arXiv subjects

Marvin Schnubel

Publications and source records attributed to Marvin Schnubel.

14 recordsLinked to original sources

arXiv2026

The Fate of Ultra-Collinear Modes in On-Shell Massive Sudakov Form Factors

Individual multi-loop diagrams for the massive Sudakov form factor contain an infinite tower of ultra-collinear momentum regions. We show that, for the on-shell form factor in QCD, these contributions cancel to all orders as a consequence of gauge invariance, so the leading-power SCET$_{\rm II}$ factorization formula is unchanged. Using the $η$ rapidity regulator, we compute the soft function and the massive jet function of the quark and gluon Sudakov form factors through two loops and resum logarithms at NNLL accuracy, including hierarchies of fermion masses. We also show that with a gauge-boson mass regulator, the infinite tower of modes is truncated and ultra-collinear and ultra-soft modes become manifest and factorize explicitly, providing a direct EFT derivation of the regulated infrared dependence.

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arXiv2025

Higgs Potential from Instantons

We propose that the Higgs potential, a key element in our understanding of Nature, is partially generated by the instantons of new confining dynamics, perhaps from a hidden sector. In this picture, while the Higgs itself is a fundamental field, it controls the strength of the non-perturbative interactions that give rise to its potential. We examine a simple setup in which this instanton contribution is augmented by a quartic term, which is sufficient for a realistic electroweak symmetry breaking mechanism. The minimum of the potential is given by the Lambert $W_0$ function, with these assumptions. We discuss the predictions of this model and how it may be tested through measurements of the Higgs self coupling. Given the connection with non-trivial dynamics, one may also consider the prospects of accessing hidden sector states at colliders; this seems to be typically challenging in our setup. Symmetry restoration in the early Universe in this scenario is briefly examined. We also comment on the possible connection of our general setup with recent work on the physics of field space end points.

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arXiv2025

Electron and Photon Structure Functions at Two Loops

We present a fully analytic computation of the complete electron and photon structure functions, or QED lepton parton distribution functions (PDFs) up to two-loop order. Our computation is performed using modern techniques of reduction to Master Integrals and solving them with the differential equation method. We obtain explicit expressions for the electron-in-electron, positron-in-electron, photon-in-electron, electron-in-photon, and photon-in-photon distributions at next-to-next-to-leading order (NNLO). Cross-checks against one-loop results, existing two-loop calculations, and a recent soft-collinear effective theory (SCET) analysis of the electron structure functions are presented.

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arXiv2024

SMEFT Matching to $Z^\prime$ Models at Dimension-8

Heavy neutral gauge bosons arise in many motivated models of Beyond the Standard Model Physics. Experimental searches require that such gauge bosons are above the TeV scale in most models which means that the tools of effective field theories, in particular the Standard Model Effective Field Theory (SMEFT), are useful. We match the SMEFT to models with heavy $Z^\prime$ bosons, including effects of dimension-8 operators, and consider the restrictions on model parameters from electroweak precision measurements and from Drell Yan invariant mass distributions and forward-backward asymmetry, $A_\text{FB}$, measurements at the LHC. The results demonstrate the model dependence of the resulting limits on SMEFT coefficients and the relatively small impact of including dimension-8 matching. In all cases, the limits from invariant mass distributions are stronger than from $A_\text{FB}$ measurements in the $Z^\prime$ models we consider.

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arXiv2024

Bringing Peccei-Quinn Mechanism Down to Earth

It is conventionally assumed that the physics underlying the Peccei-Quinn (PQ) mechanism for addressing the strong CP problem is at very high energies, orders of magnitude above the weak scale. However, this may not be the case in general and the associated PQ boson $ϕ$, besides the signature state, {\it i.e.} the ultralight axion $a$, may emerge well below the weak scale. We consider this possibility and examine some of the conditions for its viability. The example model proposed here may also provide the requisite Standard Model Higgs mass parameter, without invoking new scalars above the GeV scale. The corresponding parameter space can maintain {\it finite naturalness} against quantum corrections. This scenario, depending on choice of parameters, can potentially be constrained by flavor data. We point out that the current mild excess in $B^+\to K^+ ν\bar ν$, reported by the Belle II experiment, could be explained in this setup as $B^+\to K^+ ϕ$ and $B^+\to K^+ a$, with both $ϕ$ and $a$ escaping the detector as missing energy. For a sufficiently heavy PQ boson, in the GeV regime, one can separate these two contributions, due to the difference in $K^+$ momenta. In this case, the axion may also affect lighter meson, {\it e.g.} kaon, decays while $ϕ$ would not be a kinematically allowed final state.

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arXiv2024

Two-loop contributions of axion-like particles to electromagnetic and chromomagnetic form factors

Axions and axion-like particles emerge in many models for physics beyond the Standard Model. Thus, they have gained increasing research interest in both experimental and theoretical physics apart from their original proposition as a solution to the strong $CP$-problem. Among other aspects it has recently been shown that ALPs can potentially provide a solution to the long-lasting discrepancy between theory and experiment of the anomalous magnetic moment of the muon. Provided that the ALP has flavor-violating couplings to leptons, they can also mediate flavor-violating decays like $μ\to eγ$. Both processes are mediated through related form factors that we compute to two-loop order. We further show numerical implications of our calculations and how they might affect constraints on ALP couplings derived from experiments.

