Lepton Research Articles

Search for the lepton flavor violating decay $\tau \to$ 3$\mu$ in proton-proton collisions at $\sqrt{s} =$ 13 TeV

Results are reported from a search for the lepton flavor violating decay $\tau \to$ 3$\mu$ in proton-proton collisions at $\sqrt{s}=$ 13 TeV. The data sample corresponds to an integrated luminosity of 33.2 fb$^{-1}$ recorded by the CMS experiment at the LHC in 2016. The search exploits $\tau$ leptons produced in both W boson and heavy-flavor hadron decays. No significant excess above the expected background is observed. An upper limit on the branching fraction $\mathcal{B}(\tau \to$ 3$\mu)$ of 8.0 $\times$ 10$^{-8}$ at 90% confidence level is obtained, with an expected upper limit of 6.9 $\times$ 10$^{-8}$.

Measurement of the cross section for $\text{t}\bar{\text{t}}$ production with additional jets and b jets in pp collisions at $\sqrt{s}=$ 13 TeV

Measurements of the cross section for the production of top quark pairs in association with a pair of jets from bottom quarks $ \left({\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}}\right) $ and in association with a pair of jets from quarks of any flavor or gluons $ \left({\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{jj}}\right) $ and their ratio are presented. The data were collected in proton-proton collisions at a center-of-mass energy of 13 TeV by the CMS experiment at the LHC in 2016 and correspond to an integrated luminosity of 35.9 fb$^{−1}$. The measurements are performed in a fiducial phase space and extrapolated to the full phase space, separately for the dilepton and lepton+jets channels, where lepton corresponds to either an electron or a muon. The results of the measurements in the fiducial phase space for the dilepton and lepton+jets channels, respectively, are $ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{jj}} $ = 2.36±0.02 (stat)±0.20 (syst) pb and 31.0±0.2 (stat)±2.9 (syst) pb, and for the cross section ratio 0.017 ± 0.001 (stat) ± 0.001 (syst) and 0.020 ± 0.001 (stat) ± 0.001 (syst). The values of $ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{b}\overline{\mathrm{b}}} $ are determined from the product of the $ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}\mathrm{jj}} $ and the cross section ratio, obtaining, respectively, 0.040±0.002 (stat)±0.005 (syst) pb and 0.62±0.03 (stat)±0.07 (syst) pb. These measurements are the most precise to date and are consistent, within the uncertainties, with the standard model expectations obtained using a matrix element calculation at next-to-leading order in quantum chromodynamics matched to a parton shower.[graphic not available: see fulltext]

INVESTIGATING DOUBLY CHARGED LEPTONS AT FUTURE ENERGY FRONTIER MUON-PROTON COLLIDERS

We study the doubly charged leptons considered in extended isospin models at muon-proton colliders. We respect the lepton flavor conservation and take into consideration the single production of the doubly charged leptons related to second generation. We give the effective lagrangian describing the doubly charged lepton gauge interactions. We calculate the signal and corresponding background cross sections and analyze the kinematical distributions to obtain the suitable cuts for the discovery. We choose the W-boson hadronic decay channel to get the accessible mass limits and couplings of doubly charged leptons for various muon-proton colliders.

Search for lepton-flavour-violating decays of the $Z$ boson into a $\tau$-lepton and a light lepton with the ATLAS detector

Lepton flavour violation in the charged lepton sector is an unambiguous signal of physics beyond the Standard Model. Searches for lepton flavour violation in decays of the ZZ boson with the ATLAS detector is reported, focusing on decays into an electron or muon and a hadronically decaying \tauτ-lepton, using pppp collisions data with a centre-of-mass energy of 13 TeV. Upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95% confidence level: \mathcal{B}(Z\to e\tau) < 5.8\times10^{-5}ℬ(Z→eτ)<5.8×10−5 and \mathcal{B}(Z\to \mu\tau) < 2.4\times10^{-5}ℬ(Z→μτ)<2.4×10−5. When combined with a previous ATLAS result based on pppp collisions data with a centre-of-mass energy of 8 TeV, an upper limit of \mathcal{B}(Z\to \mu\tau) < 1.3\times10^{-5}ℬ(Z→μτ)<1.3×10−5 is obtained.

Measurement of normalized differential tt¯ cross sections in the dilepton channel from pp collisions at s√=13 TeV

Normalized differential cross sections for top quark pair production are measured in the dilepton (e^+e^−, μ^+μ^−, and μ^∓e^±) decay channels in proton-proton collisions at a center-of-mass energy of 13 TeV. The measurements are performed with data corresponding to an integrated luminosity of 2.1 fb^(−1) using the CMS detector at the LHC. The cross sections are measured differentially as a function of the kinematic properties of the leptons, jets from bottom quark hadronization, top quarks, and top quark pairs at the particle and parton levels. The results are compared to several Monte Carlo generators that implement calculations up to next-to-leading order in perturbative quantum chromodynamics interfaced with parton showering, and also to fixed-order theoretical calculations of top quark pair production up to next-to-next-to-leading order.

