We explore phenomenological constraints on, and future muon collider sensitivities to, the parameter spaces of various muonphilic portals to fermionic asymmetric dark matter (ADM). Both WEFT-level dimension-6 effective operators and two UV models based on gauged $\textit{L}$ – $\textit{L}$ are considered. One of the latter features a vector coupling to the dark matter and the other an axial vector coupling. The ADM criterion that at least 99% of the dark matter relic density is asymmetric is also imposed. We identify which of these scenarios are currently allowed by direct detection and collider constraints, and then determine how much more of the parameter space could be probed by 3 and 10 TeV muon colliders with 1 ab of data. For the UV models, the constraints from $\textit{g}$ – 2 of the muon are included. The future sensitivity curves due to neutron star heating considerations are also depicted. We present results for both the few-GeV dark matter mass regime motivated by ADM approaches to the Ω ≃ Ω /5 coincidence problem, and for larger masses in the context of more general ADM.

Australia

Arnab Roy

Raymond Volkas

Muonphilic asymmetric dark matter at a future muon collider

Recently Belle II reported the first measurement of B + → K + + invisible(inv), which is 2.7σ above the standard model (SM) prediction. If confirmed, this calls for new physics beyond SM. In the SM, the invisible particles are neutrino-anti-neutrino pairs. There are more possibilities when going beyond the SM. In this work, we focus on decays to dark matter (DM) and show that the B → K + inv excess from Belle II and DM relic density can be simultaneously explained in a simple extension of the SM. The model introduces a real scalar singlet ϕ acting as a DM candidate, and two heavy vector-like quarks Q, D with the same quantum numbers as the SM left-handed quark doublet and right-handed down-type quark singlet, respectively. All these new particles are odd under a ℤ2 symmetry while the SM particles are even. The model can successfully explain the Belle II anomaly and DM relic density for TeV-scale heavy quarks with hierarchical Yukawa couplings involving b and s quarks. At the same time, it can easily satisfy other flavour physics constraints. Direct detection searches utilizing the Migdal effect constrain some of the parameter space.

China

Xiao-Gang He

Xiao-Dong Ma

Michael Schmidt

German Valencia

Scalar dark matter explanation of the excess in the Belle II B + → K ++ invisible measurement

We provide an exhaustive classification of three-family DFSZ axion models that have no cosmological domain wall problem. This result is obtained by making the Peccei-Quinn symmetry flavour dependent in certain specific ways, thus reinforcing a possible connection between the strong CP problem and the flavour puzzle. Known DFSZ flavour variants such as the top-specific model emerge as special cases. Key features of the phenomenology of these models are briefly discussed.

Peter Cox

Matthew Dolan

Maaz Hayat

United States

Andrea Thamm

Classification of three-family flavoured DFSZ axion models that have no domain wall problem

Motivated by flavor symmetry models, we construct theories based on a low-energy limit featuring lepton flavor triality that have the flavor-violating decays <math><msup><mi>τ</mi><mo>±</mo></msup><mo>→</mo><msup><mi>μ</mi><mo>±</mo></msup><msup><mi>μ</mi><mo>±</mo></msup><msup><mi>e</mi><mo>∓</mo></msup></math> and <math><msup><mi>τ</mi><mo>±</mo></msup><mo>→</mo><msup><mi>e</mi><mo>±</mo></msup><msup><mi>e</mi><mo>±</mo></msup><msup><mi>μ</mi><mo>∓</mo></msup></math> as the main phenomenological signatures of physics beyond the standard model. These decay modes are expected to be probed in the near future with increased sensitivity by the Belle II experiment at the SuperKEKB collider. The simple standard model extensions featured have doubly charged scalars as the mediators of the above decay processes. The phenomenology of these extensions is studied here in detail.

