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Register a new userConstraining the Littlest Higgs
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Little Higgs models offer a new way to address the hierarchy problem, and give rise to a weakly-coupled Higgs sector. These theories predict the existence of new states which are necessary to cancel the quadratic divergences of the Standard Model. The simplest version of these models, the Littlest Higgs, is based on an $SU(5)/SO(5)$ non-linear sigma model and predicts that four new gauge bosons, a weak isosinglet quark, $t$, with $Q=2/3$, as well as an isotriplet scalar field exist at the TeV scale. We consider the contributions of these new states to precision electroweak observables, and examine their production at the Tevatron. We thoroughly explore the parameter space of this model and find that small regions are allowed by the precision data where the model parameters take on their natural values. These regions are, however, excluded by the Tevatron data. Combined, the direct and indirect effects of these new states constrain the `decay constant $fgsim 3.5$ TeV and $m_{t}gsim 7 $ TeV. These bounds imply that significant fine-tuning be present in order for this model to resolve the hierarchy problem.
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In this paper we discuss the consequences of including a new heavy right-handed neutrino singlet $N_R$ in the littlest Higgs model. This new state is not connected with the light neutrinos {it via} the seesaw mechanism. A very interesting property of this extended model is the full coupling of the new neutral gauge boson $A_H$ to $N_R$, giving large total cross sections and suggesting a wide range of experimental search for the $N_R$ at the p p collider CERN-LHC and future electron-positron collider ILC.
In this paper we consider the effects of the T-parity violating anomalous Wess-Zumino-Witten-Term in the Littlest Higgs model. Apart from tree level processes, the loop induced decays of the heavy mirror particles into light standard model fermions lead to a new and rich phenomenology in particular at breaking scales f below 1 TeV. Various processes are calculated and their signatures at present and future colliders are discussed. As a byproduct we find an alternative production mechanism for the Higgs boson.
In the framework of the littlest Higgs model with T-parity (LHT), we study the contributions of the T-even and T-odd particles to the branching ratio R_b. We find that the precision data of R_b can give strong constraints on the masses of T-odd fermions.
There is a recent proposal of identifying the Higgs particle of the Standard Model as a pseudo Nambu-Goldstone boson. This new broken symmetry introduces new particles and new interactions. Among these new interactions a central role to get a new physics is played by the new neutral gauge boson. We have studied the new neutral currents in the Littlest Higgs model and compared with other extended models. For high energy $e^+ + e^-$ colliders we present a clear signature for new neutral gauge bosons that can indicate the theoretical origin of these particles. Previous analysis by other authors were done at collider energies equal to the new gauge boson mass $M_{A_H}$. In this paper we show that asymmetries in fermion anti-fermion production can display model differences in the case $M_{A_H} > sqrt{s}$. For $M_{A_H} < sqrt{s}$ we show that the hard photon energy distribution in $e^+ + e^- lra gamma + f + bar f$ can present a model dependence. New bounds for the new neutral gauge boson masses are also presented.
We re-examine lepton flavor violation (LFV) in the Littlest Higgs model with T--parity (LHT) including the full T--odd (non-singlet) lepton and Goldstone sectors. The heavy leptons induce two independent sources of LFV associated with the couplings necessary to give masses to the T--odd mirror fermions and to their partners in right-handed $SO(5)$ multiplets, respectively. The latter, which have been neglected in the past, can be decoupled from gauge mediated processes but not from Higgs mediated ones and must therefore also be included in a general analysis of LFV in the LHT. We also further extend previous analyses by considering on-shell $Z$ and Higgs LFV decays together with the LFV processes at low momentum transfer. We show that current experimental limits can probe the LHT parameter space up to global symmetry breaking scales $f sim 10$ TeV. For lower $f$ values $gtrsim 1$ TeV, $mu-e$ transitions require the misalignment between the heavy and the Standard Model charged leptons to be $lesssim 1 , %$. Future LFV experiments using intense muon beams should be sensitive to misalignments below the per mille level. For $tau$ LFV transitions, which could potentially be observed at Belle II and the LHC as well as future lepton colliders, we find that generically they can not discriminate between the LHT and supersymmetric models though in some regions of parameter space this may be possible.
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