Analyticity and unitarity constrain certain classes 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 $\textit{t}$ → $\textit{qℓ}$ $\textit{ℓ}$ and identify interesting target branching ratios in the range of 10 to 10 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 $\textit{t}$ → $\textit{qeμ}$ 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.

United States

Wolfgang Altmannshofer

Zev Balme

Christopher Donohue

Stefania Gori

Siddharth Mukundhan

Targets for flavor-violating top decays

The production of $\textit{τμ}$ pairs in electron-positron collisions offers a powerful probe of lepton flavor violation. In this work, we calculate the $\textit{e}$ $\textit{e}$ → $\textit{τμ}$ cross section within the framework of the Standard Model Effective Field Theory, allowing for arbitrary $\textit{e}$ $\textit{e}$ 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 $\textit{e}$ $\textit{e}$ 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.

Pankaj Munbodh

Probing lepton flavor violation at linear electron-positron colliders

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 $\textit{μ}$ → $\textit{e}$ + $\textit{γ}$ and $\textit{τ}$ → $\textit{e/μ}$ + $\textit{γ}$, 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.

Benoît Assi

Joachim Brod

Nick Hamer

Indonesia

J. Julio

Thailand

Patipan Uttayarat

Daniil Volkov

Electron EDM and Γ($\textit{μ → eγ}$) in the 2HDM

<p>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 nonstandard Yukawa structures. A hierarchy in the vacuum expectation values of the three Higgs doublets can partially address the Standard Model 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 Cabibbo–Kobayashi–Maskawa 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, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>B</mi></math> meson, and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>D</mi></math> meson mixing as well as from the rare leptonic decays <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>B</mi><mi>s</mi></msub><mo>/</mo><msup><mi>B</mi><mn>0</mn></msup><mo>/</mo><msub><mi>K</mi><mi>L</mi></msub><mo stretchy="false">→</mo><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup><mo>/</mo><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math>. We identify regions of parameter space in which the new physics Higgs bosons can be as light as a TeV or even lighter.</p>

Kevin Toner

Flavor constraints in a generational three-Higgs-doublet model

<p>We examine the new physics sensitivity of the rare decay <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="normal">Λ</mi><mi>b</mi></msub><mo stretchy="false">→</mo><mi mathvariant="normal">Λ</mi><mi>ν</mi><mover accent="true"><mi>ν</mi><mo stretchy="false">¯</mo></mover></math>, which can be accessible at future <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi></math>-pole machines like Future Circular Collider and Circular Electron Positron Collider. We find that the longitudinal polarization of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="normal">Λ</mi><mi>b</mi></msub></math> baryons produced in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi></math> decays introduces a novel observable, the forward-backward asymmetry <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msubsup><mi>A</mi><mi>FB</mi><mo stretchy="false">↑</mo></msubsup></math> in the angle between the outgoing <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="normal">Λ</mi></math> momentum and the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="normal">Λ</mi><mi>b</mi></msub></math> spin. We provide Standard Model predictions for the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="normal">Λ</mi><mi>b</mi></msub><mo stretchy="false">→</mo><mi mathvariant="normal">Λ</mi><mi>ν</mi><mover accent="true"><mi>ν</mi><mo stretchy="false">¯</mo></mover></math> branching ratio and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msubsup><mi>A</mi><mi>FB</mi><mo stretchy="false">↑</mo></msubsup></math>, and show that future precision measurements of these observables are complementary and probe new physics scales comparable to other <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>b</mi><mo stretchy="false">→</mo><mi>s</mi><mi>ν</mi><mover accent="true"><mi>ν</mi><mo stretchy="false">¯</mo></mover></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>b</mi><mo stretchy="false">→</mo><mi>s</mi><msup><mo>ℓ</mo><mo>+</mo></msup><msup><mo>ℓ</mo><mo>−</mo></msup></math> 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.</p>

Sri Aditya Gadam

New strategies for new physics search with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi mathvariant="normal">Λ</mi></mrow><mrow><mi>b</mi></mrow></msub><mo stretchy="false">→</mo><mi mathvariant="normal">Λ</mi><mi>ν</mi><mover accent="true"><mrow><mi>ν</mi></mrow><mrow><mo stretchy="false">¯</mo></mrow></mover></mrow></math> decays

Neutrino trident production of dilepton 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 <math><mn>5</mn><mi>σ</mi></math>, improving on the previous claims at the <math><mo>∼</mo><mn>3</mn><mi>σ</mi></math> level by the CHARM-II and CCFR collaborations while accounting for additional backgrounds later identified by the NuTeV collaboration. 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 <math><mrow><mi>FASER</mi><mi>ν</mi><mn>2</mn></mrow></math> detector. We also discuss prospects for constraining beyond Standard Model contributions to neutrino trident rates at high energies.

