We discuss the structure of the momentum transfer distributions for the diffractive dissociation processes p → n π + , p → Δ ++ π − and K − → K 890 ∗0 π − . In the near-threshold mass region a clear break of slope is found around t ′KK ∼ 0.25 GeV 2 for the two baryonic channels, whereas no comparable structure is seen for the mesonic system. The K → K ∗ π differential cross section exhibits a nearly exponential behaviour up to t ′ pp ∼ 0.6 GeV 2 , falling over three orders of magnitude. The slope variations and breaks are strongly correlated both to the mass region considered and to the decay angle of the fragmentation system.

We have studied the reactions K − p → K − π + π − p and K − p → K 0 π − π 0 p at 14.3 GeV/ c using respectively 15 992 and 3723 events. Partial-wave analysis of the region 1.0 < m (K ππ ) < 1.7 GeV have been made using a modified version of the method developed at the University of Illinois.

We present experimental results on a number of K − p reactions at 14.3 GeV/ c that have three bodies in the final state. The final states are K − ω p , K − π p , Λπ + π − , Λ K + K − , Λp p , K ∗ − ω p , Λ(1520) K + K − and Λ(1520) p p . Whenever, with one exception explained by the Zweig rule, there is a K − or a proton in the final state, there is a diffractive-like threshold enhancement in the mass spectrum of the two recoiling particles. These enhancements account for a large fraction of the events in all but the Λπ + π − final state, where they cannot occur, and which is dominated by resonance production. We find evidence for the Q 1 (1300) decaying into K − ω .

We have performed a partial-wave analysis of the mainly diffractively produced low-mass (K ππ ) system in the reactions K − p → K − π + π − p and K − p → K 0 π − π 0 p at 10, 14 and 16 GeV /c . We find that the dominant 1 + S ( K ∗ π ) state has possibly a two-peak structure (around 1.27 and 1.37 GeV). In contrast the 1 + S(K ϱ ) state shows one narrow peak near thershold (around 1.27 GeV). These states are found to be of different origin. The results favour the interpretation of the 1 + S(K ϱ ) as a 1 + resonance below the (K ϱ ) threshold. The t ′ pp dependence is found to be different for the 1 + and 0 − states.

Results are presented of a bubble chamber experiment on K − p elastic scattering at 14.3 GeV/ c , in four-momentum transfer range 0.04 < | t | < 2.74 GeV 2 using an initial set of 40 000 events. The total elastic cross section is (2.96 ± 0.10) mb. The results are compared with K + p elastic scattering data at 13.8 GeV/ c , and the effective Regge trajectory is calculated using K − p data from 5 to 100 GeV/ c .

The STAR Collaboration reports on the photoproduction of $\pi^+\pi^-$ pairs in gold-gold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly-real photon emitted by one ion scatters from the other ion. We fit the $\pi^+\pi^-$ invariant mass spectrum with a combination of $\rho$ and $\omega$ resonances and a direct $\pi^+\pi^-$ continuum. This is the first observation of the $\omega$ in ultra-peripheral collisions, and the first measurement of $\rho-\omega$ interference at energies where photoproduction is dominated by Pomeron exchange. The $\omega$ amplitude is consistent with the measured $\gamma p\rightarrow \omega p$ cross section, a classical Glauber calculation and the $\omega\rightarrow\pi^+\pi^-$ branching ratio. The $\omega$ phase angle is similar to that observed at much lower energies, showing that the $\rho-\omega$ phase difference does not depend significantly on photon energy. The $\rho^0$ differential cross section $d\sigma/dt$ exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with 2 minima visible. The positions of the diffractive minima agree better with the predictions of a quantum Glauber calculation that does not include nuclear shadowing than with a calculation that does include shadowing.

Results of a high-statistics study of elastic scattering and meson resonances produced by π−p interactions at 8 GeV/c are presented. Large statistics and small systematic errors permit examination of the complete kinematic region. Total differential cross sections are given for ρ0,−, f0, g0,−, Δ±, Δ0, and N* resonances. Spin-density matrix elements and Legendre-polynomial moments are given for ρ, f, and Δ resonances. The results for ρ0 and f0 resonances are compared with the predictions of a Regge-pole-exchange model. Properties of the above resonances are compared and discussed. In particular, we present evidence that the ρ0 and f0 production mechanisms are similar. The similarity of the g0 t distribution to that of the ρ0 and f0 suggests a common production mechanism for all three resonances.

