Descripción: We perform a dispersive theoretical study of the reaction γ γ → π0 π0 emphasizing the low energy region. The large source of theoretical uncertainty to calculate the γ γ → π0 π0 total cross section for sqrt(s) ≳ 0.5   GeV within the dispersive approach is removed. This is accomplished by taking one more subtraction in the dispersion relations, where the extra subtraction constant is fixed by considering new low energy constraints, one of them further refined by taking into consideration the f0 (980) region. This allows us to make sharper predictions for the cross section for sqrt(s) ≲ 0.8   GeV, below the onset of D-wave contributions. In this way, were new more precise data on γ γ → π0 π0 available one might then distinguish between different parameterizations of the ππ isoscalar S-wave. We also elaborate on the width of the σ resonance to γγ and provide new values. © 2007 Elsevier B.V. All rights reserved.

Descripción: We compute the threshold-resummed cross section for pseudo-scalar MSSM Higgs boson production by gluon fusion at hadron colliders. The calculation is performed at next-to-next-to leading logarithmic accuracy. We present results for both the LHC and Tevatron Run II. We analyze the factorization and renormalization scale dependence of the results, finding that after performing the resummation the corresponding cross section can be computed with an accuracy better than 10%. © 2007 Elsevier B.V. All rights reserved.

Descripción: Multiple ionization cross sections of Ne atoms are reported for Li 3+ impact in the energy range of 2 to 5.8MeV. The neon ionization cross sections were separated into three collision channels, namely; direct ionization (DI), single capture (SC) and transfer ionization (TI). The direct ionization data is compared to CDW-EIS calculations as well as to other projectiles with charge state q 3+. The single capture is compared to a modified Bohr-Lindhard semi classical model.

Descripción: We compute the order αs2 corrections to the one particle inclusive electroproduction cross section of hadrons with non vanishing transverse momentum. We compare our results with H1 data on forward production of π0, and conclude that the data is well described by the DGLAP approach. within the theoretical uncertainties. © 2005 American Institute of Physics.

Descripción: Context. Magnetic clouds (MCs) are formed by magnetic flux ropes that are ejected from the Sun as coronal mass ejections. These structures generally have low plasma beta and travel through the interplanetary medium interacting with the surrounding solar wind. Thus, the dynamical evolution of the internal magnetic structure of a MC is a consequence of both the conditions of its environment and of its own dynamical laws, which are mainly dominated by magnetic forces.Aims. With in-situ observations the magnetic field is only measured along the trajectory of the spacecraft across the MC. Therefore, a magnetic model is needed to reconstruct the magnetic configuration of the encountered MC. The main aim of the present work is to extend the widely used cylindrical model to arbitrary cross-section shapes.Methods. The flux rope boundary is parametrized to account for a broad range of shapes. Then, the internal structure of the flux rope is computed by expressing the magnetic field as a series of modes of a linear force-free field.Results. We analyze the magnetic field profile along straight cuts through the flux rope, in order to simulate the spacecraft crossing through a MC. We find that the magnetic field orientation is only weakly affected by the shape of the MC boundary. Therefore, the MC axis can approximately be found by the typical methods previously used (e.g., minimum variance). The boundary shape affects the magnetic field strength most. The measurement of how much the field strength peaks along the crossing provides an estimation of the aspect ratio of the flux-rope cross-section. The asymmetry of the field strength between the front and the back of the MC, after correcting for the time evolution (i.e., its aging during the observation of the MC), provides an estimation of the cross-section global bending. A flat or/and bent cross-section requires a large anisotropy of the total pressure imposed at the MC boundary by the surrounding medium.Conclusions. The new theoretical model developed here relaxes the cylindrical symmetry hypothesis. It is designed to estimate the cross-section shape of the flux rope using the in-situ data of one spacecraft. This allows a more accurate determination of the global quantities, such as magnetic fluxes and helicity. These quantities are especially important for both linking an observed MC to its solar source and for understanding the corresponding evolution. © 2009 ESO.

Descripción: We present stopping power measurements of Zn for C and O ions and compare them with a theoretical description given by the Transport Cross Section - Extended Friedel Sum Rule (TCS-EFSR) for the valence electrons, and two different models for the inner-shells: the Shellwise Local Plasma Approximation (SLPA) and the CasP approach. The SLPA, which successfully applies to projectiles from H to B, is slightly high for C ions and clearly overestimates the data for O ions. On the other hand, total stopping results using the CasP description for the inner-shells show good agreement with the data for C and O ions, and also with the SRIM predictions. © Published under licence by IOP Publishing Ltd.

