arXiv subjects

Wolfgang Altmannshofer

Publications and source records attributed to Wolfgang Altmannshofer.

At least 19 recordsLinked to original sources

arXiv2026

Flavorful Two Higgs Doublet Models with a Twist

We explore Two Higgs Doublet Models with non-standard flavor structures. In analogy to the four, well studied, models with natural flavor conservation (type 1, type 2, lepton-specific, flipped), we identify four models that preserve an approximate $U(2)^5$ flavor symmetry acting on the first two generations. In all four models, the couplings of the 125 GeV Higgs are modified in characteristic flavor non-universal ways. The heavy neutral and charged Higgs bosons show an interesting non-standard phenomenology. We discuss their production and decay modes and identify the most sensitive search channels at the LHC. We study the effects on low energy flavor violating processes finding relevant constraints from $B_d$ and $B_s$ meson oscillations and from the rare decay $B_s \to μ^+ μ^-$. We also find that lepton flavor violating $B$ meson decays like $B_s \to τμ$ and $B \to K^{(*)} τμ$ might have branching ratios at an observable level.

hep-ph↗

arXiv2026

Ultra-Strongly Self-Interacting Dark Matter: From Phenomenology to Astrophysical Observables

We develop a minimal, testable framework for two-component self-interacting dark matter (SIDM) in which a dominant, moderately self-interacting species coexists with an ultra-strongly self-interacting subcomponent (uSIDM). A light vector mediator induces velocity-dependent self-scattering, while early-universe dynamics - standard $2 \to 2$ annihilation supplemented by interconversion $χ_1χ_1 \to χ_2χ_2$ - determine the relic abundance analytically. From observations of dwarf and low surface brightness galaxy rotation curves, as well as strong cluster lensing, we place constraints on the microphysics parameters. From these constrained regions, we map the microphysics to effective \texttt{ETHOS} parameters and evolve the linear power spectrum in \texttt{CLASS}. We identify a region where: (1) the SIDM dominant component attains $σ_{\rm{eff}}/m = 20 - 40~\text{cm}^{2}\text{g}^{-1}$ at dwarf velocities while satisfying cluster upper bounds $σ_{\rm{eff}}/m < 0.13~\rm{cm}^{2}\rm{g}^{-1}$; (2) a subpercent uSIDM fraction drives accelerated gravothermal collapse in early halos, providing seeds relevant to high-redshift quasar formation and ``little red dots''; and (3) the small-scale cutoff in the matter power spectrum remains consistent with Lyman-$α$ and satellite counts, but exhibits non-standard features, potentially discernible with future observations. The allowed space can be organized by the mediator-to-DM mass ratio and the late-time uSIDM fraction, with a narrow window singled out by the combined cosmological and astrophysical requirements.

hep-ph↗

arXiv2026

Electron EDM and $Γ(μ\to e γ)$ in the 2HDM

We present the first complete two-loop calculation of the electric dipole moment of the electron, as well as the rates of the lepton-flavor violating decays $μ\to e + γ$ and $τ\to e/μ+ γ$, in the unconstrained two-Higgs doublet model. We include the most general Yukawa interactions of the Higgs doublets with the Standard Model fermions up to quadratic order, and allow for generic phases in the Higgs potential. A python implementation of our results is provided via a public git repository.

hep-ph↗

arXiv2026

Large Hadronic Effects in $B \to K^* μμ$?

Recent results from LHCb have confirmed the long-standing $P_5^\prime$ anomaly, an intriguing discrepancy in the angular distribution of the $B \to K^* μ^+μ^-$ decay that might be a sign of new physics. In addition, the new results hint at a non-zero value for $S_7$, another observable that characterizes the $B \to K^* μ^+μ^-$ angular distribution. We stress that a non-zero $S_7$ cannot be explained by heavy new physics but instead necessarily requires a sizable hadronic effect that introduces a strong phase. We argue that, under plausible assumptions, the hadronic effect is of the correct size to also explain $P_5^\prime$. The direct CP asymmetry in $B \to K^* μ^+μ^-$ emerges in principle as a clean probe of new physics in such a scenario. We show that a combined fit of hadronic parameters and Wilson coefficients retains sensitivity to new physics and we find strong bounds on imaginary parts of new physics Wilson coefficients.

