The cross section of the pure QED process e + e − → γγ has been measured using data accumulated during the 1989 and 1990 scans of the Z 0 resonance at LEP. Both the energy dependence and the angular distribution are in good agreement with the QED prediction. Upper limits on the branching ratios of Z 0 → γγ , Z 0 → π 0 γ and Z 0 → ηγ have been set at 1.4×10 −4 , 1.4×10 −4 and 2.0×10 −4 respectively. Lower limits on the cutoff parameters of the modified electron propagator have been found to be Λ + > 117 GeV and Λ − > 110 GeV. The reaction e + e − → γγγ has also been studied and was found to be consistent with the QED prediction. An upper limit on the branching ratio of Z 0 → γγγ has been set at 6.6 × 10 −5 . All the limits are given at 95% confidence level.
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Using data samples collected at center of mass energies of $\sqrt{s}$ = 4.009, 4.226, 4.257, 4.358, 4.416 and 4.599 GeV with the BESIII detector operating at the BEPCII storage ring, we search for the isospin violating decay $Y(4260)\rightarrow J/\psi \eta \pi^{0}$. No signal is observed, and upper limits on the cross section $\sigma(e^{+}e^{-}\rightarrow J/\psi \eta \pi^{0})$ at the 90\% confidence level are determined to be 3.6, 1.7, 2.4, 1.4, 0.9 and 1.9 pb, respectively.
Results on $e^{+}e^{-}\rightarrow J/\psi\eta\pi^{0}$. Listed in the table are the integrated luminosity $\cal{L}$, radiative correction factor (1+$\delta^{r}$) taken from QED calculation assuming the $Y(4260)$ cross section follows a Breit$-$Wigner line shape, vacuum polarization factor (1+$\delta^{v}$), average efficiency ($\epsilon^{ee}{\cal B}^{ee}$ + $\epsilon^{\mu\mu}{\cal B}^{\mu\mu}$), number of observed events $N^\text{obs}$, number of estimated background events $N^\text{bkg}$, the efficiency corrected upper limits on the number of signal events $N^\text{up}$, and upper limits on the Born cross section $\sigma^\text{Born}_\text{UL}$ (at the 90 $\%$ C.L.) at each energy point.
A search for a heavy resonance decaying into a top quark and a W boson in proton-proton collisions at $\sqrt{s} =$ 13 TeV is presented. The data analyzed were recorded with the CMS detector at the LHC and correspond to an integrated luminosity of 138 fb$^{-1}$. The top quark is reconstructed as a single jet and the W boson, from its decay into an electron or muon and the corresponding neutrino. A top quark tagging technique based on jet clustering with a variable distance parameter and simultaneous jet grooming is used to identify jets from the collimated top quark decay. The results are interpreted in the context of two benchmark models, where the heavy resonance is either an excited bottom quark b$^*$ or a vector-like quark B. A statistical combination with an earlier search by the CMS Collaboration in the all-hadronic final state is performed to place upper cross section limits on these two models. The new analysis extends the lower range of resonance mass probed from 1.4 down to 0.7 TeV. For left-handed, right-handed, and vector-like couplings, b$^*$ masses up to 3.0, 3.0, and 3.2 TeV are excluded at 95% confidence level, respectively. The observed upper limits represent the most stringent constraints on the b$^*$ model to date.
Distributions of MtW in the 1b category. The data are shown by filled markers, where the horizontal bars indicate the bin widths. The individual background contributions are given by filled histograms. The expected signal for a LH b* with mb∗ = 2.4 TeV is shown by a dashed line. The shaded region is the uncertainty in the total background estimate. The lower panel shows the ratio of data to the background estimate, with the total uncertainty on the predicted background displayed as the gray band.
Distributions of MtW in the 2b category. The data are shown by filled markers, where the horizontal bars indicate the bin widths. The individual background contributions are given by filled histograms. The expected signal for a LH b* with mb∗ = 2.4 TeV is shown by a dashed line. The shaded region is the uncertainty in the total background estimate. The lower panel shows the ratio of data to the background estimate, with the total uncertainty on the predicted background displayed as the gray band.
Upper limits on the production cross section times branching fraction of the b* LH hypothesis at a 95% CL. Dashed colored lines show the expected limits from the l+jets and all-hadronic channels, where the latter start at resonance masses of 1.4 TeV. The observed and expected limits from the combination are shown as solid and dashed black lines, respectively. The green and yellow bands show the 68 and 95% confidence intervals on the combined expected limits.
