Higgs Phenomenology in Warped Extra Dimensions

This thesis is a study on phenomenology of beyond the Standard Model in the context of warped extra-dimensional (Randall-Sundrum) models. These models, through introducing a large extra space dimension along which the standard model fields can propagate, can address the hierarchy between the Plank and weak scales, provided that the geometry is suitably curved along the fifth dimension and the extra dimension is stabilized. The space-time background that is considered in this thesis is mainly in a more general form which is modified from the usual AdS5. This modification can alleviate considerably the bounds coming from precision electroweak tests and flavor physics. Of course, the usual AdS5 geometry is a special case and can be reproduced by taking the correct limits. In this thesis, we mainly consider the case where the extra dimension is bounded by two stabilized hard walls (branes) at the TeV (IR brane) and Plank (UV brane) scales. Also our principal consideration will be the case when all the standard model fields propagate in the bulk, although we comment on the case where only the Higgs is localized on the TeV brane. Within this context, after a broad review of the main concepts, we first address the phenomenology of a bulk scalar Higgs boson, and calculate its production cross section at the LHC as well as its tree-level effects on mediating flavor changing neutral currents. We perform the calculations based on two different approaches. First, we compute our predictions analytically by considering all the degrees of freedom emerging from the dimensional reduction (the infinite tower of Kaluza-Klein modes (KK)). In the second approach, we perform our calculations numerically by considering only the effects caused by the first few KK modes, present in the 4-dimensional effective theory. In the case of a Higgs leaking far from the brane, both approaches give the same predictions as the effects of the heavier KK modes decouple. However, as the Higgs boson is pushed towards the TeV brane, the two approaches seem to be equivalent only when one includes heavier and heavier degrees of freedom (which do not seem to decouple). To reconcile these results it is necessary to introduce a type of higher derivative operator which essentially encodes the effects of integrating out the heavy KK modes and dresses the brane Higgs so that it looks just like a bulk Higgs. Secondly we calculate the production rate of the Higgs boson at the LHC in the context of general 5D warped scenarios, 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 AdS5 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 modified AdS5 warped scenarios. And finally we show that in these backgrounds, high energy flavor symmetries are inherent. When these high energy symmetries are broken at lower energies, they produce the Standard Model (SM) structure including the neutrinos. This feature is completely general and depends neither on the details of the background metric, as long as it produces the required hierarchy, nor on the exact form of the symmetry, as long as it produces the required PMNS matrix. The reason for this phenomena is inherent in the structure of the exponential hierarchy factors of warped extra-dimension scenarios with bulk matter fields. While these factors produce the hierarchy of masses in quarks and charged lepton sectors, they flatten in to a plateau at larger c-parameters to accommodate the neutrinos. In the case of the quark and charged lepton sectors, these exponential hierarchy factors “wash off” the structure of the order one five dimensional Yukawa couplings, and naturally produce the hierarchical masses and the CKM matrix, while for the neutrinos sector, while for the neutrinos, the structure of the high energy symmetries are preserved to attain the SM

Fermion Masses and Mixing in General Warped Extra Dimensional Models

We analyze fermion masses and mixing in a general warped extra dimensional model, where all the Standard Model (SM) fields, including the Higgs, are allowed to propagate in the bulk. In this context, a slightly broken flavor symmetry imposed universally on all fermion fields, without distinction, can generate the full flavor structure of the SM, including quarks, charged leptons and neutrinos. For quarks and charged leptons, the exponential sensitivity of their wave-functions to small flavor breaking effects yield naturally hierarchical masses and mixing as it is usual in warped models with fermions in the bulk. In the neutrino sector, the exponential wave-function factors can be flavor-blind and thus insensitive to the small flavor symmetry breaking effects, directly linking their masses and mixing angles to the flavor symmetric structure of the 5D neutrino Yukawa couplings. The Higgs must be localized in the bulk and the model is naturally more successful in generalized warped scenarios where the metric background solution is different than $AdS_5$. We study these features in two simple frameworks, flavor complimentarily, and flavor democracy, which provide specific predictions and correlations between quarks and leptons, testable as more precise data in the neutrino sector becomes available.Comment: 33 pages, 7 figure

Unified Flavor Symmetry from warped dimensions

We propose a scenario which accommodates all the masses and mixings of the SM fermions in a model of warped extra-dimensions with all matter fields in the bulk. In this scenario, the same flavor symmetric structure is imposed on all the fermions of the Standard Model (SM), including neutrinos. Due to the exponential sensitivity on bulk fermion masses, a small breaking of the symmetry can be greatly enhanced and produce seemingly un-symmetric hierarchical masses and small mixing angles among the charged fermion zero-modes (SM quarks and charged leptons) and wash-out the obvious effects of the symmetry. With the Higgs field leaking into the bulk, and Dirac neutrinos sufficiently localized towards the UV boundary, the neutrino mass hierarchy and flavor structure will still be largely dominated by the fundamental flavor structure. The neutrino sector would then reflect the fundamental flavor structure, whereas the quark sector would probe the effects of the flavor symmetry breaking sector. As an example, we explore these features in the context of a family permutation symmetry imposed in both quark and lepton sectors.Comment: 5 pages, 1 figur

Additional Higgs bosons: Supersymmetry or warped extra dimensions?

We investigate and compare additional CP-even, CP-odd and charged scalar states appearing in two popular Beyond the Standard Model scenarios. We focus on the simplest possible Higgs sector within warped extra-dimensions and supersymmetry, with the aim to differentiate between them. In each case, we analyze the couplings of the new Higgs states, looking for distinguishing signatures. We show that the couplings of the Standard Model gauge bosons to the first Kaluza-Klein Higgs states of the extra-dimensional setup (CP-even, CP-odd and charged) are very similar to those of the heavy Higgs states of the MSSM in the decoupling region. We also find that the Yukawa couplings in the extra-dimensional scenario can mimic the different types of Yukawa couplings of general Two-Higgs Doublet Models, in particular the so-called Type-II couplings, which are similar to those in the MSSM.Comment: 7 pages, 2 figures, 3 table

PT-Symmetric Quantum Electrodynamics and Unitarity

More than 15 years ago, a new approach to quantum mechanics was suggested, in which Hermiticity of the Hamiltonian was to be replaced by invariance under a discrete symmetry, the product of parity and time-reversal symmetry, $\mathcal{PT}$. It was shown that if $\mathcal{PT}$ is unbroken, energies were, in fact, positive, and unitarity was satisifed. Since quantum mechanics is quantum field theory in 1 dimension, time, it was natural to extend this idea to higher-dimensional field theory, and in fact an apparently viable version of $\mathcal{PT}$-invariant quantum electrodynamics was proposed. However, it has proved difficult to establish that the unitarity of the scattering matrix, for example, the K\"all\'en spectral representation for the photon propagator, can be maintained in this theory. This has led to questions of whether, in fact, even quantum mechanical systems are consistent with probability conservation when Green's functions are examined, since the latter have to possess physical requirements of analyticity. The status of $\mathcal{PT}$QED will be reviewed in this report, as well as the general issue of unitarity.Comment: 13 pages, 2 figures. Revised version includes new evidence for the violation of unitarit