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Register a new userElectron-Positron Annihilation into Hadron-Antihadron Pairs
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The reactions of electron-positron to nucleon-antinucleon pairs are studied in a non-perturbative quark model. The work suggests that the two-step process, in which the primary quark-antiquark pair forms first a vector meson which in turn decays into a hadron pair, is dominant over the one-step process in which the primary quark-antiquark pair is directly dressed by additional quark-antiquark pairs to form a hadron pair. To reproduce the experimental data of the reactions of electron-positron to proton-antiproton and electron-positron to neutron-antineutron a D-wave omega-like vector meson with a mass of around 2 GeV has to be introduced.
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We investigate the production of electrons and positrons in the Milky Way within the context of dark matter annihilation. Upper limits on the relevant cross-section are obtained by combining observational data at different wavelengths (from Haslam, WMAP, and Fermi all-sky intensity maps) with recent measurements of the electron and positron spectra in the solar neighbourhood by PAMELA, Fermi, and HESS. We consider synchrotron emission in the radio and microwave bands, as well as inverse Compton scattering and final-state radiation at gamma-ray energies. According to our results, the dark matter annihilation cross-section into electron-positron pairs should not be higher than the canonical value for a thermal relic if the mass of the dark matter candidate is smaller than a few GeV. In addition, we also derive a stringent upper limit on the inner logarithmic slope, alpha, of the density profile of the Milky Way dark matter halo (alpha < 1 if m_dm < 5 GeV, alpha < 1.3 if m_dm < 100 GeV and alpha < 1.5 if m_dm < 2 TeV) assuming that cross-section = 3 x 10^(-26) cm^3 s(-1). A logarithmic slope steeper than alpha about 1.5 is hardly compatible with a thermal relic lighter than about 1 TeV, regardless of the dominant annihilation channel.
Experimental data on the total cross section of $e^+ e^-$ annihilation into hadrons are confronted with QCD and the operator product expansion using finite energy sum rules. Specifically, the power corrections in the operator product expansion, i.e. the vacuum condensates, of dimension $d = 2$, 4 and 6 are determined using recent isospin $I=0+1$ data sets. Reasonably stable results are obtained which are compatible within errors with values from $tau$-decay. However, the rather large data uncertainties, together with the current value of the strong coupling constant, lead to very large errors in the condensates. It also appears that the separation into isovector and isoscalar pieces introduces additional uncertainties and errors. In contrast, the high precision $tau$-decay data of the ALEPH collaboration in the vector channel allows for a more precise determination of the condensates. This is in spite of QCD asymptotics not quite been reached at the end of the $tau$ spectrum. We point out that isospin violation is negligible in the integrated cross sections, unlike the case of individual channels.
We provide a non-perturbative expression for the hadron production in electron-positron annihilation at zero temperature in a strongly coupled, large-Nc SU(Nc) field theory with Nf << Nc quark flavors. The resulting expressions are valid to leading order in the electromagnetic coupling constant but non-perturbatively in the SU(Nc) interactions and the mass of the quark. We obtain this quantity by computing the imaginary part of the hadronic vacuum polarization function Pi_q using holographic techniques, providing an alternative to the known method that uses the spectrum of infinitely stable mesons determined by the normalizable modes of the appropriated fields in the bulk. Our result exhibits a structure of poles localized at specific real values of q^2, which coincide with the ones found using the normalizable modes, and extends it offering the unique analytic continuation of this distribution to a function defined for values of q^2 over the complex plane. This analytic continuation permits to include a finite decay width for the mesons. By comparison with experimental data we find qualitatively good agreement on the shape of the first pole, when using the rho meson parameters and choosing a proper normalization factor. We then estimate the contribution to the anomalous magnetic moment of the muon finding an agreement within 25%, for this choice of parameters.
We calculate the cross section for the exclusive production of J^{PC}=0^{++} glueballs G_0 in association with the J/psi in e^+e^- annihilation using the pQCD factorization formalism. The required long-distance matrix element for the glueball is bounded by CUSB data from a search for resonances in radiative Upsilon decay. The cross section for e^+e^- -> J/psi+ G_0 at sqrt{s}=10.6 GeV is similar to exclusive charmonium-pair production e^+e^- -> J/psi+h for h=eta_c and chi_{c0}, and is larger by a factor 2 than that for h=eta_{c}(2S). As the subprocesses gamma^* -> (c c-bar) (c c-bar) and gamma^* -> (c c-bar) (g g) are of the same nominal order in perturbative QCD, it is possible that some portion of the anomalously large signal observed by Belle in e^+ e^- -> J/psi X may actually be due to the production of charmonium-glueball J/psi G_J pairs.
Recently, various cross sections of e+e- annihilation into hadrons were accurately measured in the energy range from 0.37 to 1.39 GeV with the CMD-2 detector at the VEPP-2M collider. In the pi+pi- channel a systematic uncertainty of 0.6% has been achieved. A Monte-Carlo Generator Photon Jets (MCGPJ) was developed to simulate events of the Bhabha scattering as well as production of two charged pions, kaons and muons. Based on the formalism of Structure Functions, the leading logarithmic contributions related to the emission of photon jets in the collinear region are incorporated into the MC generator. Radiative corrections (RC) in the first order of alpha are accounted for exactly. The theoretical precision of the cross sections with RC is estimated to be better than 0.2%. Numerous tests of the program as well as comparison with other MC generators and CMD-2 experimental data are presented.
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