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Register a new userImproved Standard-Model prediction for $pi^0to e^+e^-$
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We present an improved Standard-Model (SM) prediction for the dilepton decay of the neutral pion. The loop amplitude is determined by the pion transition form factor for $pi^0togamma^*gamma^*$, for which we employ a dispersive representation that incorporates both space-like and time-like data as well as short-distance constraints. The resulting SM branching fraction, $ text{BR}(pi^0to e^+e^-)=6.25(3)times 10^{-8}$ , sharpens constraints on physics beyond the SM, including pseudoscalar and axial-vector mediators.
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C-odd asymmetry can be studied from an accurate measurement of the angular distribution due to the interference between the S- and P-waves in e+e- --> pi+ pi- at order alpha^3. The asymmetry is dominated by the pion rescattering diagram which is enhanced by the presence of the ln(s/m^2_e), and is quite large (10% at theta=30^0 and sqrt{s} < M_{f_2}) compared to alpha/pi=0.3%. This process can also be used for alternatively measuring the size of the rescattering term and the phase of the S-wave amplitude, but does not help to solve the present discrepancy between the hadronic spectral functions from e+e- and tau-decay data.
Very recently, the {it BABAR} Collaboration indicated that there exist an explicit enhancement structure near 2.2 GeV when focusing on the $e^+e^-topi^+pi^-$ process again, which inspires our interest in studying the production of higher $rho$ mesonic states. Since the branching ratio of $pi^+pi^-$ channel of $D-$wave $rho$ states are much smaller than $S-$wave states, we choose $rho(1900)$ and $rho(2150)$ as the intermediate states in $e^+e^-topi^+pi^-$, where $rho(1900)$ and $rho(2150)$ are treated as $rho(3S)$ and $rho(4S)$ states, respectively. Our result indicates that the $BABAR$s data of $e^+e^-topi^+pi^-$ around 2 GeV can be depicted well, which shows that this enhancement structure near 2.2 GeV existing in $e^+e^-topi^+pi^-$ can be due to the contribution from two $rho$ mesons, $rho(1900)$ and $rho(2150)$. Additionally, this conclusion can be enforced by the consistence of the extracted values of $Gamma_{e^+e^-}mathcal{B}(pi^+pi^-)$ of $rho(1900)$ and $rho(2150)$ in the whole fitting processes and the corresponding theoretical calculations. The present study of $e^+e^-topi^+pi^-$ data may provide valuable information to establish the $rho$ meson family.
A search for the decays $B^0_sto e^+e^-$ and $B^0to e^+e^-$ is performed using data collected with the LHCb experiment in proton-proton collisions at center-of-mass energies of $7$, $8$ and $13,text{TeV}$, corresponding to integrated luminosities of $1$, $2$ and $2,text{fb}^{-1}$, respectively. No signal is observed. Assuming no contribution from $B^0to e^+e^-$ decays, an upper limit of $mathcal{B}(B^0_sto e^+e^-)<9.4,(11.2)times10^{-9}$ is obtained at $90,(95),%$ confidence level. If no $B^0_sto e^+e^-$ contribution is assumed, a limit of $mathcal{B}(B^0to e^+e^-)<2.5,(3.0)times10^{-9}$ is determined at $90,(95),%$ confidence level. These upper limits are more than one order of magnitude lower than the previous values.
We study muon pair production $ e^+ e^- to mu^+ mu^-$ in the noncommutative(NC) extension of the standard model using the Seiberg-Witten maps of this to the second order of the noncommutative parameter $Theta_{mu u}$. Using $mathcal{O}(Theta^2)$ Feynman rules, we find the $mathcal{O}(Theta^4)$ cross section(with all other lower order contributions simply cancelled) for the pair production. The momentum dependent $mathcal{O}(Theta^2)$ NC interaction significantly modifies the cross section and angular distributions which are different from the commuting standard model. We study the collider signatures of the space-time noncommutativity at the International Linear Collider(ILC) and find that the process $ e^+ e^- to mu^+ mu^-$ can probe the NC scale $Lambda$ in the range $0.8 - 1.0$ TeV for typical ILC energy ranges.
We study the phenomenology of a Standard Model (SM) extension with two charged singlet scalars and three right handed (RH) neutrinos at an electron-positron collider. In this model, the neutrino mass is generated radiatively at three-loop, the lightest RH neutrino is a good dark matter candidate; and the electroweak phase transition strongly first order as required for baryogenesis. We focus on the process $e^{+}+e^{-}rightarrow e^{-}mu^{+}+E_{miss}$, where the model contains new lepton flavor violating interactions that contribute to the missing energy. We investigate the feasibility of detecting this process at future $e^{-}e^{+}$ linear colliders at different center of mass energies: $E_{CM}$=250, 350, 500 GeV and 1 TeV.
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