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arXiv2023

New Physics Searches at Kaon and Hyperon Factories

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

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arXiv2022

Factorization at Next-to-Leading Power and Endpoint Divergences in $gg\to h$ Production

We derive a factorization theorem for the Higgs-boson production amplitude in gluon-gluon fusion induced by a light-quark loop, working at next-to-leading power in soft-collinear effective theory. The factorization is structurally similar to that obtained for the $h\toγγ$ decay amplitude induced by a light-quark loop, but additional complications arise because of external color charges. We show how the refactorization-based subtraction scheme developed in previous work leads to a factorization theorem free of endpoint divergences. We use renormalization-group techniques to predict the logarithmically enhanced terms in the three-loop $gg\to h$ form factor of order $α_s^3\ln^k(-M_h^2/m_b^2)$ with $k=6,5,4,3$. We also resum the first three towers of leading logarithms, $α_s^n\ln^{2n-k}(-M_h^2/m_b^2)$ with $k=0,1,2$, to all orders of perturbation theory.

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arXiv2021

Radiative Quark Jet Function with an External Gluon

Factorization theorems in soft-collinear effective theory at subleading order in power counting involve ``radiative jet functions'', defined in terms of matrix elements of collinear fields with a soft momentum emitted from inside the jet. Of particular importance are the radiative quark jet functions with an external photon or gluon, which arise e.g. in the factorization theorems for the Higgs-boson amplitudes $h\toγγ$, $h\to gg$ and $gg\to h$ induced by light-quark loops. While the photon case has been studied extensively in previous work, we present here a detailed study of the radiative jet function with an external gluon. We calculate this jet function at one- and two-loop order, derive its one-loop anomalous dimension and study its renormalization-group evolution.

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arXiv2021

Flavor probes of axion-like particles

Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model (SM). We investigate the phenomenology of an ALP with flavor-changing couplings, and present a comprehensive analysis of quark and lepton flavor-changing observables within a general ALP effective field theory. Observables studied include rare meson decays, flavor oscillations of neutral mesons, rare lepton decays, and dipole moments. We derive bounds on the general ALP couplings as a function of its mass, consistently taking into account the ALP lifetime and branching ratios. We further calculate quark flavor-changing effects that are unavoidably induced by running and matching between the new physics scale and the scale of the measurements. This allows us to derive bounds on benchmark ALP models in which only a single (flavorless or flavor-universal) ALP coupling to SM particles is present at the new physics scale, and in this context we highlight the complementarity and competitiveness of flavor bounds with constraints derived from collider, beam dump and astrophysical measurements. We find that searches for ALPs produced in meson decays provide some of the strongest constraints in the MeV-GeV mass range, even for the most flavorless of ALP models. Likewise, we discuss the interplay of flavor-conserving and flavor-violating couplings of the ALP to leptons, finding that constraints from lepton flavor-violating observables generally depend strongly on both. Additionally, we analyze whether an ALP can provide an explanation for various experimental anomalies including those observed in rare B-meson decays, measurements at the ATOMKI and KTeV experiments, and in the anomalous magnetic moments of the muon and the electron.

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arXiv2021

The Two-Loop Radiative Gluon Jet Function for $gg\to h$ via a Light Quark Loop

The Higgs-boson production channel $gg\to h$ mediated by light-quark loops receives large logarithmic corrections in QCD, which can be resummed using factorization formulae derived in soft-collinear effective theory. In these factorization formulae the radiative gluon jet function appears, which is a central object in the study of factorization beyond the leading order in scale ratios. We calculate this function at two-loop order for the first time and present the subtleties that come along with this.

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arXiv2021

The Low-Energy Effective Theory of Axions and ALPs

Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model, which interact with the known particles through higher-dimensional operators suppressed by the mass scale $Λ$ of the new-physics sector. Starting from the most general dimension-5 interactions, we discuss in detail the evolution of the ALP couplings from the new-physics scale to energies at and below the scale of electroweak symmetry breaking. We derive the relevant anomalous dimensions at two-loop order in gauge couplings and one-loop order in Yukawa interactions, carefully considering the treatment of a redundant operator involving an ALP coupling to the Higgs current. We account for one-loop (and partially two-loop) matching contributions at the weak scale, including in particular flavor-changing effects. The relations between different equivalent forms of the effective Lagrangian are discussed in detail. We also construct the effective chiral Lagrangian for an ALP interacting with photons and light pseudoscalar mesons, pointing out important differences with the corresponding Lagrangian for the QCD axion.

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arXiv2021

Consistent treatment of axions in the weak chiral Lagrangian

We present a consistent implementation of weak decays involving an axion or axion-like particle in the context of an effective chiral Lagrangian. We argue that previous treatments of such processes have used an incorrect representation of the flavor-changing quark currents in the chiral theory. As an application, we derive model-independent results for the decays $K^-\toπ^- a$ and $π^-\to e^-\barν_e a$ at leading order in the chiral expansion and for arbitrary axion couplings and mass. In particular, we find that the $K^-\toπ^- a$ branching ratio is almost 40 times larger than previously estimated.

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arXiv2019

Axion-like particles, lepton-flavor violation and a new explanation of $a_μ$ and $a_e$

Axion-like particles (ALPs) with lepton-flavor violating couplings can be probed in exotic muon and tau decays. The sensitivity of different experiments depends strongly on the ALP mass and its couplings to leptons and photons. For ALPs that can be resonantly produced, the sensitivity of three-body decays such as $μ\to 3e$ and $τ\to 3μ$ exceeds by many orders of magnitude that of radiative decays like $μ\to eγ$ and $τ\toμγ$. Searches for these two types of processes are therefore highly complementary. We discuss experimental constraints on ALPs with a single dominant lepton-flavor violating coupling. Allowing for one or more such couplings offers qualitatively new ways to explain the anomalies related to the magnetic moments of the muon and electron.

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