Test Of Lepton Universality Research Articles

Lepton universality and lepton flavor violation in Υ decays

The datasets collected at the Υ(3S) and Υ(2S) energies have recently allowed the BABAR experiment to obtain several important results in the precision tests of the Standard Model, as well as in the search for direct and indirect new physics effects. In particular, in this paper we present a test of lepton universality, measuring the ratio of decay widths Γ(Υ(1S) → τ+τ−)/Γ(Υ(1S) → μ+μ− and contributing to the search for a light pseudoscalar Higgs boson. We also show new limits on lepton flavor violating decays of the Υ(3S) and Υ(2S).

Measurement of BR(Ke2)/BR(Kμ2) in the NA62 experiment at CERN

The Measurement of the helicity suppressed ratio of charged kaon leptonic decay rates RK = Γ (K± → e±νe)/Γ(K± → μ±νμ) has long been considered as an excellent test of lepton universality and the Standard Model (SM) description of weak interactions. It was realized recently that the suppression of the SM contribution might enhance the sensitivity to SUSY-induced effects to an experimentally accessible level. The NA62 experiment at the CERN SPS has collected over 105 K → e ν decays during a dedicated run in 2007, aiming at achieving 0.5 % precision. Experimental strategy, details of the analysis and the final result on a partial set of data will be described. The result RK = (2.487 ± 0.013) × 10−5 is in agreement with the Standard Model expectation.

Status Report from Tau subgroup of the HFAG

The aim of Tau subgroup of the HFAG is to provide average values of the mass and branching fractions of the tau lepton. Using the branching fractions, we present tests of charged current lepton universality and obtain estimates for |Vus|. We also summarize the status of searches for lepton flavor violating decays of the tau lepton.

Lepton flavour universality test at the CERN NA62 experiment

A precision test of lepton universality by measurement of the ratio R K of K + → e + ν to K + → μ + ν decay rates was performed using a sample of 59963 K + → e + ν candidates with 8.8% background contamination collected by the CERN NA62 experiment. The result R K = ( 2.486 ± 0.013 ) × 10 − 5 is in agreement with the Standard Model expectation.

Searches for New Physics at the NA62 experiment

The experiment NA62 at CERN collected a large sample of K± leptonic decays in order to perform precise test of lepton universality, by measuring the helicity suppressed ratio RK = Γ(K± → e±ν)/Γ(K± → μ±ν). The final result of the analysis based on 59,963 K+ → e+ν candidates (∼ 40% of the total data set) is RK = (2.486 ± 0.013) × 10−5 and consistent with the predictions of the Standard Model. The aim for the future program of the experiment is to measure the branching ratio of the ultra-rare decay K+ → π+νν with 10% precision.

Simultaneous extraction of the Fermi constant and PMNS matrix elements in the presence of a fourth generation

Several recent studies performed on constraints of a fourth generation of quarks and leptons suffer from the ad-hoc assumption that 3 × 3 unitarity holds for the first three generations in the neutrino sector. Only under this assumption is one able to determine the Fermi constant G F from the muon lifetime measurement with the claimed precision of G F = 1.16637(1) × 10−5 GeV−2. We study how well G F can be extracted within the framework of four generations from leptonic and radiative μ and τ decays, as well as from K ℓ3 decays and leptonic decays of charged pions, and we discuss the role of lepton universality tests in this context. In the combined fit to leptonic and radiative μ and τ decays, K ℓ3 decays and leptonic decays of charged pions we find a p-value of 2.6 % for the fourth generation matrix element |U e4| = 0 of the neutrino mixing matrix. We emphasize that constraints on a fourth generation from quark and lepton flavour observables and from electroweak precision observables can only be obtained in a consistent way if these three sectors are considered simultaneously.

τ lepton decays

We discuss recent results on τ lepton physics obtained at the BABAR, Belle and KEDR detectors. They include tests of lepton universality using new measurements of τ lepton mass and some branching fractions. Also described are selected results on τ lepton hadronic decays coming from BABAR and Belle.

Test of Lepton Universality inΥ(1S)Decays atBABAR

The ratio $R_{\tau\mu}(\Upsilon(1S)) = \Gamma_{\Upsilon(1S)\to\tau^+\tau^-}/\Gamma_{\Upsilon(1S)\to\mu^+\mu^-}$ is measured using a sample of $(121.8\pm1.2)\times10^6$ $\Upsilon(3S)$ events recorded by the BABAR detector. This measurement is intended as a test of lepton universality and as a search for a possible light pseudoscalar Higgs boson. In the standard model (SM) this ratio is expected to be close to 1. Any significant deviations would violate lepton universality and could be introduced by the coupling to a light pseudoscalar Higgs boson. The analysis studies the decays $\Upsilon(3S)\to\Upsilon(1S)\pi^+\pi^-$, $\Upsilon(1S)\to l^+l^-$, where $l=\mu,\tau$. The result, $R_{\tau\mu}(\Upsilon(1S)) = 1.005 \pm 0.013(stat.) \pm 0.022(syst.)$, shows no deviation from the expected SM value, while improving the precision with respect to previous measurements.

