arXiv subjects

Ben A. Stefanek

Publications and source records attributed to Ben A. Stefanek.

At least 19 recordsLinked to original sources

arXiv2026

Minimal Flavor Protection for TeV-scale New Physics

We determine how much TeV-scale new physics can deviate from flavor universality, $U(3)^5$, while respecting stringent bounds on flavor-changing neutral currents. The minimal continuous subgroup that must be approximately preserved is identified as $SU(2)_{q} \times U(1)_{X}$. With only a few symmetry-breaking spurions of $\mathcal{O}(10^{-2})$, all observed fermion hierarchies may be reproduced, offering a new perspective on the SM flavor puzzle. Remarkably, this framework provides structural flavor protection for generic TeV-scale new physics within the SMEFT, enlarging the space of collider-accessible scenarios beyond MFV and $U(2)^5$ and allowing for richer patterns of flavor violation.

hep-ph↗

arXiv2026

Electroweak precision observables at NLL order in the SMEFT

Electroweak precision observables (EWPO) remain among the most powerful indirect probes of physics beyond the Standard Model (BSM), and future $e^+ e^-$ colliders are expected to substantially enhance their sensitivity. Within the framework of the Standard Model Effective Field Theory (SMEFT), we show how EWPO predictions can be systematically improved to next-to-leading-logarithmic (NLL) accuracy by combining fixed-order tree-level and one-loop matching with two- and one-loop renormalization-group evolution. We derive NLL expressions that encode the full dependence on all 210 independent Wilson coefficients and illustrate their impact through fits to both current and projected collider measurements. This work provides a basis for global SMEFT analyses at NLL precision, thereby maximizing the indirect reach to BSM physics through EWPO measurements. The phenomenological relevance of this framework is illustrated explicitly in both the custodial Randall-Sundrum model and a custodial composite Higgs model.

hep-ph↗

arXiv2026

The art of simulating the early Universe. Part II. Non-canonical cases & gravitational waves

We present a discussion on lattice techniques for the simulation of non-canonical field theory circumstances, complementing our previous monograph (arXiv:2006.15122) on canonical cases. We begin by reviewing basic aspects of lattice field theory, including symplectic and non-symplectic evolution algorithms. We then introduce lattice implementations of non-canonical interactions, considering scalars with a non-minimal coupling to gravity, $ϕ^2R$, non-minimal scalar kinetic theories, $\mathcal{G}_{ab}(\lbraceϕ_c\rbrace)\partial_μϕ^a\partial^μϕ^b$, and axion-like particle (ALP) interactions with Abelian gauge fields, $ϕF_{μν}\tilde F^{μν}$. Next, we discuss methods to set up special field configurations, including the creation of cosmic defect networks towards scaling (e.g. cosmic strings and domain walls), field configurations based on arbitrary power spectra or spatial profiles, and probabilistic methods as required e.g. for thermal configurations. We further extend the notion of non-canonical theories, discussing the discretization of scalar field dynamics in $d + 1$ dimensions, with $d \neq 3$. Unrelated to non-canonical aspects, we also discuss implementation(s) of gravitational wave (GW) dynamics on the lattice. This document represents the theoretical basis for the non-canonical field theory aspects (interactions, initial conditions, dimensionality) and GW dynamics implemented in ${\mathcal C}$osmo${\mathcal L}$attice v2.0, to be released in 2026.

astro-ph.CO↗

arXiv2025

Accuracy complements energy: electroweak precision tests at Tera-Z

A Tera-$Z$ factory, such as FCC-ee or CEPC, will have indirect sensitivity to heavy new physics up to the tens of TeV scale through higher-order loop contributions to precision measurements at the $Z$ pole. These indirect quantum effects may provide complementary, or even better, sensitivity to potential deviations from the Standard Model that are typically thought to best be constrained at leading order at higher energies above the $Z$ pole. We show in the SMEFT framework how accuracy complements energy for operators that modify the Higgs and gauge boson two- and three-point functions, leading to improved projected sensitivities for models such as the real singlet scalar, weakly interacting massive particles, and a custodial weak quadruplet. A thorough Tera-$Z$ programme may thus anticipate aspects of physics runs at higher energies and provide a wider scope of quantum exploration of the TeV scale than had previously been appreciated.

