European Muon Collaboration Research Articles

Pion-induced Drell-Yan processes and the flavor-dependent EMC effect

Pion-induced Drell-Yan processes are proposed as a promising tool with which to measure the flavor dependence of the European Muon Collaboration (EMC) effect, that is, the flavor-dependent modification of quark distributions in the nuclear medium. Existing pionic Drell-Yan data are compared with calculations using a recent model for nuclear quark distributions that incorporates flavor-dependent nuclear effects. While no firm conclusions can yet be drawn, we find that existing Drell-Yan data likely imply a flavor dependence of the EMC effect. We demonstrate that pion-induced Drell-Yan experiments on nuclear targets can access new aspects of the EMC effect not probed in deep inelastic scattering and can therefore provide important new constrains on the nuclear quark distributions. Predictions for possible future pion-induced Drell-Yan experiments are also presented.

Photon parton distributions in nuclei and the EMC effect

Photons, as well as quarks and gluons, are constituents of the infinite momentum frame (IMF) wave function of an energetic particle. They are mostly equivalent photons whose amplitude follows from the Lorentz transformation of the particle rest frame Coulomb field into the IMF and from the conservation of the electromagnetic current. We evaluate in a model independent way the dominant photon contribution $\ensuremath{\propto}{\ensuremath{\alpha}}_{\mathrm{em}}({Z}^{2}/{A}^{4/3})\mathrm{ln}(1/{R}_{A}{m}_{N}x)$ to the nuclear structure functions as well as the term $\ensuremath{\propto}{\ensuremath{\alpha}}_{\mathrm{em}}Z/A$. In addition we show that the definition of $x$ consistent with the exact kinematics of $\mathit{eA}$ scattering (with exact sum rules) works in the same direction as the nucleus field of equivalent photons. Combined, these effects account for the bulk of the hadronic European Muon Collaboration (EMC) effect for $x\ensuremath{\leqslant}0.5$ where Fermi motion effects are small. In particular, for these $x$ the hadronic mechanism contribution to the EMC effect does not exceed $~3%$ for all nuclei. Also, the $A$ dependence of the hadronic mechanism of the EMC effect for $x>0.5$ is significantly modified.

New Measurements of the European Muon Collaboration Effect in Very Light Nuclei

New Jefferson Lab data are presented on the nuclear dependence of the inclusive cross section from (2)H, (3)He, (4)He, (9)Be and (12)C for 0.3 < x < 0.9, Q(2) approximately 3-6 GeV(2). These data represent the first measurement of the EMC effect for (3)He at large x and a significant improvement for (4)He. The data do not support previous A-dependent or density-dependent fits to the EMC effect and suggest that the nuclear dependence of the quark distributions may depend on the local nuclear environment.

SPIN AND ORBITAL ANGULAR MOMENTUM IN THE PROTON

Since the announcement of the proton spin crisis by the European Muon Collaboration there has been considerable progress in unravelling the distribution of spin and orbital angular momentum within the proton. We review the current status of the problem, showing that not only have strong upper limits have been placed on the amount of polarized glue in the proton but that the experimental determination of the spin content has become much more precise. It is now clear that the origin of the discrepancy between experiment and the naive expectation of the fraction of spin carried by the quarks and anti-quarks in the proton lies in the non-perturbative structure of the proton. We explain how the features expected in a modern, relativistic and chirally symmetric description of nucleon structure naturally explain the current data. The consequences of this explanation for the presence of orbital angular momentum on quarks and gluons is reviewed and comparison made with recent results from lattice QCD and experimental data.

The spin of the proton

The twenty years since the announcement of the proton spin crisis by the European Muon Collaboration has seen tremendous progress in our knowledge of the distribution of spin within the proton. The problem is reviewed, beginning with the original data and the suggestion that polarized gluons may play a crucial role in resolving the problem through the U ( 1 ) axial anomaly. The discussion continues to the present day where not only have strong limits been placed on the amount of polarized glue in the proton but the experimental determination of the spin content has also become much more precise. It is now clear that the origin of the discrepancy between the experiment and the naive expectation of the fraction of spin carried by the quarks and anti-quarks in the proton lies in the non-perturabtive structure of the proton. We explain how the features expected in a modern, relativistic and chirally symmetric description of nucleon structure naturally explain the current data.

