Process Γp Research Articles

Diffraction dissociation in photoproduction at HERA

A study is presented of the process γp → XY, where there is a large rapidity gap between the systems X and Y. Measurements are made of the differential cross section as a function of the invariant mass M X of the system produced at the photon vertex. Results are presented at centre of mass energies of 〈W〉 = 187 GeV and 〈W〉 =231 GeV, both where the proton dominantly remains intact and, for the first time, where it dissociates. Both the centre of mass energy and the \(M_X^2\) dependence of HERA data and those from a fixed target experiment may simultaneously be described in a triple-Regge model. The low mass photon dissociation process is found to be dominated by diffraction, though a sizable subleading contribution is present at larger masses. The pomeron intercept is extracted and found to be αP(0) = 1.068 ± 0.016 (stat.) ± 0.022 (syst.) ± 0.041 (model), in good agreement with values obtained from total and elastic hadronic and photoproduction cross sections. The diffractive contribution to the process γp → Xp with \(M_X^2/{W^2} < 0.05\) is measured to be 22.2 ± 0.6 (stat.) ± 2.6 (syst.) ± 1.7 (model)% of the total γp cross section at 〈W〉 = 187 GeV.

Threshold kaon photo- and electroproduction in SU(3) baryon chiral perturbation theory

We calculate the processes γp → K 0 Σ +, K + Σ 0, K + Λ in three flavor heavy baryon chiral perturbation theory to one loop. Some low-energy constants are fixed from single baryon properties while others are estimated by means of resonance saturation. In addition, the shapes of the differential cross sections for K + production are used to fix the range for two of the constants. The predicted total cross sections are comparable with the few existing data points in the threshold region. The angular dependences of the recoil polarization of the Λ and Σ 0 show most features of the ones measured at ELSA. We also predict the isovector charge radius of the Σ +, 〈 r v 2〉 Σ + = (0.30 ± 0.03) fm 2.

Diffractive vector meson production at large momentum transfer

The diffractive process γp→V+X (whereV is a vector meson andX results from the dissociation of the proton) is studied. In particular, we consider the region of large momentum transfer (i.e. |t| ≫Λ QCD 2 ) and large centre-of-mass (CM) energy,s. The asymptotic (s→∞,s/|t|≫1) behaviour is derived from the BFKL equation and compared to that which is obtained in the Born approximation (two-gluon exchange). We also calculate the corrections to the Born graphs by iterating the BFKL kernel numerically. Improved convergence of the BFKL series is found by summing the logarithms which occur when an exchanged gluon goes nearly on shell. Importantly, we find evidence that the asymptotic solution to the BFKL equation is inappropriate over most of the HERA range and we provide more realistic predictions for the cross section. The predicted cross section is not too small and can be measured at HERA, up to momentum transfers |t|≈10 GeV2.

Electromagnetic form factor of the nucleon in the time-like region

We explore the possibility to verify vector meson dominance for the nucleon by measuring the half-off-shell form factor in the vector-meson mass region. Cross sections for the process γp → p + e + e − be presented.

Deep inelastic compton-scattering with polarized beam and target

The helicity difference of the photon photoproduction process γp→γx is calculated using four different polarized proton structure functions. Also discussed are the contributions originating from the Compton cross-section and the subprocesses of $$\mathcal{O}(\alpha \alpha _s )$$ induced by photon structure and fragmentation. Up-quark and gluondominance can be separated by choosing different regions of pseudorapidity η and normalized transverse momentumX′ T of the final-state photon. We show, that it is possible to distinguish between the two extreme models of proton spin: polarized gluon vice polarized strange-quark.

Large-angle compton scattering in perturbative QCD

The processes γp→γp and γn→γn are studied in perturbative QCD for large scattering angles. The unpolarized cross sections, as well as the cross sections for polarized beams and targets, are given for various choices of the baryon wavefunction. The predictions for the unpolarized cross section of γp→γp are compared with the available data. Possible future experimental verifications of the theory are discussed.

Study of D production in high-energy gamma p interactions.

We have studied D* production mechanisms using data from a photoproduction experiment at the Fermilab Tagged Photon Spectrometer. A large sample of charged D*’s was selected via the clean signature of the cascade decay D*→D0π+ and subsequently D0→K−π+ or D0→K−π+π0. The cross section for the process γp→(D*++anything)p at an average energy of 105 GeV was measured to be 88±32 nb. Only (11±7)% of D*’s were found to be consistent with being accompanied solely by a D¯* or a D¯; the remaining events contain additional particles. The distribution of the production angle of the D* in the photon-fragmentation-system center of mass is strongly anisotropic and consistent with the form f(θ*)=cos4θ*. We set a limit on the associated-production-process cross section σ(γp→(D¯*−+anything)Λc) x)<60 nb (90% C.L.).Received 17 September 1984DOI:https://doi.org/10.1103/PhysRevD.32.1053©1985 American Physical Society

POLARIZATION IN LARGE PT PHOTOPRODUCTION OF VECTOR MESONS

QCD hard scattering model predictions for the vector meson (V) spin density matrix elements in the large pT process γp → VX are discussed. These results can be a test of both the underlying probabilistic scheme of the QCD parton model and of the elementary interactions, as well as a way of learning about the spin dependence of the fragmentation process of quarks and gluons. The process e+e- → VX in the γ-Z interference region is also discussed.

Measurement of polarized target asymmetry on γp→ π+n in the third resonance region

The polarized target asymmetry for the process γp→ π +n has been measured for incident photon energies from 0.96 to 1.10 GeV, and pion c.m. production angles between 70 and 160°. π + mesons from a polarized propanediole target were detected by a magnetic spectrometer with scintillation counter hodoscopes. The radiative decay widths of nucleon resonances were found by an amplitude analysis, and are compared with theoretical predictions.