hep-ph↗

arXiv2025

Third-Family Quark-Lepton Unification and Electroweak Precision Tests

We analyze the compatibility of the hypothesis of third-family quark-lepton unification at the TeV scale with electroweak precision data, lepton flavor universality tests, and high-$p_T$ constraints. We work within the framework of the UV complete flavor non-universal 4321 gauge model, which is matched at one loop to the Standard Model Effective Field Theory. For consistency, all electroweak precision observables are also computed at one loop within the effective field theory. At tree level, the most sizeable corrections are to $W\rightarrow τν_τ$ and $Z \to ν_τν_τ$ due to integrating out a pseudo-Dirac singlet fermion required by the model for neutrino mass generation. At loop level, the new colored states of the model generate large flavor-universal contributions to the electroweak precision observables via leading- and next-to-leading log running effects, yielding a significant improvement in the electroweak fit (including an increase in the $W$-boson mass). These effects cannot be decoupled if the model addresses the charged-current $B$-meson anomalies. Overall, we find good compatibility between the data sets, while simultaneously satisfying all low- and high-energy constraints.

hep-ph↗

arXiv2025

Cornering Natural SUSY at a Tera-$Z$ Factory

The future circular $e^+ e^-$ collider (FCC-ee) stands out as the next flagship project in particle physics, dedicated to uncovering the microscopic origin of the Higgs boson. In this context, we assess indirect probes of the Minimal Supersymmetric Standard Model (MSSM), a well-established benchmark hypothesis, exploring the complementarity between Higgs measurements and electroweak precision tests at the $Z$-pole. We study three key sectors: the heavy Higgs doublet, scalar top partners, and light gauginos and higgsinos, focusing on the parameter space favored by naturalness. Remarkably, the Tera-$Z$ program consistently offers significantly greater indirect sensitivity than the Mega-$h$ run. While promising, these prospects hinge on reducing SM uncertainties. Accordingly, we highlight key precision observables for targeted theoretical work.

hep-ph↗

arXiv2025

The Higgs Self-Coupling at FCC-ee

Single Higgs production at FCC-ee probes the Higgs self-coupling at next-to-leading order (NLO). Extracting a bound requires a global analysis accounting for other possible new physics contributions up to NLO. We determine the FCC-ee sensitivity to Higgs self-coupling modifications $δκ_λ$ within the Standard Model Effective Field Theory (SMEFT) framework, including for the first time flavour, LEP, LHC, and FCC-ee observables in a global analysis with all leading NLO effects via one-loop renormalisation group evolution, as well as incorporating finite NLO contributions to electroweak precision and $ZH$ observables. The global sensitivity to $δκ_λ$ is estimated by marginalising over the effects of all other operators, bringing flavour considerations to the fore. We find that, under reasonable assumptions, FCC-ee sensitivity to $δκ_λ$ can exceed that of the HL-LHC.

hep-ph↗

arXiv2025

Non-universal probes of composite Higgs models: New bounds and prospects for FCC-ee

We study the leading loop-level phenomenology of composite Higgs models via the effective field theory of a strongly interacting light Higgs and top quark (SILH+TQ). We systematically analyze the renormalization group evolution (RGE) of tree-generated operators in the SILH+TQ scenario, finding large mixings of flavor non-universal operators into those affecting electroweak precision observables. We show that these model-independent RG contributions are more important than typical estimates for finite matching terms. Flavor non-universal effects are completely captured by examining three options for the top mixing: fully composite $q_L^3$, equal compositeness, and fully composite $t_R$. In the most phenomenologically viable case of a fully composite $t_R$, we show that the strongest bound on the natural parameter space comes from a 2-loop double-log contribution of the 4-top operator $O_{tt} = (\bar t_R γ_μt_R)(\bar t_R γ^μt_R)$ into the Peskin-Takeuchi $T$ parameter. In general, we find that this 2-loop effect allows existing electroweak precision data to give better constraints on 4-top operators than high-energy probes from top production at the LHC. Independent of the top mixing, we find that a future tera-$Z$ machine such as FCC-ee has the potential to probe composite Higgs models up to a scale of $m_* \gtrsim 25$ TeV, and test the naturalness of the electroweak scale at the $\lesssim 10^{-4}$ level.

hep-ph↗

arXiv2024

Renormalization of the primordial inflationary power spectra

It has been suggested that the effects of renormalization significantly reduce the amplitude of the inflationary spectra at scales measurable in the cosmic microwave background. Via a gauge-invariant analysis, we compute the renormalized scalar and tensor power spectra and follow their evolution in an inflating universe that undergoes a transition to an FRW phase with a growing horizon. For perturbations originating from Minkowski vacuum fluctuations, we show that the standard prediction for the spectra on superhorizon scales is a late-time attractor, while they are UV finite at all times. Our result is independent of the equation of state after inflation, showing that the standard prediction is fully robust.