Observation of ttH Production

The observation of Higgs boson production in association with a top quark-antiquark pair is reported, based on a combined analysis of proton-proton collision data at center-of-mass energies of √s = 7, 8, and 13 TeV, corresponding to integrated luminosities of up to 5.1, 19.7, and 35.9 fb^(-1), respectively. The data were collected with the CMS detector at the CERN LHC. The results of statistically independent searches for Higgs bosons produced in conjunction with a top quark-antiquark pair and decaying to pairs of W bosons, Z bosons, photons, τ leptons, or bottom quark jets are combined to maximize sensitivity. An excess of events is observed, with a significance of 5.2 standard deviations, over the expectation from the background-only hypothesis. The corresponding expected significance from the standard model for a Higgs boson mass of 125.09 GeV is 4.2 standard deviations. The combined best fit signal strength normalized to the standard model prediction is 1.26^(+0.31)_(−0.26).

Measurement of the ratio of branching fractions $\mathcalB(¯B^0 \to D^*+τ^-¯ν_τ)/\mathcalB(¯B^0 \to D^*+µ^-¯ν_µ)$

The branching fraction ratio R(D-*) = B((B) over bar (0) -> D-*(+)tau(-)(nu) over bar (tau))/B((B) over bar (0) -> D-*(+)mu(-)(nu) over bar (mu)) is measured using a sample of proton-proton collision data corresponding to 3.0 fb(-1) of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode tau(-) -> mu(-)(nu) over bar (mu)nu(tau). The semitauonic decay is sensitive to contributions from non-standard-model particles that preferentially couple to the third generation of fermions, in particular, Higgs-like charged scalars. A multidimensional fit to kinematic distributions of the candidate (B) over bar (0) decays gives R(D-*) = 0.336 +/- 0.027(stat) +/- 0.030(syst). This result, which is the first measurement of this quantity at a hadron collider, is 2.1 standard deviations larger than the value expected from lepton universality in the standard model.

G Super-Complex Space-Time Theory

The real and imaginary parts of complex physical quantities reflect the mutual contradictory relationship within the unity, including the unity and opposition, correlation dependence and mutual transformation. The quarks, leptons, fermions and bosons are same particles at different kinestate in complex space-time. The velocity vector generated by the rotation of the particles in the complex space distracts the time dimension, the vector forms the weak charge in the space projection; its rotary angular velocity vector distracts the energy dimension, the vector forms the mass charge in the space projection; Its rotary centripetal acceleration vector distracts the space dimension to form the electric charge; the acceleration jerk (variation of acceleration) generated by the rotation of the vector distracts the color dimension, the vector forms the color charge in the space projection; the projections of the electric charge, mass charge, color charge and weak charge into the 3D space form the three-nature (positive, negative and neutral electrodes), three-generation, three-boson and the past, present and future of particles; the empty space is a zero rest mass, linear and superposed, non-dispersive, holographic and open string boson state; the charge, rest mass and gravity are all changed with the temperature and kinematic velocity of particles. The universe is holographic, bounded and boundless. All information within the universe can be saved in the form of holographic image produced by the wave interference onto the horizon-surface, where the substances establish universal relation with those (mainly as the fermion state) in the universe by means of quantum entanglement.

Probe of Spin-1/2 leptons in γγ→l^+ l^- process at the CLIC

Bu calismada Kompakt Dogrusal Carpistirici (CLIC)’in spin-1/2 uyarilmis leptonlarin arastirilmasinda ki potansiyeli γγ → l + l - ( γ Weizsacker-Williams fotonu) alt sureci ile incelenecektir. Bu surec carpistircilardaki en temiz arkaplana sahip sureclerden bir tanesidir.