hep-ph↗

arXiv2026

Updated Constraints from Electric Dipole Moments in the MSSM with R-Parity Violation

We revisit the electric dipole moments (EDMs) of quarks and leptons in the Minimal Supersymmetric Standard Model (MSSM) with trilinear $R$-parity violation (RPV). In this framework, EDMs are induced at the two-loop level via RPV interactions. We perform a comprehensive recalculation of several classes of Barr-Zee type diagrams in a general $R_ξ$ gauge. While we find general agreement with previous analytic results in the literature, our work provides a valuable independent cross-check of the complicated calculations. We also point out some subtleties in the intermediate steps and in the choice of the flavor basis for the numerical evaluation of the expressions. By confronting the theoretical predictions with the latest experimental limits on EDMs, we derive updated constraints on combinations of RPV couplings. We highlight an approximate, testable correlation between the proton and neutron EDM that emerges within the considered class of RPV models, offering a distinctive signature for future EDM experiments.

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arXiv2026

Exploring Invisible New Physics with Exotic Pion Decays

We study the sensitivity of past and future stopped-pion experiments to light, invisible dark sector particles produced in exotic pion decays. We consider two-body decays involving sterile neutrinos, $π^+ \to \ell^+ N$, as well as three-body decays $π^+ \to \ell^+ ν_\ell X$, with $X$ an invisible scalar, axion-like particle, or dark vector. We recast existing limits from the PIENU experiment and project the reach of the planned PIONEER experiment using detailed simulations based on the current detector design. We find that PIONEER can improve bounds on exotic pion branching ratios by at least one order of magnitude below current limits. We compare the projected sensitivities with complementary constraints from lepton anomalous magnetic moments, mono-photon searches, and beam-dump experiments, identifying weak-violating axion-like particles as a particularly well-motivated benchmark. Our results establish PIONEER as a powerful and complementary probe of light, invisible dark sectors.

hep-ph↗

arXiv2025

Hadronic CP Violation in the 2HDM

We present the first complete two-loop calculation of the electric and chromo-electric dipole moments of the light quarks and the gluon, as well as contributions to CP-violating lepton-quark interactions, in the unconstrained two-Higgs doublet model. We include the most general Yukawa interactions of the Higgs doublets with the Standard Model fermions up to quadratic order, and allow for generic phases in the Higgs potential. We pay particular attention to a consistent treatment of all fermionic contributions in the low-energy effective theory, including a consistent renormalization-group summation of all leading-logarithmic effects. This latter part of the work is independent of the specific UV model and can generally be applied to a large class of models that do not introduce new light degrees of freedom. A python implementation of our results is provided via a public git repository.

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arXiv2025

Targets for Flavor-Violating Top Decay

Analyticity and unitarity constrain certain classe of new physics models by linking flavor-conserving and flavor-violating four-fermion interactions. In this work, we explore how these theoretical relations impact flavor-violating rare top quark decays. Building on our previous results, we present an updated analysis of the decays $t \to q \ell^+ \ell^-$ and identify interesting target branching ratios in the range of $10^{-7}$ to $10^{-6}$ once current experimental constraints from flavor-conserving processes are taken into account. We extend the analysis to top decays with lepton flavor violation, deriving correlations among the relevant Wilson coefficients and confronting them with existing limits from LEP and the LHC. Notably, we find that current searches for $t \to q e μ$ are already probing theoretically motivated regions of parameter space. These results strongly support continued efforts to explore flavor-violating top decays as a powerful probe of new physics.

hep-ph↗

arXiv2025

Flavour Physics Beyond the Standard Model

Flavour physics plays a central role in the search for physics beyond the Standard Model, posing fundamental questions whose answers may point to new physics scales far above the electroweak scale. The flavour structure of the Standard Model is strikingly hierarchical, hinting at underlying organizing principles yet to be uncovered. At the same time, flavour-changing processes rank among the most sensitive probes of new physics. In this chapter, we review how precision measurements of flavour-violating observables, in particular meson-antimeson mixing and rare decays, provide powerful indirect tests of new physics, constraining scenarios from heavy mediators to light, weakly coupled particles. We also survey theoretical frameworks proposed to explain the origin of the Standard Model flavour hierarchies. Together, these complementary aspects demonstrate how flavour physics continues to shape both experimental strategies and theoretical developments in the quest to understand the fundamental structure of particle interactions.

