A
bstract
We consider a minimal classically scale-invariant extension of the Standard Model. In this theory, the Higgs mechanism is triggered and the electroweak symmetry breaking is generated ...radiatively by the Coleman-Weinberg sector which is coupled to the SM Higgs. We extend the Higgs portal interactions of the theory to include an additional singlet which is also non-minimally coupled to gravity. This generates a single-field slow-roll inflation mechanism in the effective field theory formulation which is robust up to Planck scales. Our approach does not require integrating in any additional new physics degrees of freedom to unitarise the theory in the sub-Planckian regime where inflation happens. As a result, no large threshold corrections appear in our approach to inflation so that the electroweak scale and the SM Higgs mass are not affected. The singlet field responsible for inflation also gives a viable dark matter candidate in our model. We also discuss the relation between classical scale-invariance of the effective theory and the possible local scale invariance of the full theory and comment on the naturalness of the electroweak scale.
Motivated by advanced LIGO (aLIGO)’s recent discovery of gravitational waves, we discuss signatures of new physics that could be seen at ground- and space-based interferometers. We show that a ...first-order phase transition in a dark sector would lead to a detectable gravitational wave signal at future experiments, if the phase transition has occurred at temperatures few orders of magnitude higher than the electroweak scale. The source of gravitational waves in this case is associated with the dynamics of expanding and colliding bubbles in the early universe. At the same time we point out that topological defects, such as dark sector domain walls, may generate a detectable signal already at aLIGO. Both bubble and domain-wall scenarios are sourced by semiclassical configurations of a dark new physics sector. In the first case, the gravitational wave signal originates from bubble wall collisions and subsequent turbulence in hot plasma in the early universe, while the second case corresponds to domain walls passing through the interferometer at present and is not related to gravitational waves. We find that aLIGO at its current sensitivity can detect smoking-gun signatures from domain-wall interactions, while future proposed experiments including the fifth phase of aLIGO at design sensitivity can probe dark sector phase transitions.
A
bstract
We consider scattering processes involving massless fermions and ’t Hooft-Polyakov magnetic monopoles in a minimal SU(2) model and in the Grand Unified SU(5) theory. We construct ...expressions for on-shell amplitudes for these processes in the
J
= 0 partial wave using the spinor helicity basis consisting of single-particle and pairwise helicities. These processes are unsuppressed and are relevant for the monopole catalysis of proton decay. The amplitudes for the minimal processes involving a single fermion scattering on a monopole in the initial state and half-fermion solitons in the final state are presented for the first time and are used to obtain the amplitudes for processes involving more fermions in the initial state and integer fermion numbers in the final state. A number of such anomalous and non-anomalous processes, along with their amplitude expressions, are written down for the SU(5) GUT model.
The axion mass from 5D small instantons Gherghetta, Tony; Khoze, Valentin V.; Pomarol, Alex ...
The journal of high energy physics,
03/2020, Letnik:
2020, Številka:
3
Journal Article
Recenzirano
Odprti dostop
A
bstract
We calculate a new contribution to the axion mass that arises from gluons propagating in a 5th dimension at high energies. By uplifting the 4D instanton solution to five dimensions, the ...positive frequency modes of the Kaluza-Klein states generate a power-law term in the effective action that inversely grows with the instanton size. This causes 5D small instantons to enhance the axion mass in a way that does not spoil the axion solution to the strong CP problem. Moreover this enhancement can be much larger than the usual QCD contribution from large instantons, although it requires the 5D gauge theory to be near the non-perturbative limit. Thus our result suggests that the mass range of axions (or axion-like particles), which is important for ongoing experimental searches, can depend sensitively on the UV modification of QCD.
We introduce and discuss two inter-related mechanisms operative in the electroweak sector of the Standard Model at high energies. Higgsplosion, the first mechanism, occurs at some critical energy in ...the 25 to 103 TeV range, and leads to an exponentially growing decay rate of highly energetic particles into multiple Higgs bosons. We argue that this is a well-controlled non-perturbative phenomenon in the Higgs-sector which involves the final state Higgs multiplicities n in the regime nλ≫1 where λ is the Higgs self-coupling. If this mechanism is realised in nature, the cross-sections for producing ultra-high multiplicities of Higgs bosons are likely to become observable and even dominant in this energy range. At the same time, however, the apparent exponential growth of these cross-sections at even higher energies will be tamed and automatically cut-off by a related Higgspersion mechanism. As a result, and in contrast to previous studies, multi-Higgs production does not violate perturbative unitarity. Building on this approach, we then argue that the effects of Higgsplosion alter quantum corrections from very heavy states to the Higgs boson mass. Above a certain energy, which is much smaller than their masses, these states would rapidly decay into multiple Higgs bosons. The heavy states become unrealised as they decay much faster than they are formed. The loop integrals contributing to the Higgs mass will be cut off not by the masses of the heavy states, but by the characteristic loop momenta where their decay widths become comparable to their masses. Hence, the cut-off scale would be many orders of magnitude lower than the heavy mass scales themselves, thus suppressing their quantum corrections to the Higgs boson mass.
