Electron-muon Universality Research Articles

Implications of an enhanced B→Kνν¯ branching ratio

Rare decays mediated by b→sνν¯ transitions have been reported by the Belle II experiment. The branching ratio of the decay B+→K+νν¯ 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 B(B+→K+νν¯) exceeds 1.2×10−5(1.3×10−5) at 1σ (2σ), which tightens with a decreasing upper limit on B(B→K*νν¯), that is in reach of the Belle II experiment. In view of the strong constraints on electron-muon universality violation in |Δb|=|Δs|=1 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 Bs0→invisibles can shed light on whether new physics is light or heavy. In the former case, branching ratios can be as large as 10−5. Published by the American Physical Society 2024

Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic B-Meson Decays at Belle II.

We present the first measurement of the ratio of branching fractions of inclusive semileptonic B-meson decays, R(X_{e/μ})=B(B→Xeν)/B(B→Xμν), a precision test of electron-muon universality, using data corresponding to 189 fb^{-1} from electron-positron collisions collected with the Belle II detector. In events where the partner B meson is fully reconstructed, we use fits to the lepton momentum spectra above 1.3 GeV/c to obtain R(X_{e/μ})=1.007±0.009(stat)±0.019(syst), which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.

New perspectives for testing electron-muon universality

Intriguing results for tests of the universality of electrons and muons through measurements of rates of B → Kℓ+ℓ− and similar decays have been in the spotlight for years. The LHCb collaboration has recently reported new results which are in agreement with Lepton Flavour Universality, while the individual decay rates are found below their Standard Model predictions. In view of this new situation, we explore how much space is left for a violation of electron-muon universality. Considering new sources of CP violation and taking the new LHCb measurements into account, we show that significant differences between the short-distance coefficients for electronic and muonic final states are actually allowed by the current data. These patterns can be revealed through CP asymmetries in neutral and charged B → Kℓ+ℓ− decays. We obtain correlations between these observables and map them to the short-distance coefficients. This results in regions in New Physics parameter space with large differences between CP asymmetries of the decays with final-state electrons and muons, thereby leaving a lot of room for possible surprises in the future high-precision era.

B-anomalies from flavorful U(1)' extensions, safely

U(1)^prime extensions of the standard model with generation-dependent couplings to quarks and leptons are investigated as an explanation of anomalies in rare B-decays, with an emphasis on stability and predictivity up to the Planck scale. To these ends, we introduce three generations of vector-like standard model singlet fermions, an enlarged, flavorful scalar sector, and, possibly, right-handed neutrinos, all suitably charged under the U(1)^prime gauge interaction. We identify several gauge-anomaly free benchmarks consistent with B_s-mixing constraints, with hints for electron-muon universality violation, and the global b rightarrow s fit. We further investigate the complete two-loop running of gauge, Yukawa and quartic couplings up to the Planck scale to constrain low-energy parameters and enhance the predictive power. A characteristic of models is that the Z^prime with TeV-ish mass predominantly decays to invisibles, i.e. new fermions or neutrinos. Z^prime -production can be studied at a future muon collider. While benchmarks feature predominantly left-handed couplings C_9^{mu } and C_{10}^{mu }, right-handed ones can be accommodated as well.

Flavorful leptoquarks at the LHC and beyond: spin 1

Evidence for electron-muon universality violation that has been revealed in b → sℓℓ transitions in the observables {R}_{KK^{ast }} by the LHCb Collaboration can be explained with spin-1 leptoquarks in SU(2)L singlet V1 or triplet V3 representations in the mathcal{O} (1 − 10) TeV range. We explore the sensitivity of the high luminosity LHC (HL-LHC) and future proton-proton colliders to V1 and V3 in the parameter space connected to {R}_{KK^{ast }} -data. We consider pair production and single production in association with muons in different flavor benchmarks. Reinterpreting a recent ATLAS search for scalar leptoquarks decaying to bμ and jμ, we extract improved limits for the leptoquark masses: for gauge boson-type leptoquarks (κ = 1) we obtain {M}_{V_1} > 1.9 TeV, {M}_{V_1} > 1.9 TeV, and {M}_{V_1} > 1.7 TeV for leptoquarks decaying predominantly according to hierarchical, flipped and democratic quark flavor structure, respectively. Future sensitivity projections based on extrapolations of existing ATLAS and CMS searches are worked out. We find that for κ = 1 the mass reach for pair (single) production of V1 can be up to 3 TeV (2.1 TeV) at the HL-LHC and up to 15 TeV (19.9 TeV) at the FCC-hh with sqrt{s} = 100 TeV and 20 ab−1. The mass limits and reach for the triplet V3 are similar or higher, depending on flavor. While there is the exciting possibility that leptoquarks addressing the {R}_{KK^{ast }} -anomalies are observed at the LHC, to fully cover the parameter space pp-collisions beyond the LHC-energies are needed.

