Exclusive Photoproduction of Rho Meson with Leading Neutron at HERA

Abstract

The first measurement of exclusive photoproduction of ρ 0 mesons associated with leading neutrons at HERA is presented. The data are taken with the H1 detector in the years 2006-2007 at √ s = 319 GeV and correspond to an integrated luminosity of 1.16 pb −1 . ρ 0 mesons are reconstructed from their decays to pions in the central tracking chamber, while leading neutrons carrying a large fraction of the incoming proton momentum, x L > 0.35, are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined by the photon virtuality Q 2 < 2 GeV 2 , the total energy of the photon-proton system 20 < W γp < 100 GeV and the polar angle of the leading neutron, θ n < 0.75 mrad. The cross section of the reaction γ p → ρ 0 nY , where Y is a small mass system escaping undetected in the proton direction, is measured as a function of the neutron energy, of the effective mass of the π + π − system as well as of the transverse momentum squared, p 2 t , and of the pseudorapidity η of ρ meson. The data are interpreted in terms of the double peripheral process, involving pion exchange at the proton vertex followed by elastic photoproduction of a ρ meson on the virtual pion. In the framework of OPE approach elastic cross section of photon-pion scattering, σ el (γπ + → ρ 0 π + ) at average energy W γπ = 22 GeV is extracted.

WG2 Highlights: Small-x, Diffraction and Vector Mesons

“…The normalised cross section 1/σ dσ /dφ for the exclusive production of dijets in DDIS as a function of the azimuthal angle φ between the plane spanned by the incoming and scattered lepton momenta and the plane defined by the exchanged photon and dijet momenta in the virtual photon-pomeron rest frame (left, [24]). Cross section dσ γp /dp 2 t as a function of p 2 t of ρ mesons produced in the reaction γ * p → ρ • π + n (right, [25]). H1 has measured exclusive photoproduction of ρ • mesons associated with leading neutrons, γ * p → ρ • π + n, with the aim to extract for the first time experimentally the elastic γπ cross section [25].…”

“…Cross section dσ γp /dp 2 t as a function of p 2 t of ρ mesons produced in the reaction γ * p → ρ • π + n (right, [25]). H1 has measured exclusive photoproduction of ρ • mesons associated with leading neutrons, γ * p → ρ • π + n, with the aim to extract for the first time experimentally the elastic γπ cross section [25]. The data are interpreted in terms of the double peripheral process (DPP), involving π-exchange at the proton vertex followed by elastic scattering of this pion on the photon from the electron beam, γπ + → ρ • π + .…”

“…Figure 5:The normalised cross section 1/σ dσ /dφ for the exclusive production of dijets in DDIS as a function of the azimuthal angle φ between the plane spanned by the incoming and scattered lepton momenta and the plane defined by the exchanged photon and dijet momenta in the virtual photon-pomeron rest frame (left,[24]). Cross section dσ γp /dp 2 t as a function of p 2 t of ρ mesons produced in the reaction γ * p → ρ • π + n (right,[25]). …”