Descripción: We present updated predictions for the total cross section for Higgs boson production by gluon-gluon fusion in hadron collisions. Our calculation includes the most advanced theoretical information available at present for this observable: soft-gluon resummation up to next-to-next-to-leading logarithmic accuracy, the exact treatment of the bottom-quark contribution up to next-to-leading order, and two-loop electroweak effects. We adopt the most recent parametrization of parton distribution functions at next-to-next-to-leading order, and we evaluate the corresponding uncertainties. In comparison with our previous central predictions, at the Tevatron the difference ranges from +9% for mH = 115   GeV to -9% for mH = 200   GeV. At the LHC the cross section is instead significantly increased. The effect goes from +30% for mH = 115   GeV to +9% for mH = 300   GeV, and is mostly due to the new parton distribution functions. We also provide new predictions for the LHC at sqrt(s) = 10   TeV. © 2009 Elsevier B.V.

Descripción: We present the two-loop virtual corrections to Standard Model Higgs boson pair production via gluon fusion gg→ HH in the heavy top quark limit. Based on this result, we evaluate the corresponding cross section at the LHC at 14 TeV in the next-to-next-to-leading order soft-virtual approximation. We find an inclusive K-factor of about 2.4, resulting in an increase close to 23% with respect to the previous available calculation at next-to-leading order. As expected, we observe a considerable reduction in the renormalization and factorization scale dependence. © 2013 Elsevier B.V.

Descripción: The post-collisional interactions in ion-atom ionization collisions are studied around the electron capture to the continuum (ECC) process. For this purpose, a suitable double differential cross section is introduced, involving the longitudinal recoil-ion momentum and the projectile transverse momentum transfer. Using the fact that the ECC process is closely related to the threshold in the longitudinal momentum distribution, we study this distribution as a function of the projectile scattering angle. Using the CDW-EIS approximation we theoretically find a focusing (defocusing) effect as we get closer to the distribution threshold for proton (antiproton) impact on He atoms. © 2007 IOP Publishing Ltd.

Descripción: The nonlinear conditions for the development of helical vortex filaments in a circular tube are considered. The helical flow is assumed to be irrotational, except in a vortex filament of infinitesimal core area. By introducing an appropriate image for this helical vortex filament, the boundary condition on the material frontier is satisfied. By assuming an axisymmetric flow upstream and imposing the conservation laws, a dependence between the helix pitch and the nonlinear amplitude of the helical vortex developed downstream is obtained. Our results show that only helical flows with the pitch in a certain range of values are allowed. The dependence of this range on the swirl ratio and on the tube cross section is considered. We discuss the usefulness of the nonlinear analysis of the allowed flows to explain experimental results and to complement the usual linear study of stability. © 1999 American Institute of Physics.

Descripción: We present predictions for the inclusive cross section for Higgs boson production by gluon-gluon fusion in proton collisions at s=8 TeV. Our calculation is accurate up to next-to-next-to-leading order in QCD perturbation theory and includes soft-gluon effects up to next-to-next-to-leading logarithmic accuracy and two-loop electroweak corrections. The dependence on heavy-quark masses is taken into account exactly up to next-to-leading order and next-to-leading logarithmic accuracy, and a treatment of the Higgs boson line-shape is provided according to the complex-pole scheme. © 2012 Elsevier B.V.

Descripción: We compute the four-quark plus three-gluon and six-quark plus one-gluon tree level amplitudes using on-shell recursion relations. They are needed for the calculation of the 5-jet cross-section at the Born level, and constitute an essential ingredient for next-to-leading order 4-jet and next-to-next-to-leading order 3-jet production at hadronic colliders. Very compact expressions for all possible helicity configurations are provided, allowing direct implementation in computer codes. With the results presented in this paper, the full set of seven-parton tree amplitudes becomes available. © SISSA 2006.

Descripción: A projectile ion-recoil ion and electron-recoil ion coincidence technique has been used to study the multiple ionization process in collisions of 0.75-4.0MeV B2+ with Ne atoms. Absolute total and partial pure ionization cross sections are reported for the first time at these impact energies with the present setup. The relative contribution of each recoil charge state has been investigated. The experimental data are compared to CDW-EIS calculation and with He2+ projectiles. The data throw further light on the effect of projectile screening in the ionization of multi-electronic targets and provide a support for the validity of the CDW-EIS model in this intermediate-to low velocity regime.