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arXiv2025

Flavor Physics at the CEPC: a General Perspective

We discuss the landscape of flavor physics at the Circular Electron-Positron Collider (CEPC), based on the nominal luminosity outlined in its Technical Design Report. The CEPC is designed to operate in multiple modes to address a variety of tasks. At the $Z$ pole, the expected production of 4 Tera $Z$ bosons will provide unique and highly precise measurements of $Z$ boson couplings, while the substantial number of boosted heavy-flavored quarks and leptons produced in clean $Z$ decays will facilitate investigations into their flavor physics with unprecedented precision. We investigate the prospects of measuring various physics benchmarks and discuss their implications for particle theories and phenomenological models. Our studies indicate that, with its highlighted advantages and anticipated excellent detector performance, the CEPC can explore beauty and $τ$ physics in ways that are superior to or complementary with the Belle II and Large-Hadron-Collider-beauty experiments, potentially enabling the detection of new physics at energy scales of 10 TeV and above. This potential also extends to the observation of yet-to-be-discovered rare and exotic processes, as well as testing fundamental principles such as lepton flavor universality, lepton and baryon number conservation, etc., making the CEPC a vibrant platform for flavor physics research. The $WW$ threshold scan, Higgs-factory operation and top-pair productions of the CEPC further enhance its merits in this regard, especially for measuring the Cabibbo-Kobayashi-Maskawa matrix elements, and Flavor-Changing-Neutral-Current physics of Higgs boson and top quarks. We outline the requirements for detector performance and considerations for future development to achieve the anticipated scientific goals.

hep-ex↗

arXiv2025

Recent Progress in Flavor Model Building

The flavor puzzles remain among the most compelling open questions in particle physics. The striking hierarchies observed in the masses and mixing of charged fermions define the Standard Model (SM) flavor puzzle, a profound structural enigma pointing to physics beyond the SM. Simultaneously, the absence of deviations from SM predictions in precision measurements of flavor-changing neutral currents imposes severe constraints on new physics at the TeV scale, giving rise to the new physics flavor puzzle. This review article provides an overview of a selection of recent advancements in flavor model building, with a particular focus on attempts to address one or both of these puzzles within the quark sector.

hep-ph↗

arXiv2025

Electron EDM in the complex two-Higgs doublet model

We present the first complete two loop calculation of the electron EDM in the complex two-Higgs doublet model. We confirm gauge-independence by demonstrating analytic cancellation of the gauge parameter $ξ$ in the background field gauge and the 't Hooft $R_ξ$ gauge. We also investigate the behavior of the electron EDM near the decoupling limit, and determine the short- and long-distance contributions by matching onto an effective field theory. Compared with earlier studies of the electron EDM in the complex two-Higgs doublet model, we note disagreements in several places and provide diagnoses where possible. We also provide expressions for EDMs of light quarks.

hep-ph↗

arXiv2025

Probing Lepton Flavor Violation at Linear Electron-Positron Colliders

The production of $τμ$ pairs in electron-positron collisions offers a powerful probe of lepton flavor violation. In this work, we calculate the $e^+ e^- \to τμ$ cross section within the framework of the Standard Model Effective Field Theory, allowing for arbitrary $e^+e^-$ beam polarizations. We then estimate the sensitivities of proposed future linear colliders, ILC and CLIC, to effective lepton flavor-violating interactions. The high center-of-mass energies achievable at these machines provide particularly strong sensitivity to four-fermion operators. Furthermore, the polarization of the $e^+e^-$ beams enables novel tests of the chirality structure of these interactions. We find that our projected sensitivities not only complement but in certain scenarios surpass those achievable with low-energy tau decay measurements at Belle~II.

