共查询到20条相似文献,搜索用时 24 毫秒
1.
K.S. Kim M.K. Cheoun S.W. Hong Y. Chung 《The European Physical Journal A - Hadrons and Nuclei》2002,14(2):239-245
The Δ-resonance contribution has been included in the (e, e'p) reaction along with Coulomb distortion effects. We treat the resonance via a non-relativistic Δ current operator and use
a Dirac Hartree single-particle model for the ground-state single-particle wave function and a relativistic optical model
for the knocked-out proton wave function. It is assumed that the π-meson created by the virtual photon is absorbed in the
target nucleus following the production of a Δ-resonance. Our DWBA calculation shows that the Δ-resonance contribution to
the (e, e'p) reaction cross-section is 10-15% for an energy of 250 MeV transfered to the proton knocked out of the s-shell of 40Ca, in the parallel and perpendicular kinematics.
Received: 21 July 2001 / Accepted: 21 March 2002 相似文献
2.
Matthew A. C. Lamont 《The European Physical Journal C - Particles and Fields》2009,62(1):203-210
The nuclear wave-function is dominated at low- and medium-x by gluons. As the rapid growth of the gluon distribution towards low x, as derived from current theoretical estimates, would violate unitarity, there must be a mechanism that tames this explosive
growth. This is most efficiently studied in colliders running in e+A mode, as the nucleus is an efficient amplifier of saturation effects occurring with high gluon densities. In fact, large
A can lead to these effects manifesting themselves at energies a few orders of magnitude lower than in e+p collisions. In order to study these effects, there are proposals to build an e+A machine in the USA, operating over a large range of masses and energies. These studies will allow for an in-depth comparison
to A+A collisions where results have given tantalising hints of a new state of matter with partonic degrees of freedom. In
order to explain these results quantitatively, the gluons and their interactions must be understood fully as they are the
dominant source of hard probes at both RHIC and LHC energies. 相似文献
3.
The typical radionuclides used to label anti-CD20 in the treatment of non-Hodgkin's lymphoma are 90Y, 131I, and 188Re, with the emission of beta particles, Auger electrons, and conversion electrons for the latter two. The aim of the present work was to calculate the contribution of high linear energy transfer radiation as Auger electrons (AE) and conversion electrons (CE) of 131I and 188Re-anti-CD20 to mean specific energy into the cell nucleus by Monte Carlo simulation (MCS), so as to infer therapeutic effectiveness on a dosimetric basis. MCS was used to quantify the frequency–mean specific energy into the cell nucleus, where the cell was modeled by two concentric spheres, considering two cell models. The results showed that 10% and 33% of the mean-specific energies (z¯) per disintegration imparted to the cell nucleus for both geometries are due to AE and CE; on the other hand, if the hit of AE and CE occurs, the contribution to (z¯) is about 64% and 86% for 131I and 188Re, respectively. According to the amount of specific energy from AE and CE into the cell nucleus by positive event, they can cause catastrophic effects in the nuclear DNA in the treatment of non-Hodgkin's lymphoma with 131I, 188Re-anti-CD20. 相似文献
4.
F. Buisseret C. Semay V. Mathieu B. Silvestre-Brac 《The European Physical Journal A - Hadrons and Nuclei》2007,32(2):123-126
In the framework of quark models, hybrid mesons are either seen as two-body qˉ systems with an excited flux tube connecting the quark to the antiquark or as three-body qˉg systems including a constituent gluon. In this work we show that, starting from the three-body wave function of the qˉg hybrid meson in which the gluonic degrees of freedom are averaged, the excited flux tube picture emerges as an equivalent
qˉ potential. This equivalence between the excited flux tube and the constituent-gluon approach is confirmed for heavy hybrid
mesons but, for the first time, it is shown to hold in the light sector too, provided the contribution of the quark dynamics
is correctly taken into account. 相似文献
5.
L. Bravina A. Capella E. G. Ferreiro A. B. Kaidalov K. Tywoniuk E. Zabrodin 《The European Physical Journal C - Particles and Fields》2009,61(4):865-870
One observes strong suppression effects for hard probes, e.g. the production of J/ψ or high-p
T
particles, in nucleus–nucleus (AA) collisions at RHIC. Surprisingly, the magnitude of the suppression is quite similar to that at SPS. In order to establish
whether these features arise due to the presence of a thermalized system of quarks and gluons formed in the course of the
collision, one should investigate the impact of suppression mechanisms which do not explicitly involve such a state. We calculate
shadowing for gluons in the Glauber–Gribov theory and propose a model invoking a rapidity-dependent absorptive mechanism motivated
by energy-momentum conservation effects. Furthermore, final-state suppression due to interaction with comoving matter (hadronic
or pre-hadronic) has been shown to describe the data at SPS. We extend this model by including the backward reaction channel,
i.e. recombination of open charm, which is estimated directly from pp data at RHIC. Strong suppression of charmonium both in pA and AA collisions at LHC is predicted. This is in stark contrast with the predictions of models assuming QGP formation and thermalization
of heavy quarks. 相似文献
6.
