Knockout of charged <Emphasis Type="Italic">ρ</Emphasis> mesons from a nucleon by high-energy electrons in the quasielastic-kinematics region with the participation of transverse virtual photons

The electroproduction of charged ρ mesons on a nucleon under conditions of quasielastic kinematics (Q ² = 2−3 (GeV/c)², W = 2 GeV) is considered. Dominant diagrams are taken into account. It is shown that the transverse cross section for ρ-meson knockout, in just the same was as the respective longitudinal cross section, is dominated by the t-pole diagram involving a virtual ρ meson. This means that the leading contribution to the process comes from direct ρ-meson knockout.     

K*0 and φ meson production in proton–nucleus interactions at ?s=41.6\textGeV\sqrt{s}=41.6\text{GeV}

The inclusive production cross sections of the strange vector mesons K^*0, K̄^*0, and φ have been measured in interactions of 920 GeV protons with C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of rapidity and transverse momentum have been measured in the central rapidity region and for transverse momenta up to p_T = 3.5 GeV/c. The atomic number dependence is parametrised as σ_pA=σ_pN*A^α, where σ_pN is the proton–nucleon cross section. Within the phase space accessible, α(K^*0)=0.86±0.03, α(K̄^*0)=0.87±0.03, and α(φ)=0.96±0.02. The total proton–nucleon cross sections, determined by extrapolating the differential measurements to full phase space, are σ_pN→K*0=(5.06±0.54) mb, σ_pN→K̄*0=(4.02±0.45) mb, and σ_pN→φ=(1.17±0.11) mb. For all resonances the Cronin effect is observed; compared to the measurements of Cronin et al. for K^± mesons, the measured values of α for φ mesons coincide with those of K⁺ mesons for all transverse momenta, while the enhancement for K^*0/K̄^*0 mesons is smaller.     

Three-dimensional parton distribution functions g1T and $h_{1L}^{\perp}$ in the polarized proton–antiproton Drell–Yan process

We present predictions of the unweighted and weighted double spin asymmetries related to the transversal helicity distribution g _1T and the longitudinal transversity distribution h_1L^{^}h_{1L}^{\perp}, two of eight leading-twist transverse momentum dependent parton distributions (TMDs) or three-dimensional parton distribution functions (3dPDFs), in the polarized proton–antiproton Drell–Yan process at typical kinematics on the Facility for Antiproton and Ion Research (FAIR). We conclude that FAIR is ideal to access the new 3dPDFs toward a detailed picture of the nucleon structure.     

Cosmic-ray muons at ultrahigh energies

Fluxes of cosmic-ray muons were estimated over the energy range extending up to 10¹⁰ GeV. Data on the production of pions; kaons; η, η′, ρ, ω, and ϕ mesons; charmed particles; and J/ψ mesons from accelerator experiments devoted to studying interactions between nucleons and nuclei of air atoms up to nucleon energies of about 2 × 10⁴ GeV were used in respective calculations. At higher energies, use was made of the results obtained by calculating cross sections for the production of charmed particles and J/ψ mesons on the basis of QCD models. It is shown that, in interpreting experimental data on extensive air showers, the production of both charmed particles and J/ψ mesons in the Earth’s atmosphere must be taken into account.     

Double spin asymmetry in exclusive ρ0 muoproduction at COMPASS

The longitudinal double spin asymmetry A₁ ^ρ for exclusive leptoproduction of ρ⁰ mesons, μ+N→μ+N+ρ, is studied using the COMPASS 2002 and 2003 data. The measured reaction is incoherent exclusive ρ⁰ production on polarised deuterons. The Q² and x dependence of A₁ ^ρ is presented in a wide kinematical range, 3×10^-3<Q²< 7 (GeV/c)² and 5×10^-5<x<0.05. The results presented are the first measurements of A₁ ^ρ at small Q² (Q²< 0.1 (GeV/c)²) and small x (x<3×10^-3). The asymmetry is in general compatible with zero in the whole kinematical range. PACS 13.60.Le; 13.88.+e     

Electroproduction of <Emphasis Type="Italic">ρ</Emphasis><Superscript>0</Superscript> mesons on protons in quasielastic kinematics at intermediate energies and spin-flip mechanism of direct meson knockout