hep-ph↗

arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

hep-ph↗

arXiv2025

Flavor Constraints in a Generational Three Higgs Doublet Model

We propose a Three Higgs Doublet Model (3HDM) that goes beyond natural flavor conservation and in which each of the three Higgs doublets couples mainly to a single generation of fermions via non-standard Yukawa structures. A hierarchy in the vacuum expectation values of the three Higgs doublets can partially address the SM flavor puzzle. In light of the experimentally observed $125$ GeV Higgs boson, we primarily work within a 3HDM alignment limit such that a Standard Model-like Higgs is recovered. In order to reproduce the observed CKM mixing among quarks, the neutral Higgs bosons of the theory necessarily mediate flavor changing neutral currents at the tree level. We consider constraints from neutral kaon, $B$ meson, and $D$ meson mixing as well as from the rare leptonic decays $B_s/B^0/K_L\rightarrowμ^+μ^-/e^+e^-$. We identify regions of parameter space in which the new physics Higgs bosons can be as light as a TeV or even lighter.

hep-ph↗

arXiv2025

New Strategies for New Physics Search with $Λ_b \to Λν\barν$ Decays

We examine the new physics sensitivity of the rare decay $Λ_b \to Λν\barν$ which can be accessible at future $Z$-pole machines like FCC-ee and CEPC. We find that the longitudinal polarization of $Λ_b$ baryons produced in $Z$ decays introduces a novel observable, the forward-backward asymmetry $A_\text{FB}^\uparrow$ in the angle between the outgoing $Λ$ momentum and the $Λ_b$ spin. We provide Standard Model predictions for the $Λ_b \to Λν\barν$ branching ratio and $A_\text{FB}^\uparrow$, and show that future precision measurements of these observables are complementary and probe new physics scales comparable to other $b \to s ν\barν$ and $b \to s \ell^+ \ell^-$ processes. We also show that the zero crossing of the forward-backward asymmetry offers a robust test of form factor calculations independent of new physics.

hep-ph↗

arXiv2024

A joint explanation of the $B\to πK$ puzzle and the $B \to K ν\barν$ excess

In light of the recent branching fraction measurement of the $B^{+}\to K^{+} ν\barν$ decay by Belle II and its poor agreement with the SM expectation, we analyze the effects of an axion-like particle (ALP) in $B$ meson decays. We assume a long-lived ALP with a mass of the order of the pion mass that decays to two photons. We focus on a scenario where the ALP decay length is of the order of meters such that the ALP has a non-negligible probability to decay outside the detector volume of Belle II, mimicking the $B^{+}\to K^{+} ν\barν$ signal. Remarkably, such an arrangement is also relevant for the long-standing $B\to πK$ puzzle by noting that the measured $B^{0}\to π^{0}K^{0}$ and $B^{+}\to π^{0}K^{+}$ decays could have a $B^{0}\to a K^{0}$ and $B^{+}\to a K^{+}$ component, respectively. We also argue based on our results that the required ALP-photon effective coupling belongs to a region of parameter space that can be extensively probed in future beam dump experiments like SHiP.

hep-ph↗

arXiv2024

Discovering neutrino tridents at the Large Hadron Collider

Neutrino trident production of di-lepton pairs is well recognized as a sensitive probe of both electroweak physics and physics beyond the Standard Model. Although a rare process, it could be significantly boosted by such new physics, and it also allows the electroweak theory to be tested in a new regime. We demonstrate that the forward neutrino physics program at the Large Hadron Collider offers a promising opportunity to measure for the first time, dimuon neutrino tridents with a statistical significance exceeding $5σ$. We present predictions for various proposed experiments and outline a specific experimental strategy to identify the signal and mitigate backgrounds, based on "reverse tracking" dimuon pairs in the FASER$ν$2 detector. We also discuss prospects for constraining beyond Standard Model contributions to neutrino trident rates at high energies.

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