The thermal neutron induced fission cross section of 232Th

The thermal neutron induced fission cross section of ²³²Th was measured using the highly pure thermal neutron beam from the 87 m curved neutron guide at the High Flux Reactor of the ILL (Grenoble). An upper limit of 4 μb was obtained for the fission cross section, which is an order of magnitude smaller than previous results. This result is discussed in terms of the double-humped fission barrier theory.     

Tensor polarized γ-deuteron Compton scattering in effective field theory

The differential cross section for γ-deuteron Compton scattering from a tensor polarized deuteron is computed in an effective field theory. The first non-vanishing contributions to this differential cross section are the interference terms between the leading electric coupling diagrams and the subleading single potential pion exchange diagrams or the subleading magnetic moment coupling diagrams. At 90° photon scattering angle, only the pion term contributes at this order to the tensor polarized differential cross section. This provides a clean way to study the photon pion dynamics in the two nucleon sector. The effect is measurable for photon energies between 40 and 80 MeV provided the uncertainty in the measured cross sections are ≲ 7%.     

QCD corrections to double J/psi production in e+e- annihilation at sqrt[s]=10.6 GeV

Next-to-leading-order (NLO) QCD corrections to double J/psi production in e+e- annihilation at sqrt[s]=10.6 GeV are calculated. We find that they greatly decrease the cross section, with a K factor (NLO/LO) ranging from -0.31 to 0.25 depending on the renormalization scale. Although the renormalization scale dependence indicates a large uncertainty, when combined with the NLO QCD corrections to J/psi+etac production, it can explain why the double J/psi production could not be found at B factories while the J/psi+etac production could, despite the fact that cross section of the former is larger than that of the latter at LO by a factor of 1.8.     

Capture of ultracold neutrons in thin absorbing films

The dependence of the ultracold neutron capture cross section in targets with a thickness smaller than the neutron wavelength is calculated in the time-dependent quantum theory. It is shown that, for low velocities of neutrons, their capture cross section σ_c～v, i.e., tends to zero as the neutron velocity v tends to zero.     

Quantum limit cyclotron resonance in p-GaAs

Electron scattering by neutral acceptors has been studied through far infrared (λ=172 μm) cyclotron resonance on p-GaAs at 4.2 K. It is found that (1) The inverse relaxation time is considerably smaller in the quantum limit than that predicted by D.C. transport theory in the absence of a magnetic field. (2) The same increase linearly with neutral acceptor concentration. (3) The electron momentum transfer cross section by neutral acceptor is by more than an order of magnitude smaller than that by neutral donor even in the quantum limit.     

Improved experimental limits on the production of magnetic monopoles

We present new limits on low mass accelerator-produced pointlike Dirac magnetic monopoles trapped and bound in matter surrounding the D0 collision region of the Tevatron at Fermilab (experiment E-882). In the context of a Drell-Yan mechanism, we obtain cross section limits for the production of monopoles with magnetic charge values of 1, 2, 3, and 6 times the minimum Dirac charge of the order of picobarns, some 100 times smaller than found in similar previous Fermilab searches. Mass limits inferred from these cross section limits are presented.     

Next-to-leading-order QCD correction to e(+)e(-)-->J/psi+eta(c) at sqrt[s]=10.6 GeV

One of the most challenging open problems in heavy quarkonium physics is the double charm production in e+e- annihilation at B factories. The measured cross section of e+e- --> J/psi + eta(c) is much larger than leading order (LO) theoretical predictions. With the nonrelativistic QCD factorization formalism, we calculate the next-to-leading order (NLO) QCD correction to this process. Taking all one loop self-energy, triangle, box, and pentagon diagrams into account, and factoring the Coulomb-singular term into the cc bound state wave function, we get an ultraviolet and infrared finite correction to the cross section of e+e- --> J/psi + eta(c) at sqrt[s] = 10:6 GeV. We find that the NLO QCD correction can substantially enhance the cross section with a K factor (the ratio of NLO to LO) of about 1.8-2.1; hence, it greatly reduces the large discrepancy between theory and experiment.     

