the impacts of the LH model on the observables, such as, the total cross section, the distribution of the transverse momentum of µ−, and the forward-backward charge asymmetry of µ− in polarized and unpolarized electron-positron collision modes. Besides, we analyze the relations between the observables and the LH model parameters, and discuss the observables of process e + e − → µ+µ− in different incoming electron-positron polarization modes. We conclude that one could observe the effects contributed by the LH model on the process e + e − → µ+µ− in a reasonable parameter space, or might put more stringent constraints on the LH parameters in the future experiments at the possible Z 0-factory.

One-loop flavor change in Little Higgs models

The Little Higgs (LH) idea attempts to cure the little amount of fine-tuning necessary to bridge the gap between the Higgs mass (electroweak scale) and the new physics scale suggested by electroweak precision tests (~10 TeV). However, we show that LH models do not survive the confrontation with experimental limits on lepton flavor mixing, assuming the same naturalness arguments that motivate their introduction. Two different LH models are analyzed and several aspects of their one-loop predictions for lepton flavor-changing processes are discussed.

Extra Higgs bosons intt¯production at the LHC

The top quark has a large Yukawa coupling with the Higgs boson. In the usual extensions of the standard model the Higgs sector includes extra scalars, which also tend to couple strongly with the top quark. Unlike the Higgs, these fields have a natural mass above 2m_t, so they could introduce anomalies in ttbar production at the LHC. We study their effect on the ttbar invariant mass distribution at sqrt{s}=7 TeV. We focus on the bosons (H,A) of the minimal SUSY model and on the scalar field (r) associated to the new scale f in Little Higgs (LH) models. We show that in all cases the interference with the standard amplitude dominates over the narrow-width contribution. As a consequence, the mass difference between H and A or the contribution of an extra T-quark loop in LH models become important effects in order to determine if these fields are observable there. We find that a 1 fb^{-1} luminosity could probe the region tan beta \le 3 of SUSY and v/(sqrt{2}f) \ge 0.3 in LH models.

Radiative corrections to the Higgs potential in the LH model

In this work we compute the radiative corrections to the Higgs mass and the Higgs quartic couplings coming from the Higgs sector itself and the scalar fields $\phi$ in the Littlest Higgs (LH) model. The restrictions that the new contributions set on the parameter space of the models are also discussed. Finally this work, together with our three previous papers, complete our program addressed to compute the relevant contributions to the Higgs low-energy effective potential in the LH model and the analysis of their phenomenological consequences.

Top quark induced vacuum misalignment in little higgs models

We show that the effect of the top quark can dominate over the effect of the gauge sector in determining the vacuum alignment in little higgs (LH) models. We demonstrate that in the littlest LH model and the SU(2)xSU(2)xU(1) LH model, ensuring that the correct vacuum alignment is chosen requires that a subset of the gauge sector couplings be large to overcome the effect of the top quark. We quantify this effect by deriving bounds on the couplings in the gauge sector and demonstrate that these bounds provide a compelling theoretical reason for the gauge coupling constant hierarchy in the SU(2)xSU(2)xU(1) model that reduces the Goldstone decay constant scale to a TeV. We also argue that for a class of LH models with T parity the top quark drives the correct vacuum alignment and therefore all gauge couplings can be small.

Towards the effective potential of the littlest Higgs model

We compute the relevant parameters of the combined Higgs and φ scalar effective potential in the littlest Higgs (LH) model. These parameters are obtained as the sum of two kinds of contributions. The first one is the one-loop radiative corrections coming from fermions and gauge bosons. The second one is obtained at tree level from the higher-order effective operators needed for the ultraviolet completion of the model. Finally, we analyze the restrictions that the requirement of reproducing the standard electroweak symmetry breaking of the SM set on the LH model parameters.

