We present theoretical predictions for $\textit{μ}$ → $\textit{e}$ conversion rates using a tower of effective field theories connecting the UV to nuclear physics scales. The interactions in nuclei are described using a recently developed nonrelativistic effective theory (NRET) that organizes contributions according to bound nucleon and muon velocities, $$ {\overrightarrow{v}}_N $$ and $$ {\overrightarrow{v}}_{\mu } $$ , with $$ \left|{\overrightarrow{v}}_N\right| $$ $\textit{>}$ $$ \left|{\overrightarrow{v}}_{\mu}\right| $$ . To facilitate the top-down matching, we enlarge the set of Lorentz covariant nucleon-level interactions mapped onto the NRET operators to include those mediated by tensor interactions, in addition to the scalar and vector interactions already considered previously, and then match NRET nonperturbatively onto the Weak Effective Theory (WET). At the scale $\textit{μ}$ ≈ 2 GeV WET is formulated in terms of $\textit{u}$, $\textit{d}$, $\textit{s}$ quarks, gluons and photons as the light degrees of freedom, along with the flavor-violating leptonic current. We retain contributions from WET operators up to dimension 7, which requires the full set of 26 NRET operators. The results are encoded in the open-source Python- and Mathematica-based software suite MuonBridge, which we make available to the theoretical and experimental communities interested in $\textit{μ}$ → $\textit{e}$ conversion.

United States

Wick Haxton

Kenneth McElvain

Tony Menzo

Evan Rule

Jure Zupan

Effective theory tower for $\textit{μ}$ → $\textit{e}$ conversion

The search for charged-lepton flavor violation in muon capture on nuclei is a powerful probe of heavy new physics. A smoking gun signal for <math><mi>μ</mi><mo>→</mo><mi>e</mi></math> conversion is a monochromatic electron with energy almost equal to the muon mass. We show that light new physics can mimic this signature and that it can also lead to electrons above the <math><mi>μ</mi><mo>→</mo><mi>e</mi></math> signal peak. A concrete example of such light new physics is <math><msup><mi>μ</mi><mo>−</mo></msup></math>-nucleon annihilation into a light dark sector, which can produce an energetic <math><msup><mi>e</mi><mo>−</mo></msup></math> as well as <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> by-products. Due to the size of the muon mass, the exotic muon capture process can be kinematically allowed, while the otherwise stringent constraints, e.g., from proton decay, are kinematically forbidden. We also discuss other relevant constraints, including those from the stability of nuclei and muon capture in the interior of neutron stars.

Patrick J. Fox

Matheus Hostert

Maxim Pospelov

Muon-induced baryon number violation

We investigate the ability of <math><mi>μ</mi><mo>→</mo><mi>e</mi></math> facilities, Mu2e and COMET, to probe, or discover, new physics with their detector validation datasets. The validation of the detector response may be performed using a dedicated run with <math><msup><mi>μ</mi><mo>+</mo></msup></math>, collecting data below the Michel edge, <math><mrow><msub><mrow><mi>E</mi></mrow><mrow><mi>e</mi></mrow></msub><mo>≲</mo><mn>52</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></mrow></math>; an alternative strategy using <math><mrow><msup><mrow><mi>π</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub></mrow></math> may also be considered. We focus primarily on a search for a monoenergetic <math><msup><mi>e</mi><mo>+</mo></msup></math> produced via two-body decays <math><mrow><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><mi>X</mi></mrow></math> or <math><mrow><msup><mrow><mi>π</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><mi>X</mi></mrow></math>, with <math><mi>X</mi></math> a light new physics particle. Mu2e can potentially explore new parameter space beyond present astrophysical and laboratory constraints for a set of well motivated models including axionlike particles with flavor violating couplings (<math><mrow><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><mi>a</mi></mrow></math>), massive <math><msup><mi>Z</mi><mo>′</mo></msup></math> bosons (<math><mrow><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>→</mo><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow></math>), and heavy neutral leptons (<math><mrow><msup><mrow><mi>π</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>→</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><mi>N</mi></mrow></math>). The projected sensitivities presented herein can be achieved in a matter of days.

