In the symmetric string fragmentation recipe adopted by PYTHIA for jet simulations, the transverse momenta of successive quarks are uncorrelated. This is a simplification but has no theoretical reason. Transverse momentum correlations are naturally expected, for instance, in a covariant multiperipheral model of quark hadronization. We propose a simple recipe of string fragmentation which lead to such correlations. The definition of the jet axis and its relation with the primordial transverse momentum of the quark is also discussed.
Efficient phase space integration is important for some calculations in collider physics. Using the Altarelli-Parisi splitting functions as the underlying probability for a splitting, we developed a phase space calculation for initial state radiation, that relies on Monte Carlo integration.
The effect of orbital momentum in the strong field ionization of a hydrogen atom is considered. When the atom is in the 2P state an asymmetrical distribution in the transverse velocity v T , similar to the Collins effect in quark fragmentation, is predicted : ⟨v T ⟩ points in the direction of E × ⟨L⟩. However, the Stark efffect produces oscillations of the average orbital angular momentum ⟨L T ⟩, therefore of ⟨v T ⟩.
22 pages, 6 figures, 1 tableInternational audienceThe tunneling ionization of a hydrogen atom excited in the presence of a static electric field is investigated for the case where, before being extracted, the electron has an orbital angular momentum L perpendicular to the field E. The escaping electron has a nonzero mean transverse velocity in the direction of E cross . This asymmetry is similar to the Collins effect in the fragmentation into hadrons of a transversely polarized quark. In addition, the linear Stark effect make and oscillate in time. The degree of asymmetry is calculated at leading order in E for an initial state of maximum transverse . The conditions for the observation of this asymmetry are discussed
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