<p>Within the standard model effective field theory framework, we set indirect constraints on top quark operators that violate baryon number by one unit above the TeV scale. We find that these constraints are typically many orders of magnitude more stringent than the recently derived direct bounds from collider experiments. Therefore, direct observation of baryon number violation in these top-quark observables at the TeV scale would imply a large fine-tuning among operators across different energy scales. This possibility is not protected under universal radiative corrections or any known symmetry principles.</p>

Spain

Italy

Hector Gisbert

Antonio Rodríguez-Sánchez

Luiz Vale Silva

Constraints on baryon-number-violating top-quark operators in standard model effective field theory

Axion-mediated forces are enhanced by the presence of <math><mi>C</mi><mi>P</mi></math>-violating axion couplings, which are however tightly constrained by electric dipole moment (EDM) searches. We discuss the underlying hypotheses behind different sources of <math><mi>C</mi><mi>P</mi></math> violation at high energies and the interplay between axion-mediated force experiments and EDM observables. Specifically, we identify various mechanisms, based on new sources of <math><mi>C</mi><mi>P</mi></math> violation or Peccei-Quinn symmetry breaking, that can significantly relax EDM constraints, leading to a substantial redefinition of the QCD axion window for axion-mediated forces. By considerably enlarging the QCD axion parameter space, our results provide a well-motivated target for experiments probing scalar axion couplings to matter fields. These include fifth-force tests of gravity, as well as searches for spin-dependent forces via precision magnetometry, proton storage rings and ultracold molecules.

Luca Di Luzio

Fabrizio Nesti

Philip Sørensen

Axion window on new macroscopic forces

We perform a global analysis of |∆c| = |∆u| = 1 transitions using recent data on D 0 → μ + μ  − , D + → π + μ + μ  − , Λ c  → p μ + μ  − , and D 0 → π + π  −  μ + μ  −  decays, and work out constraints on new physics Wilson coefficients   <math> <msubsup> <mi>C</mi> <mrow> <mn>7</mn> <mo>,</mo> <mn>9</mn> <mo>,</mo> <mn>10</mn> </mrow> <mfenced> <mo>′</mo> </mfenced> </msubsup> </math>  $$ {C}_{7,9,10}^{\left(\prime \right)} $$ . While results are consistent with the standard model, we find sizeable room for new physics that can be cleanly signaled with null test observables, not probed with searches in other sectors such as kaon and b-decays. The decay D 0 → π + π  −  μ + μ  −  requires better understanding of hadronic contributions to be competitive in the current fit. Progress can be achieved by precision study of the double differential decay rate in the dipion and dimuon masses, together with improved theory modelling and D → ππ transition form factors. On the other hand, the 4-body decay is an important contributor to the future global analysis due to its angular distributions that probe complementary combinations of Wilson coefficients, and as a QCD laboratory. The decay Λ c  → pℓ + ℓ  −  is the rising star due to the simplicity of a 3-body decay with available form factors from lattice QCD, sensitivity to both 4-fermion and electromagnetic dipole couplings and its null test forward-backward asymmetry.

Germany

Gudrun Hiller

Dominik Suelmann

Effective field theory analysis of rare |∆c| = |∆u| = 1 charm decays

It has been recently claimed that the axion coupling to fermions is responsible for an oscillating electric dipole moment (EDM) in the background of axion dark matter. In this work, we re-examine the derivation of this effect. Contrary to previous studies, we point out the physical relevance of an axion boundary term, which is crucial in restoring the axion shift symmetry and drastically affects the EDM phenomenology. To describe the latter, we introduce the notion of a time-averaged effective axion EDM, which encodes the boundary term and whose magnitude depends on the oscillation regime. For slow oscillations, the boundary term washes out the standard oscillating EDM, resulting in an exact cancellation in the static limit. Conversely, during fast oscillations, the boundary term amplifies the effective EDM relatively to the standard EDM contribution. This observable is especially interesting in the case of the electron EDM. For an   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (1) axion-electron coupling, the overall size of the effective EDM in the regime of intermediate or fast oscillations is comparable to the present static EDM limit.

