Measurement of two-and three-nucleon shortrange correlation probabilities in nuclei KS The ratios of inclusive electron scattering cross sections of 4 He, 12 C, and 56 Fe to 3 He have been measured at 1 < x B < 3. At Q 2 > 1:4 GeV 2 , the ratios exhibit two separate plateaus, at 1:5 < x B < 2 and at x B > 2:25. This pattern is predicted by models that include 2-and 3-nucleon short-range correlations (SRC). Relative to A 3, the per-nucleon probabilities of 3-nucleon SRC are 2.3, 3.1, and 4.4 times larger for A 4, 12, and 56. This is the first measurement of 3-nucleon SRC probabilities in nuclei.
At forward angles, the cross-sections of photoproduction of vector mesons (ρ, ω, and φ) are well accounted for by the exchange of the Pomeron at high energies, while contributions of t channel exchange of Reggeons are significant at low energies. At large angles, the impact parameter becomes small enough to prevent their constituents to build up the exchanged Reggeons or Pomeron. Two gluon exchange appears to dominate above −t ≃ 1 GeV 2 , especially in the φ channel.
Recent studies of exclusive electroproduction of vector mesons at JLab make it possible for the first time to play with two independent hard scales: the virtuality Q 2 of the photon, which sets the observation scale, and the momentum transfer t to the hadronic system, which sets the interaction scale. They reinforce the description of hard scattering processes in terms of few effective degrees of freedom relevant to the Jlab-Hermes energy range.PACS numbers: PACS : 13.60. Le, 12.40.Nn The study of exclusive electro-production of ω mesons, recently completed at JLab [1], provides us with an original insight on the space time structure of hard scattering processes between the constituents of hadrons. The data speak for themselves in Figure 1. The high intensity of the CEBAF beam, combined with the large acceptance of CLAS, allowed us to perform measurement with an unprecedented accuracy: the two top panels show previous data, recorded 30 years ago or so with real photons at SLAC [2] or virtual photons at DESY [3], while the two bottom panels show the JLab data [1,4]. The extension to higher virtuality Q 2 of the photon reveals the underlying reaction mechanisms. At low momentum transfer −t (small angle), the variation of the cross section with Q 2 (from left to right panels) falls down as the electromagnetic form factor of the pion, the exchange of which dominates the ω channel [5] in the JLab energy range. At large −t (large angle) on the contrary, the cross section stays almost flat and points toward the coupling of the virtual photon to point-like objects.At large momentum transfers (large angles) the impact parameter (b ∼ 1/ √ −t) is small enough to force the partons to exchange the minimum number of gluons before they recombine into the final particles. These hard scattering processes are at the origin of the scaling rules [6], which have been verified in many reactions: around 90• , the cross section behaves as s N −2 , being s = W 2 the total available energy squared and N the number of active constituents. However, a quantitative understanding of experimental cross sections has been difficult to achieve. In the simplest case, Compton scattering, perturbative calculations (see e.g. [7]) fall short by an order of magnitude for the cross section and predict spin transfer coefficients with a sign opposite to experiment [8]. One is forced to rely on models based on effective partonic degrees of freedom relevant to the scale of observation, either Generalized Parton Distributions (GPDs) [9] or dressed quarks and gluons [5,10,11].The photo-production of φ meson, which is dominantly made of a pair of strange quark-antiquark, selects two gluons exchange mechanisms [5]. A fair agreement with the experiment [12] is achieved when a dressed gluon
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