Contribution of the hadronic component of a virtual photon to the structure function for charm leptoproduction at high x and Q 2

The contribution of the hadronic component of a virtual photon to the structure function for charm leptoproduction is calculated. This contribution comes from the scattering of c quarks of a virtual photon on proton quarks and gluons. A comparison of the results of our calculations for this structure function with relevant data obtained by the European Muon Collaboration (EMC) for μ+p scattering gives reasons to believe that the contribution of the resolved photon may be responsible for the excess in these EMC data over the predictions of the model of photon-gluon fusion at high momentum transfers. Therefore, it may become possible to describe the EMC data without resort to the hypothesis of a nonperturbative charmed-quark admixture in the proton wave function (intrinsic-charm hypothesis).

Nucleon spin structure functions in the chiral quark model

We show that the nucleon spin structure functions, as extracted from the recent experiments carried out by the European Muon Collaboration (EMC), the SLAC E143 and E154 Collaborations, and the Spin Muon Collaboration (SMC), can be understood quantitatively in the chiral quark model, both for the measured first moment Γ 1= ∫ 0 1 g 1(x)dx and the functional form (or the shape) of the spin structure function g 1( x) (as a function of the Bjorken scaling variable x). The success suggests strongly that the nonperturbative effects due to Goldstone bosons play a significant role, and should be taken into account, in the kinematic range of the momentum transfer squared Q 2 of several GeV 2.

Suppression of Nuclear Drell-Yan Ratio due to Energy Loss

Nuclear effect in nuclear Drell-Yan (DY) process is not clear as that in the European muon collaboration (EMC) effect, and many EMC effect models overestimate it. In this letter, the energy loss effect in nuclear DY process is studied, and it is found that the nuclear DY ratio is suppressed due to the energy loss which balances the overestimate of the DY ratio in the EMC effect models.

Mixed Rescaling Model and Direct Photon Production

Using the mixed rescaling model, the main features of the European Muon Collaboration effect can be explained. Then the ratio of cross sections, free proton(p)-bound nucleon of Iron(Fe) to free proton-free proton, for large-PT direct photon production is calculated.

Where Does the Proton Really Get Its Spin?

In 1987 the European Muon Collaboration, which had been scattering muons off polarized protons at CERN, shocked the particle physics community with the announcement that little or none of the proton's spin can be attributed to the spins of its three constituent quarks—two “up” quarks and one “down” quark. That report precipitated what became known as “the spin crisis.”

Extraction of the ratio of the neutron to proton structure functions from deep inelastic scattering

We study the nuclear ( A) dependence of the European Muon Collaboration (EMC) effect at high values of x ( x ≥ 0.6). Our approach makes use of conventional nuclear degrees of freedom within the Relativistic Impulse Approximation. By performing a nonrelativistic series expansion we demonstrate that relativistic corrections make a substantial contribution to the effect at x ≳ 0.6 and show that the ratio of neutron to proton structure functions extracted from a global fit to all nuclei is not inconsistent with values obtained from the deuteron.