Luca Luzio

On the oscillating electric dipole moment induced by axion-fermion couplings

Rare decays mediated by <math><mi>b</mi><mo>→</mo><mi>s</mi><mi>ν</mi><mover><mi>ν</mi><mo>¯</mo></mover></math> transitions have been reported by the Belle II experiment. The branching ratio of the decay <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> is found to be enhanced with respect to the standard model value. If taken at face value, the implications are profound; either lepton flavor universality is violated at the (multi)-TeV-scale, or light new physics is involved. This holds in general if <math><mi>B</mi><mo>(</mo><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><mo>)</mo></math> exceeds <math><mn>1.2</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>5</mn></mrow></msup><mo>(</mo><mn>1.3</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>5</mn></mrow></msup><mo>)</mo></math> at <math><mn>1</mn><mi>σ</mi></math> (<math><mn>2</mn><mi>σ</mi></math>), which tightens with a decreasing upper limit on <math><mi>B</mi><mo>(</mo><mi>B</mi><mo>→</mo><msup><mi>K</mi><mo>*</mo></msup><mi>ν</mi><mover><mi>ν</mi><mo>¯</mo></mover><mo>)</mo></math>, that is in reach of the Belle II experiment. In view of the strong constraints on electron-muon universality violation in <math><mo>|</mo><mi>Δ</mi><mi>b</mi><mo>|</mo><mo>=</mo><mo>|</mo><mi>Δ</mi><mi>s</mi><mo>|</mo><mo>=</mo><mn>1</mn></math> processes, viable explanations are heavy, (5–10)-TeV tree-level new physics mediators that couple only to tau-flavors, or lepton flavor violating ones. In addition, couplings of similar size to both left- and right-handed quarks are generically required, implying nonminimal beyond the standard model sectors which are carefully balanced against flavor constraints. The decay <math><msubsup><mi>B</mi><mi>s</mi><mn>0</mn></msubsup><mo>→</mo><mtext>invisibles</mtext></math> can shed light on whether new physics is light or heavy. In the former case, branching ratios can be as large as <math><msup><mn>10</mn><mrow><mo>−</mo><mn>5</mn></mrow></msup></math>.

Rigo Bause

United Kingdom

Implications of an enhanced <math><mrow><mi>B</mi><mo>→</mo><mi>K</mi><mi>ν</mi><mover><mrow><mi>ν</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math> branching ratio

The recent measurement of the <math><mi>C</mi><mi>P</mi></math> asymmetry in the decay <math><mi>D</mi><mo>→</mo><msup><mi>K</mi><mo>+</mo></msup><msup><mi>K</mi><mo>−</mo></msup></math> by LHCb, combined with <math><mi>Δ</mi><msub><mi>A</mi><mrow><mi>C</mi><mi>P</mi></mrow></msub></math>, evidences a sizable <math><mi>C</mi><mi>P</mi></math> asymmetry in the decay <math><mi>D</mi><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup></math>, which requires a dynamical enhancement of standard model higher-order contributions over tree-level ones by a factor of 2. The data furthermore imply huge <math><mi>U</mi></math>-spin breaking, about 4–5 times larger than the nominal standard model one of <math><mo>≲</mo><mn>30</mn><mo>%</mo></math> in charm. Enhanced breakdown of the two approximate symmetries points to models that violate <math><mi>U</mi></math>-spin and <math><mi>C</mi><mi>P</mi></math> and disfavors flavor singlet contributions such as chromomagnetic dipole operators as explanations of the data. We analyze the reach of flavorful <math><msup><mi>Z</mi><mo>′</mo></msup></math> models for charm <math><mi>C</mi><mi>P</mi></math> asymmetries. Models generically feature explicit <math><mi>U</mi></math>-spin and isospin breaking, allowing for correlations with <math><mi>D</mi><mo>→</mo><msup><mi>π</mi><mn>0</mn></msup><msup><mi>π</mi><mn>0</mn></msup></math> and <math><msup><mi>D</mi><mo>+</mo></msup><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mn>0</mn></msup></math> decays with corresponding <math><mi>C</mi><mi>P</mi></math> asymmetries at similar level and sign as <math><mi>D</mi><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup></math>, about <math><mi>O</mi><mo>(</mo><mn>1</mn><mo>−</mo><mn>2</mn><mo>)</mo><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>3</mn></mrow></msup></math>. Experimental and theoretical constraints very much narrow down the shape of viable models: Viable, anomaly-free models are leptophobic—or at least electron- and muophobic—with light <math><msup><mi>Z</mi><mo>′</mo></msup></math> below <math><mi>O</mi><mo>(</mo><mn>20</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> and can be searched for in low mass dijets at the LHC or in <math><mi>ϒ</mi></math> and charmonium decays, as well as dark photon searches. A <math><msup><mi>Z</mi><mo>′</mo></msup></math> around <math><mo>∼</mo><mn>3</mn></math> or <math><mrow><mo>∼</mo><mo>(</mo><mn>5</mn><mi>–</mi><mn>7</mn><mo>)</mo><mtext> </mtext><mi>GeV</mi></mrow></math> can relieve the tensions in the <math><mi>J</mi><mo>/</mo><mi>ψ</mi><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup></math> and <math><msup><mi>ψ</mi><mo>′</mo></msup><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup></math> branching ratios with pion form factor values from fits to BABAR and JLab data and simultaneously explain the charm <math><mi>C</mi><mi>P</mi></math> asymmetries. Models can also feature sizable branching ratios into light right-handed neutrinos or vectorlike dark fermions, which can be searched for in <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mo>→</mo><mrow><mi>hadrons</mi><mo>+</mo><mi>invisibles</mi></mrow></math> at Belle II and BESIII. Because of the low new physics scale, dark fermions can easily induce an early Landau pole, requiring models to be UV completed near the TeV scale.

