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Register a new userPotential Precision on Higgs Couplings and Total Width at the ILC
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We outline a systematic approach to the determination of the Standard Model-like Higgs boson total width and measurable coupling parameters in a model-independent manner at the International Linear Collider (ILC) and illustrate the complementarity for operating the ILC at $250$ GeV near the $Zh$ threshold and at $500$ GeV and $1$ TeV utilizing the $WW, ZZ$ fusion processes. We perform detailed simulations for an important contributing channel to the coupling determination and for invisible decays. Without model assumptions, and combining the information for the coupling ratios from the LHC, the total width can be determined to an accuracy of about $6%$, and the couplings for the observable channels can be measured to the $(3-5)%$ level at 250 GeV, reaching $(1-3)%$ level including the 500 GeV results, with further improvements possible with a $1$ TeV run. The best precision for the branching fraction measurement of the Higgs to invisible modes can be reached at $0.5-0.7%$ around the $Zh$ threshold. Further studies from $ZZ$ fusion at higher energies may provide significant improvement for the measurements. With modest theory assumptions, the width and coupling determinations can be further improved to the percent or sub-percent level.
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A substantial off-shell Higgs boson signal in the gluon fusion and vector boson fusion H --> ZZ and H --> WW channels at the Large Hadron Collider (LHC) facilitates a novel, complementary approach to constraining the total Higgs width Gamma_H. With LHC Run 1 data, experimental analyses by CMS and ATLAS find Gamma_H < 5.4 Gamma_H,SM and Gamma_H < [4.5,7.5] Gamma_H,SM at 95% confidence level, respectively, where Gamma_H,SM is the expected value in the Standard Model at the measured Higgs boson mass. I review the theoretical basis of the new approach and discuss its significance in comparison to other methods to bound and measure the Higgs width at the LHC and future colliders.
The recent discovery of a Higgs boson at LHC, while establishing the Higgs mechanism as the way of electroweak symmetry breaking, started an era of precision measurements involving the Higgs boson. In an effective Lagrangian framework, we consider the e+e- --> ZHH process, at an ILC running at a centre of mass energy of 800 GeV to investigate the effect of the ZZH and ZZHH couplings on the sensitivity of HHH coupling on this process. Our results show that the sensitivity of the trilinear Higgs self couplings on this process has somewhat strong dependence on the Higgs-gauge boson couplings. Single and two parameter reach of ILC with integrated luminosity of 1000 /fb are obtained on the effective couplings, c6 and cH, which are related to the HHH couplings, indicating how these limits are affected by the presence of anomalous ZZH and ZZHH couplings. The kinematic distributions studied to understand the effect of the anomalous couplings, again, show strong influence of Z-H couplings on the dependence of these distributions on HHH coupling. Similar results are indicated in the case of the process, e+e- --> nu nu HH, considered at a centre of mass energy of 2 TeV, where the cross section is large enough. The effect of WWH and WWHH couplings on the sensitivity of HHH coupling is clearely established through our analyses of this process.
We study pair-production as well as the triple self-couplings of the neutral Higgs bosons of the Minimal Supersymmetric Standard Model (MSSM) at the Future International Linear $e^{+}e^{-}$ Collider (ILC) and Compact Linear Collider (CLIC). The analysis is based on the reactions $e^{+}e^{-}to b bar b h_ih_i, t bar t h_ih_i$ with $h_i=h, H, A$. We evaluate the total cross-section for both $bbar bh_ih_i$, $tbar th_ih_i$ and calculate the total number of events considering the complete set of Feynman diagrams at tree-level. We vary the triple couplings $kappalambda_{hhh}$, $kappalambda_{Hhh}$, $kappalambda_{hAA}$, $kappalambda_{HAA}$, $kappalambda_{hHH}$ and $kappalambda_{HHH}$ within the range $kappa=-1$ and +2. The numerical computation is done for the energies expected at the ILC with a center-of-mass energy 500, 1000, 1600 $GeV$ and a luminosity 1000 $fb^{-1}$. The channels $e^{+}e^{-}to b bar b h_ih_i$ and $e^{+}e^{-}to t bar t h_ih_i$ are also discussed to a center-of-mass energy of 3 $TeV$ and luminosities of 1000 $fb^{-1}$ and 5000 $fb^{-1}$.
As one of the key properties of the Higgs boson, the Higgs total width is sensitive to global profile of the Higgs boson couplings, and thus new physics would modify the Higgs width. We investigate the total width in various new physics models, including various scalar extension, composite Higgs models, and fraternal twin Higgs model. Typically the Higgs width is smaller than the standard model value due to mixture with other scalar if the Higgs is elementary, or curved Higgs field space for the composite Higgs. On the other hand, except the possible invisible decay mode, the enhanced Yukawa coupling in the two Higgs doublet model or the exotic fermion embeddings in the composite Higgs, could enhance the Higgs width greatly. The precision measurement of the Higgs total width at the high-luminosity LHC can be used to discriminate certain new physics models.
We study the off-shell production of the Higgs boson at the LHC to probe Higgs physics at higher energy scales utilizing the process $g g rightarrow h^{*} rightarrow ZZ$. We focus on the energy scale dependence of the off-shell Higgs propagation, and of the top quark Yukawa coupling, $y_t (Q^2)$. Extending our recent study in arXiv:1710.02149, we first discuss threshold effects in the Higgs propagator due to the existence of new states, such as a gauge singlet scalar portal, and a possible continuum of states in a conformal limit, both of which would be difficult to discover in other traditional searches. We then examine the modification of $y_t (Q^2)$ from its Standard Model (SM) prediction in terms of the renormalization group running of the top Yukawa, which could be significant in the presence of large flat extra-dimensions. Finally, we explore possible strongly coupled new physics in the top-Higgs sector that can lead to the appearance of a non-local $Q^2$-dependent form factor in the effective top-Higgs vertex. We find that considerable deviations compared to the SM prediction in the invariant mass distribution of the $Z$-boson pair can be conceivable, and may be probed at a $2sigma$-level at the high-luminosity 14 TeV HL-LHC for a new physics scale up to $mathcal{O}(1 {~rm TeV})$, and at the upgraded 27 TeV HE-LHC for a scale up to $mathcal{O}(3 {~rm TeV})$. For a few favorable scenarios, $5sigma$-level observation may be possible at the HE-LHC for a scale of about $mathcal{O}(1 {~rm TeV})$.
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