Status of Pion Decay Experiments

The branching ratio of pion decays, , has provided a sensitive test of electron-muon universality in weak interactions. The uncertainty of the Standard Model prediction is at a 0.01% level. Although a recent measurement, Re/μ = (1.2344 ± 0.0023(stat) ± 0.0019(syst)) × 10-4, reduced the experimental uncertainty by a factor of two, there is room for improvement by more than an order of magnitude. The status of two experiments at TRIUMF and PSI as well as related pion decay experiments is presented.

Status of the PIENU experiment at TRIUMF

The PIENU experiment at TRIUMF aims to measure the branching ratio of pion decays R = Γ(π+ → e+νe + π+ → e+ νeγ)/Γ(π+→μ+νμ + π+ → μ+νμγ) with precision <0.1%, providing a stringent test of the Standard Model hypothesis of electron-muon universality and a search for new physics.

Rare Kaon and Pion Decays: Incisive Probes for New Physics Beyond the Standard Model

We review the current status and future prospects of rare kaon and pion decay research programs. Our emphasis is on experimental probes of New Physics beyond the Standard Model via the theoretically pristine [Formula: see text] decays and precision tests of electron-muon universality. These studies test the Standard Model at the level of its quantum-loop predictions and have the potential to uncover new interactions beyond the O(1,000 TeV) scale.

The PIENU experiment at TRIUMF : a sensitive probe for new physics

Study of rare decays is an important approach for exploring physics beyond the Standard Model (SM). The branching ratio of the helicity suppressed pion decays, R = , is one of the most accurately calculated decay process involving hadrons and has so far provided the most stringent test of the hypothesis of electron-muon universality in weak interactions. The branching ratio has been calculated in the SM to better than 0.01% accuracy to be RSM = 1.2353(1) × 10 . The PIENU experiment at TRIUMF, which started taking physics data in September 2009, aims to reach an accuracy five times better than the previous experiments, so as to confront the theoretical calculation at the level of ±0.1%. If a deviation from the RSM is found, "new physics" beyond the SM, at potentially very high mass scales (up to 1000 TeV), could be revealed. Alternatively, sensitive constraints on hypotheses can be obtained for interactions involving pseudoscalar or scalar interactions. So far, 4 million π+ → e+ νe ue events have been accumulated by PIENU. This paper will outline the physics motivations, describe the apparatus and techniques designed to achieve high precision and present the latest results.

Precision Measurements with Pions—Review

The branching ratio of pion decays, R e / μ = Γ ( π + → e + ν + e + ν γ ) / Γ ( π + → μ + ν + μ + ν γ ) , has provided the best test of electron-muon universality in weak interactions. While the Standard Model prediction is R e / μ = ( 1.2353 ± 0.0001 ) × 10 − 4 , the existing experimental results, ( 1.2265 ± 0.0056 ) × 10 − 4 (TRIUMF) and ( 1.2346 ± 0.0050 ) × 10 − 4 (PSI), have still orders of magnitude worse uncertainties. Since the decay π + → e + ν is helicity-suppressed, it is extremely sensitive to a presence of pseudo-scalar couplings arising from a wide range of new physics up to a mass scale of 1000 TeV/c 2 . Two new experiments aiming at a precision level of < 0.1 % for R e / μ , as well as related precision measurements of other pion decays, are described.

Measurement of the tau branching fractions into leptons

Using data collected with the L3 detector near the Z resonance, corresponding to an integrated luminosity of 150 pb −1, the branching fractions of the tau lepton into electron and muon are measured to be B(τ→ e ν ̄ e ν τ)=(17.806±0.104 ( stat.)±0.076 ( syst.))% , B(τ→μ ν ̄ μν τ)=(17.342±0.110 ( stat.)±0.067 ( syst.))% . From these results the ratio of the charged current coupling constants of the muon and the electron is determined to be g μ / g e=1.0007±0.0051. Assuming electron-muon universality, the Fermi constant is measured in tau lepton decays as G F =(1.1616±0.0058)×10 −5 GeV −2. Furthermore, the coupling constant of the strong interaction at the tau mass scale is obtained as α s(m τ 2)=0.322±0.009 (exp.)±0.015 (theory) .

R-parity violation in flavor-changing neutral current processes and top quark decays.

