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

We study neutrinoless double-beta decay in extensions of the Standard Model that include $\textit{n}$ right-handed neutrino singlets, with masses $\textit{m}$ below the GeV scale. Generalizing recently developed matching methods, we determine the $\textit{m}$ dependence of the short-range $\textit{nn}$ → $\textit{pp}$ couplings that appear to leading order in the chiral effective field theory description of neutrinoless double beta decay. We focus on two scenarios, corresponding to the minimal $\textit{ν}$SM and left-right symmetric models. We illustrate the impact of our new results in the case of the $\textit{ν}$SM, showing a significant impact on the neutrinoless double-beta decay half-life when $\textit{m}$ is in the 200–800 MeV range.

Wouter Dekens

Sebastián Quiroga

Neutrinoless double $\textit{β}$ decay with light sterile neutrinos: the contact terms

<p>We report an exploratory study of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><mi>α</mi><mo stretchy="false">)</mo></math> structure-dependent electromagnetic radiative corrections to unique first-forbidden nuclear beta decays. We show that the insertion of angular momentum into the nuclear matrix element by the virtual or real photon exchange opens up the decay at vanishing nuclear recoil momentum which was forbidden at tree level, leading to a dramatic change in the decay spectrum not anticipated in existing studies. We discuss its implications for precision tests on the standard model and searches for new physics.</p>

Chien-Yeah Seng

Ayala Glick-Magid

Unique Forbidden Beta Decays at Zero Momentum Transfer

We study neutrino flavor evolution in the quantum many-body approach using the full neutrino-neutrino Hamiltonian, including the usually neglected terms that mediate nonforward scattering processes. Working in the occupation number representation with plane waves as single-particle states, we explore the time evolution of simple initial states with up to <math><mi>N</mi><mo>=</mo><mn>10</mn></math> neutrinos. We discuss the time evolution of the Loschmidt echo, one body flavor and kinetic observables, and the one-body entanglement entropy. For the small systems considered, we observe “thermalization” of both flavor and momentum degrees of freedom on comparable time scales, with results converging towards expectation values computed within a microcanonical ensemble. We also observe that the inclusion of nonforward processes generates a faster flavor evolution compared to the one induced by the truncated (forward) Hamiltonian.

Srimoyee Sen

Yukari Yamauchi

Neutrino many-body flavor evolution: The full Hamiltonian

The accuracy of <math><mrow><msub><mrow><mi>V</mi></mrow><mrow><mi>u</mi><mi>d</mi></mrow></msub></mrow></math> determinations from superallowed <math><mi>β</mi></math> decays critically hinges on control over radiative corrections. Recently, substantial progress has been made on the single-nucleon, universal corrections, while nucleus-dependent effects, typically parametrized by a quantity <math><msub><mi>δ</mi><mi>NS</mi></msub></math>, are much less well constrained. Here, we lay out a program to evaluate this correction from effective field theory (EFT), highlighting the dominant terms as predicted by the EFT power counting. Moreover, we compare the results to a dispersive representation of <math><msub><mi>δ</mi><mi>NS</mi></msub></math> and show that the expected momentum scaling applies even in the case of low-lying intermediate states. Our EFT framework paves the way toward ab initio calculations of <math><msub><mi>δ</mi><mi>NS</mi></msub></math> and thereby addresses the dominant uncertainty in <math><msub><mi>V</mi><mrow><mi>u</mi><mi>d</mi></mrow></msub></math>.

Jordy de Vries

Stefano Gandolfi

Switzerland

Martin Hoferichter

Radiative Corrections to Superallowed <math><mi>β</mi></math> Decays in Effective Field Theory

We perform a loop-level analysis of charged-current (CC) processes involving light leptons and quarks within the Standard Model Effective Field Theory (SMEFT). This work is motivated by the high precision reached in experiment and Standard Model calculations for CC decays of mesons, neutron, and nuclei, and by a lingering tension in the Cabibbo universality test. We identify the SMEFT operators that induce the largest loop-level contributions to CC processes. These include four-quark and four-fermion semileptonic operators involving two third-generation quarks. We discuss the available constraints on the relevant effective couplings and along the way we derive new loop-level bounds from K →   <math> <mi>πν</mi> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \pi \nu \overline{\nu} $$  on four-quark operators involving two top quarks. We find that low-energy CC processes are quite competitive with other probes, set constraints that do not depend on flavor-symmetry assumptions, and probe operators involving third-generation quarks up to effective scales of Λ ≃ 8 TeV. Finally, we briefly discuss single-field ultraviolet completions that could induce the relevant operators.

