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Register a new userHiggs Production in a Warped Extra Dimension
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Marcela Carena
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Measurements of the Higgs-boson production cross section at the LHC are an important tool for studying electroweak symmetry breaking at the quantum level, since the main production mechanism gg-->h is loop-suppressed in the Standard Model (SM). Higgs production in extra-dimensional extensions of the SM is sensitive to the Kaluza-Klein (KK) excitations of the quarks, which can be exchanged as virtual particles in the loop. In the context of the minimal Randall-Sundrum (RS) model with bulk fields and a brane-localized Higgs sector, we derive closed analytical expressions for the gluon-gluon fusion process, finding that the effect of the infinite tower of virtual KK states can be described in terms of a simple function of the fundamental (5D) Yukawa matrices. Given a specific RS model, this will allow one to easily constrain the parameter space, once a Higgs signal has been established. We explain that discrepancies between existing calculations of Higgs production in RS models are related to the non-commutativity of two limits: taking the number of KK states to infinity and removing the regulator on the Higgs-boson profile, which is required in an intermediate step to make the relevant overlap integrals well defined. Even though the one-loop gg-->h amplitude is finite in RS scenarios with a brane-localized Higgs sector, it is important to introduce a consistent ultraviolet regulator in order to obtain the correct result.
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A comprehensive, five-dimensional calculation of Higgs-boson production in gluon fusion is performed for both the minimal and the custodially protected Randall-Sundrum (RS) model, with Standard Model fields propagating in the bulk and the scalar sector confined on or near the IR brane. For the first time, an exact expression for the gg->h amplitude in terms of the five-dimensional fermion propagator is derived, which includes the full dependence on the Higgs-boson mass. Various results in the literature are reconciled and shown to correspond to different incarnations of the RS model, in which the Higgs field is either localized on the IR brane or is described in terms of a narrow bulk state. The results in the two scenarios differ in a qualitative way: the gg->h amplitude is suppressed in models where the scalar sector is localized on the IR brane, while it tends to be enhanced in bulk Higgs models. In both cases, effects of higher-dimensional operators contributing to the gg->h amplitude at tree level are shown to be numerically suppressed under reasonable assumptions. There is no smooth cross-over between the two scenarios, since the effective field-theory description breaks down in the transition region. A detailed phenomenological analysis of Higgs production in various RS scenarios is presented, and for each scenario the regions of parameter space already excluded by LHC data are derived.
We calculate the production rate of the Higgs boson at the LHC in the context of general 5 dimensional (5D) warped scenarios with spacetime background modified from the usual $AdS_5$, and where all the SM fields, including the Higgs, propagate in the bulk. This modification can alleviate considerably the bounds coming from precision electroweak tests and flavor physics. We evaluate the Higgs production rate and show that it is generically consistent with the current experimental results from the LHC for Kaluza-Klein (KK) masses as low as 2 TeV, unlike in pure $AdS_5$ scenarios, where for the same masses, the Higgs production typically receives corrections too large to be consistent with LHC data. Thus the new pressure on warped models arising from LHC Higgs data is also alleviated in $AdS_5$-modified warped scenarios.
We calculate the production and decay rates of the Higgs boson at the LHC in the context of general 5 dimensional (5D) warped scenarios with a spacetime background modified from the usual $AdS_5$, with SM fields propagating in the bulk. We extend previous work by considering the full flavor structure of the SM, and thus including all possible flavor effects coming from mixings with heavy fermions. We proceed in three different ways, first by only including two complete Kaluza-Klein (KK) levels ($15times15$ fermion mass matrices), then including three complete KK levels ($21times21$ fermion mass matrices) and finally we compare with the effect of including the infinite (full) KK towers. We present numerical results for the Higgs production cross section via gluon fusion and Higgs decay branching fractions in both the modified metric scenario and in the usual Randall-Sundrum metric scenario.
We analyze the flavor violation in warped extra dimension due to radion mediation. We show that Delta S=2 and Delta B=2 flavor violating processes impose stringent constraints on radion mass, m_phi and the scale Lambda_phi. In particular, for Lambda_phi ~ O(1) TeV, B_d^0-bar{B}^0_d implies that m_phi ~ 65 GeV. We also study radion contributions to lepton flavor violating processes: tau -> (e,mu) phi, tau -> emu^+mu^- and B -> l_i l_j. We show that BR(B_s -> mu^+ mu^-) can be of order 10^{-8}, which is reachable at the LHCb. The radion search at LHC, through the flavor violation decays into tau mu or top-charm quarks, is also considered.
In the context of a warped extra-dimension with Standard Model fields in the bulk, we obtain the general flavor structure of the Higgs couplings to fermions. These couplings will be generically misaligned with respect to the fermion mass matrix, producing large and potentially dangerous flavor changing neutral currents (FCNCs). As recently pointed out in [arXiv:0906.1542], a similar effect is expected from the point of view of a composite Higgs sector, which corresponds to a 4D theory dual to the 5D setup by the AdS-CFT correspondence. We also point out that the effect is independent of the geographical nature of the Higgs (bulk or brane localized), and specifically that it does not go away as the Higgs is pushed towards the IR boundary. The FCNCs mediated by a light enough Higgs (specially their contribution to $epsilon_K$) could become of comparable size as the ones coming from the exchange of Kaluza-Klein (KK) gluons. Moreover, both sources of flavor violation are complementary since they have inverse dependence on the 5D Yukawa couplings, such that we cannot decouple the flavor violation effects by increasing or decreasing these couplings. We also find that for KK scales of a few TeV, the Higgs couplings to third generation fermions could experience suppressions of up to 40% while the rest of diagonal couplings would suffer much milder corrections. Potential LHC signatures like the Higgs flavor violating decays $htomutau$ or $hto tc$, or the exotic top decay channel $tto c h$, are finally addressed.
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