We show that supersymmetric $R$-parity-breaking (${R}_{p}$) interactions always result in flavor changing neutral current processes. Within a single coupling scheme, these processes can be avoided in either the charge $+\frac{2}{3}$ or the charge $\ensuremath{-}\frac{1}{3}$ quark sector, but not both. These processes are used to place constraints on ${R}_{p}$ couplings. The constraints on the first and the second generations are better than those existing in the literature. The ${R}_{p}$ interactions may result in new top quark decays. Some of these violate electron-muon universality or produce a surplus of $b$ quark events in $t\overline{t}$ decays. Results from the CDF experiment are used to bound these ${R}_{p}$ couplings.

PRECISE TESTS OF e-μ UNIVERSALITY

The status of experimental tests of electron-muon universality in weak interactions is reviewed. The average of the new measurements of the π+→e+ν branching ratio, R ave =Γ(π→eν+π→eνγ)/Γ(π→μν+π→μνγ)=(1.2312±0.0037)×10−4, is consistent with standard model calculations R th =(1.234±0.001)×10−4 and confirms the hypothesis of electron-muon universality at the 0.2% level.

Measurement of the decay [formula omitted

Using the ARGUS detector at the e +e − storage ring DORIS II at DESY we have studied the decay B − → D ∗0ℓ − λ , where ℓ − = e − or μ −, using the D 0 γ and D 0 π 0 decay modes of the D ∗0 meson. B mesons were produced in 209 000 decays of γ(4S). Assuming electron-muon universality, we obtain a value for the branching ratio of BR( B − → D ∗0ℓ − λ)= (5.8 ± 1.4 ± 1.3)% .

Measurement of the decay B 0→D −ℓ +ν

Using the ARGUS detector at the e +e − storage ring DORIS II, we have investigated the decay B 0→D −ℓ +ν, where ℓ + is e + or μ +. The B 0 mesons were produced in 150 000 ϒ(4S) decays. Assuming electron-muon universality we obtain a branching ratio BR(B 0→D −e + ν)=BR(B 0→D − μ + ν)=(1.8±0.6±0.5)%.

Measurement of the decay [formula omitted

Using the ARGUS detector at the e +e − storage ring DORIS II at DESY we have investigated the decays B 0→ D ∗−e +v and B 0→D ∗−μ +ν. The B 0 mesons were produced in 39600 ϒ(4 S)→ B 0 B 0 decays. Assuming electron-muon universality we obtain a branching ratio BR( B 0→ D ∗−e +v)= BR( B 0→ D ∗−γm +ν)=(7.0±1.2±1.9)% .

New approach to the electron-muon mass ratio

A class of models is devised which, while respecting electron-muon universality in the Lagrangian, results naturally in a small value of $\frac{{m}_{e}}{{m}_{\ensuremath{\mu}}}$, and which does not involve the introduction of exotic new gauge interactions. [The gauge group is SU(2) \ifmmode\times\else\texttimes\fi{} U(1) and the model is a relatively simple extension of the Weinberg-Salam model.] This is achieved by repeated applications of discrete symmetry. The model requires the existence of heavy leptons, and the precise value of $\frac{{m}_{e}}{{m}_{\ensuremath{\mu}}}$ depends upon the masses of these leptons.

Measuring form factors and decay parameters of heavy spin-1/2 particles produced bye−e+annihilations

The cross section for e/sup -/e/sup +/ ..-->.. U/sup -/U/sup +/ ..-->.. decay products is given for a spin-1/2 U, assuming arbitrary beam polarizations, general electromagnetic form factors, and general nonderivative, four-fermion interactions. Measurement of the momentum of one of the particles emitted in the decay specifies form factors and decay parameters of the U. In the case of heavy-lepton pair production, measurements of the form factors test the assumed absence of strong interactions, and measurements of decay parameters test the assumed decay with V +- A currents and electron-muon universality. (AIP)

Deviations from Electron-Muon Universality in the Leptonic Decays of the Intermediate Bosons

The deviations from electron-muon Universality induced by the higher-order corrections in the leptonic decays of the intermediate bosons are analyzed in the framework of the renormalizable theories of weak and electromagnetic interactions. Some general theorems are derived for the second-order weak and electromagnetic corrections which are valid for all renormalizable theories which incorporate electron-muon universality as a zeroth-order relation. The connection of our results with the theorem of cancellation of mass divergences is briefly discussed.

On the electron-muon universality in strangeness-changing processes

It is suggested that the available data are consistent with the hypothesis that the electronic and the muonic weak currents have slightly different couplings to the strangeness-changing hadronic current. The consequences of such a weak violation of the e − μ universality are studied with particular reference to the problem of the e − μ mass difference and the Cabibbo-angle determination.