It is shown that the amplitude for the direct knockout of ρ ⁰ mesons plays an important role at energies W above the resonance region, W ≳ 2 GeV, and rather high values of Q ² ≳ 1.5−2 GeV²/c ² and that it corresponds to the t pole in the channel of the virtual decay p → p + π ⁰ accompanied by quark-spin flip upon momentum transfer to the meson, π ⁰ + γ* _T → ρ ⁰. The contributions of several scalar mesons (p → p + f ₀ channel), the contribution of the tensor meson f ₂, and effects of the interference between different contributions were taken into account in addition to the contribution of the π ⁰ meson. The vectorand tensor-meson-dominance models were used to estimate the respective vertex constants. Within the proposed mechanism, the differential cross sections dσ _L /dt and dσ _T /dt were calculated for several W and Q ² values at which the respective total cross sections were measured by the CLAS Collaboration. Agreement with data on the transverse part of the total cross section, σ _T , was attained, whereby the assumption of a dominant role of magnetic spin transitions in the meson skin of the nucleon under conditions of quasielastic-knockout kinematics is confirmed. At the same time, the contribution of spin-flip transitions is suppressed in the longitudinal part of the cross section, σ _L , and this is confirmed by the results of our calculations. The behavior of the differential cross section dσ _T /dt is predicted with an eye to future experiments.     

Transversity of quarks in a nucleon

The transversity distribution of quarks in a nucleon is one of the three fundamental distributions, that characterize nucleon's properties in hard scattering processes at leading twist (twist 2). It measures the distribution of quark transverse spin in a nucleon polarized transverse to its (infinite) momentum. It is a chiral-odd twist-two distribution function — gluons do not couple to it. Quarks in a nucleon/hadron are relativistically bound and transversity is a measure of the relativistic nature of bound quarks in a nucleon. In this work, we review some important aspects of this less familiar distribution function which has not been measured experimentally so far.     

Diffractive production of two ρ<Superscript>0</Superscript><Subscript>L</Subscript> mesons in e<Superscript>+</Superscript>e<Superscript>-</Superscript> collisions

We present an estimate of the cross-section for the exclusive production of a ρ_L ⁰-meson pair in e⁺e^- scattering, which will be studied in the future high-energy International Linear Collider. For this aim, we complete calculations of the Born order approximation of the amplitudes γ^* _L,T(Q₁ ²)γ^* _L,T(Q₂ ²)→ρ_L ⁰ρ_L ⁰, for arbitrary polarization of virtual photons and longitudinally polarized mesons, in the kinematical region s≫-t,Q₁ ²,Q₂ ². These processes are completely calculable in the hard region Q₁ ²,Q₂ ²≫Λ² _QCD, and we perform most of the calculations in an analytical way. The resulting cross-section turns out to be large enough for this process to be measurable with foreseen luminosity and energy, for Q₁ ² and Q₂ ² in the range of a few GeV².     

ωN final state interactions and ω-meson production from heavy-ion collisions

We calculate the elastic and inelastic ωN→ωN, →πN, →ρN, →ρπN, →ππN, →σN reactions within a boson exchange approximation where the ωρπ coupling constant and form factor are fixed by the reaction πN→ωN in comparison to the experimental data. We find rather large ωN cross sections at low relative momenta of the ω-meson which leads to a substantial broadening of the ω-meson width in nuclear matter. The implications of the ωN final state interactions are studied for ω production in ¹²C +¹²C, ⁴⁰Ca +⁴⁰Ca and ⁵⁸Ni +⁵⁸Ni reactions at about 2 · A GeV within the HSD transport approach; the drastic changes of the transverse mass spectra relative to a general m _T-scaling (for π⁰ and η mesons) might be controlled experimentally by the TAPS Collaboration. Received: 28 April 1999 / Revised version: 7 June 1999     

The longitudinal cross section of vector meson electroproduction

We analyze electroproduction of light vector mesons (V=ρ,φ and ω) at small Bjorken-x in the handbag approach, in which the process factorizes into general parton distributions and partonic subprocesses. The latter are calculated in the modified perturbative approach in which the transverse momenta of the quark and antiquark forming the vector meson are retained and Sudakov suppression is taken into account. Modeling the generalized parton distributions through double distributions and using simple Gaussian wavefunctions for the vector mesons, we compute the longitudinal cross sections at large photon virtualities. The results are in fair agreement with the findings of recent experiments performed at HERA and HERMES.     

