arXiv subjects

Thomas Steingasser

Publications and source records attributed to Thomas Steingasser.

18 recordsLinked to original sources

arXiv2026

Darkly Charged ALPs

The established $d=5$ ALP effective Lagrangian describes the interaction of scalars with approximate shift-symmetry which carry no Standard Model (SM) charges with SM fields. It implicitly assumes that ALPs are not charged under any symmetries of the dark sector. In this paper, we remove this assumption. For ALPs carrying conserved dark charges, no $d=5$ ALP effective interaction to SM particles is possible. We build the effective Lagrangian for these darkly charged ALPs stemming from a general breaking pattern, and we show that the lowest-order shift-symmetric effective Lagrangian contains just two $d=6$ operators coupling ALPs to SM particles. We explore the model-independent phenomenological implications of these interactions, as well as the question of whether the dark matter observed in the Universe may consist of darkly charged ALPs. We identify higher order operators of the effective field theory, and determine which types of dark symmetry groups can seed darkly charged ALPs. Illustrative examples of ultraviolet completions which result in darkly charged ALPs at low-energies are provided as well. The darkly-charged ALP scenario can be generalized by including dark gauge interactions. In this paper, we have considered only the case with no such interactions.

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arXiv2026

Tunneling and tidal stripping in multifield ultralight dark matter halos

Tidal stripping is a key feature of the evolution of dark matter (DM) halos, and has major implications for the population of low-mass galaxies. In the case of ultralight DM, tidal stripping proceeds not only classically, at the tidal radius, but also via a process analogous to quantum tunneling by long-wavelength particles out of the potential of a subhalo. This modified tidal stripping behavior leads to tight constraints on the particle mass as a function of subhalo and host properties. As many models of ultralight DM predict several independent species, it is crucial to understand how these constraints can be generalized to multifield halos with different particle masses. However, numerical challenges make it difficult to directly study the tunneling process in all but the simplest multifield scenarios. We introduce a simplified approach based on semiclassical methods that entirely sidesteps the most difficult aspects of the numerical problem, and we apply this to the study of tunneling in multifield halos. Our results significantly clarify the physics of tidal stripping for ultralight DM halos even in the single-field case: we provide first-principles derivations of features of the tunneling rate previously suggested by empirical fits. We then evaluate stability bounds on two-field halos for the first time, for a wide range of density and particle mass ratios. We show that for particular parameter combinations, the stability bounds in the two-field case can be somewhat relaxed relative to the single-field case, but for much of the parameter space, the constraints become more stringent. We discuss the path towards probing realistic multifield ultralight DM halos.

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arXiv2026

On quantum tunnelling in the presence of Noether charges

We provide a complete first-principles based discussion of quantum tunnelling out of initial states carrying a conserved Noether charge. Our main result is a simple, unambiguous Euclidean-time prescription for the calculation of tunnelling rates out of such states. By relying on a combination of the direct approach and the steadyon framework for the evaluation of real-time path integrals, our derivation offers full transparency of its underlying assumptions, and is independent of any ad-hoc generalisations. This strategy also offers a simple explanation for the emergence of complex saddle points for such systems, justifying techniques postulated by earlier works. Our analysis furthermore offers the first results for initial states with both a conserved Noether charge and a non-trivial energy. We first illustrate the main conceptual points of our analysis for the simple example of a point particle in two spatial dimensions carrying a conserved angular momentum. Then, we generalise our results to the case of multiple dimensions and an arbitrary conserved Noether charge, providing an easy-to-implement prescription for the calculation of the tunnelling rate. We furthermore apply our results to the example of a complex scalar field subject to a global U(1)-symmetry with associated charge. These results provide a reliable foundation for the calculation of tunnelling rates in applications in finite-density and charge-asymmetric systems.

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arXiv2025

A new connection between WIMP dark matter and the hierarchy problem

This work proposes a direct link between the hierarchy problem and Weakly Interacting Massive Particles (WIMPs): we suggest that the small mass of the Higgs boson arises from being dynamically driven to the scale of the WIMP. Such a special electroweak vacuum is singled out by lying close to the critical boundary of a phase transition, as recently explored in a new class of cosmological solutions to the hierarchy problem. They generically predict the Higgs potential to be destabilised just above the weak scale. Intriguingly, the requirement for new physics to achieve this coincides with two independently well-motivated expectations: a split spectrum of light fermions and heavy bosons, as anticipated from naturalness, and the so-called "WIMP miracle". A WIMP with mass around the weak scale not only happens to have the correct thermal relic abundance to be the dark matter (DM), it can also give rise to the necessary critical boundary at the TeV scale through its Yukawa couplings to the Higgs. We use a higgsino-like singlet-doublet model to illustrate our Higgs-DM criticality scenario and show that if this WIMP DM mass is observed to be greater than ~1.2 TeV then it necessarily implies a strong bound on the Higgs mass and an upper bound on the scale of heavy new physics that restores vacuum stability. It can be thoroughly probed in direct detection experiments, astrophysical signals and future collider searches, further motivating a comprehensive exploration of the remaining heavy WIMP parameter space.

