Abstract

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.