Evidence for a single-spin azimuthal asymmetry in semi-inclusive pion electroproduction

Single-spin asymmetries for semi-inclusive pion production in deep-inelastic scattering have been measured for the first time. A significant target-spin asymmetry of the distribution in the azimuthal angle straight phi of the pion relative to the lepton scattering plane was formed for pi(+) electroproduction on a longitudinally polarized hydrogen target. The corresponding analyzing power in the sinstraight phi moment of the cross section is 0.022+/-0.005+/-0.003. This result can be interpreted as the effect of terms in the cross section involving chiral-odd spin distribution functions in combination with a chiral-odd fragmentation function that is sensitive to the transverse polarization of the fragmenting quark.     

A realistic calculation of the azimuthal asymmetry in semi-inclusive deep inelastic scattering

The azimuthal asymmetry in deep inelastic scattering is calculated in first order QCD for nonvanishing intrinsic transverse momentum; the hadronization is simulated with a Monte-Carlo program. It turns out that, within our scheme, the asymmetry data are mainly sensitive to the intrinsic transverse momentum. The QCD contribution to the asymmetry is small.     

Comment on azimuthal dependence in leptoproduction

It is pointed out that in the conventional calculation a large part of 〈cosφ〉 in leptoproduction comes from configurations in which the outgoing partons have small p_T. This indicates that the conventional QCD prediction for 〈cosφ〉 is not reliable in the experimentally available Q² regions at present.     

QCD predictions for semi-inclusive and inclusive leptoproduction

We give analytic predictions for angular distributions of hadrons in semi-inclusive and inclusive leptoproduction deduced from first-order calculations in QCD. The predictions allow tests of QCD and give information about gluon fragmentation functions.     

Probing transverse quark polarization via azimuthal asymmetries in leptoproduction

We consider the leading order result for polarized leptoproduction, putting emphasis on transverse momentum dependent effects appearing in azimuthal asymmetries.Measurements of weighted cross sections enable extraction of the distribution of transversely polarized quarks. We focus on the distribution in a longitudinally polarized hadron and estimate the expected asymmetries in leptoproduction.     

Flavor dependent azimuthal asymmetries in unpolarized semi-inclusive DIS at HERMES

The azimuthal cos? _h and cos2? _h modulations of the distribution of hadrons produced in unpolarized semi-inclusive deep-inelastic scattering of electrons and positrons off hydrogen and deuterium targets have been measured in the hermes experiment. For the first time these modulations were determined in a 4-Dimensional kinematic space for positively and negatively charged pions and kaons separately, as well as for unidentified hadrons. These azimuthal dependences are sensitive to the transverse motion and polarization of the quarks within the nucleon via, e.g., the Cahn, Boer-Mulders and Collins effects.     

Azimuthal asymmetry in polarized leptoproduction and spin-dependent structure functions

Asymmetry in azimuthal angle distribution of hadrons produced in polarized deep-inelastic lepton-nucleon scattering is discussed. Significant polarization effects are predicted. It is pointed out that such effects can be used as another method to measure the spin-dependent structure functions of the nucleons.     

Flavor asymmetry of polarized antiquark distributions and semi-inclusive DIS

The –expansion of QCD suggests large flavor asymmetries of the polarized antiquark distributions in the nucleon. This is confirmed by model calculations in the large– limit (chiral quark–soliton model), which give sizable results for and . We compute the contributions of these flavor asymmetries to the spin asymmetries in hadron production in semi-inclusive deep–inelastic scattering. We show that the large flavor asymmetries predicted by the chiral quark–soliton model are consistent with the recent HERMES data for spin asymmetries in charged hadron production. Received: 30 November 1999 / Published online: 17 March 2000     

On the azimuthal dependence of semi-inclusive,deep inelastic electroproduction cross sections

It is pointed out that according to the parton model, there should be no dependence on the azimuthal angle in the cross section for deep inelastic, semi-inclusive electroproduction with one detected hadron in the final state.     

On unique predictions for single spin azimuthal asymmetry

Theoretically there are two approaches to predict single spin azimuthal asymmetries. One is to take transverse momenta of the partons into account by using transverse momentum dependent parton distributions, while another is to take asymmetries as twist-3 effects. The non-perturbative effects in these approaches are parameterized with different matrix elements, and predictions can be different. Recently, gauge invariant definitions of transverse momentum dependent parton distributions were derived. With these definitions it can be shown that there are relations between non-perturbative matrix elements in these two approaches. These relations may enable us to unify two approaches and to have unique predictions for single spin azimuthal asymmetries. In this letter we derive these relations by using time-reversal symmetry and show that even with these relations the single spin azimuthal asymmetry in a Drell-Yan process is predicted differently in different approaches.Received: 2 July 2004, Published online: 24 September 2004     

Symmetry aspects of the pion leptoproduction and the upper limit of the Levelt-Mulders asymmetry

We examined the symmetry aspect of the semi-inclusive one-pion production in the deep inelastic scattering of a lepton beam off an unpolarized nucleon target, with an emphasis on the positivity restrictions on the corresponding structure functions. In combination with the Callan-Gross-type relation between two twist-two structure functions W ₁ and W ₂, we derived an upper bound on the Levelt-Mulders asymmetry, which occurs when the lepton beam is longitudinally polarized.     

