Exclusive $$\rho ^0$$ ρ 0 meson photoproduction with a leading neutron at HERA

Abstract

A first measurement is presented of exclusive photoproduction of ρ 0 mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years 2006 and 2007 at a centre-of-mass energy of √ s = 319 GeV and correspond to an integrated luminosity of 1.16 pb −1 . The ρ 0 mesons with transverse momenta p T < 1 GeV are reconstructed from their decays to charged pions, 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 γ p < 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 nπ + is mea- † Deceased a e-mail: sschmitt@mail. Sciences, sured as a function of several variables. The data are interpreted in terms of a double peripheral process, involving pion exchange at the proton vertex followed by elastic photoproduction of a ρ 0 meson on the virtual pion. In the framework of one-pion-exchange dominance the elastic cross section of photon-pion scattering, σ el (γ π + → ρ 0 π + ), is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction γ p → ρ 0 nπ + .

Strong Interactions - Experimental Overview

“…Exclusive production of π + π − in ep interactions accompanied by a forward scattered neutron can be interpreted in terms of γπ scattering. The γπ scattering cross section extracted by the H1 experiment [11] comes out to be only half of the value expected from the additive quark model and the value of γ p cross section, indicating the presence of strong absorptive corrections.…”

Exclusive vector meson production with leading neutrons in a saturation model for the dipole amplitude in mixed space

“…In this analysis we consider the HERA data on exclusive photoproduction of ρ 0 mesons, associated with leading neutrons, presented in [4]. The data covers the following kinematical ranges: 20 < W < 100 GeV and Another parameter which enters the calculations is R q , which relates the dipole-proton and dipole-pion scattering amplitude, Eq.…”

“…2, which shows the differential cross section dσðW; Q 2 ; x L Þ=dx L as a function of x L , where the range in the leading neutron transverse momentum is p T;n < 0.2 GeV, one of the kinematical ranges covered by the data in [4]. We consider the five models for the pion flux presented above and both BG and LCG models for the overlap functions.…”

“…These particles can provide us important insight on strong interactions in the soft regime and, at HERA, have been first observed in semi-inclusive reactions (e þ p → e þ n þ X, where n represents the neutron and X is a generic hadronic final state). Currently, we have access to recent high precision data on leading neutron production, in both semi-inclusive [1][2][3] and exclusive reactions [4], but already from previous HERA measurements we have learned that their production are dominated by pion exchange [5][6][7][8][9][10]. Thus, the virtual photon emitted from the incoming electron interacts with the pion (of the proton cloud) which allows us to extract the γ Ã π cross section.…”

Recent results on exclusive production at HERA

“…Exclusive photoproduction of the ρ 0 meson with a leading neutron has been studied by the H1 Collaboration [1]. In the Regge framework this process is interpreted as a Double-Regge-pole exchange reaction and is schematically shown in Fig.…”