电子与原子、离子碰撞过程的相对论效应

在玻恩近似下,建立了高能电子与原子、离于非弹性碰撞过程的相对论性理论计算方法,并以类Li等电子系列为例,阐明了电子与原子、离子非弹性碰撞过程的相对论效应.它包括:1.靶原子的相对论效应,它随原子序数的增加而增大.低z靶(如Li)的相对论效应可以忽略;对Au~(+76)靶的2_3—3p跃迁,广义振子强度相对论性计算值比相应的非相对论性值小27.1%.2.入射电子的相对论效应,它随入射电子能量的增加而增大.对高能入射电子,在特殊角度,要考虑广义振子强度横场项对微分截面的影响.在极端相对论情况下,如入射电子动能 关键词：     

Deep-inelastic inclusive ep scattering at low x and a determination of \alpha_s

A precise measurement of the inclusive deep-inelastic scattering cross section is reported in the kinematic range GeV and . The data were recorded with the H1 detector at HERA in 1996 and 1997, and correspond to an integrated luminosity of 20 pb. The double differential cross section, from which the proton structure function and the longitudinal structure function are extracted, is measured with typically 1% statistical and 3% systematic uncertainties. The measured derivative is observed to rise continuously towards small x for fixed . The cross section data are combined with published H1 measurements at high for a next-to-leading order DGLAP QCD analysis. The H1 data determine the gluon momentum distribution in the range to within an experimental accuracy of about 3% for GeV. A fit of the H1 measurements and the data of the BCDMS collaboration allows the strong coupling constant and the gluon distribution to be simultaneously determined. A value of is obtained in NLO, with an additional theoretical uncertainty of about , mainly due to the uncertainty of the renormalisation scale. Received: 21 December 2000 / Revised version: 10 May 2001 / Published online: 6 July 2001     

Rotational relaxation in ultracold He+BH collisions

Collisions of cold and ultracold BH in the v=0 level with the He atom are investigated using the quantum mechanical scattering formulation. The elastic and the inelastic cross sections are calculated using the two-dimensional ab initio potential energy surface. It is shown that the elastic cross section is larger than the inelastic one. When the collision energy is very low, the elastic cross section follows the Wigner threshold law and is one order of magnitude larger than that of He-O₂, while it is much smaller than that of He-H₂. The efficiency of the rotationally quenching state is given. The Δj=-1 transition is most efficient. The resonances are also found to occur at about the same translational energy (0.1-1 cm^-1), which gives rise to steps in the rate coefficient at temperatures around 0.1-1 K.     

Observation of the h(c)(1P) Using e+ e- collisions above the DD threshold

Using 586 pb(-1) of e+ e- collision data at E(c.m.) = 4170 MeV, produced at the Cornell Electron Storage Ring collider and collected with the CLEO-c detector, we observe the process e+ e- → π+ π- h(c)(1P). We measure its cross section to be 15.6±2.3±1.9±3.0 pb, where the third error is due to the external uncertainty on the branching fraction of ψ(2S) → π0 h(c)(1P), which we use for normalization. We also find evidence for e+ e- → ηh(c)(1P) at 4170 MeV at the 3σ level and see hints of a rise in the e+ e- → π+ π- h(c)(1P) cross section at 4260 MeV.     

Quasielastic 3He(e,e'p)2H reaction at Q2 = 1.5 GeV2 for recoil momenta up to 1 GeV/c

We have studied the quasielastic 3He(e,e(')p)2H reaction in perpendicular coplanar kinematics, with the energy and the momentum transferred by the electron fixed at 840 MeV and 1502 MeV/c, respectively. The 3He(e,e(')p)2H cross section was measured for missing momenta up to 1000 MeV/c, while the A(TL) asymmetry was extracted for missing momenta up to 660 MeV/c. For missing momenta up to 150 MeV/c, the cross section is described by variational calculations using modern 3He wave functions. For missing momenta from 150 to 750 MeV/c, strong final-state interaction effects are observed. Near 1000 MeV/c, the experimental cross section is more than an order of magnitude larger than predicted by available theories. The A(TL) asymmetry displays characteristic features of broken factorization with a structure that is similar to that generated by available models.     