Charged Current Lepton Universality and [formula omitted] using τ Lepton Decays at BaBar

Using a data sample corresponding to an integrated luminosity of L = 467 fb − 1 , collected with the BaBar detector at the SLAC PEP-II e + e − storage ring, we present preliminary measurements of B ( τ → μ ν ν ¯ ) B ( τ → e ν ν ¯ ) = ( 0.9796 ± 0.0016 ± 0.0035 ) , B ( τ → π ν ) B ( τ → e ν ν ¯ ) = ( 0.5945 ± 0.0014 ± 0.0061 ) , B ( τ → K ν ) B ( τ → e ν ν ¯ ) = ( 0.03882 ± 0.00032 ± 0.00056 ) , and B ( τ → K ν ) B ( τ → π ν ) = ( 0.06531 ± 0.00056 ± 0.00093 ) , where the uncertainties are statistical and systematic, respectively. From the branching ratio B ( τ → K ν ) B ( τ → π ν ) , | V u s | is determined to be 0.2255 ± 0.0023 . The branching ratio B ( τ → μ ν ν ¯ ) B ( τ → e ν ν ¯ ) enables a precision test of the Standard Model assumption of charged current lepton universality, | g μ g e | = 1.0036 ± 0.0020 . The branching ratios B ( τ → K ν ) B ( τ → e ν ν ¯ ) , and B ( τ → π ν ) B ( τ → e ν ν ¯ ) provide additional tests of charged current lepton universality, | g τ g μ | = 0.9859 ± 0.0057 in non-strange and | g τ g μ | = 0.9836 ± 0.0087 in strange decays.

Supersymmetric grand unification and lepton universality in [formula omitted] decays

Motivated by the prospects for an improved test of lepton universality in K → ℓ ν decays by the NA62 experiment at CERN, we study predictions for the possible lepton non-universality in K → ℓ ν decays in supersymmetric models. Violations of μ – e universality in this process may originate from mixing effects in the right-handed slepton sector, providing a unique window into this aspect of supersymmetric flavour physics in the large- tan β region. Minimal unification scenarios with universal soft supersymmetry-breaking terms at the GUT scale would predict negligible violation of lepton universality. However, lepton non-universality may be observable in non-minimal grand unified models with higher-dimensional terms contributing to fermion masses, in which case renormalization effects above the GUT scale may enhance the mixing among the right-handed sleptons. This could leads to observable lepton non-universality in K → ℓ ν decays in specific regions of the parameter space with high tan β , large A terms and small charged Higgs boson mass. Observable non-universality in K → ℓ ν decays would be correlated with a large value of BR ( τ → e γ ) . The experimental upper limit on the electric dipole moment of the electron could be reconciled with leptogenesis, if the latter occurs at a relatively low scale, which would also alleviate the cosmological gravitino problem. Even if lepton non-universality is not seen in the near future, one may nevertheless obtain significant constraints on the model parameters and unknown aspects of right-handed fermion and sfermion mixing.

KLOE results with kaons

This report presents the recent results on K mesons from the KLOE experiment at the DAFNE e + e − collider working at the ϕ peak. They include V u s determinations, the test of the unitarity of the first row of the CKM matrix and the related experimental measurements. Tests of lepton universality from leptonic and semileptonic decays will be also discussed. Limits on new physics or tests of chiral perturbation theory are also assessed by the study of rarer kaon decays.

|Vus| and lepton universality from kaon decays with the KLOE detector

KLOE has measured most decay branching ratios of KS, KL and K±-mesons. It has also measured the KL and the K± lifetime and determined the shape of the form factors involved in kaon semileptonic decays. We present in the following a description of the above measurements and a well organized compendium of all of our data, with particular attention to correlations. These data provide the basis for the determination of the CKM parameter |Vus| and a test of the unitarity of the quark flavor mixing matrix. We also test lepton universality and place bounds on new physics using measurements of |Vus| from Kl2 and Kl3 decays.

