Fermi Motion Effects Research Articles

Matching of the shape function

The shape function f( k +) describes Fermi motion effects in inclusive semi-leptonic decays such as B→ X u + e+ ν near the end-point of the lepton spectrum. We compute the leading one-loop corrections to the shape function f( k +) in the effective theory with a hard cut-off regularization. The matching constant onto full QCD is infrared safe, i.e. the leading infrared singularity represented by the term log 2 k + cancels in the difference of integrals. We compare the hard cut-off result with the result in dimensional regularization, the latter containing an additional factor of two in the coefficient of the log 2 k + term, and consequently requiring an oversubtraction.

On the extraction of the neutron spin structure functions and the Gerasimov-Drell-Hearn integral from the process [formula omitted] in the resonance region

Nuclear effects in polarized inelastic electron scattering off polarized 3He are discussed; in the kinematics of future experiments at TJNAF, Fermi motion effects are found to be much larger than in deep inelastic scattering. It is shown that improperly describing nuclear dynamics would lead to the extraction of unreliable neutron spin structure functions. On the other hand side, a simple and workable equation relating the Gerasimov-Drell-Hearn Integral for the neutron to the corresponding quantity for 3He is proposed.

Power corrections in the decay rate and distributions inB→Xsl+l−in the standard model

We investigate the leading power corrections to the decay rates and distributions in the decay $B \rightarrow X_s \ell^+ \ell^- $ in the standard model (SM) using heavy quark expansion (HQE) in $(1/m_b)$ and a phenomenological model implementing the Fermi motion effects of the b quark bound in the B hadron. In the HQE method, we find that including the leading power corrections the decay width decreases by about 4% and the branching ratio by about 1.5% from their (respective) parton model values. The dilepton invariant mass spectrum is found to be stable against power corrections over a good part of this spectrum. However, near the high-mass end-point this distribution becomes negative with the current value of the non-perturbative parameter $\lambda_2$ (the $\lambda_1$-dependent corrections are found to be inoccouos), implying the breakdown of the HQE method in this region. Our results are at variance with the existing ones in the literature in both the decay rate and the invariant dilepton mass distribution calculated in the HQE approach. As an alternative, we implement the non-perturbative effects in the decay $B \rightarrow X_s \ell^+ \ell^- $ using a phenomenologically motivated Gaussian Fermi motion model. We find small corrections to the branching ratio, but the non-perturbative effects are perceptible in both the dilepton mass distribution and the Forward-Backward asymmetry in the high dilepton mass region. Using this model for estimating the non-perturbative effects, we present the decay rates and distributions for the inclusive process $B \rightarrow X_s \ell^+ \ell^- $ in the SM.

Mesonic and binding contributions to the EMC effect in a relativistic many-body approach

We revise the conventional nuclear effects of Fermi motion, binding and pionic effects in deep inelastic lepton scattering using a relativistic formalism for an interacting Fermi sea and the local density approximation to translate results from nuclear matter to finite nuclei. In addition we also consider effects from ϱ-meson renormalization in the nucleus. The use of nucleon Green functions in terms of their spectral functions offers a precise way to account for Fermi motion and binding. On the other hand the use of many-body Feynman diagrams in a relativistic framework allows one to avoid using prescriptions given in the past to introduce relativistic corrections in a nonrelativistic formalism. We show that with realistic nucleon spectral functions and meson nucleus self-energies one can get a reasonable description of the EMC effect for x > 0.15, outside the shadowing region.

Formula omitted] structure function ratio at large x

We re-examine the large- x neutron/proton structure function ratio extracted from the latest deuteron data, taking into account the most recent developments in the treatment of Fermi motion, binding and nucleon off-shell effects in the deuteron. Our findings suggest that as x → 1 the ratio of the neutron to proton structure functions ( F 2 n F 2 p ) is consistent with the perturbative QCD expectation of 3 7 , but which is larger than the value of 1 4 obtained in earlier analyses.

