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Importance of proper renormalization scale-setting for QCD testing at colliders
Xing-Gang Wu, Sheng-Quan Wang, Stanley J. Brodsky
Front. Phys. . 2016, 11 (1 ): 111201-.
https://doi.org/10.1007/s11467-015-0518-5
A primary problem affecting perturbative quantum chromodynamic (pQCD) analyses is the lack of a method for setting the QCD running-coupling renormalization scale such that maximally precise fixed-order predictions for physical observables are obtained. The Principle of Maximum Conformality (PMC) eliminates the ambiguities associated with the conventional renormalization scale-setting procedure, yielding predictions that are independent of the choice of renormalization scheme. The QCD coupling scales and the effective number of quark flavors are set order-by-order in the pQCD series. The PMC has a solid theoretical foundation, satisfying the standard renormalization group invariance condition and all of the self-consistency conditions derived from the renormalization group. The PMC scales at each order are obtained by shifting the arguments of the strong force coupling constant α s to eliminate all non-conformal {β i } terms in the pQCD series. The {β i } terms are determined from renormalization group equations without ambiguity. The correct behavior of the running coupling at each order and at each phase-space point can then be obtained. The PMC reduces in the N C → 0 Abelian limit to the Gell-Mann-Low method. In this brief report, we summarize the results of our recent application of the PMC to a number of collider processes, emphasizing the generality and applicability of this approach. A discussion of hadronic Z decays shows that, by applying the PMC, one can achieve accurate predictions for the total and separate decay widths at each order without scale ambiguities. We also show that, if one employs the PMC to determine the top-quark pair forward-backward asymmetry at the next-to-next-to-leading order level, one obtains a comprehensive, self-consistent pQCD explanation for the Tevatron measurements of the asymmetry. This accounts for the “increasing-decreasing” behavior observed by the D0 collaboration for increasing t t ¯ invariant mass. At lower energies, the angular distributions of heavy quarks can be used to obtain a direct determination of the heavy quark potential. A discussion of the angular distributions of massive quarks and leptons is also presented, including the fermionic component of the two-loop corrections to the electromagnetic form factors. These results demonstrate that the application of the PMC systematically eliminates a major theoretical uncertainty for pQCD predictions, thus increasing collider sensitivity to possible new physics beyond the Standard Model.
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Controlling continuum wavepacket interference by two-color laser field in over-the-barrier ionization regime
Sheng-Peng Zhou, Yu-Jun Yang, Da-Jun Ding
Front. Phys. . 2016, 11 (1 ): 113201-.
https://doi.org/10.1007/s11467-015-0523-8
Continuum wavepacket interference is investigated by numerically solving the time-dependent Schröodinger equation for the interaction of hydrogen atoms with laser fields. The obtained wavepacket evolution indicates that, in the over-the-barrier ionization regime (1016 W/cm2 ), the continuum–continuum (CC) interference of ionizing electrons becomes the main process in highorder harmonics generation (HHG), compared with continuum-bound (CB) transition, as reported by Kohler et al. [Phys. Rev. Lett. 105(20), 203902 (2010)].We propose a two-color laser field scheme for controlling the quantum trajectories of ionizing electrons and for extending the CC harmonic energy. As a result, a high energy platform occurs in the HHG spectrum, which entirely originates from the CC harmonics, with a cutoff adjustable by the relative phase of the two-color fields. This provides further understanding of the dynamic feature of atoms and molecules in super intense laser fields and provides an opportunity to image the atomic or molecular potential.
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Geometries and electronic structures of the hydrogenated diamond (100) surface upon exposure to active ions: A first principles study
Feng-Bin Liu (刘峰斌), Jing-Lin Li (李景林), Wen-Bin Chen (陈文彬), Yan Cui (崔岩), Zhi-Wei Jiao (焦志伟), Hong-Juan Yan (阎红娟), Min Qu (屈敏), Jie-Jian Di (狄杰建)
Front. Phys. . 2016, 11 (1 ): 116804-.
https://doi.org/10.1007/s11467-015-0516-7
To elucidate the effects of physisorbed active ions on the geometries and electronic structures of hydrogenated diamond films, models of HCO− 3 , H3 O+ , and OH− ions physisorbed on hydrogenated diamond (100) surfaces were constructed. Density functional theory was used to calculate the geometries, adsorption energies, and partial density of states. The results showed that the geometries of the hydrogenated diamond (100) surfaces all changed to different degrees after ion adsorption. Among them, the H3 O+ ion affected the geometry of the hydrogenated diamond (100) surfaces the most. This is well consistent with the results of the calculated adsorption energies, which indicated that a strong electrostatic attraction occurs between the hydrogenated diamond (100) surface and H3 O+ ions. In addition, electrons transfer significantly from the hydrogenated diamond (100) surface to the adsorbed H3 O+ ion, which induces a downward shift in the HOMO and LUMO energy levels of the H3 O+ ion. However, for active ions like OH− and HCO− 3 , no dramatic change appears for the electronic structures of the adsorbed ions.
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