Production of Charged Scalars from the Littlest Higgs Model Associated with Top Quark at LHC

The littlest Higgs (LH) model is the most economical one among various little Higgs models, which predicts the existence of the charged scalars Φ±. In this paper, we study the production of the charged Higgs boson Φ− with single top quark via the process gb → tΦ− at the CERN Large Hadron Collider (LHC). The numerical results show that the production cross section is smaller than 0.2 pb in most of the parameters space, it is very difficult to observe the signatures of the charged scalars via the process pp → gb + X → tΦ− + X at the LHC experiments. However, it can open a window to distinguish the top-pions in the TC2 model or charged Higgs in the MSSM from Φ±.

Probing the Lightest New Gauge Boson BH in the Littlest Higgs Model via Processes γγ → ff̄BH at ILC

The neutral gauge boson BH with the mass of hundreds GeV is the lightest particle predicted by the littlest Higgs (LH) model, and such particle should be the first signal of the LH model at the planed ILC if it exists indeed. In this paper, we study some processes of the BH production associated with the fermion pair at the ILC, i.e., γγ → ff̄BH. The studies show that the most promising processes to detect BH among γγ → ff̄BH are γγ → l′+l′−BH l′ = e, μ), and they can produce the sufficient signals in most parameter space preferred by the electroweak precision data at the ILC. On the other hand, the signal produced via the certain BH decay modes is typical and such signal can be easily identified from the SM background. Therefore, BH, the lightest gauge boson in the LH model, would be detectable at the photon collider realized at the ILC.

Little Higgs model effects in γγ → γγ

Though the predictions of the standard model (SM) are in excellent agreement with experiments, there are still several theoretical problems associated with the Higgs sector of the SM, where it is widely believed that some new physics will take over at the TeV scale. One beyond the SM theory which resolves these problems is the Little Higgs (LH) model. In this work we have investigated the effects of the LH model on γγ → γγ scattering [1].

Correction of the Littlest Higgs Model to HiggsProduction Process e−γ→νeW−Hin e−γ Collisions

The littlest Higgs model is the most economical one amongvarious little Higgs models. In the context of the littlestHiggs (LH) model, we study the process e−γ→νeW−H and calculate the contributions of the LH model tothe cross section of this process. The results show that in most ofthe parameter spaces preferred by the electroweak precision data, thevalue of the relative correction is larger than 10%. Suchcorrection to the process e−γ→νeW−H islarge enough to be detected via e−γ collisions in thefuture high energy linear e+e− collider (ILC) experimentwith the c.m. energy (s)1/2 = 500 GeV and a yearly integratedluminosity £ = 100 fb−1, which will give an ideal way totest the model.

Effects of the little Higgs models ontt¯h0production viaγγcollision at linear colliders

In the frameworks of the littlest Higgs (LH) model and its extension with T parity (LHT), we studied the associated $t\overline{t}{h}^{0}$ production process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}t\overline{t}{h}^{0}$ at the future ${e}^{+}{e}^{\ensuremath{-}}$ linear colliders up to QCD next-to-leading order. We present the regions of $\sqrt{s}\ensuremath{-}f$ parameter space in which the LH and LHT effects can and cannot be discovered with the criteria assumed in this paper. The production rates of process $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}t\overline{t}{h}^{0}$ in different photon polarization collision modes are also discussed. We conclude that one could observe the effects contributed by the LH or LHT model on the cross section for the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}t\overline{t}{h}^{0}$ in a reasonable parameter space, or might put more stringent constraints on the $\mathrm{LH}/\mathrm{LHT}$ parameters in the future experiments at linear colliders.

Single production of the doubly charged scalar in the littlest Higgs model

Single production of the doubly charged scalars Φ ± ± via eγ, ep and pp collisions is studied in the context of the little Higgs ( LH ) model. Our numerical results show that the new particles Φ ± ± can be abundantly produced and their possible signatures should be detected in future high energy linear e + e − collider ( ILC ) . The cross section for single production of Φ ± ± at the LHC is much smaller than that at the ILC or the THERA.