Tim Höhne

Daniel F. Litim

Tom Steudtner

<math><mi>U</mi></math>-spin-<math><mi>C</mi><mi>P</mi></math> anomaly in charm

We perform a model-independent analysis of  $$|\Delta b|=|\Delta d|=1$$  <math> <mrow> <mo>|</mo> <mi>Δ</mi> <mi>b</mi> <mo>|</mo> <mo>=</mo> <mo>|</mo> <mi>Δ</mi> <mi>d</mi> <mo>|</mo> <mo>=</mo> <mn>1</mn> </mrow> </math>   processes to test the standard model and probe flavor patterns of new physics. Constraints on Wilson coefficients are obtained from global fits to  $$B^+ \rightarrow \pi ^+ \,\mu ^+\mu ^-$$  <math> <mrow> <msup> <mi>B</mi> <mo>+</mo> </msup> <mo>→</mo> <msup> <mi>π</mi> <mo>+</mo> </msup> <mspace width="0.166667em"></mspace> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> </mrow> </math>  ,  $$B^0_s\rightarrow \bar{K}^{*0}\, \mu ^+\mu ^-$$  <math> <mrow> <msubsup> <mi>B</mi> <mi>s</mi> <mn>0</mn> </msubsup> <mo>→</mo> <msup> <mrow> <mover> <mrow> <mi>K</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> </mrow> <mrow> <mrow></mrow> <mo>∗</mo> <mn>0</mn> </mrow> </msup> <mspace width="0.166667em"></mspace> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> </mrow> </math>  ,  $$B^0\rightarrow \mu ^+\mu ^-$$  <math> <mrow> <msup> <mi>B</mi> <mn>0</mn> </msup> <mo>→</mo> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> </mrow> </math>  , and radiative  $$B \rightarrow X_d \,\gamma $$  <math> <mrow> <mi>B</mi> <mo>→</mo> <msub> <mi>X</mi> <mi>d</mi> </msub> <mspace width="0.166667em"></mspace> <mi>γ</mi> </mrow> </math>   decays data. The fits are consistent with the standard model but leave sizable room for new physics. Besides higher-statistics measurements and more data in theory-friendly bins of the dilepton mass, further complementary observables such as angular distributions of  $$B_s^0 \rightarrow \bar{K}^{*0}\,\ell ^+ \ell ^-$$  <math> <mrow> <msubsup> <mi>B</mi> <mi>s</mi> <mn>0</mn> </msubsup> <mo>→</mo> <msup> <mrow> <mover> <mrow> <mi>K</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> </mrow> <mrow> <mrow></mrow> <mo>∗</mo> <mn>0</mn> </mrow> </msup> <mspace width="0.166667em"></mspace> <msup> <mi>ℓ</mi> <mo>+</mo> </msup> <msup> <mi>ℓ</mi> <mo>-</mo> </msup> </mrow> </math>  ,  $$B \rightarrow \rho \,\ell ^+ \ell ^-$$  <math> <mrow> <mi>B</mi> <mo>→</mo> <mi>ρ</mi> <mspace width="0.166667em"></mspace> <msup> <mi>ℓ</mi> <mo>+</mo> </msup> <msup> <mi>ℓ</mi> <mo>-</mo> </msup> </mrow> </math>   or the baryonic modes  $$\Xi _b \rightarrow \Sigma \,\ell ^+ \ell ^-$$  <math> <mrow> <msub> <mi>Ξ</mi> <mi>b</mi> </msub> <mo>→</mo> <mi>Σ</mi> <mspace width="0.166667em"></mspace> <msup> <mi>ℓ</mi> <mo>+</mo> </msup> <msup> <mi>ℓ</mi> <mo>-</mo> </msup> </mrow> </math>  ,  $$\Omega _b^- \rightarrow \Xi ^- \, \ell ^+ \ell ^-$$  <math> <mrow> <msubsup> <mi>Ω</mi> <mi>b</mi> <mo>-</mo> </msubsup> <mo>→</mo> <msup> <mi>Ξ</mi> <mo>-</mo> </msup> <mspace width="0.166667em"></mspace> <msup> <mi>ℓ</mi> <mo>+</mo> </msup> <msup> <mi>ℓ</mi> <mo>-</mo> </msup> </mrow> </math>   are necessary to resolve the significant degeneracy in the fit for the semileptonic four-fermion operators. Assuming minimal quark flavor violation, the  $$b \rightarrow s$$  <math> <mrow> <mi>b</mi> <mo>→</mo> <mi>s</mi> </mrow> </math>   global fit implies tight constraints on the  $$b \rightarrow d$$  <math> <mrow> <mi>b</mi> <mo>→</mo> <mi>d</mi> </mrow> </math>   couplings, and hence allows to test this paradigm with improved data. Another benefit from  $$|\Delta b|=|\Delta d|=1$$  <math> <mrow> <mo>|</mo> <mi>Δ</mi> <mi>b</mi> <mo>|</mo> <mo>=</mo> <mo>|</mo> <mi>Δ</mi> <mi>d</mi> <mo>|</mo> <mo>=</mo> <mn>1</mn> </mrow> </math>   processes is to shed light on the B-anomalies in  $$|\Delta b|=|\Delta s|=1$$  <math> <mrow> <mo>|</mo> <mi>Δ</mi> <mi>b</mi> <mo>|</mo> <mo>=</mo> <mo>|</mo> <mi>Δ</mi> <mi>s</mi> <mo>|</mo> <mo>=</mo> <mn>1</mn> </mrow> </math>   modes from a new angle. Specifically, studies of lepton flavor-specific and dineutrino modes are informative on the lepton flavor structure. Rare  $$b \rightarrow d \, \ell \ell , \nu \bar{\nu }$$  <math> <mrow> <mi>b</mi> <mo>→</mo> <mi>d</mi> <mspace width="0.166667em"></mspace> <mi>ℓ</mi> <mi>ℓ</mi> <mo>,</mo> <mi>ν</mi> <mover> <mrow> <mi>ν</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> </mrow> </math>   decays can be studied at high luminosity flavor facilities LHCb, Belle II, and a future Z-factory.

