Unnatural Parity Exchange Research Articles

Spin density matrix elements in exclusive omega meson muoproduction

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive omega meson muoproduction on the proton at COMPASS using 160 GeV/c polarised mu ^{+} and mu ^{-} beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/c^2< W< 17.0 GeV/c^2, with the average kinematics langle Q^{2} rangle = 2.1 (GeV/c)^2, langle W rangle = 7.6 GeV/c^2, and langle p^{2}_{mathrm{T}} rangle = 0.16 (GeV/c)^2. Here, Q^2 denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and p_T the transverse momentum of the omega meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (gamma ^*_T rightarrow V_L) indicate a violation of s-channel helicity conservation. Additionally, we observe a sizeable contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing W. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow to evaluate in a model-dependent way the contribution of UPE transitions and assess the role of parton helicity-flip GPDs in exclusive omega production.

Spin structure of exclusive ω muoproduction at COMPASS

Spin Density Matrix Elements (SDMEs) are determined for exclusive ω meson production on unpolarized protons, in the COMPASS kinematic region of 1.0 (GeV/c)2 < Q2 < 10.0 (GeV/c)2, 5.0 GeV/c2 < W < 17.0 GeV/c2 and . Using extracted the preliminary SDMEs values the hypothesis of S-Channel Helicity Conservation (SCHC) is studied. Certain matrix elements that correspond to the transition indicate violation of SCHC in exclusive ω production. A sizable contribution of unnatural parity exchange amplitudes is found for exclusive ω meson muoproduction, and there is a clear indication of its decrease with increasing W. The extracted longitudinal-to-transverse cross section ratio is 0.553 ± 0.044 ± 0.020.

Differential Cross Section and Photon-Beam Asymmetry for the γ[over →]p → π^{-}Δ^{++}(1232) Reaction at Forward π^{-} Angles for E_{γ}=1.5-2.95 GeV.

Differential cross sections and photon-beam asymmetries for the γ[over →]p→π^{-}Δ^{++}(1232) reaction have been measured for 0.7<cosθ_{π}^{c.m.}<1 and E_{γ}=1.5-2.95 GeV at SPring-8/LEPS. The first-ever high statistics cross-section data are obtained in this kinematical region, and the asymmetry data for 1.5<E_{γ}(GeV)<2.8 are obtained for the first time. This reaction has a unique feature for studying the production mechanisms of a pure uu[over ¯] quark pair in the final state from the proton. Although there is no distinct peak structure in the cross sections, a non-negligible excess over the theoretical predictions is observed at E_{γ}=1.5-1.8 GeV. The asymmetries are found to be negative in most of the present kinematical regions, suggesting the dominance of π exchange in the t channel. The negative asymmetries at forward meson production angles are different from the asymmetries previously measured for the photoproduction reactions producing a dd[over ¯] or an ss[over ¯] quark pair in the final state. Advanced theoretical models introducing nucleon resonances and additional unnatural-parity exchanges are needed to reproduce the present data.

First measurement of coherent ϕ -meson photoproduction from He4 near threshold

The differential cross sections and decay angular distributions for coherent ϕ-meson photoproduction from helium-4 are measured for the first time at forward angles with linearly polarized photons in the energy range Eγ=1.685–2.385GeV. Thanks to the target with spin-parity JP=0+, unnatural-parity exchanges are absent, and thus natural-parity exchanges can be investigated clearly. The decay asymmetry with respect to photon polarization is shown to be very close to the maximal value. This ensures the strong dominance (>94%) of natural-parity exchanges in this reaction. To evaluate the contribution from natural-parity exchanges to the forward cross section (θ=0∘) for the γp→ϕp reaction near threshold, the energy dependence of the forward cross section (θ=0∘) for the γHe4→ϕHe4 reaction is analyzed. The comparison to γp→ϕp data suggests that enhancement of the forward cross section arising from natural-parity exchanges and/or destructive interference between natural-parity and unnatural-parity exchanges is needed in the γp→ϕp reaction near threshold.Received 3 November 2017Revised 21 January 2018DOI:https://doi.org/10.1103/PhysRevC.97.035208©2018 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle interactionsParticle productionPhotonuclear reactionsParticles & FieldsNuclear Physics