Strange Sea Polarization in the Proton and a Constituent Quark Model

The valence-quark and q q -sea polarizations in the polarized proton are calculated in the frame­ work of a constituent quark model. It is pointed out that the configuration mixing of the symmetric three-quark state with the quark-diquark state gives a good fit to existing data on the strange sea polarization in the proton. Recently, the Spin Muon Collaboration (SMC) reported on the measurement of the spin-dependent structure function of the deuteron 1 > and pointed out that its first moment is smaller than the prediction of the Ellis-Jaffe sum rule 2 > as already found by the European Muon Collaboration (EMC). 3 > Namely, they conclude that the quarks do not carry the spin of the nucleon and the strange sea polarization is large and negative. On the other hand, the SLAC group (E142) 4 > reported the opposite result from the measurement at Q 2 =2 GeV 2 , i.e., the guarks carry half the nucleon spin and the strange sea polarization is consistent with the one expected by the naive parton model. 2 > This apparent discrepancy is an open question. In this paper, we calculate the valence-quark and qq-sea polarizations in the polarized proton in the framework of a constituent quark model. 5 >,G> It is shown that the amount of the strange sea polarization is strongly dependent on the structure of constituents and the SUA3) symmetry breaking parameter in the qq sea polarization. It is pointed out that the configuration mixing of the simple three-quark state with the quark-diquark state gives a good fit to existing data on the strange sea polarization in the proton below Q 2 =12 GeV 2 • We allow a distinction between the valence and sea quarks in the proton with positive helicity. For the up- and down-quark polarizations, we write Llu=Lluv+ Llus and Lld=Lldv+Llds where Llu(Lld) denotes the polarization carried by the u quark (d quark). The subscripts V and S denote valence and sea, respectively. We introduce the valence-quark spin l:v(=Lluv+Lldv).7) We assume the isospin invariance of the sea-quark with respect to the up- and down-quark but the broken SUf(3) symmetry so that

A microscopic understanding of the structure functions of finite nuclei

A number of relatively successful quark model descriptions of nuclear matter have recently been developed, based on a mean-field description of non-overlapping nucleon bags bound by the self-consistent exchange of scalar (σ) and vector (ω) mesons. The meson fields modify the internal quark motion in the nucleon bag in medium and this induces a saturation mechanism for nuclear matter. The model gives a nuclear compressibility around 220 MeV and provides a rather satisfactory interpretation of nuclear matter properties. Furthermore, it has recently been shown that the twist-2 valence quark distributions of the free nucleon calculated in the MIT bag model are in reasonable agreement with world data. By using a local density approximation we combine these developments to microscopically investigate the structure functions of finite nuclei. The effect of nuclear shadowing and antishadowing is taken into account phenomenologically. The model can quantitatively reproduce the existing experimental data - the so-called European Muon Collaboration (EMC) effect. We show that the quark degrees of freedom in nuclei and the final state interaction between the debris of the struck nucleon and the residual nucleus both play important roles in the nuclear structure functions. Thus the conventional treatment based on nucleon binding within the impulse approximation is quantitatively unreliable. We also study the effects of the self-coupling terms of the σ field and nucleon-nucleon correlations on some nuclear matter properties and the nuclear structure functions.

Proton spin structure functions and generalized Sullivan processes.

We employ the time-ordered perturbation field theory in the infinite momentum limit, as developed extensively by Drell, Levy, and Yan, to derive the convolution formulas for the proton spin structure functions ${g}_{1}(x,{Q}^{2})$ and ${g}_{2}(x,{Q}^{2})$ suitable for the generalized Sullivan processes, in which the meson in the cloud or the recoiling baryon is struck and smashed by the virtual photon. Following a previous suggestion that the sea quark distributions of a nucleon at low and moderate ${Q}^{2}$ may be attributed primarily to such Sullivan processes, we use the convolution formulas to obtain the first moments ${\ensuremath{\Gamma}}_{p}\ensuremath{\equiv}\ensuremath{\int}{0}^{1}{g}_{1}^{p}(x,{Q}^{2})dx$ and ${\ensuremath{\Gamma}}_{n}\ensuremath{\equiv}\ensuremath{\int}{0}^{1}{g}_{1}^{n}(x,{Q}^{2})dx$, as well as to investigate ${g}_{1}^{p}(x,{Q}^{2})$ and ${g}_{1}^{n}(x,{Q}^{2})$ as a function of $x$. Our results indicate that both the data on ${g}_{1}^{p}(x,{Q}^{2})$ obtained previously by the European Muon Collaboration and the recent data on the deuteron spin structure function ${g}_{1}^{d}(x,{Q}^{2})$ obtained by the Spin Muon Collaboration can be understood and the Bjorken sum rule is satisfied. A comparison between our prediction on ${g}_{1}^{n}(x,{Q}^{2})$ and the recent SLAC data is also given.