We have carried out a partial-wave analysis (PWA) of three-pion systems produced in the coherent dissociation of π+ mesons on nuclear targets. The data have been analyzed for copper and lead targets at an incident π+ energy of 202.5 GeV. This PWA provides further evidence for resonant contributions to JP=1+ and 0− waves at 3π masses below 1.5 GeV, which can be plausibly identified with A1 and π′ mesons. The contribution from electromagnetic production of the A2 has also been extracted, and an estimate for Coulomb production and radiative width of the A1 has been obtained.

Photon diffractive dissociation, $\gamma p \to Xp$, has been studied at HERA with the ZEUS detector using $ep$ interactions where the virtuality $Q^2$ of the exchanged photon is smaller than 0.02 GeV$^2$. The squared four-momentum $t$ exchanged at the proton vertex was determined in the range $0.073<|t|<0.40$ GeV$^2$ by measuring the scattered proton in the ZEUS Leading Proton Spectrometer. In the photon-proton centre-of-mass energy interval $176<W<225$ GeV and for masses of the dissociated photon system $4<M_X<32$ GeV, the $t$ distribution has an exponential shape, $dN/d|t| \propto \exp{(-b|t|)}$, with a slope parameter $b=6.8 \pm 0.9$~(stat.)~$ ^{+1.2}_{-1.1}$~(syst.)~GeV$^{-2}$.

We have studied the diffractive dissociation into di-jets of 500 GeV/c pions scattering coherently from carbon and platinum targets. Extrapolating to asymptotically high energies (where t_{min} approaches 0) we find that when the per-nucleus cross-section for this process is parameterized as $ \sigma = \sigma_0 A^{\alpha} $, $ \alpha $ has values near 1.6, the exact result depending on jet transverse momentum. These values are in agreement with those predicted by theoretical calculations of color-transparency.

We have carried out a systematic study of the coherent dissociation of pions into 3 pions using nuclear targets. The experiment was performed at Fermilab using a high resolution forward spectrometer. Data were taken with carbon, copper and lead targets at an incident momentum of 202.5 GeV/c. Results are presented on momentum transfers, 3-pion masses, and on the nuclearA-dependence of the production cross section.

Diffractive dissociation of neutrons and N ∗ production are studied in the reaction π − n → π − π − p at 15 GeV/ c . The reaction is dominated by a broad, low-mass diffractive enhancement in the pπ − mass. Evidence is presented for the production of at least one N ∗ resonance in the mass region 1.4–1.8 GeV. Comparison with ISR data suggest that this N ∗ resonance is produced by pomeron exchange. The N ∗ production occurs predominantly at t ′ > 0.1 GeV 2 which suggests a different coupling from the usual diffractive reactions. The non-resonant diffractive background is compared with a double-Regge model and the statistical dissociation model.

A study is presented of the process gamma p -->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 mx of the system produced at the photon vertex. Results are presented at centre of mass energies of W_gp = 187 GeV and W_gp = 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 mx~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 alpha_pom(0) = 1.068 \pm 0.016 (stat.) \pm 0.022 (syst.) \pm 0.041 (model), in good agreement with values obtained from total and elastic hadronic and photoproduction cross sections. The diffractive contribution to the process gamma p --> Xp with mx~2 / W_gp~2 < 0.05 is measured to be 22.2 \pm 0.6 (stat.) \pm 2.6 (syst.) \pm 1.7 (model) % of the total gamma p cross section at W_gp = 187 GeV.

We report a measurement of the diffraction dissociation differential cross section d2σSD/dM2dt for p¯p→p¯X at √s =546 and 1800 GeV, M2/s<0.2 and 0≤-t≤0.4 GeV2. Our results are compared to theoretical predictions and to extrapolations from experimental results at lower energies.

The reaction γp→ρfast0pπ+π− has been studied with the linearly polarized 20-GeV monochromatic photon beam at the SLAC Hybrid Facility to test the prediction of s-channel helicity conservation in inelastic diffraction for t’<0.4 (GeV/c)2. In a sample of 1934 events from this reaction, the ρ0 decay-angular distributions and spin-density-matrix elements are consistent with s-channel helicity conservation, the π+π− mass shape displays the same skewing as seen in the reaction γp→pπ+π−, and the pπ+π− mass distribution compares well and scales according to the vector dominance model with that produced in π±p→πfast±pπ+π−.