Hard scattering and Δ σ exchange in backward photoproduction of Δ ++π −

A tagged photon beam and multiparticle spectrometer have been used to measure the backward photoproduction process γp → Δ ++π −. The energy dependence of the production cross section between 2.8 and 4.8 GeV is studied and found to exhibit shrinkage in excess of that expected for Δ σ dominance. An interpretation of the production mechanism in terms of an incoherent mixture of Δ σ exchange and a “hard scattering” contribution is presented.

Measurements of the polarisation parameters G and H in positive pion photoproduction

Measurements have been made of the polarisation parameters G and H for the process γp→ π + n in the photon energy range 600–1875 MeV and pion c.m. angles between 30° and 100°. These data were obtained in a double polarisation experiment, in which the polarised photon beam from the Daresbury electron synchrotron was incident upon a polarised proton target. Theoretical predictions from a current analysis are compared with the data.

QCD predictions for γp collisions with a large pT real photon in the final state

The processes γp → γ + jet + X and γp → π 0 + jet + X are discussed in detail using perturbative QCD. The possibilities of experimental tests are also discussed.

Polarisation parameters in neutral pion photoproduction

The polarisation parameters Σ, P and T have been measured for the process γp→ π 0p in the photon energy range 1300–2100 MeV and c.m. angles between 30° and 110°, in an experiment with a polarised beam and polarised target. The results are compared with a recent theoretical analysis which fits data from threshold to 16 GeV. The new data are in general agreement with the analysis, but with some significant discrepancies in detail.

Polarisation parameters in positive pion photoproduction

Measurements have been made of the polarisation parameters Σ, T and P for the process γp → π +n in the photon energy range 520–2250 MeV at c.m. angles between 30° and 120°. These data were obtained in a double polarisation experiment, using the polarised photon beam from the Daresbury electron synchrotron incident on a polarised proton target. The data are compared with predictions from current theoretical analyses.

Measurement of polarized target asymmetry in γp → π+n below 1.02 GeV

The polarized target asymmetry for the process γp → π +n has been measured for incident photon energies below 1.02 GeV over a range of c.m. angles from 40° to 160°. π + mesons from a polarized butanol target were detected by a magnetic spectrometer. The results are compared with predictions given by existing analyses. A tentative interpretation of the data is performed, and a larger contribution of S-wave resonances is suggested. The photocouplings of dominant resonances were hardly changed by the inclusion of new data and they seem to be almost uniquely determined.

The measurement of polarized target asymmetry in γp → π0p between 0.4 and 1.0 GeV around 100°

The polarized target asymmetry for the process γp → π 0p was measured in the energies from 0.4 to 1.0 GeV at c.m. angles around 100° detecting both the gamma from π 0 and the recoil protons. A proton range telescope was used to detect the recoil protons. Observed asymmetries agree well with the existing data at overlapping data points. From the comparison between the present data and the available recoil proton polarization data, the contribution of the invariant amplitude A 3 can be examined. The experimental results are valuable in the determination of the photocouplings of nucleon resonances.

Photoproduction of πΔ(1236) from protons below 1.1 GeV

The π + and π − momentum spectrum of the reaction γp → ππN have been measured at lab angles 20°, 40°, 60° and 90° and photon energies 0.744, 0.844, 0.944 and 1.044 GeV with a magnetic spectrometer using the photon subtraction method. Both spectra show a peak corresponding to the production of the πΔ(1236) state. The analysis has been carried out with amplitudes for the quasi two-body process γp → πΔ(1236) and a phase-space background so as to reproduce both π + and π − spectra. The ratio of cross sections σ( γp → π + Δ 0)/ ( γp → π − Δ ++) is found to vary 0.0 to 1.8, depending on kinematical conditions.

Differential cross sections for the photoproduction of eta mesons from protons near 2 GeV

Differential cross sections for the process γp → p η have been measuredd at c.m. angles of 28°, 35° and 42° at incident energies between 2 GeV and 2.8 GeV. Data from an earlier experiment below 2 GeV have been re-analysed and corrected. The whole set of data is compared with the form ( s− M 2) 2 d σ/d t, which fits data at higher energies. There is good agreement above 2 GeV, particularly at the largest angle, but strong departures below.

An eikonal model for neutral pion photoproduction including reggeon-reggeon cuts

We have fitted the experimental data on the high-energy processes γp → π 0p, γn → π 0n and γp → ηp with just the ϱ and ω Regge poles, and strong absorptive cuts generated by the P+P′ exchange amplitude from πN elastic scattering. We obtain excellent agreement with all the data, including target and photon asymmetries, without invoking secondary reggeons such as the B or H trajectories. This result, together with earlier work on the πN charge exchange scattering, leads us to conclude that the addition of P′ ⊗ reggeon cuts can solve many of the phase problems of the traditional SCRAM model.

A global model of K + photoproduction

We extend the use of resonance-saturated fixed- t dispersion relations to the associated photoproduction process γp→K + Λ. In contrast with previous analyses we are able to demonstrate that the Born term couplings in this process are compatible with those determined from K ±p data, namely ( g 2 Λ + 0.8 g 2 Σ )/4 π = 12.2±2.5. We find that the distinctive structure of the data at backward angles in the low energy region indicates substantial contributions from the elusive D 13(1700) resonance. With the introduction of a very simple model for the imaginary parts of the high-energy amplitudes we are able to fit the 5–16 GeV differential cross section data and obtain a qualitative description of the polarisation data at 5 GeV.