Spin Density Matrix Elements in exclusive production ofωmesons at Hermes

Spin density matrix elements have been determined for exclusiveω meson production on hydrogen and deuterium targets, in the kinematic region of 1.0 < Q 2 < 10.0 GeV 2 , 3.0 < W < 6.3 GeV and−t ' < 0.2 GeV 2 . The data, from which SDMEs are determined, were accumulated with the HERMES forward spectrometer during the running period of 1996 to 2007 using the 27.6 GeV electron or positron beam of HERA. A sizable contribution of unnatural parity exchange amplitudes is found for exclusiveω meson production.

Polarization degrees of freedom in near-threshold photoproduction of $ \omega$ -mesons in the $ \pi^{0}_{}$ $ \gamma$ decay channel

We study polarization variables in the photoproduction of \( \omega\) -mesons with subsequent \( \omega\) → \( \gamma\)\( \pi^{0}_{}\) decay. Single and double polarization observables are calculated as a function of different final-state angles. Reaction models include pomeron (natural parity) and \( \pi^{0}_{}\) (unnatural parity) exchange in the t -channel. In addition, the contribution of s -channel resonances is considered. The sensitivity of the polarization observables to the reaction dynamics is discussed.

Study of spin density matrix in exclusive diffractive [formula omitted] meson production at HERMES

Spin Density Matrix Elements (SDMEs) are determined from exclusive diffractive ρ 0 electroproduction in the HERMES experiment at 27.5 GeV beam energy. A set of 23 SDMEs is extracted from the data on hydrogen and deuterium targets in the kinematic region 1 Q 2 5 GeV 2 , 3 W 6.3 GeV , and 0 | t ′ | 0.4 GeV 2 using the maximum likelihood method. A statistically significant violation of s -channel helicity conservation is established. A signal of unnatural parity exchange is observed at the 2 σ level.

Threshold production of φ mesons off protons by linearly polarized photons at SPring-8/LEPS

The LEPS/SPring-8 experiment measured the reactiopn of γp → φp at E γ = 1.5 – 2.4 GeV from December 2000 to January 2001. Preliminary results of azimuthal K + decay angle distribution show that natural-parity exchange is relatively stronger than unnatural-parity exchange at energy E γ = 2.3 GeV in the forward direction.

Study of theηπ0spectrum and search for aJPC=1−+exotic meson

A partial wave analysis ~PWA! of the hp 0 system ~where h!gg) produced in the charge exchange reaction p 2 p!hp 0 n at an incident momentum of 18 GeV/c is presented as a function of the hp 0 invariant mass mhp 0 and momentum transfer squared t p2!hp , from the incident p 2 to the outgoing hp 0 system. S, P, and D waves were included in the PWA. The a0(980) and a2(1320) states are clearly observed in the overall hp 0 effective mass distribution as well as in the amplitudes associated with S wave and D waves, respectively, after partial wave decomposition. The observed distributions in moments ~averages of spherical harmonics! were compared to the results from the PWA and the two are consistent. The distribution in t p2!hp for individual D waves associated with natural and unnatural parity exchange in the t channel are consistent with Regge phenomenology. Of particular interest in this study is the P wave since this leads to an exotic J PC 51 21 for the hp system. A P wave is present in the data; however, attempts to describe the mass dependence of the amplitude and phase motion with respect to the D wave as a Breit-Wigner resonance are problematic. This has implications regarding the existence of a reported exotic J PC 51 21 meson decaying into hp 0 with a mass near 1.4 GeV/c 2 .

Coherentφandωmeson photoproduction from deuteron and nondiffractive channels

For coherent photoproduction of phi and omega mesons from deuteron at forward angles, the isovector pi-exchange amplitude is strongly suppressed. We show a possibility to study the non-diffractive channels associated with the unnatural parity exchange in phi-photoproduction and with the baryon resonance excitations in omega-photoproduction by measuring the spin observables.

