39 research outputs found
Spontaneous Charge Breaking in the NMSSM - Dangerous or not?
We investigate the impact of charge-breaking minima on the vacuum stability
of the NMSSM. We find that, in contrast to Two-Higgs-Doublet Models like the
MSSM, at both tree- and loop-level there exists global charge-breaking minima.
Consequently, many regions of parameter space are rendered metastable, which
otherwise would have been considered stable if these charge-breaking minima
were neglected. However, the inclusion of these new scalar field directions has
little impact on otherwise metastable vacuum configurations.Comment: 7 pages, 4 figure
Ricci Reheating
We present a model for viable gravitational reheating involving a scalar
field directly coupled to the Ricci curvature scalar. Crucial to the model is a
period of kination after inflation, which causes the Ricci scalar to change
sign thus inducing a tachyonic effective mass for the
scalar field. The resulting tachyonic growth of the scalar field provides the
energy for reheating, allowing for temperatures high enough for thermal
leptogenesis. Additionally, the required period of kination necessarily leads
to a blue-tilted primordial gravitational wave spectrum with the potential to
be detected by future experiments. We find that for reheating temperatures
GeV, the possibility exists for the Higgs field to play
the role of the scalar field.Comment: 9 pages, 6 figure
The Ultraviolet Landscape of Two-Higgs Doublet Models
We study the predictions of generic ultraviolet completions of two-Higgs
doublet models. We assume that at the matching scale between the two-Higgs
doublet model and a ultraviolet complete theory -- which can be anywhere
between the TeV and the Planck scale -- arbitrary but perturbative values for
the quartic couplings are present. We evaluate the couplings down from the
matching scale to the weak scale and study the predictions for the scalar mass
spectrum. In particular, we show the importance of radiative corrections which
are essential for both an accurate Higgs mass calculation as well as
determining the stability of the electroweak vacuum. We study the relation
between the mass splitting of the heavy Higgs states and the size of the
quartic couplings at the matching scale, finding that only a small class of
models exhibit a sizeable mass splitting between the heavy scalars at the weak
scale. Moreover, we find a clear correlation between the maximal size of the
couplings and the considered matching scale.Comment: 16 pages, 10 figure
Sarah Goes Left and Right Looking Beyond the Standard Model and Meets Susy
Progress in the search for physics beyond the Standard Model (BSM) proceeds through two main avenues. The frst requires the development of models that address the host of theoretical and experimental deficiencies of the Standard Model (SM). The second avenue requires scrutinising these models against all available data as well as checks for theoretical consistency. Unfortunately there exists a large number of strongly motivated models as well as an absence of any signs illuminating the correct path nature has chosen. With the lack of a clear direction, automated tools provide an effective means to test as many models as possible. In this thesis we demonstrate how the SARAH framework can be used in this context as well as its adaptability for confronting unexpected hints of new physics, such as the diphoton excess, that have arisen at the Large Hadron Collider (LHC) over the previous years. We then turn to more theoretical constraints namely, studying the stability of the electroweak vacuum in minimal supersymmetric models. Here we studied the impact of previously neglected directions when including non-standard vacuum expectation values. In the second half of this thesis we consider low-scale left-right symmetric models both with and without supersymmetry. In the non-supersymmetric case we consider constraints arising from charged lepton flavour violation. We have significantly improved existing parametrisations allowing for the new Yukawa couplings to be determined as a function of the underlying model parameters. The last scenario we consider is a model based on SO(10) unification at the high-scale. We build a complete model with TeV-scale breaking of the left-right phase studying in detail the phenomenology
Soft Gamma Rays from Heavy WIMPs
We propose an explanation of the galactic center gamma ray excess by
supersymmetric WIMPs as heavy as 500 GeV. The lightest neutralino annihilates
into vector-like leptons or quarks which cascade decay through intermediate
Higgs bosons. Due to the long decay chains, the gamma ray spectrum is much
softer than naively expected and peaks at GeV energies. The model predicts
correlated diboson and dijet signatures to be tested at the LHC.Comment: 8 pages, 8 figures; v2: focus on gamma ray excess, matches published
versio
A constrained supersymmetric left-right model
We present a supersymmetric left-right model which predicts gauge coupling
unification close to the string scale and extra vector bosons at the TeV scale.
