52 research outputs found
Formation and Evolution of Primordial Black Hole Binaries in the Early Universe
The abundance of primordial black holes (PBHs) in the mass range can potentially be tested by gravitational wave observations due to
the large merger rate of PBH binaries formed in the early universe. To put the
estimates of the latter on a firmer footing, we first derive analytical PBH
merger rate for general PBH mass functions while imposing a minimal initial
comoving distance between the binary and the PBH nearest to it, in order to
pick only initial configurations where the binary would not get disrupted. We
then study the formation and evolution of PBH binaries before recombination by
performing N-body simulations. We find that the analytical estimate based on
the tidally perturbed 2-body system strongly overestimates the present merger
rate when PBHs comprise all dark matter, as most initial binaries are disrupted
by the surrounding PBHs. This is mostly due to the formation of compact N-body
systems at matter-radiation equality. However, if PBHs make up a small fraction
of the dark matter, , these estimates become more
reliable. In that case, the merger rate observed by LIGO imposes the strongest
constraint on the PBH abundance in the mass range . Finally,
we argue that, even if most initial PBH binaries are perturbed, the present
BH-BH merger rate of binaries formed in the early universe is larger than
Comment: 32pages, 12 figures, typos corrected, references added, figures
updated, matches version published in JCA
Evidence for Dark Matter Self-Interactions via Collisionless Shocks in Cluster Mergers
While dark matter self-interactions may solve several problems with structure
formation, so far only the effects of two-body scatterings of dark matter
particles have been considered. We show that, if a subdominant component of
dark matter is charged under an unbroken gauge group, collective dark
plasma effects need to be taken into account to understand its dynamics. Plasma
instabilities can lead to collisionless dark matter shocks in galaxy cluster
mergers which might have been already observed in the Abell 3827 and 520
clusters. As a concrete model we propose a thermally produced dark pair plasma
of vectorlike fermions. In this scenario the interacting dark matter component
is expected to be separated from the stars and the non-interacting dark matter
halos in cluster collisions. In addition, the missing satellite problem is
softened, while constraints from all other astrophysical and cosmological
observations are avoided.Comment: Matches the version to be published in Physics Letters
Equilibrium ion distribution in the presence of clearing electrodes and its influence on electron dynamics
Here we compute the ion distribution produced by an electron beam when
ion-clearing electrodes are installed. This ion density is established as an
equilibrium between gas ionization and ion clearing. The transverse ion
distributions are shown to strongly peak in the beam's center, producing very
nonlinear forces on the electron beam. We will analyze perturbations to the
beam properties by these nonlinear fields. To obtain reasonable simulation
speeds, we develop fast algorithms that take advantage of adiabatic invariants
and scaling properties of Maxwell's equations and the Lorentz force.
Our results are very relevant for high current Energy Recovery Linacs, where
ions are produced relatively quickly, and where clearing gaps in the electron
beam cannot easily be used for ion elimination. The examples in this paper
therefore use parameters of the Cornell Energy Recovery Linac project. For
simplicity we only consider the case of a circular electron beam of changing
diameter. However, we parameterize this model to approximate non-round beams
well. We find suitable places for clearing electrodes and compute the
equilibrium ion density and its effect on electron-emittance growth and halo
development. We find that it is not sufficient to place clearing electrodes
only at the minimum of the electron beam potential where ions are accumulated
Towards Completing the Standard Model: Vacuum Stability, EWSB and Dark Matter
We study the standard model (SM) in its full perturbative validity range
between and the Landau pole, assuming that a yet unknown
gravitational theory in the UV does not introduce additional particle
thresholds, as suggested by the tiny cosmological constant and the absence of
new stabilising physics at the EW scale. We find that, due to dimensional
transmutation, the SM Higgs potential has a global minimum at 10^26 GeV,
invalidating the SM as a phenomenologically acceptable model in this energy
range. We show that extending the classically scale invariant SM with one
complex singlet scalar S allows us to: (i) stabilise the SM Higgs potential;
(ii) induce a scale in the singlet sector via dimensional transmutation that
generates the negative SM Higgs mass term via the Higgs portal; (iii) provide a
stable CP-odd singlet as the thermal relic dark matter due to CP-conservation
of the scalar potential; (iv) provide a degree of freedom that can act as an
inflaton in the form of the CP-even singlet. The logarithmic behaviour of
dimensional transmutation allows one to accommodate the large hierarchy between
the electroweak scale and the Landau pole, while understanding the latter
requires a new non-perturbative view on the SM.Comment: 15 pages, 9 figures. Final version to be published in Physical Review
Anomalous Higgs-boson coupling effects in HWW production at the LHC
We study the LHC associated production of a Higgs boson and a W^+W^-
vector-boson pair at 14 TeV, in the Standard Model and beyond. We consider
different signatures corresponding to the cleanest H and W decay channels, and
discuss the potential of the high-luminosity phase of the LHC. In particular,
we investigate the sensitivity of the HWW production to possible anomalous
Higgs couplings to vector bosons and fermions. Since the b-quark initiated
partonic channel contributes significantly to this process, we find a moderate
sensitivity to both the size and sign of an anomalous top-quark Yukawa
coupling, because perturbative unitarity in the standard model implies a
destructive interference in the b b-bar subprocess. We show that a combination
of various signatures can reach a ~9 standard-deviation sensitivity in the
presently allowed negative region of the top-Higgs coupling, if not previously
excluded.Comment: 13 pages, 3 figure
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