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 ν  M  SM is defined as the SM extended to include dimension-5 operators. In this model neutrino masses violate lepton number, and two parameters of the lepton mixing matrix, the Majorana phases, are yet to be constrained. One combination of these phases and the neutrino masses, often denoted by m  ee , is probed by neutrinoless double beta decays (0νββ). We explore what information may be obtained beyond 0νββ, and how it depends on the lightest neutrino mass. We point out that with current central values of the mixing parameters, ∆L  e  = 2 and ∆L  e  = ∆L  μ  = 1 (or ∆L  e  = 2 and ∆L  μ  = 2) processes cannot simultaneously vanish, providing a no-lose theorem, in principle, for excluding the ν  M  SM, even in the case of normal mass ordering.

Avital Dery

Yuval Grossman

Zoltan Ligeti

Majorana phases beyond neutrinoless double beta decay

We analyze the capability of the DarkQuest proton beam-dump experiment at Fermilab to discover new light resonances decaying into photons. As an example model, we focus on muon-philic scalar particles that decay to photons. This is one of the few minimal models that can address the <math><mrow><mo>(</mo><mi>g</mi><mo>−</mo><mn>2</mn><msub><mrow><mo>)</mo></mrow><mrow><mi>μ</mi></mrow></msub></mrow></math> anomaly at low mass. These scalars can be copiously produced by meson decays and muon bremsstrahlung. We point out that thanks to DarkQuest’s compact geometry, muons can propagate through the dump and efficiently produce dark scalars near the end of the dump. This mechanism enables DarkQuest to be sensitive to both long-lived and prompt scalars. At the same time, diphoton signatures are generically not background free, and we discuss in detail the different sources of background and strategies to mitigate them. We find that the backgrounds can be sufficiently reduced for DarkQuest to test currently viable <math><mo>(</mo><mi>g</mi><mo>−</mo><mn>2</mn><msub><mo>)</mo><mi>μ</mi></msub></math> parameter space.

Canada

Nikita Blinov

Nick Hamer

Diphoton signals of muon-philic scalars at DarkQuest

A QCD axion with a decay constant below 1011 GeV is a strongly-motivated extension to the Standard Model, though its relic abundance from the misalignment mechanism or decay of cosmic defects is insufficient to explain the origin of dark matter. Nevertheless, such an axion may still play an important role in setting the dark matter density if it mediates a force between the SM and the dark sector. In this work, we explore QCD axion-mediated freeze-out and freeze-in scenarios, finding that the axion can play a critical role for setting the dark matter density. Assuming the axion solves the strong CP problem makes this framework highly predictive, and we comment on experimental targets.

Jeff Dror

Pankaj Munbodh

QCD axion-mediated dark matter

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

New physics in the rare top decays <math><mi>t</mi><mo>→</mo><mi>q</mi><msup><mo>ℓ</mo><mo>+</mo></msup><msup><mo>ℓ</mo><mo>−</mo></msup></math> is currently very weakly constrained. We show that in a large class of Standard Model extensions, existing experimental constraints on new physics in flavor-conserving processes imply strong indirect bounds on new physics contributions to flavor-violating processes of the form <math><mi>t</mi><mo>→</mo><mi>q</mi><msup><mo>ℓ</mo><mo>+</mo></msup><msup><mo>ℓ</mo><mo>−</mo></msup></math>. These indirect bounds arise from basic principles of quantum field theory together with a few generic conditions on the UV structure of the theory, and are roughly an order of magnitude stronger than the present experimental bounds on the same processes. These constraints provide a theoretically motivated target for experimental searches for <math><mi>t</mi><mo>→</mo><mi>q</mi><msup><mo>ℓ</mo><mo>+</mo></msup><msup><mo>ℓ</mo><mo>−</mo></msup></math>: violation of these bounds would exclude a large class of new physics models, and would provide nontrivial insight into the UV behavior of the new physics.

Benjamin V. Lehmann

Jianhong Zuo

UV physics from IR features: New prospects from top flavor violation

We study the signal and background that arise in nuclear magnetic resonance searches for axion dark matter, finding key differences with the existing literature. We find that spin-precession instruments are much more sensitive than what has been previously estimated in a sizable range of axion masses, with sensitivity improvement of up to a factor of 100 using a <math><mrow><mmultiscripts><mrow><mi>Xe</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>129</mn></mrow></mmultiscripts></mrow></math> sample. This improves the detection prospects for the QCD axion, and we estimate the experimental requirements to reach this motivated target. Our results apply to both the axion electric and magnetic dipole moment operators.

Germany

Jacob M. Leedom

CERN

Nicholas L. Rodd

Sensitivity of Spin-Precession Axion Experiments

A new beam dump experiment that utilizes the beam of future high energy electron-positron colliders could be an excellent avenue to search for dark sector particles due to its unprecedented high energy and intensity. We consider heavy neutral leptons (HNLs) as a specific example to demonstrate the sensitivity of searches for dark sector particles at future electron-positron collider beam dump experiments. This includes the study of the reach at the International Linear Collider (ILC), the Cool Copper Collider (C3), and the Compact Linear Collider (CLIC). We comprehensively examine the HNL production and detector acceptance at these electron beam dump experiments. We show that these experiments will probe regions of HNL parameter space, not yet probed by past experiments, as well as by future approved experiments. Our study also motivates a more detailed analysis of heavy meson productions in high-energy electron-nucleon collisions in thick targets.

Pierce Giffin

Yu-Dai Tsai

Douglas Tuckler

Heavy neutral leptons at beam dump experiments of future lepton colliders

We study dark matter freeze-in scenarios where the mass of the mediator particle that couples dark matter to the Standard Model is larger than the reheat temperature, T  RH , in the early Universe. In such setups, the standard approach is to work with an effective field theory (EFT) where the mediator is integrated out. We examine the validity of this approach in various generic s- and t-channel mediator frameworks. We find that the EFT approach breaks down when the mediator mass is between one to two orders of magnitude larger than T  RH  due to various effects such as s-channel resonance, a small thermally-suppressed abundance of the mediator, or decays of Standard Model particles through loops induced by the mediator. This highlights the necessity of including these contributions in such dark matter freeze-in studies. We also discuss the collider phenomenology of the heavy mediators, which is qualitatively different from standard freeze-in scenarios. We highlight that, due to the low T  RH , the Standard Model-dark matter coupling in these scenarios can be relatively larger than in standard freeze-in scenarios, improving the testability prospects of these setups.

Evan Frangipane

Bibhushan Shakya

Dark matter freeze-in with a heavy mediator: beyond the EFT approach

We present a supersymmetric extension of the Standard Model in which the new physics contributions to the anomalous magnetic moment of the muon can be more than an order of magnitude larger than in the minimal supersymmetric Standard Model. The extended electroweak symmetry breaking sector of the model can consistently accommodate Higgs bosons and Higgsinos with O(1) couplings to muons. We find that sleptons with masses in the multi-TeV range can comfortably explain the recently confirmed discrepancy in the anomalous magnetic moment of the muon. We discuss additional phenomenological aspects of the model, including its effects on tau flavor changing decays.