Jack Alster 《Molecular physics》2013,111(6):1235-1239
The field gradient fluctuation at the Li+ nucleus in dilute aqueous solution is calculated via Monte Carlo simulations of Li+ + nH2O clusters (n = 6, 50 and 150). The intermolecular potentials and the lithium field gradient function, necessary for the simulations, are based on ab initio quantum mechanical calculations. It is found that the dominant contribution to the field gradient fluctuation at the lithium nucleus comes from the water molecules in the first shell. Furthermore, it is the lateral displacement of these molecules that causes the largest fluctuation. The contribution from the rotation of a water molecule is of minor importance. The field gradient at a lithium nucleus arising from a water molecule is badly described in a simple electrostatic model. 相似文献
7.
A. H. Moudden 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,64(2):173-183
Several structures for off-stoichiometric beryllium diboride dicarbide Be1−xB2C2 have been designed, and their properties studied from first-principles density functional methods. Among the most stable
phases examined, the layered hexagonal structures are shown to exhibit various features in the electronic properties and in
the lattice dynamics reminiscent of the superconducting magnesium diboride and alkaline earth-intercalated graphites. For
substoichiometric composition x ∼ 1/3, the system is found metallic with a moderately strong electron-phonon coupling through a predominant contribution
arising from high frequency streching modes modulating the σ-bonding of the B-C network, and a weaker contribution at medium frequency range of the phonon spectra, arising from the intercalent
motion coupled to the π-bonding states. Further, anharmonicities emerging from the proximity of the Fermi level to the σ-band edge, contributes to reduce the phonon softening hence stabilizing the structure. All these effects appear to combine
favourably to produce a high temperature phonon-superconductivity. 相似文献
8.
P. Kopietz A. Völker 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):397-405
A time-dependent electric field gives rise to a stationary non-equilibrium current I
(2)
around a mesoscopic metal ring threaded by a magnetic flux. We show that this current, which is proportional to the intensity
of the field, is closely related to the exchange part of the interaction contribution to the equilibrium persistent current, and that the corresponding non-linear conductivity directly measures the weak localization correction
to the polarization. We explicitly calculate the disorder average of I
(2)
in the diffusive regime as function of the frequency of the electric field and the static flux piercing the ring, and suggest
an experiment to test our theory.
Received: 5 September 1997 / Accepted: 4 November 1997 相似文献
9.
K. Kumerički D. Müller K. Passek-Kumerički 《The European Physical Journal C - Particles and Fields》2008,58(2):193-215
To leading order approximation, the physical content of generalized parton distributions (GPDs) that is accessible in deep
virtual electroproduction of photons or mesons is contained in their value on the cross-over trajectory. This trajectory separates
the t-channel and s-channel dominated GPD regions. The underlying Lorentz covariance implies correspondence between these two regions through
their relation to GPDs on the cross-over trajectory. This point of view leads to a family of GPD sum rules which are a quark
analogue of finite energy sum rules and it guides us to a new phenomenological GPD concept. As an example, we discuss the
constraints from the JLab/Hall A data on the dominant u-quark GPD H. The question arises whether GPDs are governed by some kind of holographic principle. 相似文献
10.
The light weakly bound nucleus 7Li is studied within a dicluster α + t picture. Different observables obtained within our simple model are compared with previous calculations and experiments showing
good agreement. In particular, we calculate dipole and quadrupole electromagnetic response to the continuum. The energy distribution
of B(Eλ) values are consistent with the energy-weighted molecular sum rule and display a sizable contribution of non-resonant character
arising from the weak binding property. The corresponding form factors for excitations to the continuum are used in a semiclassical
coupled-channel scheme to get estimates for the breakup cross-section in a heavy-ion reaction. The nuclear contribution is
found to play an important role in the process for bombarding energies around the Coulomb barrier. The masses and charges
ratios of the two clusters are shown to lead to features of the cluster halo that may significantly differ from the one usually
associated with one-nucleon haloes. 相似文献
11.
