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Register a new userNon-Perturbative Effects in $mu to e gamma$
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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.
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The triplet-like QED processes $gamma + ell_i to ell^+_j ell^-_j + ell_i$ with $i eq j,$ and $i=e, mu,$ $j=e, mu,tau$ has been investigated as the reactions where a dark photon, $A$, is produced as a virtual state with subsequent decay into a $ell^+_j ell^-_j-$-pair. This effect arises due to the so-called kinetic mixing and is characterized by the small parameter $epsilon$ describing the coupling strength relative to the electric charge $e$. The main advantage of searching $A$ in these processes is that the background to the $A$ signal is pure QED. Concerning $A$, we consider its contribution in the Compton-type diagrams only since, in this case, the virtual dark photon has time-like nature and its propagator has the Breit-Wigner form. So, near the resonance, $A$ can manifest itself. The contribution of $A$ in the Borsellino diagrams is negligible since, in this case, the virtual dark photon is space-like, the $A$ propagator does not peak and the effect is proportional, at least, to $epsilon^2$. We calculate the distributions over the invariant mass of the produced $ell^+_j ell^-_j-$ pair and search for the kinematical region where the Compton-type diagrams contribution is not suppressed with respect to the Borsellino ones. The value of the parameter $epsilon$ is estimated as a function of the dark photon mass for a given number of events.
We consider the case that $mu$-$e$ conversion signal is discovered but other charged lepton flavor violating (cLFV) processes will never be found. In such a case, we need other approaches to confirm the $mu$-$e$ conversion and its underlying physics without conventional cLFV searches. We study R-parity violating (RPV) SUSY models as a benchmark. We briefly review that our interesting case is realized in RPV SUSY models with reasonable settings according to current theoretical/experimental status. We focus on the exotic collider signatures at the LHC ($pp to mu^- e^+$ and $pp to jj$) as the other approaches. We show the correlations between the branching ratio of $mu$-$e$ conversion process and cross sections of these processes. It is first time that the correlations are graphically shown. We exhibit the RPV parameter dependence of the branching ratio and the cross sections, and discuss the feasibility to determine the parameters.
We study infrared contributions to semihard parton-parton interactions by considering an effective charge whose finite infrared behavior is constrained by a dynamical mass scale. Using an eikonal QCD-based model in order to connect this semihard parton-level dynamics to the hadron-hadron scattering, we obtain predictions for the proton-proton ($pp$) and antiproton-proton ($bar{p}p$) total cross sections, $sigma_{tot}^{pp,bar{p}p}$, and the ratios of the real to imaginary part of the forward scattering amplitude, $rho^{pp,bar{p}p}$. We discuss the theoretical aspects of this formalism and consider the phenomenological implications of a class of energy-dependent form factors in the high-energy behavior of the forward amplitude. We introduce integral dispersion relations specially tailored to relate the real and imaginary parts of eikonals with energy-dependent form factors. Our results, obtained using a group of updated sets of parton distribution functions (PDFs), are consistent with the recent data from the TOTEM, AUGER and Telescope Array experiments.
In this present paper, we investigate the muon pairs production in the interaction between two quasireal photons in $e^+e^-$ collision. The total and differential cross section of the process $gamma gamma to mu^+mu^-$ at a beam energy of photons from 3 GeV to 40 GeV in the center-of-mass and for different values of muon transverse momentum and the muon rapidity and the muon angle are calculated. We also study the total cross section, as a function of the $e^+ e^-$ center-of-mass energy $sqrt {s}$ in the region 5 GeV $leq sqrt {s} leq$ 209 GeV process of the $e^+ +e^- to e^+ + e^- +mu^+ + mu^-$ by the two-photon mechanism. The obtained our results are in satisfactory agreement with the experimental data.
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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