Maria Dawid

One-loop analysis of β decays in SMEFT

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.

Germany

Tom Tong

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

It was recently estimated that the strong isospin-symmetry breaking (ISB) corrections to the Fermi matrix element in free neutron decay could be of the order <math><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>4</mn></mrow></msup></math>, one order of magnitude larger than the naïve estimate based on the Behrends-Sirlin-Ademollo-Gatto theorem. To investigate this claim, we derive a general expression of the leading ISB correction to hadronic Fermi matrix elements, which takes the form of a four-point correlation function in lattice gauge theory and is straightforward to compute from first principles. Our formalism paves the way for the first determination of such correction in the neutron sector with fully-controlled theory uncertainties.

China

Xu Feng

Mikhail Gorchtein

Luchang Jin

Gerald A. Miller

Quark mass difference effects in hadronic Fermi matrix elements from first principles

We present a lattice QCD study of the contribution of the isovector quark chromoelectric dipole moment (qcEDM) operator to the nucleon electric dipole moments (nEDM). The calculation was carried out on four <math><mrow><mn>2</mn><mo>+</mo><mn>1</mn><mo>+</mo><mn>1</mn></mrow></math> flavor highly improved staggered quark (HISQ) ensembles generated by the MILC Collaboration. Wilson-clover quarks were used to construct correlation functions. This clover-on-HISQ formulation is not fully <math><mi>O</mi><mo>(</mo><mi>a</mi><mo>)</mo></math> improved, and gives rise to additional systematics over and above those due to removing excited state contributions to getting ground-state matrix elements, and the final chiral and continuum extrapolations to get the physical result. We use the nonsinglet axial Ward identity (AWI) including corrections up to <math><mrow><mi>O</mi><mo>(</mo><mi>a</mi><mo>)</mo></mrow></math> to show how to control the power-divergent mixing of the isovector qcEDM operator with the lower-dimensional pseudoscalar operator. The residual corrections are observed to give rise to <math><mi>O</mi><mo>(</mo><mn>25</mn><mo>%</mo><mo>)</mo></math> violations in relations arising from the AWI. We devise three methods attempting to control the resulting uncertainty in the <math><mi>C</mi><mi>P</mi></math>-violating form factor; each of these, however, can have large <math><mi>O</mi><mo>(</mo><msup><mi>a</mi><mn>2</mn></msup><mo>)</mo></math> corrections. Preliminary results for the nEDM due to qcEDM are presented choosing the method giving the most uniform behavior.

Tanmoy Bhattacharya

Rajan Gupta

Jun-Sik Yoo

Boram Yoon

Quark chromoelectric dipole moment operator on the lattice

We study radiative corrections to low-energy charged-current processes involving nucleons, such as neutron beta decay and (anti)neutrino-nucleon scattering within a top-down effective-field-theory approach. We first match the Standard Model to the low-energy effective theory valid below the weak scale and, using renormalization group equations with anomalous dimensions of <math><mrow><mi>O</mi><mo>(</mo><mi>α</mi><mo>,</mo><mi>α</mi><msub><mrow><mi>α</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>,</mo><msup><mrow><mi>α</mi></mrow><mrow><mn>2</mn></mrow></msup><mo>)</mo></mrow></math>, evolve the resulting effective coupling down to the hadronic scale. Here, we first match to heavy-baryon chiral perturbation theory and subsequently, below the pion-mass scale, to a pionless effective theory, evolving the effective vector coupling with anomalous dimensions of <math><mi>O</mi><mo>(</mo><mi>α</mi><mo>,</mo><msup><mi>α</mi><mn>2</mn></msup><mo>)</mo></math> all the way down to the scale of the electron mass, relevant for beta decays. We thus provide a new evaluation of the “inner” radiative corrections to the vector coupling constant and to the neutron decay rate, discussing differences with the previous literature. Using our new result for the radiative corrections, we update the extraction of the Cabibbo-Kobayashi-Maskawa matrix element <math><msub><mi>V</mi><mrow><mi>u</mi><mi>d</mi></mrow></msub></math> from the neutron decay.

Oleksandr Tomalak

Effective field theory for radiative corrections to charged-current processes: Vector coupling