gr-qc↗

arXiv2024

Dynamics of Non-minimally Coupled Scalar Fields in the Jordan Frame

The presence of scalar fields with non-minimal gravitational interactions of the form $ξ|ϕ|^2 R$ may have important implications for the physics of the early universe. While many studies solve the dynamics of non-minimally coupled scalars in the Einstein frame, where gravity is simply described by the Einstein-Hilbert action, we instead propose a procedure to solve the dynamics directly in the original Jordan frame where the non-minimal couplings are maintained explicitly. Our algorithm can be applied to scenarios that include minimally coupled fields and an arbitrary number of non-minimally coupled scalars, with the expansion of the universe sourced by all fields present. This includes situations when the dynamics become fully inhomogeneous, fully non-linear (due to e.g.~backreaction or mode rescattering effects), and/or when the expansion of the universe is dominated by non-minimally coupled species. As an example, we study geometric preheating with a non-minimally coupled scalar spectator field when the inflaton oscillates following the end of inflation. In the future, our technique may be used to shed light on aspects of the equivalence of the Jordan and Einstein frames at the quantum level.

astro-ph.CO↗

arXiv2024

Ricci Reheating on the Lattice

We study the dynamics of a non-minimally coupled (NMC) scalar spectator field in non-oscillatory inflationary scenarios, where there is a transition from inflation to kination domination (KD). Engineering a realistic finite-duration transition through a CMB-compatible inflaton potential, we calculate the initial tachyonic growth of the NMC field during KD and perform lattice simulations of the subsequent non-linear dynamics. We characterize the regularization effect on the tachyonic growth, either due to self-interactions, or via gravitational backreaction when the NMC field grows to dominate the energy of the universe. Our study provides the first realistic treatment of the dynamics, with significant improvements compared to previous work, where one or more of the following aspects were assumed: ($i$) the background expansion can be neglected during the tachyonic growth, ($ii$) coherence of the NMC field, ($iii$) coherence of the inflaton, ($iv$) instantaneous transition, and ($v$) a KD equation of state of exactly $w = 1$. Using our methodology, which requires none of the above assumptions, we determine the conditions to achieve proper reheating, i.e. energetic dominance of the NMC field over the inflaton. We characterize the time and energy scales of the problem, either for backreaction due to self-interactions, or (as a novelty of this work) due to gravitational effects. Finally, we calculate $\mathcal{O}(1)$ lattice correction factors to analytic scaling relations derived by some of us in previous work. This enables simple future studies without the need to run lattice simulations.

astro-ph.CO↗

arXiv2024

U(2) is Right for Leptons and Left for Quarks

We posit that the distinct patterns observed in fermion masses and mixings are due to a minimally broken $\mathrm{U}(2)_{q+e}$ flavor symmetry acting on left-handed quarks and right-handed charged leptons, giving rise to an accidental $\mathrm{U}(2)^5$ symmetry at the renormalizable level without imposing selection rules on the Weinberg operator. We show that the symmetry can be consistently gauged by explicit examples and comment on realizations in $\mathrm{SU}(5)$ unification. Via a model-independent SMEFT analysis, we find that selection rules due to $\mathrm{U}(2)_{q+e}$ enhance the importance of charged lepton flavor violation as a probe, where significant experimental progress is expected in the near future.

hep-ph↗

arXiv2023

New Physics in the Third Generation: A Comprehensive SMEFT Analysis and Future Prospects

We present a comprehensive analysis of electroweak, flavor, and collider bounds on the complete set of dimension-six SMEFT operators in the $U(2)^5$-symmetric limit. This operator basis provides a consistent framework to describe a wide class of new physics models and, in particular, the motivated class of models where the new degrees of freedom couple mostly to the third generation. By analyzing observables from all three sectors, and consistently including renormalization group evolution, we provide bounds on the effective scale of all 124 $U(2)^5$-invariant operators. The relation between flavor-conserving and flavor-violating observables is analyzed taking into account the leading $U(2)^5$ breaking in the Yukawa sector, which is responsible for heavy-light quark mixing. We show that under simple, motivated, and non-tuned hypotheses for the parametric size of the Wilson coefficients at the high scale, all present bounds are consistent with an effective scale as low as 1.5 TeV. We also show that a future circular $e^+ e^-$ collider program such as FCC-ee would push most of these bounds by an order of magnitude. This would rule out or provide clear evidence for a wide class of compelling new physics models that are fully compatible with present data.