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arXiv2025

Path integral predictions for pre-asymptotic false vacuum decay

When tunneling occurs out of generic initial states, a significant fraction of probability is lost at early times during which the dynamics is governed by excited resonance states. However, first-principles analyses based on path integrals have only captured the leading asymptotic behavior during which the tunneling rate is dominated by the false vacuum contribution. In this work, we discuss the behavior in the pre-asymptotic regime from a first-principles path integral perspective. We demonstrate how the relevant expressions can be evaluated systematically through semi-classical methods in the recently developed steadyon picture. This approach allows one to trace the role of the relevant physical scales, making transparent the underlying assumptions and approximations and offering a clear path to establishing a systematically improvable framework to evaluate tunneling rates non-perturbatively.

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arXiv2025

Quantum tunneling from excited states in the steadyon picture

Recent developments in the understanding of real-time path integrals led to the development of the ``steadyon picture'' for the semi-classical calculation of quantum tunneling rates. We discuss tunneling out of a generic localized initial state in this picture and present its application for the important example of a resonance state in a one-dimensional point particle potential. We find that the steadyon picture indeed reproduces existing results obtained using the WKB method. Our analysis furthermore demonstrates how applying this picture to physical states naturally addresses open conceptual questions regarding this framework. Finally, we perform a numerical study for a specific potential. We demonstrate in particular the existence of regimes in which the tunneling rate is dominated by higher resonances, rather than the false vacuum, as well as their importance.

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arXiv2025

Precision Unitarity Calculations in Inflationary Models

We revisit perturbative unitarity in scalar field inflation with a nonminimal coupling, with Higgs inflation serving as the most prominent example. Although such models are phenomenologically successful, it is critical to examine whether or not unitarity violations spoil their theoretical self-consistency. The analysis of these issues has so far typically relied on order-of-magnitude estimates of scattering amplitudes, which are appropriate for generic parameters. It is not evident that these methods apply to scenarios relying on a near-critical inflationary potential, for which an interplay of both small scalar self-couplings and nonminimal couplings could partially alleviate the unitarity issues. To allow for an exploration of this possibility, we consider the full $S$-matrix for the relevant scattering processes, taking into account important phase space volume factors, leading to a precise evaluation of the cut-off scale. In the single-field case, we demonstrate that near-criticality raises the cut-off scale considerably, compared to previous estimates. In the multifield case, momentum-dependent self-interactions in the kinetic sector lower the cut-off compared to the single-field case to a value comparable to but slightly larger than previous estimates. We carefully study both the single-field and multifield cases in metric and metric-affine (Palatini) formulations of gravity, as well as introduce a new phenomenologically viable model with a canonical kinetic term and a significantly raised cut-off, and discuss the importance of background field effects.

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arXiv2025

Higgs Criticality and the Metastability Bound: a target for future colliders

New physics at the TeV scale or lower may destabilise the electroweak vacuum. How low could the vacuum instability scale be? This fundamental question may be tied to a deeper understanding of the Higgs potential and its associated hierarchy problem. The scale of vacuum instability can be viewed as an upper bound on the Higgs mass-the so-called vacuum metastability bound-and criticality of the Higgs potential through some underlying mechanism then places our universe at this metastable point. In this report, we summarise recent work developing this eminently testable hypothesis. If the vacuum metastability bound plays a role in determining the properties of the Higgs boson, the new physics responsible will likely be discovered or excluded in the entire natural region of parameter space at future facilities. This makes it a tantalising and attractive target for future colliders.

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arXiv2025

Higgs near-criticality at future colliders

The so-called metastability bound on the Higgs mass suggests that the smallness of the Higgs mass may be a byproduct of the metastability of the electroweak vacuum. A significantly strong bound requires new physics capable of lowering the scale where the Higgs quartic coupling turns negative through renormalization group effects, without destabilizing the electroweak vacuum entirely. We analyze in this context the low-scale Majoron model of neutrino masses, which automatically contains two key elements for a viable scenario: heavy fermions to lower the instability scale and a extended scalar sector to stabilize the potential and achieve realistic lifetimes for the electroweak vacuum. We show how the metastability bound can be generalized to theories with multiple scalars and present an efficient way of calculating the tunneling rate in such potentials. We also demonstrate that FCC will probe regions of the parameter space relevant for metastability: large regions of the fermionic sector at FCC-ee and some reach to the scalar sector at FCC-hh.

