Measurement of elastic ω photoproduction at HERA ZEUS Collaboration

The reaction γp →ωp(ω → π⁺π^?π⁰ and π⁰ → γ γ) has been studied in ep interactions using the ZEUS detector at photon-proton centre-of-mass energies between 70 and 90GeV and ∣t∣ < 0.6GeV², where t is the squared four momentum transferred at the proton vertex. The elastic ω photoproduction cross section has been measured to be σ_{γp → ωp} = 1.21 ±0.12 ±0.23 μb. The differential cross section dσ_γp→ωp/d¦t¦ has an exponential shape e^?b∣t∣ with a slope b =10.0 ± 1.2 ± 1.3 GeV^?2. The angular distributions of the decay pions are consistent with s-channel helicity conservation. When compared to low energy data, the features of ω photoproduction as measured at HERA energies are in agreement with those of a soft diffractive process. Previous measurements of the ρ⁰ and ? photoproduction cross sections at HERA show a similar behaviour.     

Theoretical study of deuteron polarization in γ+d → π0+d

The state of polarization of the recoil deuteron in γ+d → π⁰+d on unpolarized and target polarized deuterons is analyzed in detail. A new set of statistical tensors is introduced to describe non-oriented systems, such as the recoil deuteron in γ+d → π⁰+d, and their relationships to the vector and tensor polarization parameters are established. The new statistical tensors reduce, in the case of oriented systems, to Fano's statistical tensors. Numerical results are computed using two different deuteron models for polarized photon asymmetry, polarized target asymmetry and the vector and tensor polarizations of the recoil deuteron, and these are presented at various c.m. pion production angles at different energies in the region of the first π-N resonance. The study of these parameters is suggested as a promising source of information on isoscalar pion photoproduction amplitudes.     

Can HERA data improve the LEP constraints on the WWV vertex?

The combined results from ep → νWX, ep → eWX, and ep → νγX processes at HERA are used to constrain anomalous three-boson couplings. The effective model for anomalous couplings where there is no light Higgs boson and where interactions responsible for the breakdown of electroweak symmetry are strongly coupled is considered. Bounds on the couplings L _9L and L _9R , which parametrize contributions from the anomalous WWγ (WWZ) vertices, attainable from an analysis of the distributions for the processes in question are presented. The results are compared with the bounds resulting from the LEP I and LEP II data. It is shown that the bounds coming from HERA significantly reduce the parameter region allowed by the analysis of the LEP I and LEP II data.     

Cross-section asymmetries in vector-meson electroproduction and in inelastic ep scattering with polarized beam and target

We discuss the cross-section asymmetries for forward diffractive vector meson (ρ⁰) production by inelastic scattering of longitudinally polarized electrons (or muons) on polarized protons. Only for the case when both the beam and the target are polarized can considerable effects be expected. On the basis of the off-diagonal GVDM we give relations between the cross section asymmetries in ep → epρ⁰ (forward direction) and in ep → eX. For a longitudinally polarized target the cross-section asymmetries in ep → epρ⁰ are predicted to be roughly twice as large as in ep → eX.     

Deeply virtual compton scattering at HERMES

Generalized Parton Distributions(GPDs)provide a way to access total angular momenta of partons and give a multidimensional picture of the nucleon structure.Deeply Virtual Compton Scattering(DVCS)is the most direct exclusive process to study GPDs.Different azimuthal cross-section asymmetries with respect to beam helicity,beam charge,and target polarization have been measured in the HERMES experiment.A recoil detector was installed at HERMES to directly detect the recoil proton.     

Photoproduction of π+ mesons on polarized protons

The asymmetry of the cross sections for the photoproduction of π⁺ mesons on polarized protons γ + p↑ → π⁺ + n has been studied in the four-momentum transfer range 0.1 ? |t| ? 1.25 (GeV/c)² for photon energies of 2.5, 3.4 and 5.0 GeV. The measurements were carried out on a polarized butanol target. Both particles in the final state were detected: the pion by a magnetic spectrometer, the recoil nucleon in a scintillation counter matrix. The asymmetry was found to be negative with values around ?0.4.     

