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Register a new userSearching for string resonances in e^+e^- and gamma gamma collisions
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If the fundamental mass scale of superstring theory is as low as few TeVs, the massive modes of vibrating strings, Regge excitations, will be copiously produced at the Large Hadron Collider (LHC). We discuss the complementary signals of low mass superstrings at the proposed electron-positron facility (CLIC), in e^+e^- and gamma gamma collisions. We examine all relevant four-particle amplitudes evaluated at the center of mass energies near the mass of lightest Regge excitations and extract the corresponding pole terms. The Regge poles of all four-point amplitudes, in particular the spin content of the resonances, are completely model independent, universal properties of the entire landscape of string compactifications. We show that gamma gamma to e^+ e^- scattering proceeds only through a spin-2 Regge state. We estimate that for this particular channel, string scales as high as 4 TeV can be discovered at the 11sigma level with the first fb^{-1} of data collected at a center-of-mass energy approx 5 TeV. We also show that for e^+e^- annihilation into fermion-antifermion pairs, string theory predicts the precise value, equal 1/3, of the relative weight of spin 2 and spin 1 contributions. This yields a dimuon angular distribution with a pronounced forward-backward asymmetry, which will help distinguishing between low mass strings and other beyond the standard model scenarios.
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We compute the non-perturbative contribution of semileptonic tensor operators $(bar q sigma^{mu u} q)(bar ell sigma_{mu u} ell)$ to the purely leptonic process $mu to e gamma$ and to the electric and magnetic dipole moments of charged leptons by matching onto chiral perturbation theory at low energies. This matching procedure has been used extensively to study semileptonic and leptonic weak decays of hadrons. In this paper, we apply it to observables that contain no strongly interacting external particles. The non-perturbative contribution to $mu to e $ processes is used to extract the best current bound on lepton-flavor-violating semileptonic tensor operators, $Lambda_text{BSM} gtrsim 450$ TeV. We briefly discuss how the same method applies to dark-matter interactions.
$J/Psi$ particles are abundantly produced at the Beijing Electron Positron Collider (BEPC). The $J/Psi$ decays provide an excellent place for studying $N^*$ resonances. For $J/Psitobar NNpi$ and $bar NNpipi$, the $pi N$ and $pipi N$ systems are limited to be pure isospin 1/2 due to isospin conservation. This is a big advantage in studying $N^*$ resonances from $J/Psi$ decays, compared with $pi N$ and $gamma N$ experiments which suffer difficulty on the isospin decomposition of 1/2 and 3/2. All other $N^*$ decay channels which are presently under investigation at CEBAF(JLab, USA), ELSA(Bonn,Germany) and GRAAL(Grenoble, France) with real photon or space-like virtual photon can also be studied at BEPC complementally with the time-like virtual photon. The process $J/Psitobar NN^*$ or $Nbar N^*$ provides a new way to probe the internal structure of the $N^*$ resonances. The recent results and outlook of our new $N^*$ program at BEPC are presented.
In this paper we study transverse polarization of $Lambda$ hyperons in single-inclusive leptonic annihilation. We show that when the transverse momentum of the $Lambda$ baryon is measured with respect to the thrust axis, a transverse momentum dependent (TMD) factorization formalism is required and the polarization is generated by the TMD polarizing fragmentation function (TMD PFF), $D_{1T}^perp$. However, when the transverse momentum of the $Lambda$ baryon is measured with respect to the momentum of the initial leptons, a collinear twist-3 formalism is required and the polarization is generated by the intrinsic collinear twist-3 fragmentation function $D_{T}$. Thus while these measurements differ from one another only by a change in the measurement axis, they probe different distribution functions. Recently, Belle measured a significant polarization in single-inclusive $Lambda$ baryon production as a function of the transverse momentum with respect to the thrust axis. However, this data can in principle be re-analyzed to measure the polarization as a function of the transverse momentum of the $Lambda$ baryon with respect to the lepton pair. This observable could be the first significant probe of the function, $D_{T}$. In this paper, we first develop a TMD formalism for $Lambda$ polarization; we then present a recent twist-3 formalism that was established to describe $Lambda$ polarization. Using the TMD formalism, we demonstrate that the $Lambda$ polarization at OPAL and Belle can be described using the twist-2 TMD factorization formalism. Finally, we make a theoretical prediction for this polarization in the collinear twist-3 formalism at Belle.
Results of the SND experiment at the VEPP-2M e+ e- collider on the QED processes e+ e- --> e+ e- gamma and e+ e- --> e+ e- gamma gamma with production at large angles are presented. Energy and angular distributions of the final particles were studied. No deviations from QED with an accuracy of 3.8% for the first process and 10.3% for the second were found.
We analyze the potential of the e+e- Linear Colliders, operating in the e-gamma and gamma-gamma modes, to probe anomalous quartic vector--boson interactions through the multiple production of Ws and Zs. We examine all $SU(2)_L otimes U(1)_Y$ chiral operators of order p^4 that lead to new four--gauge--boson interactions but do not alter trilinear vertices. We show that the e-gamma and gamma-gamma modes are able not only to establish the existence of a strongly interacting symmetry breaking sector but also to probe for anomalous quartic couplings of the order of 10^{-2} at 90% CL. Moreover, the information gathered in the e-gamma mode can be used to reduced the ambiguities of the e+e- mode.
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