Evidence for quark spin-flip in pomeron exchange

Abstract

Spin-parity analyses of the ωπ system in the reaction γp → (ωπ)p for photon laboratory energies from 20 to 70 GeV have shown that production of the J P = 1 + b1(1235) meson dominates, with a J P = 1 − background at the level of 20%. Using vector-meson dominance arguments, this background is shown to be consistent with the data on e + e − → ωπ. The energy dependence of the data imply that the mechanism is a combination of Reggeon and Pomeron exchange. Assuming that the latter is relevant only for the J P = 1 − component and extrapolating to W = 200 GeV, it is argued that this accounts for most of the preliminary ωπ signal observed by the H1 Collaboration in the same reaction. A residual peak can be ascribed to the b1(1235), which requires a quark spin-flip from Pomeron exchange. Precisely the same mechanism occurs in the reaction πp → a1(1260)p.

The target asymmetry in hard vector-meson electroproduction and parton angular momenta

“…As is wellknown the familiar soft Pomeron exchange dominantly couples to the proton helicity non-flip vertex while its flip coupling is very small. In fact, it is hard to find convincing evidence for a non-zero flip coupling phenomenologically [23]. Supposing that this behavior also holds for the hard gluonic Regge exchange controlling H g and E g at low-x, we are forced to infer that E g ≃ 0.…”

“…On the other hand, for β u val > β d val (22) is larger than (23). Note that in the chiral soliton model one finds e uv 20 + e dv 20 = 0 in the large-N c limit [19].…”

The role of the quark and gluon GPDs in hard vector-meson electroproduction

“…As is well-known this trajectory couples mainly to the proton helicity non-flip vertex while the flip coupling is very small. It is hard to find phenomenological evidence for a non-zero flip coupling [54]. Thus, the relative importance of gluon and valence quark GPDs is very different for E and H. It seems unlikely that E g plays an analogously prominent role as H g .…”

Exclusive hard Reactions and QCD

“…The target helicity-flip E CFF might be associated with the spin-flip "pomeron" [76]. It might be sizeable, however, in the cross section (3.6) it is suppressed by the factor ⟨⟨|t|⟩⟩/4M 2 p ∼ 0.05.…”