Leptonic widths of high ψ -resonances in a unitary coupled-channel model

The leptonic widths of high $\ensuremath{\psi}$-resonances are calculated in a coupled-channel model with unitary inelasticity, where analytical expressions for the mixing angles between $(n+1){^{3}S}_{1}$ and $n{^{3}D}_{1}$ states and probabilities ${Z}_{i}$ of the $c\overline{c}$ component are derived. These factors depend on energy (mass) and can be different for $\ensuremath{\psi}(4040)$ and $\ensuremath{\psi}(4160)$. However, our calculations give a small difference between the mixing angles, $\ensuremath{\theta}(\ensuremath{\psi}(4040))=(2{8}_{\ensuremath{-}2}^{+1})\ifmmode^\circ\else\textdegree\fi{}$ and $\ensuremath{\theta}(\ensuremath{\psi}(4160))=(2{9}_{\ensuremath{-}3}^{+2})\ifmmode^\circ\else\textdegree\fi{}$, and $\ensuremath{\sim}10%$ difference between the probabilities ${Z}_{1}(\ensuremath{\psi}(4040))=0.8{5}_{\ensuremath{-}0.02}^{+0.05}$ and ${Z}_{2}(\ensuremath{\psi}(4160))=0.79\ifmmode\pm\else\textpm\fi{}0.01$. It provides the leptonic widths ${\mathrm{\ensuremath{\Gamma}}}_{ee}(\ensuremath{\psi}(4040))=\phantom{\rule{0ex}{0ex}}(1.0\ifmmode\pm\else\textpm\fi{}0.1)\text{ }\text{ }\mathrm{keV}$ and ${\mathrm{\ensuremath{\Gamma}}}_{ee}(\ensuremath{\psi}(4160))=(0.62\ifmmode\pm\else\textpm\fi{}0.0.07)\text{ }\text{ }\mathrm{keV}$ in agreement with experiment; for $\ensuremath{\psi}(4415)$ the value ${\mathrm{\ensuremath{\Gamma}}}_{ee}(\ensuremath{\psi}(4415))=(0.66\ifmmode\pm\else\textpm\fi{}0.06)\text{ }\text{ }\mathrm{keV}$ is obtained, while for the missing resonance $\ensuremath{\psi}(4510)$, we predict its mass, $M(\ensuremath{\psi}(4500))=(4512\ifmmode\pm\else\textpm\fi{}2)\text{ }\text{ }\mathrm{MeV}$, and ${\mathrm{\ensuremath{\Gamma}}}_{ee}(\ensuremath{\psi}(4510))=(0.68\ifmmode\pm\else\textpm\fi{}0.14)\text{ }\text{ }\mathrm{keV}$.

Weak currents in nondiagonal leptonic processes

Nondiagonal leptonic processes are studied on the basis of neutral (NC) and charged (CC) weak current interactions. NC and CC descriptions of recoil electron spectra upon the elastic scattering of neutral (anti)leptons, including arbitrary vector and axial-vector current constants that can be real and complex or purely imaginary, are presented. Based on the NC description, the spectrum of muons originating upon the inelastic scattering of a muonic neutrino by a polarized target electron, the total cross section of this process and neutron conversion of an electron–muon pair, as well as the charged and neutral components of the electron spectrum emitted in muon decay are obtained. The possibility of forbidden nondiagonal neutral electron–muon currents emerging in these processes is analyzed.

Lepton polarization andCP-violating effects in theB¯→K¯0*(1430)ℓ+ℓ−decay in the standard and the two Higgs doublet model

In this paper we analyze the dilepton mass square $q^2$ dependency of single lepton polarization asymmetries and CP violation for $\overline{B}\rightarrow \overline{K}_0^{*}(1430) \ell^+\ell^-, \ell=\mu,\tau)$ in the 2HDM context. Also, we study the averages of these asymmetries in the domain $4 m_{\ell}^2<q^2< (m_B-m_{{K}_0^{*}})^2$. Our study manifests that the investigation of the above-mentioned asymmetries for $\overline{B}\rightarrow \overline{K}_0^*(1430) \ell^+\ell^-$ processes could provide useful information for probing new Higgs bosons in the future B-physics experiments.

Characterization of a prototype for the electromagnetic calorimeter of the Mu2e experiment

The Mu2e experiment at Fermilab searches the neutrinoless conversion of the muon into electron in the field of an Aluminum nucleus. The observation of this process would be a proof of the Charged Lepton Flavor Violation (CLFV). In case of no observation, the upper limit will be set to R_(μe) < 6×10^(−17) @ 90% CL, improving by a factor of 4 the previous best determination. The Mu2e detector apparatus consists of a straw tubes tracker that will measure the electrons momentum, and an electromagnetic calorimeter that provides a tracking-independent measurement of the electron energy, time and position. In this paper, we describe the baseline project of the EMC and present results in terms of performances and R&D.

Standard model fermions andN=8supergravity

In a scheme originally proposed by M. Gell-Mann, and subsequently shown to be realized at the SU(3)xU(1) stationary point of maximal gauged SO(8) supergravity by N. Warner and one of the present authors, the 48 spin 1/2 fermions of the theory remaining after the removal of eight Goldstinos can be identified with the 48 quarks and leptons (including right-chiral neutrinos) of the Standard Model, provided one identifies the residual SU(3) with the diagonal subgroup of the color group SU(3)_c and a family symmetry SU(3)_f. However, there remained a systematic mismatch in the electric charges by a spurion charge of $\pm$1/6. We here identify the `missing' U(1) that rectifies this mismatch, and that takes a surprisingly simple, though unexpected form.

