Inclusive Asymmetries Research Articles

Analyzing the nucleon spin in weak interaction processes at HERA energies

We investigate the possibility to measure the spin content carried by the different quark flavors in a nucleon by means of polarized deep inelastic scattering with W ± exchange at HERA. Such measurements require a polarized proton beam. The expected inclusive and semi-inclusive asymmetries are sizable and for realistic luminosities the expected statistical accuracies are good enough to extract new and relevant information on the valence quark and the strange sea distributions.

Charm-anticharm asymmetries in high energy photoproduction

We report measurements of charm particle production asymmetries from the Fermilab photoproduction experiment E687. An asymmetry in the rate of production of charm versus anticharm particles is expected to arise primarily from fragmentation effects. We observe statistically significant asymmetries in the photoproduction of D +, D ∗+ and D 0 mesons and find small (but statistically weak) asymmetries in the production of the D s + meson and the Λ c + baryon. Our inclusive photoproduction asymmetries are compared to predictions from nonperturbative models of charm quark fragmentation.

Polarisation of valence and non-strange sea quarks in the nucleon from semi-inclusive spin asymmetries

We present a measurement of semi-inclusive spin asymmetries for positively and negatively charged hadrons from deep inelastic scattering of polarised muons on polarised protons and deuterons in the range 0.003 < x < 0.7. From these asymmetries and the previously published inclusive spin asymmetries we determine, for the first time, the x-dependent spin distributions for up and down valence quarks and for non-strange sea quarks. We find that the first moments of the valence quark spin distributions are Δu v = 1.01 ± 0.19 ± 0.14 and Δd v = −0.57 ± 0.22 ± 0.11. The spin distribution function of non-strange sea quarks is consistent with zero over the measured range of x and the first moment is Δ u = Δ d = −0.02 ± 0.09 ± 0.03 .

Radiative Decay of Polarized τ-Leptons, τ ⇛ ντ + π + γ

Using the standard form of the matrix element, which contains the so-called structural part as well as Inner Bremsstrahlung (IB) contribution, we calculate the structure functions for τ− → ντ + π− + γ in terms of the vector and axial form factors of the transition W → π− + γ. We work out the Dalitz plot for all three possible exclusive asymmetries and show that they are all non-zero. We discuss the consequences of the radiative zero in the IB amplitude. We show that inclusive asymmetries in τ− → ντ + π− + γ can be calculated analytically without knowledge of any concrete parametrization of the form factors v(t) and a(t). The t-dependence of v(t) and a(t) is computed in the framework of the vector dominance model using the conservation of vector current hypothesis for the v(0) and the existing experimental data for the ratio γ = a(0)/v(0) from π− → e− + ν(e) + γ decay. We demonstrate the sensitivity of exclusive and inclusive asymmetries to the sign of the vector form factor. The predicted polarization effects are large in magnitude.

Sensitivity of the asymmetry of the polarized b-quark radiative b → sγg decay to the model FCNC

The effects induced by the initial b-quark longitudinal polarization in the rare b → sγg decay in the standard (SM) and two Higgs doublet (2HDM) models are investigated. Using the structure functions formalism we calculate different asymmetries in b → sγg decay in terms of the magnetic dipole type form factors, which describe the b → sγ and b → sg transitions. It is shown that the general expressions for the structure functions are model independent; the model dependent information is contained in the corresponding magnetic dipole form factors. It is obtained that the inclusive asymmetry, corresponding to the detection of the photon only, is sensitive to the QCD corrections and to the choice of the FCNC model.

Tests of the Electroweak Theory at LEP

LEP offers a rich choice of tests of the electroweak theory such as the measurement of hadronic and leptonic cross sections, leptonic forward-backward asymmetries, τ polarization asymmetries, partial widths and forward-backward asymmetries of heavy quark flavours, of the inclusive qq charge asymmetry and of final state radiation in hadronic events. We discuss experimental aspects of these measurements and their theoretical parametrization and summarize the results available so far. We present several analyses which reveal specific aspects of the results, such as their constraints on Standard Model parameters and on new particles, the sensitivity to deviations from the Standard Model multiplet structure and an analysis in a framework which provides a model independent search for new physics.