Exclusive meson production at HERMES

Hard exclusive production of mesons in deep-inelastic scattering allows one to probe the so-far unknown Generalized Parton Distributions (GPDs) of the nucleon. The HERMES experiment has measured several different observables in exclusive meson production by scattering the 27.6GeV HERA lepton beam off an internal fixed gaseous target. Recent results on exclusive π ⁺, ρ ⁰ and pion pair production will be presented.     

Next-to-leading order corrections in exclusive meson production

We analyze in detail the size of next-to-leading order corrections to hard exclusive meson production within the collinear factorization approach. Corrections to the cross section are found to be huge at small x_B and substantial in typical fixed-target kinematics. With the models we take for nucleon helicity-flip distributions, the transverse target polarization asymmetry in vector meson production is strongly affected by radiative corrections, except at large x_B. Its overall size is very small for ρ production but can be large in the ω channel. PACS 12.38.Bx; 13.60.Le     

Composite-meson model and meson exchange currents

We study the meson exchange currents (MEC) mechanism for the pion double-charge exchange (DCX) reaction in a composite-meson model. The model assumes that the mesons are two-quark systems and can interact with each other only through quark loops. The contributions of the ρ, σ, and f₀ mesons, the four-quark box diagram as well as a contact diagram has been taken into account. It is shown that the contribution of the ρ, σ, and f₀ mesons increases the forward scattering cross-section in an average by 25% and decreases with energy.     

Can the total angular momentum of u-quarks in the nucleon be accessed at HERMES?

We investigate the possibility of acquiring information on the generalized parton distribution E and, through a model for E, also on the u-quark total angular momentum J_u by studying deeply virtual Compton scattering and hard exclusive ρ⁰ electroproduction on a transversely polarized hydrogen target at HERMES. It is found that a change in J_u from zero to 0.4 corresponds to a 4σ (2σ) difference in the calculated transverse target-spin asymmetry in deeply virtual Compton scattering (ρ⁰ electroproduction), where σ is the total experimental uncertainty. PACS 12.38.Bx, 13.60.Le     

The comparison of characteristics of π− mesons produced in central Mg-Mg interactions with the quark gluon string model predictions

A detailed study of pion production in central Mg-Mg collisions at a momentum of 4.3 GeV/c per incident nucleon was carried out with use of the setup GIBS. The average kinematical characteristics of pions (multiplicity n ₋, momentum P, transverse momentum P _T, emission angle Θ, rapidity Y) and corresponding distributions have been obtained. The experimental results have been compared with the predictions of the Quark Gluon String Model (QGSM) and satisfactory agreement between the experimental data and the model has been found. The QGSM reproduces also the dependence of average P _T on n ₋. The temperatures of π⁻ mesons have been estimated in the rapidity interval of 0.5 ≤ Y ≤ 2.1. A satisfactory fit for π⁻ mesons has been achieved by using a form involving two temperatures T ₁ and T ₂. It is found that the QGSM underestimates T ₂ by (10−15)%. The data have been analyzed using the transverse momentum technique. The observed dependence of the <P _x ^′(Y)> on Y shows the S-shape behaviour. The slope at midrapidity F has been determined. The QGSM reproduces the <P _x> distribution satisfactorily, but underestimates the parameter F. Received: 2 August 1997 / Revised version: 17 November 1997     

Diffractive photoproduction of D*±(2010) at HERA

Diffractive photoproduction of D^*±(2010) mesons was measured with the ZEUS detector at the ep collider HERA, using an integrated luminosity of 78.6 pb^-1. The D^* mesons were reconstructed in the kinematic range: transverse momentum p_T(D^*) > 1.9 GeV and pseudorapidity |η(D^*)|<1.6, using the decay D^*+→D⁰π⁺ _s followed by D⁰→K^-π⁺(+c.c.). Diffractive events were identified by a large gap in pseudorapidity between the produced hadronic state and the outgoing proton. Cross sections are reported for photon–proton centre-of-mass energies in the range 130 < W < 300 GeV and for photon virtualities Q² < 1 GeV², in two ranges of the Pomeron fractional momentum x_IP<0.035 and x_IP<0.01. The relative contribution of diffractive events to the inclusive D^*±(2010) photoproduction cross section is about 6%. The data are in agreement with perturbative QCD calculations based on various parameterisations of diffractive parton distribution functions. The results are consistent with diffractive QCD factorisation.     