The subtleties in constructing a model which is in agreement with the measured
quark masses and mixing for such a low left-right breaking scale are discussed.
It is shown that in the constrained version of this model radiative breaking of
the gauge symmetries is possible and a SM-like Higgs is obtained. Additional
CP-even scalars of a similar mass or even much lighter are possible. The
expected mass hierarchies for the supersymmetric states differ clearly from
those of the constrained MSSM. In particular, the lightest down-type squark,
which is a mixture of the sbottom and extra vector-like states, is always
lighter than the stop. We also comment on the model's capability to explain
current anomalies observed at the LHC.Comment: 21 pages, 5 figures; v2: references added, matches published versio
The Supercooling Window at Weak and Strong Coupling
Supercooled first order phase transitions are typical of theories where
conformal symmetry is predominantly spontaneously broken. In these theories the
fate of the flat scalar direction is highly sensitive to the size and the
scaling dimension of the explicit breaking deformations. For a given
deformation, the coupling must lie in a particular region to realize a
supercooled first order phase transition. We identify the supercooling window
in weakly coupled theories and derive a fully analytical understanding of its
boundaries. Mapping these boundaries allows us to identify the deformations
enlarging the supercooling window and to characterize their dynamics
analytically. For completeness we also discuss strongly coupled conformal field
theories with an holographic dual, where the complete characterization of the
supercooling window is challenged by calculability issues.Comment: 16 pages + appendices, 12 figures; v2: minor typo correcte
R-Parity Violation at the LHC
We investigate the phenomenology of the MSSM extended by a single R-parity
violating coupling at the unification scale. For all R-parity violating
couplings, we discuss the evolution of the particle spectra through the
renormalization group equations and the nature of the lightest supersymmetric
particle (LSP) within the CMSSM, as an example of a specific complete
supersymmetric model. We use the nature of the LSP to classify the possible
signatures. For each possible scenario we present in detail the current LHC
bounds on the supersymmetric particle masses, typically obtained using
simplified models. From this we determine the present coverage of R-parity
violating models at the LHC. We find several gaps, in particular for a
stau-LSP, which is easily obtained in R-parity violating models. Using the
program CheckMATE we recast existing LHC searches to set limits on the
parameters of all R-parity violating CMSSMs. We find that virtually all of them
are either more strongly constrained or similarly constrained in comparison to
the R-parity conserving CMSSM, including the models. For
each R-parity violating CMSSM we then give the explicit lower mass bounds on
all relevant supersymmetric particles.Comment: 43 pages, 13 tables, 17 figures; updated Figs. 11-17 and Tab. 12
including NLO corrections; version accepted for publication in EPJ
N-loop running should be combined with N-loop matching
We investigate the high-scale behaviour of Higgs sectors beyond the Standard
Model, pointing out that the proper matching of the quartic couplings before
applying the renormalisation group equations (RGEs) is of crucial importance
for reliable predictions at larger energy scales. In particular, the common
practice of leading-order parameters in the RGE evolution is insufficient to
make precise statements on a given model's UV behaviour, typically resulting in
uncertainties of many orders of magnitude. We argue that, before applying
N-loop RGEs, a matching should even be performed at N-loop order in contrast to
common lore. We show both analytical and numerical results where the impact is
sizeable for three minimal extensions of the Standard Model: a singlet
extension, a second Higgs doublet and finally vector-like quarks. We highlight
that the known two-loop RGEs tend to moderate the running of their one-loop
counterparts, typically delaying the appearance of Landau poles. For the
addition of vector-like quarks we show that the complete two-loop matching and
RGE evolution hints at a stabilisation of the electroweak vacuum at high
energies, in contrast to results in the literature.Comment: 16 pages, 11 figures; v2: title changed, accepted for publication in
PR