7,296 research outputs found
Perturbativity Constraints in BSM Models
Phenomenological studies performed for non-supersymmetric extensions of the
Standard Model usually use tree-level parameters as input to define the scalar
sector of the model. This implicitly assumes that a full on-shell calculation
of the scalar sector is possible - and meaningful. However, this doesn't have
to be the case as we show explicitly at the example of the Georgi-Machacek
model. This model comes with an appealing custodial symmetry to explain the
smallness of the parameter. However, the model cannot be renormalised
on-shell without breaking the custodial symmetry. Moreover, we find that it can
often happen that the radiative corrections are so large that any consideration
based on a perturbative expansion appears to be meaningless: counter-terms to
quartic couplings can become much larger than and/or two-loop mass
corrections can become larger than the one-loop ones. Therefore, conditions are
necessary to single out parameter regions which cannot be treated
perturbatively. We propose and discuss different sets of such perturbativity
conditions and show their impact on the parameter space of the Georgi-Machacek
model. Moreover, the proposed conditions are general enough that they can be
applied to other models as well. We also point out that the vacuum stability
constraints in the Georgi-Machacek model, which have so far only been applied
at the tree level, receive crucial radiative corrections. We show that large
regions of the parameter space which feature a stable electroweak vacuum at the
loop level would have been - wrongly - ruled out by the tree-level conditions.Comment: 64 pages, 20 figure
Dark Energy and the Hubble Age
I point out that an effective upper limit of approximately 20 Gyr (for a
Hubble constant of 72 km/s/Mpc) or alternatively on the -independent
quantity , exists on the age of the Universe, essentially
independent of the unknown equation of state of the dominant dark energy
component in the Universe. Unless astrophysical constraints on the age of the
Universe can convincingly reduce the upper limit to below this value no useful
lower limit on the equation of state parameter for this component can be
obtained. Direct dating by stars does not provide a useful constraint, but
model-dependent cosmological limits from supernovae and the CMB observations
may. For a constant value of , a bound Comment: 4 pages, submitted to Ap. J. Lett (analytic asymptotic upper bound
now added
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
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
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