Probing meson spectral functions with double differential dilepton spectra in heavy-ion collisions

The double differential dilepton spectrum d/(d ² d _⊥ ²) at fixed transverse mass M _⊥ allows a direct access to the vector meson spectral functions. Within a fireball model the sensitivity of d/(d ² d _⊥ ²) against variations of both the in-medium properties of mesons and the dynamics of the fireball is investigated. In contrast to the integrated invariant-mass spectrum d/d ², in the spectrum d/(d ² d _⊥ ²) with fixed M _⊥ the ω signal is clearly seen as bump riding on the ρ background even in case of strong in-medium modifications.[3mm] Received: 16 November 2000 / Accepted: 16 January 2001     

Flavour structure of low-energy hadron-pair photoproduction

We consider the process γγ→H₁H̄₂, where H₁ and H₂ are either mesons or baryons. The experimental findings for such quantities as the pp̄ and K_SK_S differential cross sections, in the energy range currently probed, are found often to be in disparity with the scaling behaviour expected from hard constituent scattering. We discuss the long-distance pole–resonance contribution in understanding the origin of these phenomena, as well as the amplitude relations governing the short-distance contribution, which we model as a scaling contribution. When considering the latter, we argue that the difference found for the K_SK_S and the K⁺K^- integrated cross sections can be attributed to the s-channel isovector component. This corresponds to the ρω→a subprocess in VMD (vector-meson-dominance) language. The ratio of the two cross sections is enhanced by the suppression of the φ component, and it is hence constrained. We give similar constraints to a number of other hadron-pair production channels. After writing down the scaling and pole–resonance contributions respectively, the direct summation of the two contributions is found to reproduce some salient features of the pp̄ and K⁺K^- data. PACS 11.30.Hv; 12.40.-y; 12.40.Nn; 12.40.Vv; 13.66.Bc     

Highlights and perspectives of the JLab spin physics program

Nucleon spin structure has been an active, exciting and intriguing subject of interest for the last three decades. Recent precision spin-structure data from Jefferson Lab have significantly advanced our knowledge of nucleon structure in the valence quark (high-x) region and improved our understanding of higher-twist effects, spin sum rules and quark-hadron duality. First, results of spin sum rules and polarizabilities in the low to intermediate Q ² region are presented. Comparison with theoretical calculations, in particular with Chiral Perturbation Theory (ChPT) calculations, are discussed. Surprising disagreements of ChPT calculations with experimental results on the generalized spin polarizability, δ _LT, were found. Then, precision measurements of the spin asymmetry, A ₁, in the high-x region are presented. They provide crucial input for global fits to world data to extract polarized parton distribution functions. The up and down quark spin distributions in the nucleon were extracted. The results for Δd/d disagree with the leading-order pQCD prediction assuming hadron helicity conservation. Results of precision measurements of the g ₂ structure function to study higher-twist effects are presented. The data indicate a significant higher-twist (twist-3 or higher) effect. The second moment of the spin structure functions and the twist-3 matrix element d ₂ results were extracted. The high Q ² result was compared with a Lattice QCD calculation. Results on the resonance spin-structure functions in the intermediate Q ² range are presented, which, in combination with DIS data, enable a detailed study of quark-hadron duality in spin-structure functions. Finally, an experiment to study neutron transversity and transverse spin asymmetries is discussed. A future plan with the 12 GeV energy upgrade at JLab is briefly outlined.     

Measurement of light mesons at RHIC by the PHENIX experiment

The PHENIX experiment at RHIC has measured a variety of light neutral mesons (π ⁰, K _S ⁰, η, ω, η ^′, φ) via multi-particle decay channels over a wide range of transverse momentum. A review of the recent results on the production rates of light mesons in p + p and their nuclear modification factors in d + Au, Cu + Cu and Au + Au collisions at different energies is presented.