Poland

Toni Mäkelä

United Kingdom

Subir Sarkar

Sebastian Trojanowski

Keping Xie

Bei Zhou

Discovering neutrino tridents at the Large Hadron Collider

The study of the rare decays <math><mi>B</mi><mo>→</mo><msup><mi>K</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup><mi>ν</mi><mover><mi>ν</mi><mo>¯</mo></mover></math> offers a window into the dynamics operating at the electroweak scale, allowing studies of the Standard Model and searches for heavy new physics. However, the analysis of these decays is also potentially sensitive to the on shell production of new light bosons <math><mi>X</mi></math> through the process <math><mi>B</mi><mo>→</mo><msup><mi>K</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup><mi>X</mi></math>. In particular, Belle II has recently measured <math><msup><mi>B</mi><mo>+</mo></msup><mo>→</mo><msup><mi>K</mi><mo>+</mo></msup><mi>ν</mi><mover><mi>ν</mi><mo>¯</mo></mover></math>, finding a <math><mn>2.8</mn><mi>σ</mi></math> excess under the assumption of heavy new physics. Since this excess is rather localized in the kaon energy, a fit that includes the decay mode <math><msup><mi>B</mi><mo>+</mo></msup><mo>→</mo><msup><mi>K</mi><mo>+</mo></msup><mi>X</mi></math> to the kinematic distributions prefers <math><msub><mi>m</mi><mi>X</mi></msub><mo>≈</mo><mn>2</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> with branching fraction <math><mi>Br</mi><mo>[</mo><mi>B</mi><mo>→</mo><mi>K</mi><mi>X</mi><mo>]</mo><mo>=</mo><mo>(</mo><mn>8.8</mn><mo>±</mo><mn>2.5</mn><mo>)</mo><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>6</mn></mrow></msup></math> and a significance of <math><mo>≈</mo><mn>3.6</mn><mi>σ</mi></math>. However, no excess was found in the BABAR measurements of <math><mi>B</mi><mo>→</mo><msup><mi>K</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup><mi>ν</mi><mover><mi>ν</mi><mo>¯</mo></mover></math>, and a global analysis of the Belle II and BABAR data leads to <math><mi>Br</mi><mo>[</mo><mi>B</mi><mo>→</mo><mi>K</mi><mi>X</mi><mo>]</mo><mo>=</mo><mo>(</mo><mn>5.1</mn><mo>±</mo><mn>2.1</mn><mo>)</mo><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>6</mn></mrow></msup></math> with a reduced significance of <math><mo>≈</mo><mn>2.4</mn><mi>σ</mi></math>. We then study various simplified dark-flavored models and present a possible UV completion based on a gauged <math><msub><mi>B</mi><mn>3</mn></msub><mo>−</mo><msub><mi>L</mi><mn>3</mn></msub></math> symmetry, highlighting the discovery potential of dedicated searches for <math><mi>B</mi><mo>→</mo><msup><mi>K</mi><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup><mi>X</mi></math> at Belle II.

Switzerland

Andreas Crivellin

Austria

Huw Haigh

Gianluca Inguglia

Spain

Jorge Martin Camalich

Light new physics in <math><mrow><mi>B</mi><mo>→</mo><msup><mrow><mi>K</mi></mrow><mrow><mo>(</mo><mo>*</mo><mo>)</mo></mrow></msup><mi>ν</mi><mover><mrow><mi>ν</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math>?

We show that bottom-strange production at a high-energy muon collider, <math><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup><mo>→</mo><mi>b</mi><mi>s</mi></math>, is a sensitive probe of new physics. We consider the full set of four-fermion contact interactions that contribute to this process at dimension 6, and discuss the complementarity of a muon collider and of the study of rare <math><mi>B</mi></math> meson decays that also probe said new physics. If a signal were to be found at a muon collider, the forward-backward asymmetry of the <math><mi>b</mi></math>-jet provides diagnostics about the underlying chirality structure of the new physics. In the absence of a signal at a center of mass energy of 10 TeV, <math><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup><mo>→</mo><mi>b</mi><mi>s</mi></math> can indirectly probe new physics at scales close to 100 TeV. We also discuss the impact that beam polarization has on the muon collider sensitivity performance.

Stefano Profumo

Probing new physics with <math><mrow><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup><mo>→</mo><mi>b</mi><mi>s</mi></mrow></math> at a muon collider

Lepton flavor violation is one of the cleanest probes of physics beyond the standard model. In this work, we explore the sensitivity of the process e + e  −  → τμ to new physics above the TeV scale at the proposed circular electron-positron colliders FCC-ee and CEPC. We compute the e + e  −  → τμ cross-section in the Standard Model Effective Field Theory and assess the relevant backgrounds. We compare our sensitivity projections to existing and expected constraints from tau decays and Z decays and find that the future electron-positron colliders provide competitive probes of new physics. We highlight the complementarity of searches for resonant e + e  −  → Z → τμ production on the Z pole and searches for non-resonant e + e  −  → τμ at higher center-of-mass energies.

Talise Oh

Probing lepton flavor violation at Circular Electron-Positron Colliders

We revisit the theory and constraints on axionlike particles (ALPs) interacting with leptons. We clarify some subtleties in the constraints on ALP parameter space and find several new opportunities for ALP detection. We identify a qualitative difference between weak-violating and weak-preserving ALPs, which dramatically change the current constraints due to possible “energy enhancements” in various processes. This new understanding leads to additional opportunities for ALP detection through charged meson decays (e.g., <math><msup><mi>π</mi><mo>+</mo></msup><mo>→</mo><msup><mi>e</mi><mo>+</mo></msup><mi>ν</mi><mi>a</mi></math>, <math><msup><mi>K</mi><mo>+</mo></msup><mo>→</mo><msup><mi>e</mi><mo>+</mo></msup><mi>ν</mi><mi>a</mi></math>) and <math><mi>W</mi></math> boson decays. The new bounds impact both weak-preserving and weak-violating ALPs and have implications for the QCD axion and addressing experimental anomalies using ALPs.

Jeff A. Dror

New Opportunities for Detecting Axion-Lepton Interactions