E. L. Medeiros M. M. N. Rodrigues S. B. Duarte O. A. P. Tavares 《The European Physical Journal A - Hadrons and Nuclei》2007,34(4):417-427
Half-life measurements for both ground-state and isomeric transitions in proton radioactivity are systematized by using a
semiempirical, one-parameter model based on tunneling through a potential barrier, where the centrifugal and overlapping effects
are taken into account within the spherical nucleus approximation. This approach, which has been successfully applied to alpha
decay cases covering ∼ 30 orders of magnitude in half-life, has shown, in addition, very adequate at fitting all existing
data on partial half-life, T
1/2p , of proton emission from nuclei. Nearly 70 measured half-life values have been analysed, and the data could be described
by two straight lines relating the pure Coulomb contribution to half-life with the quantity
(Z
d is the atomic number of the daughter nucleus, μ0 is the reduced mass, and Q
p is the total nuclear energy available for decay). These straight lines are shown to correspond to different degrees of deformation,
namely, very prolate ( δ ≳ 0.1 , and other shaped ( δ ≲ 0.1 parent nuclei. The goodness in reproducing the data attained
in the present systematics allows for half-life predictions for a few possible cases of proton radioactivity not yet experimentally
accessed. 相似文献
12.
Zhi-Gang Wang 《The European Physical Journal C - Particles and Fields》2008,58(2):245-250
We study the final-state rescattering effects in the decay B
0→η
c
K
*. The numerical results indicate that the corrections are comparable with the contribution from the naive factorizable amplitude,
and the total amplitudes can accommodate the experimental data. 相似文献
13.
We re-evaluate the non-perturbative contribution to the thrust distribution in e
+
e
−→ hadrons, in the light of the latest experimental data and the recent NNLO perturbative calculation of this quantity. By
extending the calculation to NNLO+NLL accuracy, we perform the most detailed study to date of the effects of non-perturbative
physics on this observable. In particular, we investigate how well a model based on a low-scale QCD effective coupling can
account for such effects. We find that the difference between the improved perturbative distribution and the experimental
data is consistent with a 1/Q-dependent non-perturbative shift in the distribution, as predicted by the effective coupling model. Best fit values of α
s
(91.2 GeV)=0.1164−0.0026+0.0028 and α
0(2 GeV)=0.59±0.03 are obtained with χ
2/d.o.f.=1.09. This is consistent with NLO+NLL results but the quality of fit is improved. The agreement in α
0 is non-trivial because a part of the 1/Q-dependent contribution (the infrared renormalon) is included in the NNLO perturbative correction.
Address after 1 October 2008: Rudolf Peierls Centre for Theoretical Physics, 1 Keble Road, Oxford OX1 3NP, UK. 相似文献
14.
Recent studies of the dissociative adsorption of methane on clean Ni(111), Ni(100), Ni(110), and sulfur-modified Ni(100), as well as ethane, propane, and n-butane on Ni(100) have been carried out under the high incident flux conditions of 1.00 Torr methane, 0.10 Torr ethane, 0.01 Torr propane, and 0.001 Torr n-butane, respectively. It has been found that the activation energies for these processes range from 3.1±1.0 to 13.3±1.5 kcal mol–1. A comparison with the results of corresponding molecular beam studies suggests that the effects of vibrational energy on sticking probabilities must be accounted for and the sticking probabilities of molecules with very low normal kinetic energies must be accurately known when attempting to model high pressure processes using molecular beam techniques. While dissociation of ethane, propane, and n-butane on Ni(100) is believed to proceed primarily via a trapped molecular precursor, the results on sulfur-modified Ni(100) surface indicate that the direct channel to methane dissociation likely dominates and the contribution from the trapped molecular precursor mechanism is likely relatively small, with the sulfur atoms poisoning this reaction by a simple site blocking mechanism. 相似文献
15.
Starting from an analytical macroscopic/phenomenological model yielding the self-bias voltage as a function of the absorbed
radio-frequency (rf) power of an asymmetric capacitively coupled discharge in NF3 this paper studies the dependence of the ion flux onto the powered electrode on the gas pressure. An essential feature of
the model is the assumption that the ions' drift velocity in the sheath near the powered electrode is proportional to E
α, where E=−ΔU (U being the self-bias potential), and α is a coefficient depending on the gas pressure and cross section of elastic ion-neutral
collisions. The model also considers the role of γ-electrons, stochastic heating as well as the contribution of the active
electron current to the global discharge power balance. Numerically solving the model's basic equations one can extract the
magnitude of the ion flux (at three different gas pressures) in a technological etching device (Alcatel GIR 220) by using
easily measurable quantities, notably the self-bias voltage and absorbed rf power. 相似文献
16.