New approach to polarized deep inelastic scattering

We show, in a simple way, that the flavor asymmetry in the light-quark sea of the proton discovered by the New Muon Collaboration suggests also a large flavor asymmetry for the corresponding polarized distributions. This accounts for the violation of the Ellis-Jaffe proton sum rule reported by the European Muon Collaboration, but it implies no violation of the Ellis-Jaffe neutron sum rule, which seems confirmed from the very recent SLAC data.

Unified Description of the EMC Effect and Nuclear Shadowing Effect

The parton evolution model is improved by using the modified Alteralli-Parisi equation which can describe the nuclear shadowing and antishadowing effects in the small x region. Within the theoretical framework of the improved parton evolution model, the unified description of the European Muon Collaboration (EMC) effect, and nuclear shadowing and antishadowing effects is given without introducing the nuclear shadowing factor.

The proton spin and the Wigner rotation

It is shown that in both the gluonic and strange sea explanations of the Ellis-Jaffe sum rule violation discovered by the European Muon Collaboration (EMC), the spin of the proton, when viewed in in its rest reference frame, could by fully provided by quarks and antiquarks within a simple quark model picture, taken into account the relativistic effect from the Wigner rotation.

Shadowing and antishadowing of the photon structure function of the hydrogen molecule.

The photon structure function of the hydrogen molecule is calculated using the known approximate nonrelativistic wave function and the photon-emission vertex of the canonical light-cone Hamiltonian of quantum electrodynamics. The obtained photon structure function is compared to the photon structure function of the hydrogen atom. Typical European Muon Collaboration (EMC)-like effects such as shadowing for small [ital x][sub [ital B][ital j]] and antishadowing for larger [ital x][sub [ital B][ital j]] are found. Shadowing saturates for smallest [ital x][sub [ital B][ital j]] and depends only weakly on resolution [Lambda][sup 2]. For larger [ital x][sub [ital B][ital j]] the EMC ratio of the structure functions oscillates. We find that the period is inversely proportional to the proton-proton distance in the hydrogen molecule. Also the saturation of shadowing for low [ital x][sub [ital B][ital j]] is understood analytically. Qualitatively the calculated effects in photon structure function of the H[sub 2] molecule are similar to those observed in the quark structure function measured in deep inelastic scattering of leptons on nuclei.

Direct-photon production in heavy-ion collisions.

To study plasma photons in relativistic heavy-ion collisions one must account for direct photons from all possible sources. The aim of this work is the examination of the effect that initial-state parton distributions in nuclei, distorted to accommodate the European Muon Collaboration (EMC) effect, will have on the direct-photon spectrum produced relative to the basic QCD subprocesses in free nucleon-nucleon collisions. Since this parton distortion has been quite satisfactorily described by an expansion of the nuclear state in multiquark color-singlet (cluster) basis states, this «quark cluster model» should give useful predictions for the reflection of the EMC effect in the nucleus-nucleus direct photons

Heavy-quark and lepton-pair production on nuclei.

We present a description of the [ital A] dependence of heavy-quark and Drell-Yan pair production in hadron-nucleus collisions, based on the Reggeon approach to strong interactions at high energies and the parton model. The same method has been successfully applied to the production of light-quark states. The existence of a new energy scale, which depends on the mass and [ital x][sub [ital F]] of the heavy system, is emphasized. A phenomenological model, which includes both the shadowing corrections to the nucleon structure function and the rescattering of the heavy-quark states, is proposed. The model quantitatively describes the available experimental data on [ital J]/[psi] and heavy lepton-pair production in [ital hA] collisions as well as the European Muon Collaboration effect at low [ital x].