Photon proton cross sections for elastic light vector meson production, σelνp, inelastic diffractive production, σndνp, non-diffractive production, σdνp, as well as the total cross section, σtotνp, have been measured at an average υp center of mass energy of 180 GeV with the ZEUS detector at HERA. The resulting values are σelνp = 18 ± 7 μb, σdνp = 33 ± 8 μb, σndνp = 91 ± 11 μb, and σtotνp 143 ± 17 μb, where the errors include statistical and systematic errors added in quadrature.

We present results from a measurement of double diffraction dissociation in $\bar pp$ collisions at the Fermilab Tevatron collider. The production cross section for events with a central pseudorapidity gap of width $\Delta\eta^0>3$ (overlapping $\eta=0$) is found to be $4.43\pm 0.02{(stat)}{\pm 1.18}{(syst) mb}$ [$3.42\pm 0.01{(stat)}{\pm 1.09}{(syst) mb}$] at $\sqrt{s}=1800$ [630] GeV. Our results are compared with previous measurements and with predictions based on Regge theory and factorization.

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We report the first observation of diffractively produced W bosons. In a sample of W -> e nu events produced in p-barp collisions at sqrt{s}=1.8 TeV, we find an excess of events with a forward rapidity gap, which is attributed to diffraction. The probability that this excess is consistent with non-diffractive production is 1.1 10^{-4} (3.8 sigma). The relatively low fraction of W+Jet events observed within this excess implies that mainly quarks from the pomeron, which mediates diffraction, participate in W production. The diffractive to non-diffractive W production ratio is found to be R_W=(1.15 +/- 0.55)%.

Differential cross sections of p p forward elastic scattering were measured between 400 and 730 MeV/ c , and the real-to-imaginary ratio, ϱ, of the forward amplitude was deduced. We found that ρ increases from ∼ 0.1 to ∼ 0.4 in this momentum range. A dispersion-relation analysis shows the existence of a pole-like structure in the real part of the p p amplitude near threshold.

The first double diffractive cross-section measurement in the very forward region has been carried out by the TOTEM experiment at the LHC with center-of-mass energy of sqrt(s)=7 TeV. By utilizing the very forward TOTEM tracking detectors T1 and T2, which extend up to |eta|=6.5, a clean sample of double diffractive pp events was extracted. From these events, we measured the cross-section sigma_DD =(116 +- 25) mub for events where both diffractive systems have 4.7 <|eta|_min < 6.5 .

We have analysed the reaction π + p → pπ + π + π − at 16 GeV/c by means of the prism plot analysis (PPA) as proposed by Pless et al. We have separated ten reaction channels contributing to the final state pπ + π + π − and present the results in terms of partial and differential cross sections, invariant mass and decay angular distributions. We show that the PPA is a self-controlling method which is demonstrated by the emergence of a broad (3π) + enhancement around 1800 MeV decaying into ρ 0 π + .

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with $\xl>0.97$, where $\xl$ is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of $t$, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section on $t$ is measured in the interval $0.073<|t|<0.4$~$\gevtwo$ and is found to be described by an exponential shape with the slope parameter $b=\tslopeerr$. The diffractive structure function $\ftwodfour$ is presented as a function of $\xpom \simeq 1-\xl$ and $\beta$, the momentum fraction of the struck quark with respect to $\xpom$, and averaged over the $t$ interval $0.073<|t|<\ftwodfourtmax$~$\gevtwo$ and the photon virtuality range $5<Q^2<20~\gevtwo$. In the kinematic range $4 \times 10^{-4} < \xpom < 0.03$ and $0.015<\beta<0.5$, the $\xpom$ dependence of $\ftwodfour$ is fitted with a form $\xpoma$, yielding $a= \ftwodfouraerr$. Upon integration over $t$, the structure function $\ftwod$ is determined in a kinematic range extending to higher $\xpom$ and lower $\beta$ compared to our previous analysis; the results are discussed within the framework of Regge theory.

A search for charm production in the coherent diffractive dissociation reaction pSi→XSi was carried out for the modes D 0 → K − π + , D 0 → K − π + π + π − , and D + → K − π + π + . No charm signals were observed, and the 90% confidence level upper limit for coherent charm pair production was determined to be 26 μ b per silicon nucleus. The results are interpreted as an upper limit of 0.2% on the amount of intrinsic charm in the proton.

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