Higgs effective potential in the littlest Higgs model at the one-loop level

In this work we compute the contributions to the Higgs effective potential coming from the fermion and gauge boson sectors at the one-loop level in the context of the $SU(5)/SO(5)$ littlest Higgs (LH) model using a cutoff $\ensuremath{\Lambda}$ and including all finite parts. We consider both, the $(SU(2)\ifmmode\times\else\texttimes\fi{}U(1){)}_{1}\ifmmode\times\else\texttimes\fi{}(SU(2)\ifmmode\times\else\texttimes\fi{}U(1){)}_{2}$ and the $(SU(2)\ifmmode\times\else\texttimes\fi{}U(1){)}_{1}\ifmmode\times\else\texttimes\fi{}(SU(2)\ifmmode\times\else\texttimes\fi{}U(1))$ gauge group versions of the LH model. We also show that the Goldstone bosons present in the model do not contribute to the effective potential at the one-loop level. Finally, by neglecting the contribution of higher dimensional operators, we discuss the restrictions that the new one-loop contributions set on the parameter space of the LH model and the need to include higher loop corrections to the Higgs potential.

Lepton flavor violating signals of a little Higgs model at high energy linear e+e- colliders

The littlest Higgs (LH) model predicts the existence of the doubly charged scalars $\Phi^{\pm\pm}$ , which generally have large flavor changing couplings to leptons. We calculate the contributions of Φ±± to the lepton flavor violating (LFV) processes li→ljγ and li→ljlklk, and we compare our numerical results with the current experimental upper limits on these processes. We find that some of these processes can give severe constraints on the coupling constant Yij and the mass parameter MΦ. Taking into account the constraints on these free parameters, we further discuss the possible lepton flavor violating signals of $\Phi^{\pm\pm}$ at high energy linear e+e- collider (ILC) experiments. Our numerical results show that the possible signals of $\Phi^{\pm\pm}$ might be detected via the subprocesses e±e±→l±l± in future ILC experiments.

Higgs boson pair production process e+e− → ZHH in the littlest Higgs model at the ILC

In this paper, we calculate the contribution of the littlest Higgs (LH) model to the process e+e− → ZHH at the future high energy e+e− collider (ILC). The results show that, within the parameter spaces preferred by the electroweak precision, the deviation of the total cross sections from its SM value varies from a few per cent to tens of per cent. The correction of the LH model to the process might be detected at the future ILC experiments in the favourable parameter space. On the other hand, we find that the correction of the LH model is sensitive to the trilinear Higgs coupling in some case and the process can also provide us a chance to probe such coupling in the LH model.

RareKandBdecays in the littlest Higgs model without T-parity

We analyze rare K and B decays in the Littlest Higgs (LH) model without T-parity. We find that the final result for the Z^0-penguin contribution contains a divergence that is generated by the one-loop radiative corrections to the currents corresponding to the dynamically broken generators. Including an estimate of these logarithmically enhanced terms, we calculate the branching ratios for the decays K^+ -> pi^+ nu bar nu, K_L -> pi^0 nu bar nu, B_{s,d} -> mu^+ mu^- and B -> X_{s,d} nu bar nu. We find that for the high energy scale f=O(2-3) TeV, as required by the electroweak precision studies, the enhancement of all branching ratios amounts to at most 15% over the SM values. On the technical side we identify a number of errors in the existing Feynman rules in the LH model without T-parity that could have some impact on other analyses present in the literature. Calculating penguin and box diagrams in the unitary gauge, we find divergences in both contributions that are cancelled in the sum except for the divergence mentioned above.

Process e−γ→e−H in Littlest HiggsModel

In the context of the littlest Higgs (LH) model, weestimate the production cross section of the processe−γ→e−H in the future high-energy lineare+e− collider (ILC) with the center-of-mass (CM)energy (S)1/2 = 500 GeV. Our numerical results show that thecontributions of the LH model to this process are very small inmost of the parameter space allowed by the electroweak precisionmeasurement data, and are very difficult to be detected in thefuture ILC experiments.