A
bstract
We consider classically scale-invariant extensions of the Standard Model (
CSI ESM
) which stabilise the Higgs potential and have good dark matter candidates. In this framework all mass ...scales, including electroweak and dark matter masses, are generated dynamically and have a common origin. We consider Abelian and non-Abelian hidden sectors portally coupled to the SM with and without a real singlet scalar. We perform a careful analysis of RG running to determine regions in the parameter space where the SM Higgs vacuum is stabilised. After combining this with the LHC Higgs constraints, in models without a singlet, none of the regained parameter space in Abelian ESMs, and only a small section in the non-Abelian ESM survives. However, in all singlet-extended models we find that the Higgs vacuum can be stabilised in all of the parameter space consistent with the LHC constraints. These models naturally contain two dark matter candidates: the real singlet and the dark gauge boson in non-Abelian models. We determine the viable range of parameters in the CSI ESM framework by computing the relic abundance, imposing direct detection constraints and combining with the LHC Higgs constraints. In addition to being instrumental in Higgs stabilisation, we find that the singlet component is required to explain the observed dark matter density.
A
bstract
In a scalar theory which we use as a simplified model for the Higgs sector, we adopt the semiclassical formalism of Son for computations of
n
-particle production cross-sections in the ...high-multiplicity
n
→ ∞ weak-coupling
λ
→ 0 regime with the value of λ
n
held fixed and large. The approach relies on the use of singular classical solutions to a certain boundary value problem. In the past this formalism has been successfully used and verified in computations of perturbative multi-particle processes at tree-level, and also at the next-to-leading order level in the small
λn
expansion near the multi-particle mass threshold. We apply this singular solutions formalism in the regime of ultra-high multiplicities where
λn
≫ 1, and compute the leading positive ∼
n
λ
n
contribution to the exponent of the multi-particle rate in this large
λn
limit. The computation is carried out near the multi-particle mass threshold where the multiplicity
n
approaches its maximal value allowed by kinematics. This calculation relies on the idea of Gorsky and Voloshin to use a thin wall approximation for the singular solutions that resemble critical bubbles. This approximation is justified in precisely the high-multiplicity
λ
n
→
∞
regime of interest. Based on our results we show that the scalar theory with a spontaneous symmetry breaking used here as a simplified model for the Higgs sector, is very likely to realise the high-energy Higgsplosion phenomenon.
Multiparticle amplitudes in a scalar EFT Khoze, Valentin V.; Schenk, Sebastian
The journal of high energy physics,
05/2022, Letnik:
2022, Številka:
5
Journal Article
Recenzirano
Odprti dostop
A
bstract
At sufficiently high energies the production of a very large number of particles is kinematically allowed. However, it is well-known that already in the simplest case of a weakly-coupled ...massive
λφ
4
theory,
n
-particle amplitudes become non-perturbative in the limit where
n
scales with energy. In this case, the effective expansion parameter,
λn
, is no longer small and the perturbative approach breaks down. In general, the associated
n
-particle production rates were argued to be described by an exponential that, depending on the specifics of the underlying Quantum Field Theory model, could be either growing or decaying in the large-
n
regime. We investigate such processes in general settings of Effective Field Theory (EFT), involving arbitrary higher-dimensional operators of
φ
. We perform the resummation of all leading loop corrections arising from EFT vertices for amplitudes at the multiparticle threshold. We find that the net effect of higher-dimensional operators amounts to an exponentially growing factor. We show that if an exponential growth was already generated by the renormalizable interactions, it would then be further enhanced by the EFT contributions. On the other hand, if the multiparticle rates computed in the renormalizable part of the theory were suppressed, this suppression would not be lifted in the EFT.
A
bstract
The Standard Model with an added Higgs portal interaction and no explicit mass terms is a classically scale-invariant theory. In this case the scale of electroweak symmetry breaking can be ...induced radiatively by the Coleman-Weinberg mechanism operational in a hidden sector, and then transmitted to the Standard Model through the Higgs portal. The smallness of the generated values for the Higgs vev and mass, compared to the UV cutoff of our classically scale-invariant effective theory, is naturally explained by this mechanism.
We show how these classically conformal models can generate the baryon asymmetry of the Universe without the need of introducing mass scales by hand or their resonant fine-tuning. The minimal model we consider is the Standard Model coupled to the Coleman-Weinberg scalar field charged under the U(1)
B−L
gauge group. Anomaly cancellation requires automatic inclusion of three generations of right-handed neutrinos. Their GeV-scale Majorana masses are induced by the Coleman-Weinberg field and lead to the generation of active neutrino masses through the standard see-saw mechanism. Leptogenesis occurs via flavour oscillations of right-handed sterile neutrinos and is converted to the baryon asymmetry by electroweak sphalerons.
A
bstract
In a secluded dark sector which is coupled to the Standard Model via a Higgs portal interaction we arrange for the existence of ’t Hooft-Polyakov magnetic monopoles and study their ...implications for cosmology. We point out that a dark sector which can accommodate stable monopoles will also contain massless dark photons
γ
′
as well as charged massive vector bosons
W
±
′
. The dark matter in this scenario will be a combination of magnetically and electrically charged species under the unbroken U(1) subgroup of the dark sector. We estimate the cosmological production rate of monopoles and the rate of monopole-anti-monopole annihilation and conclude that monopoles with masses of few hundred TeV or greater, can produce sizeable contributions to the observed dark matter relic density. We scan over the parameter space and compute the relic density for monopoles and vector bosons. Turning to dark photons, we compute their contribution to the measured density of relativistic particles
N
eff
and also apply observational constraints from the Bullet cluster and other large scale galaxies on long-range interactions of monopoles and of dark vector bosons. At scales relevant for dwarf galaxies we identify regions on the parameter space where self-interacting monopole and vector dark mater components can aid solving the core-vs-cusp and the too-big-to-fail problems.
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