Cross Sections For Such Processes Research Articles

Relaxed constraints on the heavy scalar masses in 2HDM

In the wake of new scalar searches at LHC in various channels, it is interesting to investigate the sacrosanctity of the constraints on the masses and couplings of the heavier scalars in a two-Higgs-doublet model (2HDM). We consider the effects of new physics beyond a 2HDM encoded in terms of bosonic dim-6 operators. Although these constraints are mostly immune to such new physics, we demonstrate for a specific class of bosonic operators, the constraints on the masses of the exotic scalars from cascade decays can get substantially relaxed. We present such effects for both degenerate and hierarchical mass spectra of the heavier scalars in 2HDM. Some decay channels of the new scalars vanish at the alignment limit in the tree-level 2HDM. But the inclusion of dim-6 terms can lead to significant cross-sections for such processes. It is also pointed out that observation of such processes can no longer rule out the alignment limit if such dim-6 operators are present.

ELECTRONIC EXCITATION OF METASTABLE STATES OF RARE GAS ATOMS

The important role of the scientific school of Prof. I. Zapisochny in the study of electron-atom collisions and its determination of most important parameters in this field are emphasized. The processes of electron-impact excitation to and from the metastable states of rare gases atoms are examined. The methods of measurement of the cross sections for such processes are described and the influence of cascade population on the metastable cross sections discussed also. The cross section data are presented. Keywords: electron-atom collisions, metastable states,

Using scalars to probe theories of low scale quantum gravity

Arkani-Hamed, Dimopoulos and Dvali have recently suggested that gravity may become strong at energies near 1 TeV which would remove the hierarchy problem. Such a scenario can be tested at present and future colliders since the exchange of towers of Kaluza-Klein gravitons leads to a set of new dimension-8 operators that can play important phenomenological roles. In this paper we examine how the production of pairs of scalars at $e^+e^-$, $\gamma \gamma$ and hadron colliders can be used to further probe the effects of graviton tower exchange. In particular we examine the tree-level production of pairs of identical Higgs fields which occurs only at the loop level in both the Standard Model and its extension to the Minimal Supersymmetric Standard Model. Cross sections for such processes are found to be potentially large at the LHC and the next generation of linear colliders. For the $\gamma\gamma$ case the role of polarization in improving sensitivity to graviton exchange is emphasized.

Direct semiexclusive production of energetic mesons.

The formation of color-singlet hadrons in hard-scattering processes is governed by process-independent distribution amplitudes. Such amplitudes may be measured in a class of processes we call semiexclusive, where one hadron is formed directly in hard scattering and the other QCD partons hadronize in jets. These processes can be identified experimentally by requiring a large angular or rapidity gap around a directly produced meson. We present cross sections for such processes, and show that experimental results, especially the ratio of charged to neutral $K$ production, are highly sensitive to the hadronic distribution amplitude. We discuss methods and prospects for thus gaining information about hadronic wave functions at present and future colliders.

Nonperturbative electromagnetic lepton-pair production in peripheral relativistic heavy-ion collisions

We discuss a nonperturbative treatment of lepton-pair production caused by the strong and sharply pulsed electromagnetic fields generated in peripheral relativistic heavy-ion collisions with an emphasis on the capture process into the atomic K shell. We calculate, in a field-theoretical framework, impact-parameter-dependent probabilities and cross sections for such processes by solving the time-dependent Dirac equation on a three-dimensional Cartesian lattice using the basis-spline collocation method. We give a full discussion of the stationary states used in computing S-matrix elements. Use of the axial gauge for the electromagnetic potentials produces an interaction easier to implement on the lattice than the Lorentz gauge. Preliminary calculations are given for muon-pair production with capture into the K shell in collisions of $^{197}\mathrm{Au}$${+}^{197}$Au at collider energies per nucleon of 2 and 100 GeV.

Electron-hydrogen collisions in the presence of a laser field: one-photon excitation

The authors present a new calculation of the excitation cross sections of atomic hydrogen by the impact of fast electrons in the presence of a laser field. The computation is performed within the framework of perturbation theory to lowest non-vanishing order. In contrast with previous work, the authors do not restrict themselves to the Bethe-Born approximation for the electron-atom interaction; this opens many new channels for the scattering process. The Coulomb Green's function is represented accurately and efficiently in either of two L2 basis sets, one of Slater-Laguerre type and one Sturmian, rather than as a sum and integral over intermediate states. One result is to show that the Bethe-Born approximation does not provide reliable estimates of the cross sections for such processes. Another interesting feature of the authors' model is the prediction of deep minima in the cross sections. Their origin, as well as the magnitudes of the different terms contributing to the overall transition amplitudes, is discussed.

Interpretation of collisional ionization rates in flames

This paper discusses semi-quantitatively the ionization process, A + M*→ A++ e–+ M*′, where A is an atom and M* and M*′ are differently excited states of a molecule. This process is particularly relevant to the discussion of the collisional ionization rates in flame systems, and estimates of the cross-sections for such processes together with a correct treatment of the number of states involved can readily account for the anomalously high cross-sections for such processes previously inferred from ionization rate constants. The discussion raises other related problems which suggest that a complete understanding of the process will not be readily obtained until more complex experiments correlating flame composition, ionization rates, electron densities and temperature variations are performed.