 Main
Higgs Physics and Cosmology
 Roberts, Alex
 Advisor(s): Nomura, Yasunori
Abstract
Recently, a new framework for describing the multiverse has
been proposed which is based on the principles of quantum mechanics.
The framework allows for welldefined predictions, both regarding global
properties of the universe and outcomes of particular experiments,
according to a single probability formula. This provides complete
unification of the eternally inflating multiverse and many worlds
in quantum mechanics. We elucidate how cosmological
parameters can be calculated in this framework, and study the probability
distribution for the value of the cosmological constant. We consider
both positive and negative values, and find that the observed value
is consistent with the calculated distribution at an order of magnitude
level. In particular, in contrast to the case of earlier measure
proposals, our framework prefers a positive cosmological constant over
a negative one. These results depend only moderately on how we model
galaxy formation and life evolution therein.
We explore supersymmetric theories in which the Higgs mass is boosted by the nondecoupling Dterms of an extended $U(1)_X$ gauge symmetry, defined here to be a general linear combination of hypercharge, baryon number, and lepton number. Crucially, the gauge coupling, $g_X$, is bounded from below to accommodate the Higgs mass, while the quarks and leptons are required by gauge invariance to carry nonzero charge under $U(1)_X$. This induces an irreducible rate, $\sigma$BR, for $pp \rightarrow X \rightarrow \ell\ell$ relevant to existing and future resonance searches, and gives rise to higher dimension operators that are stringently constrained by precision electroweak measurements. Combined, these bounds define a maximally allowed region in the space of observables, ($\sigma$BR, $m_X$), outside of which is excluded by naturalness and experimental limits. If natural supersymmetry utilizes nondecoupling Dterms, then the associated $X$ boson can only be observed within this window, providing a model independent `litmus test' for this broad class of scenarios at the LHC. Comparing limits, we find that current LHC results only exclude regions in parameter space which were already disfavored by precision electroweak data..
Recent LHC data, together with the electroweak naturalness
argument, suggest that the top squarks may be significantly lighter
than the other sfermions. We present supersymmetric models in which
such a split spectrum is obtained through ``geometries'': being
``close to'' electroweak symmetry breaking implies being ``away from''
supersymmetry breaking, and vice versa. In particular, we present
models in 5D warped spacetime, in which supersymmetry breaking and
Higgs fields are located on the ultraviolet and infrared branes,
respectively, and the top multiplets are localized to the infrared
brane. The hierarchy of the Yukawa matrices can be obtained while
keeping near flavor degeneracy between the first two generation sfermions, avoiding stringent constraints from flavor and $CP$ violation. Through the AdS/CFT correspondence, the models can be interpreted as purely 4D theories in which the top and Higgs multiplets are composites of
some strongly interacting sector exhibiting nontrivial dynamics at
a low energy. Because of the compositeness of the Higgs and top
multiplets, Landau pole constraints for the Higgs and top couplings
apply only up to the dynamical scale, allowing for a relatively heavy
Higgs boson, including $m_h = 125~{\rm GeV}$ as suggested by the
recent LHC data. We analyze electroweak symmetry breaking for
a wellmotivated subset of these models, and find that finetuning
in electroweak symmetry breaking is indeed ameliorated. We also
discuss a flat space realization of the scenario in which supersymmetry
is broken by boundary conditions, with the top multiplets localized
to a brane while other matter multiplets delocalized in the bulk.
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