Measurement of theKL→ πμν form factor parameters with the KLOE detector

Using 328 pb^{-1}of data collected at DAFNE corresponding to $\sim$ 1.8 million $K_L\to \pi\mu\nu$ decays, we have measured the $K_{\mu 3}$ form factor parameters. The structure of the $K-\pi$ vector-current provides information about the dynamics of the strong interaction; its knowledge is necessary for evaluation of the phase-space integral required for measuring the CKM matrix element $V_{us}$ and for testing lepton universality in kaon decays. Using a new parametrization for the vector and scalar form factors, we find $\lambda_+$=\pt(25.7\pm 0.6),-3, and $\lambda_0$=\pt(14.0\pm 2.1),-3,. Our result for $\lambda_0$, together with recent lattice calculations of $f_\pi$, $f_K$ and $f(0)$, satisfies the Callan-Trieman relation

Testing LFV measuring the ratio [formula omitted] between the Branching Ratio of [formula omitted] and [formula omitted] in NA48/2 experiment: Measurement and Perspectives

The measurement of the ratio R K = Br [ K ± → e ± ν ( γ ) ] / Br [ K ± → μ ± ν ( γ ) ] between the Branching Ratios of K ± → e ± ν ( γ ) and K → μ ± ν ( γ ) decays is a sensitive test of lepton universality and new physics. This ratio is predicted by the Standard Model with high precision [M. Finkemeier, Phys. Lett. B 387 (1996) 391, hep-ph/9505434 ], while its present measurement accuracy is limited by the lack of a statistically proportionate sample of K ± → e ± ν ( γ ) decays [A.R. Clark et al., Phys. Rev. Lett. 29 (1972) 1274] [K.S. Heard et al., Phys. Lett. B 55 (1975) 324] [K.S. Heard et al., Phys. Lett. B 60 (1976) 302]. The NA48/2 experiment [J.R. Batley et al., CERN/SPSC 2000-003 (2000)], located at CERN SPS accelerator, collected two different data sets of Kaon leptonic decays. The former sample has been collected in 2003 data taking and the latter in 2004 data taking. After the description of the most relevant features of the data collected in 2003 and 2004, a preliminary result based on 2003 data set will be presented: R K = ( 2.416 ± 0.043 ( stat ) ± 0.024 ( syst ) ) ⋅ 10 −5 Future perspectives to improve the measurement precision are also discussed.

Proposal for Testing Lepton Universality in Upsilon Decays at a B Factory Running at Υ(3S)

A proposal is presented for detecting new physics at a B factory running at the Υ(3 S ) resonance by testing lepton universality to the few percent level in the leptonic decays of the Υ(1 S ) and Υ(2 S ) resonances tagged by the dipion in the chain decay: Υ(3 S )→π + π - Υ(1 S ,2 S ); Υ(1 S ,2 S )→l + l - , l= e ,µ,τ.

The B - TAU FCNC Connection in SUSY Unified Theories

In the context of SUSY grand unification a link is established between the hadronic and leptonic soft breaking sectors. Such relation is here exploited in particular for FCNC processes in B physics. It is shown how bounds on leptonic FCNC involving the third generation translate into constraints on FC B decays. In the second part of the contribution we show that tests of lepton universality in K and B decays can represent an interesting handle to obtain relevant information on the amount of FCNC in the second and third fermion generation.

TAU PHYSICS FROM B FACTORIES

Some recent τ-physics results are presented from the BaBar and Belle experiments at the SLAC and KEK B factories, which produce copious numbers of τ-lepton pairs. Measurements of the tau mass and lifetime allow to test lepton universality and CPT invariance, while searches for lepton-flavour violation in tau decays are powerful ways to look for physics beyond the Standard Model. In semihadronic, non-strange tau decays, the vector hadronic final state is particularly important in helping determine the hadronic corrections to the anomalous magnetic moment of the muon, while studies of strange final states are the best available ways to measure the CKM matrix element V us and the mass of the strange quark.

Measurement of the leptonic decay widths of the ϕ-meson with the KLOE detector

The ϕ-meson leptonic widths, Γee and Γμμ, are obtained, respectively, from the e+e− forward–backward asymmetry and the μ+μ− cross section around the ϕ-mass energy. We find Γee=1.32±0.05±0.03 keV and ΓeeΓμμ=1.320±0.018±0.017 keV. These results, compatible with Γee=Γμμ, provide a precise test of lepton universality. Combining the two results gives Γℓℓ(ϕ)=1.320±0.023 keV.

New results on τ lepton

This is a review of new results on the Tau lepton. The lifetime has been measured with improved precision, allowing the test of lepton universality to 0.2% level. There are several new measurements of the branching ratios for the hadronic decays, including much improved measurements for the Cabibbo-suppressed decays. All results are consistent with the Standard Model expectations.

CLEO contributions to tau physics

We review many of the contributions of the CLEO experiment to tau physics. Topics discussed are: branching fractions for major decay modes and tests of lepton universality; rare decays; forbidden decays; Michel parameters and spin physics; hadronic sub-structure and resonance parameters; the tau mass, tau lifetime, and tau neutrino mass; searches for CP violation in tau decay; tau pair production, dipole moments, and CP violating EDM; and tau physics at CLEO-III and at OLEO-c.