Innes Bigaran

Taiwan

Michael A. Schmidt

Lepton-flavor-violating tau decays from triality

We analyse the experimental limits on the breaking scale of Pati-Salam extensions of the Standard Model. These arise from the experimental limits on rare-meson decay processes mediated at tree-level by the vector leptoquark in the model. This leptoquark ordinarily couples to both left- and right-handed SM fermions and therefore the meson decays do not experience a helicity suppression. We find that the current limits vary from   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (80–2500) TeV depending on the choice of matrix structure appearing in the relevant three-generational charged-current interactions. We extensively analyse scenarios where additional fermionic degrees of freedom are introduced, transforming as complete Pati-Salam multiplets. These can lower the scales of Pati-Salam breaking through mass-mixing within the charged-lepton and down-quark sectors, leading to a helicity suppression of the meson decay widths which constrain Pati-Salam breaking. We find four multiplets with varying degrees of viability for this purpose: an SU(2) L/R  bidoublet, a pair of SU(4) decuplets and either an SU(2) L  or SU(2) R  triplet all of which contain heavy exotic versions of the SM charged leptons. We find that the Pati-Salam limits can be as low as   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (5–150) TeV with the addition of these four multiplets. We also identify an interesting possible connection between the smallness of the neutrino masses and a helicity suppression of the Pati-Salam limits for three of the four multiplets.

Tomasz Dutka

Lowering the scale of Pati-Salam breaking through seesaw mixing

Recent measurements of the Higgs-muon coupling are directly probing muon mass generation for the first time. We classify minimal models with a one-loop radiative mass mechanism and show that benchmark models are consistent with current experimental results. We find that these models are best probed by measurements of (g − 2) μ , even when taking into account the precision of Higgs measurements expected at future colliders. The current (g − 2) μ  anomaly, if confirmed, could therefore be a first hint that the muon mass has a radiative origin.

Michael Baker

Radiative muon mass models and (g − 2) μ

Precision measurements of the Higgs couplings are, for the first time, directly probing the mechanism of fermion mass generation. The purpose of this work is to determine to what extent these measurements can distinguish between the tree-level mechanism of the Standard Model and the theoretically motivated alternative of radiative mass generation. Focusing on the third-family, we classify the minimal one-loop models and find that they fall into two general classes. By exploring several benchmark models in detail, we demonstrate that a radiative origin for the tau-lepton and bottom-quark masses is consistent with current observations. While future colliders will not be able to rule out a radiative origin, they can probe interesting regions of parameter space.

Has the origin of the third-family fermion masses been determined?

We examine the collider and dark matter phenomenology of the Standard Model extended by a hypercharge-zero SU(2) triplet scalar and gauge singlet scalar. In particular, we study the scenario where the singlet and triplet are both charged under a single ℤ2 symmetry. We find that such an extension is capable of generating the observed dark matter density, while also modifying the collider phenomenology such that the lower bound on the mass of the triplet is smaller than in minimal triplet scalar extensions to the Standard Model. A high triplet mass is in tension with the parameter space that leads to novel electroweak phase transitions in the early universe. Therefore, the lower triplet masses that are permitted in this extended model are of particular importance for the prospects of successful electroweak baryogenesis and the generation of gravitational waves from early universe phase transitions.

Nicole Bell

Leon Friedrich

Michael Ramsey-Musolf

A real triplet-singlet extended Standard Model: dark matter and collider phenomenology

Building UV completions of lepton-number-violating effective operators has proved to be a useful way of studying and classifying models of Majorana neutrino mass. In this paper we describe and implement an algorithm that systematises this model-building procedure. We use the algorithm to generate computational representations of all of the tree-level completions of the operators up to and including mass-dimension 11. Almost all of these correspond to models of radiative neutrino mass. Our work includes operators involving derivatives, updated estimates for the bounds on the new-physics scale associated with each operator, an analysis of various features of the models, and a look at some examples. We find that a number of operators do not admit any completions not also generating lower-dimensional operators or larger contributions to the neutrino mass, ruling them out as playing a dominant role in the neutrino-mass generation. Additionally, we show that there are at most five models containing three or fewer exotic multiplets that predict new physics that must lie below 100 TeV. Accompanying this work we also make available a searchable database containing all of our results and the code used to find the completions. We emphasise that our methods extend beyond the study of neutrino-mass models, and may be useful for generating completions of high-dimensional operators in other effective field theories. Example code: ref. [37].

John Gargalionis

Exploding operators for Majorana neutrino masses and beyond

We study the phenomenology of a hypercharge-zero SU (2) triplet scalar whose existence is motivated by two-step electroweak symmetry-breaking. We consider both the possibility that the triplets are stable and contribute to the dark matter density, or that they decay via mixing with the standard model Higgs boson. The former is constrained by disappearing charged track searches at the LHC and by dark matter direct detection experiments, while the latter is constrained by existing multilepton collider searches. We find that a two-step electroweak phase transition involving a stable triplet with a negative quadratic term is ruled out by direct detection searches, while an unstable triplet with a mass less than 230 GeV is excluded at 95% confidence level.