48 research outputs found
Is a Higgs Vacuum Instability Fatal for High-Scale Inflation?
We study the inflationary evolution of a scalar field with an unstable
potential for the case where the Hubble parameter during inflation is
larger than the instability scale of the potential. Quantum
fluctuations in the field of size imply that
the unstable part of the potential is sampled during inflation. We investigate
the evolution of these fluctuations to the unstable regime, and in particular
whether they generate cosmological defects or even terminate inflation. We
apply the results of a toy scalar model to the case of the Standard Model (SM)
Higgs boson, whose quartic evolves to negative values at high scales, and
extend previous analyses of Higgs dynamics during inflation utilizing
statistical methods to a perturbative and fully gauge-invariant formulation. We
show that the dynamics are controlled by the renormalization group-improved
quartic coupling evaluated at a scale , such that Higgs
fluctuations are enhanced by the instability if . Even if , the instability in the SM Higgs potential does not end inflation;
instead the universe slowly sloughs off crunching patches of space that never
come to dominate the evolution. As inflation proceeds past 50 -folds, a
significant proportion of patches exit inflation in the unstable vacuum, and as
much as 1% of the spacetime can rapidly evolve to a defect. Depending on the
nature of these defects, however, the resulting universe could still be
compatible with ours.Comment: 31 pages, 3 figures; v2: references added, journal versio
Spacetime Dynamics of a Higgs Vacuum Instability During Inflation
A remarkable prediction of the Standard Model is that, in the absence of
corrections lifting the energy density, the Higgs potential becomes negative at
large field values. If the Higgs field samples this part of the potential
during inflation, the negative energy density may locally destabilize the
spacetime. We use numerical simulations of the Einstein equations to study the
evolution of inflation-induced Higgs fluctuations as they grow towards the true
(negative-energy) minimum. These simulations show that forming a single patch
of true vacuum in our past light cone during inflation is incompatible with the
existence of our Universe; the boundary of the true vacuum region grows outward
in a causally disconnected manner from the crunching interior, which forms a
black hole. We also find that these black hole horizons may be arbitrarily
elongated---even forming black strings---in violation of the hoop conjecture.
By extending the numerical solution of the Fokker-Planck equation to the
exponentially suppressed tails of the field distribution at large field values,
we derive a rigorous correlation between a future measurement of the
tensor-to-scalar ratio and the scale at which the Higgs potential must receive
stabilizing corrections in order for the Universe to have survived inflation
until today.Comment: 36 pages, 11 figures; revised to match published versio
Isocurvature Perturbations and Non-Gaussianity of Gravitationally Produced Nonthermal Dark Matter
Gravitational particle production naturally occurs during the transition from
the inflationary phase to the non-inflationary phase. If the particles are
stable and very weakly interacting, they are natural nonthermal dark matter
candidates. We show that such nonthermal dark matter particles can produce
local non-Gaussianities large enough to be observed by ongoing and near future
experiments without being in conflict with the existing isocurvature bounds. Of
particular interest is the fact that these particles can be observable through
local non-Gaussianities even when they form a very small fraction of the total
dark matter content.Comment: 18 pages, 4 figures, version accepted by PR
Extended Axion Dark Matter Search Using the CAPP18T Haloscope
We report an extended search for the axion dark matter using the CAPP18T
haloscope. The CAPP18T experiment adopts innovative technologies of a
high-temperature superconducting magnet and a Josephson parametric converter.
The CAPP18T detector was reconstructed after an unexpected incident of the
high-temperature superconducting magnet quenching. The system reconstruction
includes rebuilding the magnet, improving the impedance matching in the
microwave chain, and mechanically readjusting the tuning rod to the cavity for
improved thermal contact. The total system noise temperature is 0.6\,K.
The coupling between the cavity and the strong antenna is maintained at to enhance the axion search scanning speed. The scan frequency range
is from 4.8077 to 4.8181 GHz. No significant indication of the axion dark
matter signature is observed. The results set the best upper bound of the
axion-photon-photon coupling () in the mass ranges of 19.883
to 19.926\,eV at 0.7 or
1.9 with 90\,\% confidence
level. The results demonstrate that a reliable search of the high-mass dark
matter axions can be achieved beyond the benchmark models using the technology
adopted in CAPP18T.Comment: 7 pages and 4 figure