Using data samples collected at center-of-mass energies of $\sqrt{s}$ = 4.009, 4.230, 4.260, and 4.360 GeV with the BESIII detector operating at the BEPCII collider, we perform a search for the process $e^+e^-\to\gamma\chi_{cJ}$ $(J = 0, 1, 2)$ and find evidence for $e^+e^-\to\gamma\chi_{c1}$ and $e^+e^-\to\gamma\chi_{c2}$ with statistical significances of 3.0$\sigma$ and 3.4$\sigma$, respectively. The Born cross sections $\sigma^{B}(e^+e^-\to\gamma\chi_{cJ})$, as well as their upper limits at the 90% confidence level are determined at each center-of-mass energy.
The results on $e^+e^-\to\gamma\chi_{c0}$ Born cross section measurement. Shown in the table are the significance $\sigma$, detection efficiency $\epsilon$, number of signal events from the fits N$^{\rm obs}$, radiative correction factor ($1+\delta^{r}$), vacuum polarization factor ($1+\delta^{v}$), upper limit (at the 90$\%$ C.L.) on the number of signal events N$^{\rm UP}$, Born cross section $\sigma^{B}$ and upper limit (at the 90$\%$ C.L.) on the Born cross section $\sigma^{\rm UP}$ at different CME points. Numbers taken from journal version: some slight differences with respect to arXiv:1411.6336v1 in last two columns.
The results on $e^+e^-\to\gamma\chi_{c1}$ Born cross section measurement. Shown in the table are the significance $\sigma$, detection efficiency $\epsilon$, number of signal events from the fits N$^{\rm obs}$, radiative correction factor ($1+\delta^{r}$), vacuum polarization factor ($1+\delta^{v}$), upper limit (at the 90$\%$ C.L.) on the number of signal events N$^{\rm UP}$, Born cross section $\sigma^{B}$ and upper limit (at the 90$\%$ C.L.) on the Born cross section $\sigma^{\rm UP}$ at different CME points.
The results on $e^+e^-\to\gamma\chi_{c2}$ Born cross section measurement. Shown in the table are the significance $\sigma$, detection efficiency $\epsilon$, number of signal events from the fits N$^{\rm obs}$, radiative correction factor ($1+\delta^{r}$), vacuum polarization factor ($1+\delta^{v}$), upper limit (at the 90$\%$ C.L.) on the number of signal events N$^{\rm UP}$, Born cross section $\sigma^{B}$ and upper limit (at the 90$\%$ C.L.) on the Born cross section $\sigma^{\rm UP}$ at different CME points.
In an analysis of a 2.92~fb$^{-1}$ data sample taken at 3.773~GeV with the BESIII detector operated at the BEPCII collider, we measure the absolute decay branching fractions to be $\mathcal B(D^0 \to K^-e^+\nu_e)=(3.505\pm 0.014 \pm 0.033)\%$ and $\mathcal B(D^0 \to \pi^-e^+\nu_e)=(0.295\pm 0.004\pm 0.003)\%$. From a study of the differential decay rates we obtain the products of hadronic form factor and the magnitude of the CKM matrix element $f_{+}^K(0)|V_{cs}|=0.7172\pm0.0025\pm 0.0035$ and $f_{+}^{\pi}(0)|V_{cd}|=0.1435\pm0.0018\pm 0.0009$. Combining these products with the values of $|V_{cs(d)}|$ from the SM constraint fit, we extract the hadronic form factors $f^K_+(0) = 0.7368\pm0.0026\pm 0.0036$ and $f^\pi_+(0) = 0.6372\pm0.0080\pm 0.0044$, and their ratio $f_+^{\pi}(0)/f_+^{K}(0)=0.8649\pm 0.0112\pm 0.0073$. These form factors and their ratio are used to test unquenched Lattice QCD calculations of the form factors and a light cone sum rule (LCSR) calculation of their ratio. The measured value of $f_+^{K(\pi)}(0) |V_{cs(d)}|$ and the lattice QCD value for $f^{K(\pi)}_+(0)$ are used to extract values of the CKM matrix elements of $|V_{cs}|=0.9601 \pm 0.0033 \pm 0.0047 \pm 0.0239$ and $|V_{cd}|=0.2155 \pm 0.0027 \pm 0.0014 \pm 0.0094$, where the third errors are due to the uncertainties in lattice QCD calculations of the form factors. Using the LCSR value for $f_+^\pi(0)/f_+^K(0)$, we determine the ratio $|V_{cd}|/|V_{cs}|=0.238\pm 0.004\pm 0.002\pm 0.011$, where the third error is from the uncertainty in the LCSR normalization. In addition, we measure form factor parameters for three different theoretical models that describe the weak hadronic charged currents for these two semileptonic decays. All of these measurements are the most precise to date.