Bremsstrahlung in heavy-ion collisions

The bremsstrahlung background emission of γ-rays in equal-mass heavy-ion collisions is calculated. The cross section is found to be about an order of magnitude smaller than preliminary upper limits of ¹²C-¹²C experiments in progress.     

Next-to-leading order QCD corrections to the production of two bottom-antibottom pairs at the LHC

We report the results of a computation of the full next-to-leading order QCD corrections to the production of two bb pairs at the LHC. This calculation at the parton level provides predictions for well separated b jets. The results show that the next-to-leading order corrections lead to an enhancement of the cross section for the central scale choice by roughly 50% with respect to the leading order result. The theoretical uncertainty estimated by variation of the renormalization and factorization scales is strongly reduced by the inclusion of next-to-leading order corrections.     

An exclusive event generator for e~+ e~- scan experiments

An exclusive event generator is designed for e+e-scan experiments,including initial state radiation effects up to the second order correction.The generator is coded within the framework of BesEvtGen.There are seventy hadronic decay modes available,with effective center-of-mass energy coverage from the two pion mass threshold up to about 6 GeV.The accuracy achieved for the initial state radiation correction reaches the level achieved by the KKMC generator.The uncertainty associated with the calculation of the correction factor to the initial state radiation is dominated by the measurements of the energy-dependent Born cross section.     

Determining the uncertainty on the total heavy-flavor cross section

We assess the theoretical uncertainties on the total heavy-quark cross section. We discuss the importance of the quark mass, the choice of the scale, the number of light flavors and the parton densities on the estimate of the uncertainty. At first glance, the uncertainty bands on the total charm cross sections obtained by integrating the FONLL inclusive cross section and by integrating the partonic total cross sections appear to be incompatible. We explain how this apparent difference arises and describe how the two results can be reconciled. The small mass of the charm quark amplifies the effect of varying the other parameters in the calculation, making the uncertainty on the total charm cross section difficult to quantify. On the other hand, the bottom-quark total cross section is under much better theoretical control, and differences between the two approaches are small.     

Electron-nucleon interaction in quasi-free scattering: (I). Non-relativistic Hamiltonian through second order

In the frame of a program aiming at examining the approximations involved in the theoretical treatment of (e, e′p) reactions, the electron-nucleon interaction producing the knock-out mechanism is considered in the non-relativistic approach of McVoy and Van Hove and compared with the relativistic one based on the one-photon-exchange Feynman graph. In this paper the non-relativistic Hamiltonian through second order in the nucleon recoil velocity is adopted; in the next paper higher order terms are included. The two approaches appear equivalent as far as the usual DWIA cross section is concerned, where a factorization occurs between the electron-nucleon cross section and the nucleon momentum distribution. However the unfactorized DWIA cross section behaves differently. This is attributed to a different off-shell behaviour of the two scattering amplitudes.     

Proton-nucleus inelastic cross section at ultra high energy

Energy dependence of proton-nucleus reaction cross section at very high energy is discussed. It is stressed that depending on the gluon distribution near the nuclear surface, proton-nucleous total cross section increases much more rapidly compared to the usual Glauber independent nucleon estimate. The recent observation of smaller X _max than the expected value at UHECR domain can be an indication for such a mechanism.     

Model differential cross section of elastic scattering of electrons by atoms in the Monte Carlo simulation of the passage of electrons in a substance

A new model differential cross section is proposed for describing elastic scattering of electrons in simulating the passage of electrons through a substance by the Monte Carlo method. This differential cross section correctly describes the first and second transport scattering cross sections, but is characterized by the total elastic scattering cross section much smaller than the actual value of the total elastic scattering cross section. The application of this differential cross section makes it possible to considerably reduce the number of elastic collisions in the Monte Carlo simulation of passage of electrons in a substance and to model the passage of high-energy electrons using the individual collision model.