On a possible manifestation of f1 trajectory in J/ψ photoproduction

We analyze a possible manifestation of f_1-trajectory in elastic J/psi photoproduction at high energy and large momentum transfer. Inspite of the small contribution of f_1-trajectory in total cross sections, it becomes significant in various spin observables. In particular, we show that the crucial test for f_1-exchange can be made by measuring the single beam- and double parity- and beam-target asymmetries at large momentum transfers, where a strong deviation from the exchange of conventional Pomerons is expected. This effect is caused by the interference of natural (Pomeron) and unnatural (f_1) parity exchange parts of amplitude in the region where their contributions become comparable to each other and might be interesting to observe in forthcoming experiments, if feasible. .

Nucleonic resonance excitations with linearly polarized photons inγp→ωp

In this work, an improved quark model approach to the $\ensuremath{\omega}$ meson photoproduction with an effective Lagrangian is presented. The t-channel natural-parity exchange is taken into account through the Pomeron exchange, while the unnatural-parity exchange is described by the ${\ensuremath{\pi}}^{0}$ exchange. With a very limited number of parameters, the available experimental data in the low energy regime can be consistently accounted for. We find that the beam polarization observables show sensitivities to some s-channel individual resonances in the $\mathrm{SU}(6)\ensuremath{\bigotimes}\mathrm{O}(3)$ quark model symmetry limit. Especially, the two resonances ${P}_{13}(1720)$ and ${F}_{15}(1680),$ which belong to the representation $[56{,}^{2}8,2,2,J],$ have dominant contributions over other excited states. Concerning the essential motivation of searching for ``missing resonances'' in meson photoproduction, this approach provides a feasible framework, on which systematic investigations can be done.

Nucleonic resonance excitations and “missing resonances” in the ω meson photoproduction

In this work, an improved quark model approach to the ω meson photoproduction with an effective Lagrangian is reported. The t-channel natural parity exchange is consistently taken into account through the Pomeron exchange, while the unnatural parity exchange is described by the π 0 exchange. With very limited number of parameters, the available data in the low energy regime can be consistently reproduced in this framework. Effects from the intermediate nucleonic resonances are investigated in several polarization observables. The sensitivities of these observables to the resonance effects are probably the best way for identifying those so-called “missing resonances” in the baryon spectrum.

Resonances in ηππ and ωω systems studied on the VES setup

The ηππ and ωω systems produced in the π − Be interactions with 37GeV/ c π − beam have been studied at the VES spectrometer. For the unnatural-parity exchange, the isoscalar channels demonstrate similar structures in ηπ − π + and ωω final states, while the amplitudes with I = 1 have the same behaviour for ηπ + π − π 0. The I = 0 J PC = 2 ++ amplitudes show the signal of f 2(1565) meson which decays to a 2(1320) π − and ωω. The f 2(1920) meson is observed in ωω. In the I = 0 J PC = 4 ++ intensities there are signals from f 4(2050) and f 4(2330) mesons. The I = 1 J PC = 3 −− waves demonstrate the presence of p 3(1690) meson in a 2(1320) π and pη decay modes. 3 −− waves at higher masses have complex behaviour and can be modelled by the further 3 −− resonances.

φphotoproduction near threshold with Okubo-Zweig-Iizuka evadingφNNinteractions

Existing intermediate and high energy $\ensuremath{\varphi}$-photoproduction data is consistent with purely diffractive production (i.e., Pomeron exchange). However, near threshold $(1.574 \mathrm{GeV}l{E}_{\ensuremath{\gamma}}^{\mathrm{lab}}l~2.0 \mathrm{GeV})$ the energy dependence of the diffractive amplitude is not well known and nondiffractive amplitudes such as ${\ensuremath{\pi}}^{0}$ and $\ensuremath{\eta}$ exchange, $s\overline{s}$ knockout, and nonzero $\ensuremath{\varphi}\mathrm{NN}$ couplings can give sizable contributions to the cross section and polarization density matrix elements of the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\varphi}}{K}^{+}{K}^{\ensuremath{-}}$ decay angular distribution. We stress the importance of measurements with linearly polarized photons near the $\ensuremath{\varphi}$ threshold to separate natural and unnatural parity exchange mechanisms. Approved and planned $\ensuremath{\varphi}$ photoproduction and electroproduction experiments at Jefferson Lab will help establish the relative dynamical contributions near threshold and clarify outstanding theoretical issues related to apparent Okubo-Zweig-Iizuka violations.