Marcel Golz

Model-independent analysis of  $$\varvec{b \rightarrow d}$$  <math> <mrow> <mi>b</mi> <mo>→</mo> <mi>d</mi> </mrow> </math>   processes

We present the contributions to electric dipole moments (EDMs) induced by the Yukawa couplings of an additional electroweak doublet of colour-octet scalars. The full set of one-loop diagrams and the enhanced higher-order effects from Barr-Zee diagrams are computed for the quark (chromo-)EDM, along with the two-loop contributions to the Weinberg operator. Using the stringent experimental upper limits on the neutron EDM, constraints on the parameter space of the Manohar-Wise model are derived.

Víctor Miralles

Joan Ruiz-Vidal

Electric dipole moments from colour-octet scalars

$$SU(2)_L$$  <math> <mrow> <mi>S</mi> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>2</mn> <mo>)</mo> </mrow> <mi>L</mi> </msub> </mrow> </math>  -invariance links charged dilepton and dineutrino couplings. Phenomenological implications are worked out for flavor changing neutral currents involving strange, charm, beauty and top quark transitions in a model-independent way. We put forward novel tests of lepton universality and lepton flavor conservation in  $$|\varDelta c|= |\varDelta u|=1$$  <math> <mrow> <mo>|</mo> <mi>Δ</mi> <mi>c</mi> <mo>|</mo> <mo>=</mo> <mo>|</mo> <mi>Δ</mi> <mi>u</mi> <mo>|</mo> <mo>=</mo> <mn>1</mn> </mrow> </math>   and  $$|\varDelta b|= |\varDelta q|=1$$  <math> <mrow> <mo>|</mo> <mi>Δ</mi> <mi>b</mi> <mo>|</mo> <mo>=</mo> <mo>|</mo> <mi>Δ</mi> <mi>q</mi> <mo>|</mo> <mo>=</mo> <mn>1</mn> </mrow> </math>  ,  $$q=d,s$$  <math> <mrow> <mi>q</mi> <mo>=</mo> <mi>d</mi> <mo>,</mo> <mi>s</mi> </mrow> </math>  , transitions suitable for the experiments Belle II and BES III. Single top production plus dileptons uniquely probes semileptonic four-fermion  $$|\varDelta t|= |\varDelta q'|=1$$  <math> <mrow> <mrow> <mo>|</mo> <mi>Δ</mi> <mi>t</mi> <mo>|</mo> <mo>=</mo> <mo>|</mo> <mi>Δ</mi> </mrow> <msup> <mi>q</mi> <mo>′</mo> </msup> <mrow> <mo>|</mo> <mo>=</mo> <mn>1</mn> </mrow> </mrow> </math>  ,  $$q'=u,c$$  <math> <mrow> <msup> <mi>q</mi> <mo>′</mo> </msup> <mo>=</mo> <mi>u</mi> <mo>,</mo> <mi>c</mi> </mrow> </math>  , couplings and further study at the LHC is encouraged. Tests with single top production associated with dileptons and missing energy are suitable for study at future  $$e^+ e^-$$  <math> <mrow> <msup> <mi>e</mi> <mo>+</mo> </msup> <msup> <mi>e</mi> <mo>-</mo> </msup> </mrow> </math>   or muon colliders.

Lepton universality and lepton flavor conservation tests with dineutrino modes

We present a systematic global analysis of dineutrino modes b → qν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$ , q = d, s, and charged dilepton b → qℓ + ℓ  −  transitions. We derive improved or even entirely new limits on dineutrino branching ratios including decays B 0 → (K 0 , X  s )ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$ , B  s  → ϕν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$  and B 0 → (π 0 , ρ 0)ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$  from dineutrino modes which presently are best constrained: B + → (K + , π + , ρ +)ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$  and B 0 → K *0 ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$ . Using SMEFT we obtain new flavor constraints from the dineutrino modes, which are stronger than the corresponding ones from charged dilepton rare b-decay or Drell-Yan data, for eτ and ττ final states, as well as for μτ ones in b → s processes. The method also allows to put novel constraints on semileptonic four-fermion operators with top quarks. Implications for ditau modes b → sτ + τ  −  and b → dτ + τ  −  are worked out. Even stronger constraints are obtained in simplified BSM frameworks such as leptoquarks and Z′-models. Furthermore, the interplay between dineutrinos and charged dileptons allows for concrete, novel tests of lepton universality in rare B-decays. Performing a global fit to b → sμ + μ  −  , sγ transitions we find that lepton universality predicts the ratio of the B 0 → K *0 ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$  to B 0 → K 0 ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$  (B + → K + ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$ ) branching fractions to be within 1.7 to 2.6 (1.6 to 2.4) at 1σ, a region that includes the standard model, and that can be narrowed with improved charged dilepton data. There is sizable room outside this region where universality is broken and that can be probed with the Belle II experiment. Using results of a fit to B 0 → μ + μ  − ,   <math> <msubsup> <mi>B</mi> <mi>s</mi> <mn>0</mn> </msubsup> </math>  $$ {B}_s^0 $$  →   <math> <mover> <mi>K</mi> <mo>¯</mo> </mover> </math>  $$ \overline{K} $$  *0 μ + μ  −  and B + → π + μ + μ  −  data we obtain an analogous relation for |∆b| = |∆d| = 1 transitions: if lepton universality holds the ratio of the B 0 → ρ 0 ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$  to B 0 → π 0 ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$  (B + → π + ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$ ) branching fractions is within 2.5 to 5.7 (1.2 to 2.6) at 1 σ. Putting upper limits on   <math> <mi>B</mi> </math>  $$ \mathcal{B} $$ (B  s  → ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$ ) at the level of 10 −5 and   <math> <mi>B</mi> </math>  $$ \mathcal{B} $$ (B 0 → ν   <math> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ \overline{\nu} $$ ) below 10 −6 would allow to control backgrounds from (pseudo-)scalar operators such as those induced by light right-handed neutrinos.