We investigate two-neutrino double beta decay (2$\textit{νββ}$) in chiral effective field theory. We find contributions from weak magnetism and double-weak pion-exchange at next-to-leading-order in the chiral power counting. We discuss the impact of the chiral corrections on the electron spectra and find that they should be included in analyses of 2$\textit{νββ}$ decay that aim to uncover new physics signatures in the electron spectrum. We illustrate this point by revisiting the effect of sterile neutrinos and non-standard charged interactions. We also find that the pion-exchange contributions involve nuclear matrix elements that are related to those appearing in neutrinoless double beta decay (0$\textit{νββ}$). We investigate whether the 0$\textit{νββ}$ nuclear matrix elements can be obtained from detailed measurements of the energy spectrum of the outgoing electrons in 2$\textit{νββ}$ transitions.

Netherlands

Saad Morabit

United States

Ryan Bouabid

Vincenzo Cirigliano

Jordy Vries

Czechia

Lukáš Gráf

Emanuele Mereghetti

2$\textit{νββ}$ spectrum in chiral effective field theory

An investigation of relatively light (GeV-scale), long-lived right-handed neutrinos is performed within minimal left-right symmetric models using the neutrino-extended Standard Model Effective Field Theory framework. Light sterile neutrinos can be produced through rare decays of kaons, $\textit{D}$-mesons, and $\textit{B}$-mesons at the Large Hadron Collider (LHC) and the Long-Baseline Neutrino Facility (LBNF) of Fermilab. Their decays could result in displaced vertices, which can be reconstructed. By performing Monte-Carlo simulations, we assess the sensitivities of the future LHC far-detector experiments ANUBIS, CODEX-b, FACET, FASER(2), MoEDAL-MAPP1(2), MATHUSLA, the recently approved beam-dump experiment SHiP, and the upcoming neutrino experiment DUNE at the LBNF, to the right-handed gauge-boson mass $$ {M}_{W_R} $$ as functions of neutrino masses. We find that DUNE and SHiP could be sensitive to right-handed gauge-boson masses up to ~ 25 TeV. We compare this reach to indirect searches such as neutrinoless double beta decay, finding that displaced-vertex searches are very competitive.

Germany

Herbi Dreiner

Jelle Groot

Julian Günther

China

Zeren Wang

Probing light sterile neutrinos in left-right symmetric models with displaced vertices and neutrinoless double beta decay

Recent developments in the Standard Model analysis of semileptonic charged-current processes involving light quarks have revealed ~ 3σ tensions in Cabibbo universality tests involving meson, neutron, and nuclear beta decays. In this paper, we explore beyond the Standard Model explanations of this so-called Cabibbo Angle Anomaly in the framework of the Standard Model Effective Field Theory (SMEFT), including not only low-energy charged current processes (‘L’), but also electroweak precision observables (‘EW’) and Drell-Yan collider processes (‘C’) that probe the same underlying physics across a broad range of energy scales. The resulting ‘CLEW’ framework not only allows one to test explanations of the Cabibbo Angle Anomaly, but is set up to provide near model-independent analyses with minimal assumptions on the flavor structure of the SMEFT operators. Besides the global analysis, we consider a large number of simpler scenarios, each with a subset of SMEFT operators, and investigate how much they improve upon the Standard Model fit. We find that the most favored scenarios, as judged by the Akaike Information Criterion, are those that involve right-handed charged currents. Additional interactions, namely oblique operators, terms modifying the Fermi constant, and operators involving right-handed neutral currents, play a role if the CDF determination of the W mass is included in the analysis.

Wouter Dekens

Tom Tong

Anomalies in global SMEFT analyses. A case study of first-row CKM unitarity

At the Deep Underground Neutrino Experiment (DUNE), a proton beam hits a fixed target leading to large production rates of mesons. These mesons can decay and potentially provide a source of long-lived neutral fermions. Examples of such long-lived fermions are heavy neutral leptons which can mix with the standard-model active neutrinos, and the bino-like lightest neutralino in R-parity-violating supersymmetry. We show that the Standard Model Effective Field Theory extended with right-handed singlet neutrinos can simultaneously describe heavy neutral leptons and bino-like neutralinos in a unified manner. We use the effective-field-theory framework to determine the sensitivity reach of the DUNE near detector in probing various scenarios of long-lived neutral fermions.