Summary of the range of each $q^2$ bin, the number of the observed events $N_{\rm observed}$, the number of produced events $N_{\rm produced}$, and the partial decay rate $\Delta\Gamma$ in each $q^2$ bin for $D^0\to K^-e^+\nu_e$ decays.
Summary of the range of each $q^2$ bin, the number of the observed events $N_{\rm observed}$, the number of produced events $N_{\rm produced}$, and the partial decay rate $\Delta\Gamma$ in each $q^2$ bin for $D^0\to \pi^-e^+\nu_e$ decays.
Using data samples collected with the BESIII detector operating at the BEPCII collider at center-of-mass energies from 3.810 to 4.600 GeV, we perform a study of $e^{+}e^{-} \to \eta J/\psi$ and $\pi^0 J/\psi$. Statistically significant signals of $e^{+}e^{-} \to \eta J/\psi$ are observed at $\sqrt{s}$ = 4.190, 4.210, 4.220, 4.230, 4.245, 4.260, 4.360 and 4.420 GeV, while no signals of $e^{+}e^{-} \to \pi^{0} J/\psi$ are observed. The measured energy-dependent Born cross section for $e^{+}e^{-} \to \eta J/\psi$ shows an enhancement around 4.2~GeV. The measurement is compatible with an earlier measurement by Belle, but with a significantly improved precision.
Results on $e^{+}e^{-}\to\eta J/\psi$ in data samples in which a signal is observed with a statistical significance larger than $5\sigma$. The table shows the CM energy $\sqrt{s}$, integrated luminosity $\mathcal{L}_\mathrm{int}$, number of observed $\eta$ events $N^\mathrm{obs}_{\eta}(\mu^{+}\mu^{-})$/$N^\mathrm{obs}_{\eta}(e^{+}e^{-})$ from the fit, efficiency $\epsilon_{\mu}/\epsilon_{e}$, radiative correction factor $(1+\delta^{r})$, vacuum polarization factor $(1+\delta^{v})$, Born cross section $\sigma^{B}(\mu^{+}\mu^{-})$/$\sigma^{B}(e^{+}e^{-})$ and combined Born cross section $\sigma^{B}_\mathrm{Com}$. The first uncertainties are statistical and the second systematic.
Upper limits of $e^{+}e^{-} \to \eta J/\psi$ using the $\mu^{+}\mu^{-}$ mode. The table shows the CM energy $\sqrt{s}$, integrated luminosity $\mathcal{L}_\mathrm{int}$, number of observed $\eta$ events $N^\mathrm{sg}_{\eta}$, number of background from $\eta$ sideband $N^\mathrm{sb}_{\eta}$, and from $J/\psi$ sideband $N^\mathrm{sb}_{J/\psi}$, efficiency $\epsilon$, upper limit of signal number with the consideration of selection efficiency $N^\mathrm{up}_{\eta}/\epsilon$ (at the $90\%$ C.L.), radiative correction factor $(1+\delta^{r})$, vacuum polarization factor $(1+\delta^{v})$, Born cross section $\sigma^{B}$ and upper limit on the Born cross sections $\sigma^{B}_\mathrm{up}$ (at the $90\%$ C.L.). The first uncertainties are statistical and the second systematic.
Upper limits of $e^{+}e^{-} \to \pi^{0} J/\psi$. The table shows the number of observed events in the $\pi^{0}$ signal region $N^\mathrm{sg}$, number of events in $\pi^{0}$ sideband $N^\mathrm{sb}_{\pi^{0}}$, and in $J/\psi$ sideband $N^\mathrm{sb}_{J/\psi}$, efficiency $\epsilon$, the upper limit of signal events with the consideration of the selection efficiency $N^\mathrm{up}(\mu^{+}\mu^{-})/\epsilon$ (at the $90\%$ C.L.) and the upper limit of Born cross sections $\sigma^{B}_\mathrm{up}$ (at the $90\%$ C.L.).