Radiative and dilepton decays of the hadronic moleculeZc+(3900)

The newly observed hidden-charm meson ${Z}_{c}^{+}(3900)$ with quantum numbers ${J}^{P}={1}^{+}$ is considered as a hadronic molecule composed of $\overline{D}{D}^{*}$. We give detailed predictions for the decay modes ${Z}_{c}^{+}\ensuremath{\rightarrow}J/\ensuremath{\psi}{\ensuremath{\pi}}^{+}\ensuremath{\gamma}$ and ${Z}_{c}^{+}\ensuremath{\rightarrow}J/\ensuremath{\psi}{\ensuremath{\pi}}^{+}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}\text{ }(\ensuremath{\ell}=e,\ensuremath{\mu})$ in a phenomenological Lagrangian approach.

Hypothesized lepton inner discreteness

Hypothesized lepton inner discreteness

Tuning Effect in Particle Masses, Nuclear Data and Standard Model Parameters

Recent determination of the scalar boson mass and results of the analysis of nuclear excitations collected in the new nuclear data compilation confirmed the presence of a common tuning effect in particle masses, in parameters of the Standard Model and in nuclear data. This tuning effect includes the distinguished character of estimates of constituent quark masses and the presence of long-range correlations in nucleon masses and nuclear data with the parameters close to (or rational to) the electromagnetic mass splitting of the electron and the nucleon. One of SM-parameters of the tuning effect is the QED radiative correction. Its value coincides with the ratios between masses of both leptons and masses of the constituent quark and Z-boson. It means that the lepton ratio (L=mµ/me=MZ/Mq with Mq=441 MeV) has the universal character.

Search for excited leptons in pp collisions at (square root) s (equivalent to) 7 TeV

Search for excited leptons in pp collisions at (square root) s (equivalent to) 7 TeV

Study ofη–η′mixing from radiative decay processes

We perform a thorough analysis of the $VP\ensuremath{\gamma}({\ensuremath{\gamma}}^{*})$ and $P\ensuremath{\gamma}\ensuremath{\gamma}({\ensuremath{\gamma}}^{*})$ decays in the resonance chiral theory, where $V$ stands for the vector resonances $\ensuremath{\rho}$, ${K}^{*}$, $\ensuremath{\omega}$, and $\ensuremath{\phi}$; $P$ stands for $\ensuremath{\pi}$, $K$, $\ensuremath{\eta}$, and ${\ensuremath{\eta}}^{\ensuremath{'}}$; and ${\ensuremath{\gamma}}^{*}$ subsequently decays into lepton pairs. Upon imposing QCD short-distance constraints on resonance couplings, the $\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\gamma}({\ensuremath{\gamma}}^{*})$, $\ensuremath{\rho}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\gamma}({\ensuremath{\gamma}}^{*})$, ${K}^{*0}\ensuremath{\rightarrow}{K}^{0}\ensuremath{\gamma}$ processes only depend on one free parameter and $\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}({\ensuremath{\gamma}}^{*})$ can be completely predicted. The four mixing parameters of the $\ensuremath{\eta}--{\ensuremath{\eta}}^{\ensuremath{'}}$ system, i.e. two mixing angles ${\ensuremath{\theta}}_{8}$, ${\ensuremath{\theta}}_{0}$ and two decay constants ${F}_{8}$, ${F}_{0}$, are determined from radiative decays involving $\ensuremath{\eta}$ or ${\ensuremath{\eta}}^{\ensuremath{'}}$. The higher order low energy constants of the pseudo-Goldstone Lagrangian in the chiral anomaly sector are predicted by integrating out heavy resonances. We also predict the decay widths of $\ensuremath{\rho}\ensuremath{\rightarrow}\ensuremath{\pi}{e}^{+}{e}^{\ensuremath{-}}$, ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\gamma}{e}^{+}{e}^{\ensuremath{-}}$ and $\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{\eta}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, which can be compared with the future measurement in these channels.

Lepton flavour violation in D and B decays at Belle

We search for the flavour-changing neutral current decays D 0 → μ + μ − and D 0 → e + e − , and for the lepton-flavour violating decays D 0 → e ± μ ∓ using 660 fb − of data. We also search for lepton-number violating decays B + → D − e + e + , B + → D − μ + μ + and B + → D − μ + e + using 710 fb − of data. All the data is collected with the Belle detector at the KEKB asymmetric-energy e + e − collider. We find no evidence for any of these decays. We obtain significantly improved upper limits on the branching fractions: B ( D 0 → μ + μ − ) 1.4 × 10 − 7 , B ( D 0 → e + e − ) 7.9 × 10 − 8 , B ( D 0 → e + μ − ) + B ( D 0 → μ + e − ) 2.6 × 10 − 7 , B ( B + → D − e + e + ) 2.6 × 10 − 6 , B ( B + → D − e + μ + ) 1.6 × 10 − 6 and B ( B + → D − μ + μ + ) 0.9 × 10 − 6 at 90% confidence level.