Descripción: We present a simple hydrodynamic model to obtain the profiles of the relevant physical quantities along a nozzle of arbitrary cross-section in a cutting torch. The model uses a two-zone approximation (a hot central plasma carrying the discharge current wrapped by a relatively cold gas which thermally isolates the nozzle wall from the plasma). Seeking for a solution with sonic conditions at the nozzle exit, the model allows expressing all the profiles in terms of the externally controlled parameters of the torch (geometry of the torch, discharge current, mass flow of the gas and plenum pressure) and the values of the arc and gas temperatures at the nozzle entrance. These last two values can be estimated simply appealing to energy conservation in the cathode-nozzle region. The model contains additional features compared with previous reported models, while retaining simplicity. The detailed consideration of an arc region coupled to the surrounding gas dynamics allows determining voltage drops and consequent delivered power with less assumptions than those found in other published works, and at the same time reduces the set of parameters needed to determine the solution.

Descripción: We consider the transverse-momentum (qT) distribution of Higgs bosons produced at hadron colliders. We use a formalism that uniformly treats both the small-qT and large-qT regions in QCD perturbation theory. At small qT (qT ≪ MH, MH being the mass of the Higgs boson), we implement an all-order resummation of logarithmically-enhanced contributions up to next-to-next-to-leading logarithmic accuracy. At large qT (qT ≳ MH), we use fixed-order perturbation theory up to next-to-leading order. The resummed and fixed-order approaches are consistently matched by avoiding double-counting in the intermediate-qT region. In this region, the introduction of unjustified higher-order terms is avoided by imposing unitarity constraints, so that the integral of the qT spectrum exactly reproduces the perturbative result for the total cross section up to next-to-next-to-leading order. Numerical results at the LHC are presented. These show that the main features of the qT distribution are quite stable with respect to perturbative QCD uncertainties. © 2003 Elsevier Science B.V. All rights reserved.

Descripción: The fluorescence emission of merocyanine 540 (MC540) in 95% ethanol was studied under continuous irradiation. Fluorescence spectra from excited states of both normal (N) and photoisomeric (P) species are identical. The laser fluence dependence of the fluorescence intensity is interpreted on the basis of a photochromic system involving N and P ground states and first excited singlet states. Common flash photolysis equations are generalized in order to include a photoequilibrium between isomers. The emission data are used together with previous flash photolysis and optoacoustic results to obtain P fluorescence and photoisomerization quantum yields as 0.07 ± 0.02 and 0.20 ± 0.04, respectively, P absorption cross section at the maximum (560 nm) as 4.74 × 10-16 cm2 (125 × 103 M-1·cm-1), and the energy difference between the ground states as 165 kJ·mol-1. Internal conversion, which can take place as a direct process or through a twisted state, is the main deactivation pathway for excited singlet P. © 1990 American Chemical Society.

Descripción: We discuss the production of two hadrons in e+e- annihilation within the framework of perturbative QCD. The cross section for this process is calculated to next-to-leading order accuracy with a selection of variables that allows the consideration of events where the two hadrons are detected in the same jet. In this configuration we contemplate the possibility that the hadrons come from a double fragmentation of a single parton. The double-fragmentation functions required to describe the transition of a parton to two hadrons, are also necessary to completely factorize all collinear singularities. We explicitly show that factorization applies to order αs in the case of two-hadron production. © 2003 Elsevier B.V. All rights reserved.

Descripción: We consider the transverse-momentum (qT) distribution of Drell-Yan lepton pairs produced in hadron collisions. At small values of qT, we resum the logarithmically-enhanced perturbative QCD contributions up to next-to-next-to-leading logarithmic accuracy. At intermediate and large values of qT, we consistently combine resummation with the known next-to-leading order perturbative result. All perturbative terms up to order α S 2 are included in our computation which, after integration over qT, reproduces the known next-to-next-to-leading order result for the Drell-Yan total cross section. We show and discuss the reduction in the scale dependence of the results with respect to lower-order calculations, estimating the corresponding perturbative uncertainty. We present a preliminary comparison with Tevatron Run II data. © 2010 Elsevier B.V.

Descripción: Results of a detailed geologic survey of the Río Borbollón area, located in the Cordillera Principal of the province of Mendoza are presented. The stratigraphic record of the area corresponds to a Jurassic-early Cretaceous sedimentary succession, the products of Neogene igneous activity and Quaternary deposits. The Tres Esquinas and La Manga Formations of Jurassic age were recognized for the first time in the study area. The main structural features of the area are described and interpreted based on the obtained data and previous works. A balanced structural cross section of the Malargüe fold and thrust belt at 34°15'S depicts the structural style and the amount of orogenic shortening in this region of the Andes.