Low energy [formula omitted]N interaction in nuclear matter

We investigate the low-energy K N interaction in nuclear matter including Pauli blocking, Fermi motion and binding effects. We use a coupled-channel approach based on the Chiral SU(3) Effective Lagrangian which describes all available low energy data of the coupled K N, πΣ, πΛ system. Due to the dynamics of the Λ (1405) resonance we find a strong non-linear density dependence of the K −p scattering amplitude in nuclear matter. The real part of the K −p scattering length changes sign already at a small fraction of nuclear matter density, less than 0.2 p o . This may explain the striking behaviour of the K − -nuclear optical potential found in the analysis of kaonic atom data.

Structure functions of bound nucleons: From the EMC effect to nuclear shadowing

We describe the nuclear structure functions in the whole range of the Bjorken variablex, by combining various effects in a many-step procedure. First, we present a QCD motivated model of nucleons, treated, in the limit of vanishingQ2, as bound states of three relativistic constituent quarks. Gluons and sea quarks are generated radiatively from the input valence quarks. All parton distributions are described in terms of the confinement (or nucleon's) radius. The results for free nucleons are in agreement with the experimental determinations. The structure functions of bound nucleons are calculated by assuming that the main effect of nucleon binding is stretching of nucleons. The larger size of bound nucleons lowers the valence momentum and enhances the radiatively generated glue and sea densities. In the small-x region the competitive mechanism of nuclear shadowing takes place. It also depends on the size of the nucleons. By combining stretching, shadowing and Fermi motion effects (the latter confined to very largex), the structure function ratio is well reproduced. Results are also presented for theA-dependence of the momentum integral of charged partons, the nuclear gluon distribution and the hadron-nuclei cross sections.

The Fermi-motion effect in the signal for color transparency in (e, e'p) scattering

The signal for color transparency depends on the composition and intranuclear rescattering of the ejectile wave packet emerging from the hard ep vertex. The electron scattering kinematics, in conjunction with the Fermi motion of bound nucleons, allows one to control the composition of the ejectile wave packet. Using the diffraction operator technique developed by us recently we derive the dependence of the color transparency signal on the Bjorken variable x of the scattered electron. The Fermi-motion bias is found to be much weaker than has been claimed recently.

Nuclear Fermi motion effect on pion double charge exchange

The influence of nuclear Fermi motion on the conventional mechanism of pion double charge exchange (DCX) is examined. Concrete calculations of the 0° excitation function over the energy region 0–300 MeV are performed for the double-isobaric-analog-state (DIAS) DCX transition on 14C. The calculated results coincide very well with the experimental data for energies above 100 MeV, but are about a factor of 2 smaller than the experimental data around 50 MeV. It indicates that the pure conventional mechanism seems incapable to explain fully the anomalous behavior of the low-energy DCX, and some new mechanisms are needed.

European Muon Collaboration effect in deuteron and in three-body nuclei.

We investigate the European Muon Collaboration (EMC) effect in deuteron and three-body nuclei. The binding and the Fermi motion effects of nucleons are studied in detail. They reduce the structure function ratios from unity by less than 5%. Roles of excess pion are also discussed in connection with the momentum sum rule, which is violated by the binding effect.

Nucleus-nucleus optical potential in an optimal approximation.

The first-order optical potential for elastic nucleus-nucleus scattering is derived, taking into account binding and Fermi motion effects through an optimal approximation. The resulting potential is expressed as a product of the projectile elastic form factor times the nucleon-nucleus elastic amplitude (rather than the nucleon-nucleus potential as in the folding model). The effective energy in this amplitude rises with angle and agrees with the usual choice of the folding model for forward angles. The form of the spin-orbit nucleus-nucleus potential for projectiles with spin is discussed as well.

New cascade model calculation of pion multiplicity in high-energy heavy-ion collisions

Pion multiplicities in nuclear collisions are calculated using the nuclear cascade model developed by Kitazoe et al. This cascade model differs in particular from previous models in the way binding, Pauli principle, and Fermi motion effects are treated. The treatment of such details in the cascade is found to have significant influence on the pion multiplicities. Close agreement with the Ar + KCl central collision data is obtained without introducing the compressional energy effect postulated by Stock et al.

Electron Scattering from Nuclear Targets and Quark Distributions in Nuclei

The deep-inelastic electromagnetic structure functions of steel, deuterium, and hydrogen nuclei have been measured with use of the high-energy electron beam at the Stanford Linear Accelerator Center. The ratio of the structure functions of steel and deuterium cannot be understood simply by corrections due to Fermi-motion effects. The data indicate that the quark momentum distributions in the nucleon become distorted in the nucleus. The present results are consistent with recent measurements with high-energy muon beams.