Xiang Liu Zheng-Tao Wei Xue-Qian Li 《The European Physical Journal C - Particles and Fields》2009,59(3):683-689
To test the validity of perturbative QCD (pQCD) and investigate its range of application, one should look for a suitable process.
B→J/ψ
D is a promising candidate. The linear momentum of the products is relatively small, so that there may exist a region where
exchanged gluons are soft and the perturbative treatment may fail, so that the non-perturbative effect would be significant.
We attribute such non-perturbative QCD effects to the long-distance final-state interaction (FSI) which is estimated in this
work. We find that the contribution from the FSI to the branching ratio is indeed sizable and may span the rather wide range
of 10−6∼10−5 and cover a region where the pQCD prediction is of the same order. A more accurate measurement of its branching ratio may
provide important information about the application region of pQCD and help to clarify the picture of inelastic rescattering
(i.e. FSI), which is generally believed to play an important role in B decays. 相似文献
17.
D. Amrath M. Diehl J. P. Lansberg 《The European Physical Journal C - Particles and Fields》2008,58(2):179-192
We study deeply virtual Compton scattering on a virtual pion that is emitted by a proton. Using a range of models for the
generalized parton distributions of the pion, we evaluate the cross section, as well as the beam spin and beam charge asymmetries
in the leading-twist approximation. Studying Compton scattering on the pion in suitable kinematics puts high demands on both
beam energy and luminosity, and we find that the corresponding requirements will first be met after the energy upgrade at
Jefferson Laboratory. As a by-product of our study, we construct a parameterization of pion generalized parton distributions
that has a non-trivial interplay between the x and t dependence and is in good agreement with form factor data and lattice calculations. 相似文献
18.
N. T. Khai T. D. Thiep T. T. An P. V. Cuong N. T. Vinh A. G. Belov O. D. Maslov 《Physics of Particles and Nuclei Letters》2008,5(5):437-440
Bremsstrahlung emission, or radiation loss, is the dominant mechanism of energy dissipation of electrons at relativistic energies
greater than a few MeV when it is subjected to acceleration in the field of the nucleus or of the electrons. In this study,
the Monte Carlo calculations for bremsstrahlung spectra have been described for the case of a thick tungsten target with incident
electron beams from 10 to 50 MeV, where secondary interactions induced by the electrons and photons in the target, such as
energy loss, absorption, scattering, and (e
+, e
−)-pair production effects, were taken into account.
The text was submitted by the authors in English. 相似文献
19.
Manoj Kumar Sharma H. D. Bhardwaj Unnati Pushpendra P. Singh B. P. Singh R. Prasad 《The European Physical Journal A - Hadrons and Nuclei》2007,31(1):43-51
With a view to study the pre-equilibrium emission mechanism in α-induced reactions the excitation functions for 93Nb(α, n)96m
Tc, 93Nb(α, n)96Tc, 93Nb(α, 2n)95m
Tc, 93Nb(α, 2n)95g
Tc and 93Nb(α, 3n)94Tc reactions have been measured in the energy range threshold to ≈ 10MeV/nucleon using the activation technique. The measured
excitation functions have also been compared with theoretical predictions based on the semi-classical code, which takes into
account compound nucleus as well as pre-equilibrium emission. The analysis of the data indicates significant contribution
from pre-equilibrium emission at these energies particularly in the high-energy tail portion of EFs. The effect of the variation
of the parameters used in the code has been studied. The isomeric cross-section ratios have also been measured. It has been
observed that the pre-equilibrium fraction increases rapidly with the increase in α-particle bombarding energy. 相似文献
20.
E. G. Ferreiro F. Fleuret A. Rakotozafindrabe 《The European Physical Journal C - Particles and Fields》2009,61(4):859-864
We present a new approach to estimate the effect of gluon shadowing in nucleus + nucleus collisions and its consequences on
the J/ψ production yield. Using kinematical information available from the measured J/ψ production in proton + proton collisions at
GeV, we build a Glauber Monte Carlo code which takes into account shadowing in two alternative ways: multiple-scattering
corrections or Q
2 evolution of parton densities. We exploit the dependence of these different parameterizations to the J/ψ transverse momentum and we give the first predictions on the resulting p
T dependence of the nuclear modification factor in deuteron + gold collisions at the same energy. 相似文献