Richard J. Hill

Ryan Plestid

Searching for new physics at <math><mi>μ</mi><mo>→</mo><mi>e</mi></math> facilities with <math><msup><mi>μ</mi><mo>+</mo></msup></math> and <math><msup><mi>π</mi><mo>+</mo></msup></math> decays at rest

Recent advances in <math><mi>b</mi></math>, <math><mi>c</mi></math>, and <math><mi>s</mi></math> quark tagging coupled with novel statistical analysis techniques will allow future high energy and high statistics electron-positron colliders, such as the FCC-ee, to place phenomenologically relevant bounds on flavor violating Higgs and <math><mi>Z</mi></math> decays to quarks. We assess the FCC-ee reach for <math><mrow><mi>Z</mi><mo>/</mo><mi>h</mi><mo>→</mo><mi>b</mi><mi>s</mi><mo>,</mo><mi>c</mi><mi>u</mi></mrow></math> decays as a function of jet tagging performance. We also update the standard model (SM) predictions for the corresponding branching ratios, as well as the indirect constraints on the flavor violating Higgs and <math><mi>Z</mi></math> couplings to quarks. Using the type III two Higgs doublet model as an example of beyond the standard model physics, we show that the searches for <math><mi>h</mi><mo>→</mo><mi>b</mi><mi>s</mi><mo>,</mo><mi>c</mi><mi>u</mi></math> decays at FCC-ee can probe new parameter space not excluded by indirect searches. We also reinterpret the FCC-ee reach for <math><mi>Z</mi><mo>→</mo><mi>b</mi><mi>s</mi><mo>,</mo><mi>c</mi><mi>u</mi></math> in terms of the constraints on models with vectorlike quarks.

Slovenia

Jernej F. Kamenik

Arman Korajac

Manuel Szewc

Michele Tammaro

Flavor-violating Higgs and <math><mi>Z</mi></math> boson decays at a future circular lepton collider

We study the exotic muon decays with five charged tracks in the final state. First, we investigate the Standard Model rate for μ + → 3e + 2e  −  2ν (  <math> <mi>B</mi> </math>  $$ \mathcal{B} $$  = 4.0 × 10 −10) and find that the Mu3e experiment should have tens to hundreds of signal events per 1015 μ + decays, depending on the signal selection strategy. We then turn to a neutrinoless μ + → 3e + 2e  −  decay that may arise in new-physics models with lepton-flavor-violating effective operators involving a dark Higgs h  d . Following its production in μ + → e + h  d  decays, the dark Higgs can undergo a decay cascade to two e + e  −  pairs through two dark photons, h  d  → γ  d  γ  d  → 2(e + e  − ). We show that a μ + → 3e + 2e  −  search at the Mu3e experiment, with potential sensitivity to the branching ratio at the   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (10 −12) level or below, can explore new regions of parameter space and new physics scales as high as Λ ∼ 1015 GeV.

Canada

New physics in multi-electron muon decays

$$B^\pm \rightarrow DK^\pm $$  <math> <mrow> <msup> <mi>B</mi> <mo>±</mo> </msup> <mo>→</mo> <mi>D</mi> <msup> <mi>K</mi> <mo>±</mo> </msup> </mrow> </math>   transitions are known to provide theoretically clean information about the CKM angle  $$\gamma $$  <math> <mi>γ</mi> </math>  , with the most precise available methods exploiting the cascade decay of the neutral D into CP self-conjugate states. Such analyses currently require binning in the D decay Dalitz plot, while a recently proposed method replaces this binning with the truncation of a Fourier series expansion. In this paper, we present a proof of principle of a novel alternative to these two methods, in which no approximations at the level of the data representation are required. In particular, our new strategy makes no assumptions about the amplitude and strong phase variation over the Dalitz plot. This comes at the cost of a degree of ambiguity in the choice of test statistic quantifying the compatibility of the data with a given value of  $$\gamma $$  <math> <mi>γ</mi> </math>  , with improved choices of test statistic yielding higher sensitivity. While our current proof-of-principle implementation does not demonstrate optimal sensitivity to  $$\gamma $$  <math> <mi>γ</mi> </math>  , its conceptually novel approach opens the door to new strategies for  $$\gamma $$  <math> <mi>γ</mi> </math>   extraction. More studies are required to see if these can be competitive with the existing methods.