Fermi motion and Pauli exclusion principle effects ind(π±, π± p)nin theΔ-resonance region

An ab initio method of taking Fermi motion effects in pion scattering into account combined with proper antisymmetrization of the two nucleon final state in $d({\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}} {\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}, p)n$ shows that the reaction amplitude consists of not only a contribution from pion scattering on a proton, but also pion scattering on a neutron, and that the two amplitudes differ considerably in their $\ensuremath{\pi}\ensuremath{-}N$ c.m. energies. The neutron scattering contribution to the differential cross section exceeds 10% in as many as 51 out of 195 ${\ensuremath{\pi}}^{\ensuremath{-}}$ events recorded in a recent kinematically complete experiment by Hoftiezer et al. in the $\ensuremath{\Delta}$-resonance region. In several kinematical situations the neutron contribution exceeds 100% and could even be as high as \ensuremath{\sim}400%.NUCLEAR REACTIONS $d({\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}, {\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}} p)n$, ab initio method for Fermi motion effects, Pauli principle, effective mass, exchange amplitude.

On the Nuclear Effects in Nucleon Decays

Various nuclear effects upon nucleon decays, which occur in a nucleus, are extensively studied with an emphasis on a huge water Cerenkov detector. The Monte Carlo calculation is carried out for the typical decay process nucleon– positron+pion. The virtual meson exchange process is also taken into consideration. Results show that apart from the Fermi motion effect about 50% of nucleon decay signals are completely free from nuclear effects in the case of 13 O nucleus. The advantage of using material made from light nuclei is discussed.

West'sβcorrection revisited

It is shown that the existence of West's $\ensuremath{\beta}$ correction in the conventional approach to the Fermi-motion effects on deuterium targets is due to the use of the spectator approximation in deriving the normalization condition for the deuteron vertex. It is absent, however, in the nucleon-parton approach which uses the Brodsky parametrization method.

Gauge invariance and nucleonic Fermi motion in radiative pion capture on16O

Partial transition rates and the gamma -ray spectrum for the radiative pion capture reaction on 16O have been evaluated using the gauge-invariant effective Hamiltonian which also contains the nucleonic Fermi motion effects. Compared with the results obtained for the conventional effective Hamiltonian, a 15-25% reduction in the yield of the gamma -ray spectrum is observed. This is almost completely related to a new energy term and kinematic renormalisation of the constants of the effective Hamiltonian results in essential changes in the weak partial transitions.

Further studies of Fermi-motion effects in lepton scattering from nuclear targets

We have calculated the ratio of deep-inelastic structure functions of nuclear targets to the sum of free-neutron and - proton structure functions. The calculations incorporate structure-function fits which are based on quantum-chromodynamic considerations. This paper is an addendum to an earlier publication in which we calculated the Fermi-motion corrections using other fits to the nucleon structure functions.

(π,πn) puzzle above the (3,3) resonance

We calculate the corrections due to pion production, Fermi motion, and the Pauli principle on the ratio of $^{12}\mathrm{C}({\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}},\ensuremath{\pi}N)^{11}\mathrm{C}$ total cross sections for particle-stable final states. These corrections are important above 400 MeV. When they are combined with those estimated earlier for final state nucleon charge exchange interactions, good agreement is obtained with the data, which cover the energy region from 40 to 550 MeV.[NUCLEAR REACTIONS $^{12}\mathrm{C}({\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}},\ensuremath{\pi}N)^{11}\mathrm{C}$, calculated ratio of knockout cross sections 40 - 1000 MeV, impulse approximation, nucleon final state charge exchange interactions, pion production corrections, Pauli principle and Fermi motion effects.]

Pauli blocking and Fermi motion effects in ion-ion collisions

A geometrical model for Pauli blocking and Fermi motion effects in ion-ion collisions is presented. The results, given as effective nucleon-nucleon total cross sections, incicate that the Pauli blocking reduces the nucleon-nucleon cross section in the ion-ion environment by a larger amount than previously estimated.