Probing the polarized sea by inclusive leptoproduction of hadrons.

We give predictions for the inclusive production of {pi} and {ital K} in polarized leptoproduction from polarized nucleons. As {ital E}{sub {pi},}{ital K}{sup lab}{r arrow}0 the hadron inclusive asymmetry equals that of the total cross section; for {ital E}{sub {pi},}{ital K}{sup lab}{gt}0 new information obtains. We show that these processes can provide sensitive tests for a polarized sea in a polarized nucleon.

Inclusive forward-backward asymmetry for light quarks.

The energy-differential forward-backward asymmetry in the reaction ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}M+ \mathrm{anything}$, $M$ being a leading, light-quark-constituted meson in a jet, is considered. The measurement of this quantity yields rank-1 and higher-rank fragmentation functions for light quarks hadronizing into light mesons, separately. It is also possible, in the case of a large event sample and good particle identification capabilities at high energies, to obtain information on $Z$ couplings to light quarks.

Inclusive electrodisintegration of polarised deuterons

The general structure of the asymmetries of longitudinally polarised electron scattering by deuterons with vector and tensor polarisations has been established for the process e-+d to e-+n+p using the structure function formalism and considering the deuteron as an elementary particle with a unit spin and a positive space parity. The analysis has been performed in terms of the one-photon approximation using the conservation of the hadron electromagnetic current and P-invariance of the hadron electromagnetic interaction. The inclusive scattering of longitudinally polarised electrons by a target with a vector polarisation is described by two real structure functions, while the scattering of unpolarised electrons by a target with a tensor polarisation is described by four structure functions. Both the vector and tensor target polarisations allow, in principle, studies of possible T-odd effects in hadron electrodynamics. The sensitivity of inclusive asymmetries to the neutron electric form factor and the spin deuteron wavefunction has also been investigated.

Lepton inclusive distributions and asymmetries from the decay of scalar lepton pairs in e+e− collisions with polarized beams

We present a detailed analysis of the inclusive spectral and angular distributions of a lepton coming from the decay of a scalar lepton pair—produced in electron-positron collisions with polarized beams, in a simple softly broken supersymmetric model of electroweak interactions. We analyse in detail also the longitudinal and transverse polarization asymmetries in various cases (degenerate or not degenerate sleptons, massive or massless photino) pointing out the corresponding signatures.

Quantum- chromodynamic predictions for inclusive spin-spin asymmetries at large transverse momentum

We discuss predictions for the asymmetry ${A}_{\mathrm{LL}}$ in the inclusive production at large ${p}_{T}$ of charged and neutral pions by longitudinally polarized protons. We work in the framework of a hard-scattering model based on perturbative quantum chromodynamics. Various assumptions for the distribution of the proton's spin among its constituents---quarks, ocean antiquarks, and gluons---and the effects of scaling violations on the parton distributions are considered.

Asymmetries in inclusive proton-nucleon scattering at 11.75 GeV/c

We have measured the left-right asymmetry for the inclusive production of ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$, ${K}^{\ifmmode\pm\else\textpm\fi{}}$, and protons in proton-proton and proton-deuteron collisions at 11.75 GeV/c. The measurements, utilizing the polarized proton beam at the Argonne zero-gradient synchrotron, cover a wide range of kinematic variables [$u$ between 0.6 and -1.5 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ and $x$ between 0.1 and 0.9 for meson production, $t$ between -0.2 and -2.0 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ and $x$ between -0.25 and 0.75 for proton production]. There is considerable angular structure in the pion-production data but not in the kaon or proton processes. Proton production alone is clearly sensitive to the isotopic spin of the target particle. A simple phenomenological model of baryon-exchange dominance successfully relates the inclusive pion asymmetries to the asymmetries in backward pion-nucleon elastic scattering. A triple-Regge model for the inclusive proton asymmetries is also discussed.