Taiwan

Guanghui Zhou

Long-lived neutral fermions at the DUNE near detector

We present νDoBe, a Python tool for the computation of neutrinoless double beta decay (0νββ) rates in terms of lepton-number-violating operators in the Standard Model Effective Field Theory (SMEFT). The tool can be used for automated calculations of 0νββ rates, electron spectra and angular correlations for all isotopes of experimental interest, for lepton-number-violating operators up to and including dimension 9. The tool takes care of renormalization-group running to lower energies and provides the matching to the low-energy effective field theory and, at lower scales, to a chiral effective field theory description of 0νββ rates. The user can specify different sets of nuclear matrix elements from various many-body methods and hadronic low-energy constants. The tool can be used to quickly generate analytical and numerical expressions for 0νββ rates and to generate a large variety of plots. In this work, we provide examples of possible use along with a detailed code documentation. The code can be accessed through:

Oliver Scholer

νDoBe — A Python tool for neutrinoless double beta decay

A perturbative scheme is applied to calculate corrections to the leading, exponentially small (beyond-all-orders) amplitude of the “trailing” wave asymptotics of weakly localized solitons. The model considered is a Korteweg–de Vries equation modified by a fifth order derivative term, <math><msup><mi>ε</mi><mn>2</mn></msup><msubsup><mo>∂</mo><mi>x</mi><mn>5</mn></msubsup></math> with <math><mi>ε</mi><mo>≪</mo><mn>1</mn></math> (fKdV). The leading order corrections to the tail amplitude are calculated up to <math><mi>O</mi><mo>(</mo><msup><mi>ε</mi><mn>5</mn></msup><mo>)</mo></math>. An arbitrary precision numerical code is implemented to solve the fKdV equation and to check the perturbative results. Excellent agreement is found between the numerical and analytical results. Our work also clarifies the origin of a long-standing disagreement between the <math><mi>O</mi><mo>(</mo><msup><mi>ε</mi><mn>2</mn></msup><mo>)</mo></math> perturbative result of Grimshaw and Joshi [SIAM J. Appl. Math. 55, 124 (1995)] and the numerical results of Boyd [Comput. Phys. 9, 324 (1995)].

Hungary

Gyula Fodor

France

Péter Forgács

Muneeb Mushtaq

Higher order corrections to beyond-all-order effects in a fifth order Korteweg–de Vries equation

We investigate neutrinoless double beta (0νββ) decay rates in minimal left-right symmetric models in presence of relatively light right-handed neutrinos. By use of an effective field theory approach, we systematically include all contributions in the model as well as the dependence of the decay amplitude on the masses of right-handed neutrinos. In type-I and type-II seesaw scenarios, we analyze the impact of right-handed neutrinos heavier than about 10 MeV, showing that this effect can lead to a detection of 0νββ decay in the next-generation experiments even for the normal hierarchy and a relatively large right-handed scale set by the mass of hypothetical right-handed gauge bosons. Finally, we comment on a possible connection between light right-handed neutrinos and the strong CP problem.

Gang Li

Michael Ramsey-Musolf

Juan Vasquez

Light sterile neutrinos, left-right symmetry, and 0νββ decay

Axions are introduced to explain the observed smallness of the   <math> <mover> <mi>θ</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\theta} $$  term of QCD. Standard Model extensions typically contain new sources of CP violation, for instance to account for the baryon asymmetry of the universe. In the presence of additional CP-violating sources a Peccei-Quinn mechanism does not remove all CP violation, leading to CP-odd interactions among axions and Standard Model fields. In this work, we use effective field theory to parametrize generic sources of beyond-the-Standard-Model CP violation. We systematically compute the resulting CP-odd couplings of axions to leptons and hadrons by using chiral perturbation theory. We discuss in detail the phenomenology of the CP-odd axion couplings and compare limits from axion searches, such as fifth force and monopole-dipole searches and astrophysics, to direct limits on the CP-violating operators from electric dipole moment experiments. While limits from electric dipole moment searches are tight, the proposed ARIADNE experiment can potentially improve the existing constraints in a window of axion masses.

Sachin Shain

CP-violating axion interactions in effective field theory

Large numbers of τ leptons are produced at Belle II. These could potentially decay into sterile neutrinos that, for the mass range under consideration, are typically long-lived, leading to displaced-vertex signatures. Here, we study a displaced-vertex search in the context of the sterile-neutrino-extended Standard Model Effective Field Theory. The production and decay of the sterile neutrinos can be realized via either the standard active-sterile neutrino mixing or higher-dimensional operators in the effective Lagrangian. We perform Monte-Carlo simulations to estimate the Belle II sensitivities to such interactions. We find that Belle II can probe non-renormalizable dimension-six operators involving a single sterile neutrino up to a few TeV in the new-physics scale.

Long-lived sterile neutrinos at Belle II in effective field theory

We investigate contributions to the anomalous magnetic moments of charged leptons in the neutrino-extended Standard Model Effective Field Theory (νSMEFT). We discuss how νSMEFT operators can contribute to a lepton’s magnetic moment at one- and two-loop order. We show that only one operator can account for existing electronic and muonic discrepancies, assuming new physics appears above 1 TeV. In particular, we find that a right-handed charged current in combination with minimal sterile-active mixing can explain the discrepancy for sterile neutrino masses of   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (100) GeV while avoiding direct and indirect constraints. We discuss how searches for sterile neutrino production at the (HL-)LHC, measurements of h → μ + μ  −  and searches for h → e + e  − , neutrinoless double beta decay experiments, and improved unitarity tests of the CKM matrix can further probe the relevant parameter space.

Kaori Fuyuto

Poland

Richard Ruiz

Leptonic anomalous magnetic moments in ν SMEFT