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Register a new userSuppression of Electroweak Instanton Processes in High-energy Collisions
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Electroweak instantons are a prediction of the Standard Model and have been studied in great detail in the past although they have not been observed. Earlier calculations of the instanton production cross section at colliders revealed that it was exponentially suppressed at low energies, but may grow large at energies (much) above the sphaleron mass. Such calculations faced difficulty in the breakdown of the instanton perturbation theory in the high-energy regime. In this paper we review the calculation for the electroweak instanton cross section using the optical theorem, including quantum effects arising from interactions in the initial state and show that this leads to an exponential suppression of the cross section at all energies, rendering the process unobservable.
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The transition between the broken and unbroken phases of massive gauge theories, namely the rearrangement of longitudinal and Goldstone degrees of freedom that occurs at high energy, is not manifestly smooth in the standard formalism. The lack of smoothness concretely shows up as an anomalous growth with energy of the longitudinal polarization vectors, as they emerge in Feynman rules both for real on-shell external particles and for virtual particles from the decomposition of the gauge field propagator. This makes the characterization of Feynman amplitudes in the high-energy limit quite cumbersome, which in turn poses peculiar challenges in the study of Electroweak processes at energies much above the Electroweak scale. We develop a Lorentz-covariant formalism where polarization vectors are well-behaved and, consequently, energy power-counting is manifest at the level of individual Feynman diagrams. This allows us to prove the validity of the Effective W Approximation and, more generally, the factorization of collinear emissions and to compute the corresponding splitting functions at the tree-level order. Our formalism applies at all orders in perturbation theory, for arbitrary gauge groups and generic linear gauge-fixing functionals. It can be used to simplify Standard Model loop calculations by performing the high-energy expansion directly on the Feynman diagrams. This is illustrated by computing the radiative corrections to the decay of the top quark.
The recent MoEDAL experiment at LHC to detect the electroweak monopole makes the theoretical prediction of the monopole mass an urgent issue. We discuss different ways to estimate the mass of the electroweak monopole. We first present a scaling argument which indicates that the mass of the electroweak monopole to be around 4 TeV. To justify this we construct finite energy analytic dyon solutions which could be viewed as the regularized Cho-Maison dyon, modifying the coupling strengths of the electromagnetic interaction of $W$-boson in the standard model. Our result demonstrates that a genuine electroweak monopole whose mass scale is much smaller than the grand unification scale can exist, which can actually be detected at the present LHC.
We present an effective action for the electroweak sector of the Standard Model valid for the calculation of scattering amplitudes in the high energy (Regge) limit. Gauge invariant Wilson lines are introduced to describe reggeized degrees of freedom whose interactions are generated by effective emission vertices. From this approach previous results at leading logarithmic accuracy for electroweak boson Regge trajectories are reproduced together with the corresponding interaction kernels. The proposed framework lays the path for calculations at higher orders in perturbation theory.
We review the evolution of the studies of diffractive processes in the strong interaction over the last 60 years. First, we briefly outline the early developments of the theory based on analyticity and unitarity of the S-matrix, including the derivation and exploration of the Regge trajectories and related moving cuts. Special attention is paid to the concept of the Pomeron trajectory introduced for description of total, elastic and diffractive cross sections at high energies and to the emergence of the dynamics of multi-Pomeron interactions.The role of large longitudinal distances and color coherent phenomena for the understanding of inelastic diffraction in hadron-hadron scattering and deep inelastic scattering is emphasized. The connection of these phenomena to the cancellation of the contribution of the Glauber approximation in hadron-nucleus collisions and to the understanding of the Gribov-Glauber approximation is explained. The presence of different scales in perturbative QCD due to masses of heavy quarks has led to the emergence of numerous new phenomena including non-universality of the slopes of Regge trajectories made of light and heavy quarks and non-universal energy dependence of elastic cross sections. The application of the perturbative QCD techniques allowed us to calculate from the first principles the interaction of small transverse size color singlets with hadrons leading to the development of the quantitative theory of hard exclusive reactions and to the successful prediction of many regularities in hard large mass diffraction. It also led to the prediction of the phenomenon of complete transparency of nuclear matter in QCD in special processes. The conflict of perturbative QCD with probability conservation for high energy processes of virtual photon-nucleon scattering is explained. Some properties of the new QCD regime are outlined.
It is shown that hadron abundances in high energy e+e-, pp and p{bar p} collisions, calculated by assuming that particles originate in hadron gas fireballs at thermal and partial chemical equilibrium, are in very good agreement with the data. The freeze-out temperature of the hadron gas fireballs turns out to be nearly constant over a large center of mass energy range and not dependent on the initial colliding system. The only deviation from chemical equilibrium resides in the incomplete strangeness phase space saturation. Preliminary results of an analysis of hadron abundances in S+S and S+Ag heavy ion collisions are presented.
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