Jeffrey Backus

Marat Freytsis

Yuval Grossman

United Kingdom

Stefan Schacht

Toward extracting  $$\gamma $$  <math> <mi>γ</mi> </math>   from  $$B\rightarrow DK$$  <math> <mrow> <mi>B</mi> <mo>→</mo> <mi>D</mi> <mi>K</mi> </mrow> </math>   without binning

Germany

Joachim Brod

Aaron Gootjes-Dreesbach

Erratum to: Effective field theory for dark matter direct detection up to dimension seven

We introduce a new unbinned two sample test statistic sensitive to CP violation utilizing the optimal transport plan associated with the Wasserstein (earth mover’s) distance. The efficacy of the test statistic is shown via two examples of CP asymmetric distributions with varying sample sizes: the Dalitz distributions of B 0 → K + π  −  π 0 and of D 0 → π + π  −  π 0 decays. The windowed version of the Wasserstein distance test statistic is shown to have comparable sensitivity to CP violation as the commonly used energy test statistic, but also retains information about the localized distributions of CP asymmetry over the Dalitz plot. For large statistic datasets we introduce two modified Wasserstein distance based test statistics — the binned and the sliced Wasserstein distance statistics, which show comparable sensitivity to CP violation, but improved computing time and memory scalings. Finally, general extensions and applications of the introduced statistics are discussed.

Adam Davis

Ahmed Youssef

Earth mover’s distance as a measure of CP violation

Fundamental physical constants are determined from a collection of precision measurements of elementary particles, atoms, and molecules. This is usually done under the assumption of the standard model (SM) of particle physics. Allowing for light new physics (NP) beyond the SM modifies the extraction of fundamental physical constants. Consequently, setting NP bounds using these data, and at the same time assuming the Committee on Data of the International Science Council recommended values for the fundamental physical constants, is not reliable. As we show in this Letter, both SM and NP parameters can be simultaneously determined in a consistent way from a global fit. For light vectors with QED-like couplings, such as the dark photon, we provide a prescription that recovers the degeneracy with the photon in the massless limit and requires calculations only at leading order in the small new physics couplings. At present, the data show tensions partially related to the proton charge radius determination. We show that these can be alleviated by including contributions from a light scalar with flavor nonuniversal couplings.

France

CERN

Cédric Delaunay

Jean-Philippe Karr

China

Japan

Teppei Kitahara

Netherlands

Jeroen C. J. Koelemeij

Israel

Yotam Soreq

Self-Consistent Extraction of Spectroscopic Bounds on Light New Physics

We investigate an economical explanation for the (g − 2) μ  anomaly with a neutral vector boson from a spontaneously broken U(1) X  gauge symmetry. The Standard Model fermion content is minimally extended by 3 right-handed neutrinos. Using a battery of complementary constraints, we perform a thorough investigation of the renormalizable, quark flavor-universal, vector-like U(1) X  models, allowing for arbitrary kinetic mixing. Out of 419 models with integer charges not greater than ten, only 7 models are viable solutions, describing a narrow region in model space. These are either L  μ  − L  τ  or models with a ratio of electron to baryon number close to −2. The key complementary constraints are from the searches for nonstandard neutrino interactions. Furthermore, we comment on the severe challenges to chiral U(1) X  solutions and show the severe constraints on a particularly promising such candidate.