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arXiv2025

Gauge hierarchy and metastability from Higgs-driven crunching

We present a new solution to the Higgs hierarchy problem based on dynamical vacuum selection in a landscape scanning the Higgs mass. In patches where the Higgs mass parameter takes a natural value, the Higgs potential only admits a minimum with a large and negative energy density. This causes a cosmological crunch, removing such patches from the landscape. Conversely, in patches where the Higgs mass parameter is smaller than a critical value, the Higgs potential admits a metastable minimum with the standard cosmological history. This critical value is determined by the instability scale, where the quartic coupling turns negative due to its running. The ability of this mechanism to explain the observed Higgs mass hinges on new physics at the TeV scale, such as vector-like fermions. We study two simple realizations of this scenario in a heavy neutral lepton model and in the singlet-doublet model, the latter mimicking a Higgsino-bino system. We show that the relevant parts of their parameter spaces can be probed by proposed future colliders, such as the FCC-ee or a muon collider.

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arXiv2024

Spontaneous symmetry breaking, gauge hierarchy and electroweak vacuum metastability

The so-called metastability bound asserts that an unnaturally small Higgs mass is a necessary condition for electroweak vacuum metastability, offering a new approach towards solving the hierarchy problem. So far, this result relies on the assumption of a negative Higgs mass parameter, or equivalently, on electroweak spontaneous symmetry breaking. We derive a new, corresponding bound for the case of a positive mass parameter. When the new bound is significantly more restrictive than or comparable to its established counterpart, it may offer an explanation for the sign of the Higgs mass parameter, and thus, spontaneous symmetry breaking itself. New physics at scales $\mathcal{O} (1-10)$ TeV can lower these bounds as far as the TeV-scale. As an illustration, we consider vacuum stability in the presence of additional TeV-scale fermions with Yukawa couplings to the Higgs, as well as a dimension-six term parameterizing new physics in the UV. This scenario requires new physics that couples strongly to the Higgs, and can potentially be probed at future colliders. Finally, to allow for comparison with concrete mechanisms predicting metastability, we provide the mass-dependent lifetime of the electroweak vacuum for this model.

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arXiv2024

Higgs criticality in and beyond the Standard Model

The properties of the Higgs potential are determined by three parameters: the mass parameter, the quartic self-coupling, and a constant term. Remarkably, all three of these parameters seem subject to a significant amount of fine-tuning. All these tunings can be seen as their corresponding parameters being close to critical values marking quantum phase transitions. While such behavior is surprising from a conventional particle physics perspective, it is a common feature of dynamical systems. This has motivated the conjecture that the values of the Higgs' parameters are the result of some dynamical mechanism. This possibility suggests the construction of mechanisms dynamically choosing sets of Higgs parameters. In these notes, I discuss a complementary approach. Taking seriously that such a mechanism could exist in nature, it is plausible to assume that it also influences Beyond-Standard-Model physics. This suggests considering near-criticality in any model of interest and investigating its consequences more generally, in particular independent of a concrete mechanism responsible for it. I first explain what it means for the parameters of the Higgs potential to be near-critical. This includes a discussion of the recently discovered "metastability bound" on the Higgs mass, which can be understood through a critical point. I then review two concrete examples of mechanisms in which the parameters of the Higgs potential are dynamically driven towards critical values. These mechanisms also serve as an important proof on concept for the feasibility of the assumptions at the foundation of these notes. Using a simple example for concreteness, the final part of these notes then explicitly demonstrates how to approach a given model in the light of the near-criticality conjecture.

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arXiv2024

Toward quantum tunneling from excited states: Recovering imaginary-time instantons from a real-time analysis

We revisit the path integral description of quantum tunneling and lay the groundwork for its generalization to excites states through real-time path integral techniques. For clarity, we focus on the simple toy model of a point particle in a double-well potential, for which we perform all steps explicitly. Instead of performing the familiar Wick rotation from physical to imaginary time -- which is inconsistent with the requisite boundary conditions when treating tunneling from states other than the false vacuum -- we regularize the path integral by adding an infinitesimal complex contribution to the Hamiltonian, while keeping time strictly real. We find that this gives rise to a complex stationary-phase solution, in agreement with recent insights from Picard-Lefshitz theory. We then show that there exists a class of analytic solutions for the corresponding equations of motion, which can be made to match the appropriate boundary conditions in the physically relevant limits of a vanishing regulator and an infinite physical time. We provide a detailed discussion of this non-trivial limit. We find that, for systems without an explicit time-dependence, our approach reproduces the picture of an instanton-like solution defined on a finite Euclidean-time interval. Lastly, we discuss the generalization of our approach to broader classes of systems, for which it serves as a reliable framework for high-precision calculations.

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arXiv2023

Finite-Temperature Instantons from First Principles

We derive the finite-temperature quantum-tunneling rate from first principles. The rate depends on both real- and imaginary-time; we demonstrate that the relevant instantons should therefore be defined on a Schwinger-Keldysh contour, and how the familiar Euclidean-time result arises from it in the limit of large physical times. We generalize previous results for general initial states, and identify distinct behavior in the high- and low-temperature limits, incorporating effects from background fields. We construct a consistent perturbative scheme that incorporates large finite-temperature effects.