Using a data sample collected with the BESIII detector operating at the BEPCII storage ring, we observe a new neutral state $Z_c(3900)^{0}$ with a significance of $10.4\sigma$. The mass and width are measured to be $3894.8\pm2.3\pm3.2$ MeV/$c^2$ and $29.6\pm8.2\pm8.2$~MeV, respectively, where the first error is statistical and the second systematic. The Born cross section for $e^+e^-\to\pi^0\pi^0 J/\psi$ and the fraction of it attributable to $\pi^0 Z_c(3900)^{0}\to\pi^0\pi^0 J/\psi$ in the range $E_{cm}=4.19-4.42$ GeV are also determined. We interpret this state as the neutral partner of the four-quark candidate $Z_c(3900)^\pm$.
Efficiencies, yields, $R=\frac{\sigma(e^+e^-\to\pi^0 Z_c(3900)^{0}\to\pi^0\pi^0 J/\psi)}{\sigma(e^+e^-\to\pi^0\pi^0 J/\psi)}$, and $\pi^0\pi^0 J/\psi$ Born cross sections at each energy point. For $N(Z_c^0)$ and $N(\pi^0\pi^0 J/\psi)$ errors and upper limits are statistical only. For $R$ and $\sigma_{\rm Born}$, the first errors and statistical and second errors are systematic. The statistical uncertainties on the efficiencies are negligible. Upper limits of $R$ (90$\%$ confidence level) include systematic errors.
We report the first observation of the Dalitz decay $\eta' \to \gamma e^+e^-$, based on a data sample of 1.31 billion $J/\psi$ events collected with the BESIII detector. The $\eta'$ mesons are produced via the $J/\psi \to \gamma \eta'$ decay process. The ratio $\Gamma(\eta' \to \gamma e^+ e^-)/\Gamma(\eta'\to\gamma\gamma)$ is measured to be $(2.13\pm0.09(\text{stat.})\pm0.07(\text{sys.}))\times10^{-2}$. This corresponds to a branching fraction ${\cal B}(\eta' \to \gamma e^+e^-)= (4.69 \pm0.20(\text{stat.})\pm0.23(\text{sys.}))\times10^{-4}$. The transition form factor is extracted and different expressions are compared to the measured dependence on the $e^+e^-$ invariant mass. The results are consistent with the prediction of the Vector Meson Dominance model.
We describe the sample of energetic single-photon events ( E γ > 15 GeV) collected by L3 in the 1991–1993 LEP runs. The event distributions agree with expectations from the Standard Model. The data are used to constrain the ZZ γ coupling and to set an upper limit of 4.1 × 10 −6 , μ B (90% C.L.) on the the magnetic moment of the τ neutrino.
The number of events expected from Standard Model is 8.2. Here UNSPEC is 'invisible' particle.
90 PCT C.L. limit on an anomalous magnetic moment for tau-neutrino from '1GAMMA + nothing' events. Magnetic moment in Bohr magnetons.
A search is made for a vector-like $T$ quark decaying into a Higgs boson and a top quark in 13 TeV proton-proton collisions using the ATLAS detector at the Large Hadron Collider with a data sample corresponding to an integrated luminosity of 139 fb$^{-1}$. The Higgs-boson and top-quark candidates are identified in the all-hadronic decay mode, where $H\to b\bar{b}$ and $t\to b W \to b q \bar{q}^\prime$ are reconstructed as large-radius jets. The candidate Higgs boson, top quark, and associated B-hadrons are identified using tagging algorithms. No significant excess is observed above the background, so limits are set on the production cross-section of a singlet $T$ quark at 95% confidence level, depending on the mass, $m_T$, and coupling, $\kappa_T$, of the vector-like $T$ quark to Standard Model particles. In the considered mass range between 1.0 and 2.3 TeV, the upper limit on the allowed coupling values increases with $m_T$ from a minimum value of 0.35 for 1.07 < $m_T$ < 1.4 TeV to 1.6 for $m_T$ = 2.3 TeV.
Dijet invariant mass distribution for the $SR$ showing the results of the model when fitted to the data. A $T$-quark hypothesis with $m_{T} = 1.6$ TeV and $\kappa_{T} = 0.5$ is used in the fit.
Dijet invariant mass distribution for the $ttNR$ showing the results of the model when fitted to the data. A $T$-quark hypothesis with $m_{T} = 1.6$ TeV and $\kappa_{T} = 0.5$ is used in the fit.
Observed and expected 95% CL upper limits on the single $T$-quark coupling $\kappa_{T}$ as a function of $m_{T}$ are shown.
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