Evaluation of the Integrated Cross Sections of Reactions of Exclusive Pion and Kaon Electroproduction off Protons in the Resonance Region at Photon Virtualities from 5 to 12 GeV<Superscript>2</Superscript>

The integrated cross sections of the ep → eπ⁺n, ep → eπ⁰p, ep → eK⁺Λ, and ep → eK⁺Σ⁰ reactions are evaluated in the energy range of nucleon resonance excitation at photon virtualities Q² from 5 to 12 GeV². These exclusive channels will be explored at photon virtualities Q² > 5 GeV² for the first time in future experiments with the CLAS12 detector. The cross-section evaluation is based on the extrapolation of exclusive contributions to the inclusive structure functions F₁ and F₂ from a region of Q² < 5 GeV², in which the experimental data are available, to the region of higher Q². This evaluation of cross sections is of particular importance in the development of the program of experiments with the CLAS12 detector for studying the structure of the ground and excited nucleon states, which may reveal the dynamics of strong interactions in the nonperturbative regime.     

The ZEUS Leading Proton Spectrometer and its use in the measurement of elastic ρ0 photoproduction at HERA

The differential cross section dσ/dt for elastic ρ⁰ photoproduction, γp → ρ⁰p (ρ⁰ → π⁺π^?), has been measured in ep interactions at HERA. The squared four-momentum exchanged at the proton vertex, t, has been determined directly by measuring the momentum of the scattered proton using the ZEUS Leading Proton Spectrometer (LPS), a large scale system of silicon micro-strip detectors operating close to the HERA proton beam. The LPS allows the measurement of the momentum of high energy protons scattered at small angles with accuracies of 0.4% for the longitudinal momentum and 5 MeV for the transverse momentum. Photoproduction of ρ⁰ mesons has been investigated in the interval 0.073 < ¦t¦< 0.40 GeV², for photon virtualities Q² < 1 GeV² and photon-proton centre-of-mass energies W between 50 and 100 GeV. In the measured range, the t distribution exhibits an exponential shape with a slope parameter b = 9.8 ± 0.8 (stat.) ±1.1 (syst.) GeV^?2. The use of the LPS eliminates the contamination from events with diffractive dissociation of the proton into low mass states.     

Influence of the transverse beam sizes on theep→epγ cross section at the HERA and a FUTURE CERN electron-proton collider

In the processep→epγ, proposed for luminosity measurements at HERA, impact parameters occur which are larger than the transverse beam sizes in theep-colliders of HERA and a CERN option (LHC+LEP). This decreases the number of observed photons compared to the standard QED calculation. The difference is larger than 10% at photon energiesE _γ<0.4E _e for the CERN option andE _γ<0.01E _e for HERA.     

K*0Σ+ Photoproduction from Threshold to 3.0 GeV

The CLAS collaboration and the CBELSA/TAPS collaboration reported the differential cross section data for K*⁰ photoproduction, and found that the t-channel K exchange dominates the production amplitudes at small scattering angles. The measurement of forward-angle parity spin asymmetry for the γ p → K*⁰Σ⁺ reaction, where the theoretical calculations show an unambiguous signal, would establish the role of the κ(800). The γ p → K*⁰Σ⁺ reaction, where the Σ⁺ was identified through missing mass, on a liquid hydrogen target was measured at the LEPS detector in the energy from threshold to 3.0 GeV by using a linearly polarized photon beam at SPring-8. Decay angular distributions in the rest frame of the K ⁺ π ^? system were fit to extract spin-density matrix elements of the K*⁰ decay.     

The process e + e − → ωπ0 near the Φ resonance

The reaction e ⁺ e ^?→ωπ⁰ near a Φ resonance was studied with a spherical neutral detector at VEPP-2M e ⁺ e ^? collider. Both main modes of decay of a ω meson were investigated: ω→π⁺π^?π⁰ and ω→π ⁰γ. The probability of decay Φ→ωπ⁰ was obtained from the magnitude of the interference wave in the cross section for the reaction e ⁺ e ^?→ωπ⁰→ π⁺π^?π⁰π⁰: B(Φ→ωπ⁰) = (5.5 _?1.4 ^+1.6 ±0.3) ×10^?5. The ratio of the partial widths of the ω meson was obtained from the ratio of the cross sections for the two modes: Γ(ω→π⁰γ )/Γ(ω→π⁺π^?π⁰) = 0.0994 ± 0.0036 ± 0.0038.     

Chiral anomaly in Kγ → Kπ reactions

Implications of the chiral anomaly for the cross sections of the reactions K ⁺γ → K ⁺π⁰, K ⁰γ → K ⁰π⁰, K ⁺γ → K ⁰π⁺, and K ⁰γ → K ⁺π^? are investigated. Near the threshold, the contribution of the chiral anomaly dominates the cross sections for the first and the second reaction. These cross sections are compared with the cross sections for the third and the fourth reaction, which receive no contribution from the chiral anomaly. Prospects for an experimental study of the KKπγ vertex in the Coulomb production of π mesons by a K-meson beam are discussed.     