hep-ph↗

arXiv2023

Deconstructed Hypercharge: A Natural Model of Flavour

The flavour puzzle is one of the greatest mysteries in particle physics. A `flavour deconstruction' of the electroweak gauge symmetry, by promoting at least part of it to the product of a third family factor (under which the Higgs is charged) times a light family factor, allows one to address the flavour puzzle at a low scale due to accidentally realised $U(2)^5$ flavour symmetries. The unavoidable consequence is new heavy gauge bosons with direct couplings to the Higgs, threatening the stability of the electroweak scale. In this work, we propose a UV complete model of flavour based on deconstructing only hypercharge. We find that the model satisfies finite naturalness criteria, benefiting from the smallness of the hypercharge gauge coupling in controlling radiative Higgs mass corrections and passing phenomenological bounds. Our setup allows one to begin explaining flavour at the TeV scale, while dynamics solving the large hierarchy problem can lie at a higher scale up to around 10 TeV - without worsening the unavoidable little hierarchy problem. The low-energy phenomenology of the model is dominated by a single $Z'$ gauge boson with chiral and flavour non-universal couplings, with mass as light as a few TeV thanks to the $U(2)^5$ symmetry. The natural parameter space of the model will be probed by the HL-LHC and unavoidably leads to large positive shifts in the $W$-boson mass, as well as an enhancement in $\text{Br}(B_{s,d} \to μ^+ μ^-)$. Finally, we show that a future electroweak precision machine such as FCC-ee easily has the reach to fully exclude the model.

hep-ph↗

arXiv2023

Explaining the $B_{d,s}\rightarrow {K^{()}\bar K^{()}}$ non-leptonic puzzle and charged-current $B$-anomalies via scalar leptoquarks

We present a model based on $S_1$ scalar leptoquarks to solve the tension observed in the recently proposed non-leptonic optimized observables $L_{K^{*} \bar{K}^{*}}$ and $L_{K \bar{K}}$. These observables are constructed as ratios of U-spin related decays based on $B_{d,s}^0\rightarrow {K^{(*)0}\bar K^{(*)0}}$. The model gives a one-loop contribution to the Wilson coefficient of the chromomagnetic dipole operator needed to explain the tension in both non-leptonic observables, while naturally avoiding large contributions to the corresponding electromagnetic dipoles. The necessary chiral enhancement comes from an $O(1)$ Yukawa coupling with a TeV-scale right-handed neutrino running in the loop. We endow the model with a $U(2)$ flavor symmetry, necessary to protect light-family flavor observables that otherwise would be in tension. Furthermore, we show that the same $S_1$ scalar leptoquark is capable of simultaneously explaining the hints of lepton flavor universality violation observed in charged-current $B$-decays. The model therefore provides a potential link between two puzzles in $B$-physics and TeV-scale neutrino mass generation. Finally, the combined explanation of the $B$-physics puzzles unavoidably results in an enhancement of $\mathcal{B}(B\rightarrow K ν\bar ν)$, yielding a value close to present bounds.

hep-ph↗

arXiv2023

Confronting the vector leptoquark hypothesis with new low- and high-energy data

In light of new data we present an updated phenomenological analysis of the simplified $U_1$-leptoquark model addressing charged-current $B$-meson anomalies. The analysis shows a good compatibility of low-energy data (dominated by the lepton flavor universality ratios $R_D$ and $R_{D^*}$) with the high-energy constraints posed by $pp\to τ\barτ$ Drell-Yan data. We also show that present data are well compatible with a framework where the leptoquark couples with similar strength to both left- and right-handed third-generation fermions, a scenario that is well-motivated from a model building perspective. We find that the high-energy implications of this setup will be probed at the 95% confidence level in the high-luminosity phase of the LHC.

hep-ph↗

arXiv2022

Flavor Non-universal Vector Leptoquark Imprints in $K\to πν\bar ν$ and $ΔF = 2$ Transitions

We analyze $K\to πν\bar ν$ rates in a model with a TeV-scale leptoquark addressing $B$-meson anomalies, based on the flavor non-universal 4321 gauge group featuring third-generation quark-lepton unification. We show that, together with the tight bounds imposed by $ΔF = 2$ amplitudes, the present measurement of $\mathcal{B}(K^+ \to π^+ ν\barν)$ already provides a non-trivial constraint on the model parameter space. In the minimal version of the model, the deviations from the Standard Model in $\mathcal{B}(K^+ \to π^+ ν\barν)$ are predicted to be in close correlation with non-standard effects in the Lepton Flavor Universality ratios $R_D$ and $R_{D^*}$. With the help of future data, these correlations can provide a decisive test of the model.

hep-ph↗

arXiv2022

Multi-Scale 5D Models for Flavor Hierarchies and Anomalies

So-called 4321 gauge models at the TeV scale with hierarchical couplings reminiscent of the Standard Model Yukawas offer a coherent combined explanation of the recent $B$-meson anomalies. In these proceedings, based on arXiv:2203.01952, we discuss how such models could arise from a multi-scale theory of flavor, based on a warped fifth dimension with three branes. This higher dimensional construction provides a natural description of flavor hierarchies, addresses the electroweak hierarchy problem, and allows for the Higgs to be identified as a pseudo-Nambu-Goldstone boson emerging from the same dynamics responsible for the breaking of 4321 gauge symmetry.

hep-ph↗