Diffractive production of vector mesons in muon-proton scattering at 150 and 100 GeV

Abstract

We have studied the production of p and $ J mesons from muon scattering on a liquidhydrogen target at 150 and 100 GeV. For the p we observe a skewed mass distribution which becomes somewhat more normal with increasing Q 2 (the square of the fourmomentum transferred from the muon), and an exponential distribution in t (the square of the four-momentum transferred to the target proton) with a slope which is consistent with a slight decrease as Q2 increases. The dependence of the cross section on Q2 follows that of the square of the p propagator with little contribution from longitudinal p production. The angular distribution of the p decay confirms the smallness of the contribution from longitudinal p production at our energies. The cross section when extrapolated to Q2=0 agrees with that measured in real photoproduction. The decay angular distribution of the p decay shows that s-channel helicity is largely conserved, although we detect a helicity-single-flip contribution at the 10-15 %.level. Natural-parity exchange in the t channel dominates, and the transverse and longitudinal amplitudes are found to be in phase. These characteristics are consistent with the diffractive nature of the vectordominance model. The t distribution of # production is also exponential, although less steep than that for the p. We observe an elastically produced four-pion state at a mass of approximately 1600 MeV. We identify this state with the pf(1600), and find it to be produced with a distribution exponential in t.

Diffractive electroproduction of ρ and ϕ mesons at HERA

“…The observation at low energy [35,36,43] of dominant natural parity exchange (NPE) supports the attribution of the vacuum quantum numbers (J P C = 0 ++ ) to the pomeron; the recent observation by the HERMES collaboration [42] of the presence at low energy JHEP05(2010)032 of a small contribution (about 6%) of unnatural parity exchange is attributed to quark exchange (π, a 1 or b 1 exchange). At high energy, the modeling of diffraction as two gluon exchange implies a NPE character, in particular in the GK GPD model [78].…”

“…Results at lower energy have been published, in particular for ρ electroproduction, by the DESYGlasgow [35], CHIO [36], NMC [37,38], E665 [39] and HERMES [40][41][42] collaborations. The experimental and theoretical status of diffractive VM production before the high energy fixed target and HERA experiments is presented in detail in the review [43].…”

“…When integrated over |t|, the T 01 amplitude in the present kinematic domain is larger for low Q 2 than for large Q 2 , as shown by the r 5 00 matrix element measurement in figure 33. At low energy and for Q 2 around 0.5 GeV 2 , the T 01 amplitude is non-zero, with |T 01 | / |T 00 | 2 + |T 11 | 2 = 15 to 20% for W about 2.5 GeV [35] and 11 to 14% for 10 ≤ W ≤ 16 GeV [36].…”

Color transparency: Past, present and future

“…the hadronic qq structure of high-energy photons [93,94,95,96,97,98] arising from the virtual photon fluctuations to short-lived quark-antiquark states. These qq states of mass M qq can propagate over a distance l f r -photon coherence length -without expansion (the frozen approximation) 5 .…”

Exclusive $\rho^0$ Meson Photoproduction with a Leading Neutron at HERA

“…, ππ, which may be misidentified as ρ 0 candidates, are estimated using MC simulations with the relative yield normalisation fixed to previously measured and published values: [61][62][63][64]. The resulting overall background contamination in the analysis region 0.6 < M ππ < 1.1 GeV is found to be (1.5 ± 0.7) %.…”