Dispersion theory of parity non-conservation in pion-nucleon scattering at low energies

We examine the possibility for testing the weak parity non-conserving (pnc) interaction in pion-nucleon scattering at low energies. We start out analysis by assuming the existence of a pnc πNN coupling which conserves time reversal invariance and has the isospin selection rule ΔI = 1, and the existence of fixed-t dispersion relations obeyed by the pnc invariant amplitudes. By using the nonrelativistic expansion, we then derive dispersion relations for the partial-wave amplitudes.We calculate parity-odd observables in π^?p → π^?p and in π^?p → π⁰n, e.g., the up-down asymmetry α in the total cross section for a polarized target and the longitudinal polarization P of the recoil nucleon. In the conventional Cabibbo model for the weak interaction P and α are found to be of order 10^?7 to 10^?8.We have also examined the t-channel pnc ?-exchange process which has the isospin selection rule ΔI = 0, 2 and found it to contribute dominantly to P in the charge-exchange channel π^?p → π⁰n.     

Photoproduction of jets and heavy flavors at future polarized ep-colliders

We study photoproduction of jets and heavy flavors in a polarized collider mode of HERA and in polarized ep collisions at √S ≈ GeV. We examine the sensitivity of the cross sections and their asymmetries to the proton’s polarized gluon distribution and to the completely unknown parton distributions of longitudinally polarized photons.     

Measurement of the e + e − → ηπ+π− cross section in the \sqrt s = 1.04–1.38-GeV energy range with a spherical neutral detector at the VEPP-2M collider

The e ⁺ e ^? → ηπ⁺π^? cross section in the η → 2γ channel has been measured in the $ \sqrt s $ = 1.04- to 1.38-GeV energy range. The data with a total integrated luminosity of 9.05 pb^?1 were collected on a spherical neutral detector at the VEPP-2M collider. The results are consistent with the BaBar and CMD2 data and have a higher statistical accuracy.     

Search for e + e − → f 0(600)γ, f 0(980)γ, f 0(1350)γ, and f 2(1270)γ processes in the energy range from 1.05 to 1.38 GeV

In the energy range from 1.05 to 1.38 GeV, the upper limits for the cross sections of e ⁺ e ^? → f ₀(600)γ, f ₀(980)γ, f ₀(1350)γ, and f ₂(1270)γ → π⁰π⁰γ processes have been established. Measurements are performed in accordance with the statistics gathered in experiment with a spherical neutral detector (SND) on the VEPP-2M e ⁺ e ^? collider. The resultant upper limits vary from 6 to 42 pb depending on the energy and the model.     

Polarization of recoil neutrons from single-pion photoproduction off protons in the resonance region

The polarization of the recoil neutrons from the reaction γp→π⁺n was measured at a c.m. angle of 105° for incident photon energies between 675 and 1125 MeV. A scattered π⁺ meson and a recoil neutron were detected in coincidence with a magnetic spectrometer and a neutron polarimeter system using liquid hydrogen. Results are compared with recent phenomenological analyses and the other existing data measured by the double polarization measurement technique.     

^3 \overrightarrow {He} (γ, π°)^3 \overrightarrow {He} as the nuclear probeas the nuclear probe

The state of polarization of the recoil nucleus inγ+³He→π°+³He as well as the asymmetry in the differential cross-section when the initial³He is polarized are studied together with the differential cross-section taking into consideration theS, S′ andD-state admixtures in the nuclear wavefunctions. In view of the considerable spin dependence in the photoproduction amplitudes these observables are found to be quite sensitive to the small admixtures ofS′ andD-states in the nuclear wave-functions.     

<Emphasis Type="Italic">η</Emphasis> Physics with the crystal ball detector

The experimental results obtained with the Crystal Ball detector for the physics of η-meson decays are reviewed. The η → 3π ⁰ Dalitz plot slope and the η → π ⁰ γγ decay width are measured with a high accuracy using 3 × 10⁷ η decays. Upper limits on the η → 3γ, η → 2π ⁰ γ, and η → 3π ⁰ γ C-forbidden decays are found. The sensitivity of these upper limits to C violation is estimated. An upper limit on the η → 4π ⁰ C P-forbidden decay is found for the first time and the sensitivity of this value to C P violation is estimated.     

Regge pole model for invariant functions. IV

TheNN→NN andγN→πN differential cross sections for polarized targets are discussed in terms of invariant functions. In former fits of high energy experimental data certain of these invariant functions came out to be small. Based on these fits thepn→np,p¯p→ n¯n, γp→ nπ ⁺ andγn→ pπ ^? differential cross sections are predicted to be independent of the target polarization.