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arXiv2023

Gravity-improved metastability bounds for the Type-I Seesaw Mechanism

Right-handed neutrinos (RHN) destabilize the electroweak vacuum by increasing its decay rate. In the SM, the latter is dominated by physics at the RG scale at which $λ$ reaches its minimum, $μ_*^{\text{SM}} \sim 10^{17}$ GeV. For large neutrino Yukawa coupling $Y_ν$, RHNs can push $μ_*$ beyond the Planck scale, implying that gravitational effects need to be taken into account. In this work, we perform the first comprehensive study of electroweak vacuum metastability in the type-I seesaw mechanism including these effects. Our analysis covers both low- and high-scale seesaw models, with two as well as three RHNs and for multiple values of the Higgs' non-minimal coupling to gravity. We find that gravitational effects can significantly stabilize the vacuum, leading to weaker metastability bounds. We show that metastability sets the strongest bounds for low-scale seesaws with $M_N>1$ TeV. For high-scale seesaws, we find upper bounds on the allowed masses for the RHNs, which are relevant for high-scale leptogenesis. We also point out that $\text{Tr}(Y_ν^\dagger Y_ν)$, which is commonly used to express these metastability bounds, cannot be used for all of parameter space. Instead, we argue that bounds can always be expressed reliably through $\text{Tr}(Y_ν^\dagger Y_ν\,Y_ν^\dagger Y_ν)$. Lastly, we use this insight to develop a new technique for an easier RG analysis applicable to scenarios with degenerate RHN masses.

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arXiv2023

Higgs Criticality beyond the Standard Model

Both parameters in the Higgs field's potential, its mass and quartic coupling, appear fine-tuned to near-critical values, which gives rise to the hierarchy problem and the metastability of the electroweak vacuum. Whereas such behavior appears puzzling in the context of particle physics, it is a common feature of dynamical systems, which has led to the suggestion that the parameters of the Higgs potential could be set through some dynamical process. In this article, we discuss how this notion could be extended to physics beyond the Standard Model (SM). We first review in which sense the SM Higgs parameters can be understood as near-critical and show that this notion can be extrapolated in a unique way for a generic class of SM extensions. Our main result is a prediction for the parameters of such models in terms of their corresponding Standard Model effective field theory Wilson coefficients and corresponding matching scale. For generic models, our result suggests that the scale of new (bosonic) physics lies close to the instability scale. We explore the potentially observable consequences of this connection, and illustrate aspects of our analysis with a concrete example. Lastly, we discuss implications of our results for several mechanisms of dynamical vacuum selection associated with various Beyond-Standard-Model (BSM) constructions.

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arXiv2022

Gauge hierarchy from electroweak vacuum metastability

We consider the possibility that the gauge hierarchy is a byproduct of the metastability of the electroweak vacuum, i.e., that whatever mechanism is responsible for the latter also sets the running Higgs mass to a value smaller than its natural value by many orders of magnitude. This perspective is motivated by the early-time framework for eternal inflation put forth recently, which favors vacua that are relatively short-lived, but applies more generally to any theoretical approach predicting that our vacuum should be metastable. We find that the metastability of the electroweak vacuum, together with the requirement that such a non-trivial vacuum exists, requires the Higgs mass to be smaller than the instability scale by around one order of magnitude. While this bound is quite weak in the Standard Model (SM), as the instability scale is $\sim 10^{11}$ GeV, simple and well-motivated extensions of the SM - concretely, the $ν$MSM with an approximate $B-\tilde{L}$ symmetry and the minimal SU(4)/Sp(4) composite Higgs model - can significantly tighten the bound by lowering the instability scale. We find that the bound can be brought down to $\simeq 10$ TeV where our perturbative treatment of the decay rate becomes unreliable. Our results imply that, assuming the SM symmetry breaking pattern, small running Higgs masses are a universal property of theories giving rise to metastability, suggesting a common origin of the two underlying fine-tunings and providing a strong constraint on any attempt to explain metastability.

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arXiv2020

On the domain of moduli fields

The concept of the moduli space allows for a simple, universally applicable description of the low-energy dynamics of topological solitons. This description is remarkably insensitive to the properties of the underlying theory, whose details only manifest themselves via the moduli space metric. This article presents a generalization of this concept, which allows to transfer its most intriguing features to configurations of any energy captured by the theory giving rise to the soliton, given that these are localized sufficiently close to the soliton's center. The resulting theory is capable of describing all dynamics within its range of applicability by just one family of fields, with all the information about the underlying theory entering via a finite number of background functions, which can be linked to physical properties of the present soliton.

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