On the possibilities to study the exotic state 1−+ in photoproduction of the πη, πη′ systems

We show that photoproduction experiments provide propitious conditions to study the exotic state 1−+ in the πη and πη′ channels. For unpolarized photons contributions from natural and unnatural parity exchanges do not interfere with each other, a fact which permits to estimate the correctness of phase shift analyses. In photoproduction of the π+ η system in the mass range ofa 2(1320) a more stringent limit on the product Λ(1−+→πγ)Br(1−+→πη) can be obtained than in experiments on Primakoff production on nuclei. Particularly favourable is the study of the state 1−+ in the coherent photoproduction of the π0 η and π0 η′ systems on nuclei, with significantly enlarged statistics and a simplified phase shift analysis.

A study of the K s0K s0 system in the reaction K −p→K s0K s0Λ at 11 GeV/ c

A 4.1 event/nb sample of 11 GeV/ c K −p interactions in the LASS spectrometer at SLAC has been used in an analysis of the exclusive K s 0K s 0Λ final state. The K s 0K s 0 mass distribution exhibits a small cluster of events near threshold, but the mass spectrum is dominated by the f 2 ′(1525), which an amplitude analysis shows to be produced by both natural and unnatural parity exchange. This analysis also provides evidence for an S-wave state in the f 2 ′(1525) region, while production of the f 2(1720) is not observed. However, there is structure at ∼ 2.2 GeV/ c 2 which is similar in shape to that observed in the region of the X(2220) in radiative J/ ψ decay; an analysis of the angular distribution indicates a spin of at least 2 in this region. This, together with results from other analyses, indicates that the X(2220) may be the 2 ++ or 4 ++, mainly s s , state expected near this mass in the conventional quark model.

Study of the reaction p

We present density-matrix elements and single-spin correlations for the reaction ${p}_{\ensuremath{\uparrow}}$p\ensuremath{\rightarrow}p${\ensuremath{\pi}}^{+}$n at 3, 4, 6, and 11.75 GeV/c, using both longitudinal and transverse beam polarizations. For small momentum transfers, the spin correlations are mainly due to off-shell ${\ensuremath{\pi}}^{+}$p elastic scattering, while for larger t there are large polarization effects associated with the production dynamics for ${p}_{\ensuremath{\uparrow}}$p\ensuremath{\rightarrow}${\ensuremath{\Delta}}^{++}$n. Comparison of longitudinal and transverse polarization correlations suggests that the ${\ensuremath{\Delta}}^{++}$-production spin effects are due mainly to unnatural-parity exchanges. We present a model-dependent amplitude analysis, and extract the energy dependence of the natural- and unnatural-parity-exchange contributions.

A study of theKπ system in the reaction $$K^ - p \to \bar K^0 \pi ^ - p$$ at 8.25 GeV/c

The production ofK* resonances has been studied in the reaction\(K^ - p \to \bar K^0 \pi ^ - p\) at 8.25 GeV/c. The data comes from a high statistics bubble chamber experiment (180 events/μb). Masses, widths and production cross-sections have been determined for the first threeK*'s. The contributions from natural and unnatural parity exchange have been obtained for theK*(890) and theK*(1420). A partial wave analysis of theK π system from threshold to 1.9 GeV provides evidence for a 0+ enhancement near 1.4 GeV which could be interpreted as the κ(1350).

Production of f'(1520) in 8.25 GeV/c K −p interactions

We present results for the hypercharge exchange reaction K −p→f' λ from a high statistics experiment at 8.25 GeV/ c using the CERN 2m HBC. The total and differential cross sections have been measured; the polarisation of the Λ hyperon and the f' density matrix elements have been calculated as functions of momentum transfer. We also present detailed information on the relative strength of the natural and unnatural parity exchange contributions to the production mechanism.