Liberated color in leptoproduction

Within a spontaneously broken color gauge theory of integer charge quarks (icq) one expects standard “confined” QCD predictions for leptoproduction experiments (e.g.,μN→μX) to hold for energies below the threshold for color production; above such a threshold, however, the color degree of freedom must be excited leading to a threshold rise in structure functions. We predict the expected rises for μN and charged current (cc) νN and $\text{ν}\text{N}$ scattering within the “naive” parton model. Comparing these predictions with the available data on μN, cc νN and $\text{ν}\text{N}$ as well as e^?e⁺ experiments, we deduce that the lightest spin-parity 1^? color octet meson (like the gluon) as well as the lightest color octet (8, 8) baryon must lie above 7–8 GeV. This in turn closes the only window in the light mass region <3.1 GeV (i.e., the 1.1 to 1.8 GeVregion), whichhadsofarbeenleftopenasapossiblevalueforthephysicalmassofthegluon. Wenextobservethatthetrendofthemostrecent high-Q² high-W Michigan State-Fermilab experiment on μN scattering exhibiting a relatively sharp rise in F₂ above the expectations of confined QCD of order 15% at low x agrees very well with our predictions based on the gauge model of icq. This rise, if confirmed, would favor the hypothesis of gauge icq, and simultaneously imply a threshold for the liberated color octet (8, 8) state at nearly (9±0.6) GeV and a mass for the liberated gluon in the region 8 to 9.5 GeV. We remark that within a left-right symmetric gauge structure leading to icq, charged current scattering of laboratory neutrinos (νN→μ^?X) will excite color only provided that the charged gluons V⁺ mix with V-A gauge particles W_L⁺mrather then with the V+A gauge particles W_R⁺. Thus it is possible that the μN, e^?e⁺ and neutral current scattering of neutrinos (i.e., νN→νX) show signs of color excitation, while the charged current scattering of neutrinos does not. We discuss briefly the implications of the threshold for liberated color octet states possibly lying around 8–10 GeV or higher on future e^?e⁺ experiments as well as $\text{ν}\text{N}\text{,}\text{ν}\text{N}\text{,}\text{p}\text{p}\text{-}\text{and}\text{p}\text{p}\text{-}\text{produced}$ dilepton-search experiments.     

Scaling in semi-inclusive experiments

In bubble chamber experiments (in contrast to counter experiments) the semi-inclusive experiment a+b→c+(n−1) charged + anything neutral is more natural than the corresponding inclusive experiment a+b→c+anything. We argue that the cross sections for semi-inclusive experiments have very neat scaling properties.     

High p(T) azimuthal asymmetry in noncentral A + A at RHIC

The high p(T)>3 GeV azimuthal asymmetry, v(2)(p(T)), in noncentral nuclear collisions at RHIC is shown to be a sensitive measure of the initial parton density distribution of the produced quark-gluon plasma. A generalization of the Gyulassy-Lévai-Vitev non-Abelian energy loss formalism including Bjorken (1+1)D expansion as well as important kinematic constraints is used.     

Asymmetry in semi-inclusive annihilation

Interference between one- and two-photon processes for e⁺e^? annihilation into hadrons in a two-jet parton model leads to a charge asymmetry of detected final state hadrons along the directions of the incident leptons. The asymmetry near the lepton axis grows as $\text{2}\text{ln}{}^2\text{1}\text{2}\text{θ - 4}\text{ln}\text{1}\text{2}\text{θ}\text{ln}\text{s/ΔE}{}^2$, so that in spite of the O(α) suppression relative to the Born cross section, the asymmetry can amount to a 2% effect at θ = 2° for π^± or K^± inclusive measurements in a typical experiment. The precise size of the asymmetry depends on the parton charges. Our results are only meant to apply in the region ω ? 0.5,where the parton model appears to be relevant at present experimental energies.     

Parton showers in leptoproduction events

We present a model that includes the production of arbitrarily many jets in lepton-hadron events, using the leading log formalism for parton shower evolution. The main problem encountered here, which has not previously been illuminated by studies ofe ⁺ e ^? annihilation or Drell-Yan/Z ⁰/W ^± production, is the choice of kinematics in the space-like shower evolution. In our preferred solution, the standard definition of Bjorkenx is preserved during the construction of initial and final state showers—a nontrivial constraint. The resulting model is described in detail, including some first investigations of its properties.     

Double-hadron leptoproduction in the nuclear medium

The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.     

气体放电等离子体特性测量I-V曲线不对称性的研究

利用气体放电双探针法研究了等离子体的I-V曲线中的电流I相对于电压V轴交点的不对称性,并提出2种可能的解释:一认为是由于两探针表面积不同引起的;二认为是由于探针所在处等离子体电位不等引起的.本文利用仪器的工艺误差和调换放电管电压的方法,对提出的2种可能原因分别进行验证,并指出第二种解释的合理性,并对其进行了理论分析.