Study of the pion trajectory in the photoproduction of leading neutrons at HERA
Introduction
Several studies of leading neutron production in ep interactions at HERA have been reported previously [1], [2], [3], [4], [5]. Many features of the data are described by pion-exchange models, in which the incoming proton fluctuates into a neutron–pion state and the pion interacts with the incoming electron or positron. The kinematic variables t, the square of the four-momentum transfer at the proton–neutron vertex, and , the energy fraction of the proton carried by the neutron, are convenient variables for studying energy-angle correlations. In this study of semi-inclusive photoproduction, , where the photon is quasi-real, the energy distribution of leading neutrons is measured as a function of t, which is determined using a new position detector to measure the angle of the neutron. The distribution is presented as a function of t for large () and small (). The results are interpreted in the context of pion exchange, in order to provide a test of the consistency of this picture of leading neutron production.
Section snippets
Experimental set-up and kinematics
The data used for this measurement were collected in the year 2000 at the ep collider HERA with the ZEUS detector, during a short run period in which a special trigger was implemented. The data set corresponds to an integrated luminosity of 9 pb−1. During this period HERA collided 27.5 GeV positrons with 920 GeV protons at a center-of-mass energy of 318 GeV.
Charged particles are tracked in the central tracking detector [6], which operates in a magnetic field of 1.43 T provided by a thin
Event selection
The data sample was collected using a trigger that required at least 5 GeV in the LUMI electron calorimeter in coincidence with at least 0.5 GeV in the rear part of the CAL. In addition, the trigger required an energy deposit in the FNC corresponding to . The trigger efficiency of the FNC was close to 100% for the range under consideration in this Letter ().
Photoproduction events were selected offline using cuts based on the reconstructed vertex position and calorimeter
Neutron efficiencies and correction factors
The efficiencies and correction factors for the leading neutron were calculated with a single-particle Monte Carlo (MC) simulation. The MC program included the geometry of the proton beam-line magnets which define the geometric acceptance, the details of the absorbing material as obtained from survey measurements, the proton beam divergence, and the measured energy (FNC) and position (FNT) resolutions for hadronic showers. The MC program accounts for the different amount of absorbing material
Results
The events were binned in and t. The bins were chosen to be well within the acceptance in the –t plane and contained 12 523 events. The corrected distributions of leading neutrons as a function of t are shown in Fig. 2. The distributions are consistent with a power-law dependence in of the form . For each t bin the power was obtained by a least-squares fit of this function to the observed distribution. Only statistical errors were used in the fits because
Discussion
Previous experiments ([3] and references therein) have shown that leading neutron production in lepton–hadron and hadron–hadron experiments can be described by pion-exchange models. The consistency of this description can be tested by assuming that pion exchange is the dominant mechanism and deriving the pion Regge trajectory from the measured values of .
The pion “flux”, the splitting function of a proton to a neutron and pion (), can be written [15] as
Summary
The dependence of the energy distribution of photoproduced leading neutrons on the momentum transfer at the proton–neutron vertex has been studied at an average photon–proton center-of-mass energy of 220 GeV. The distributions in bins of t are described by a power law, , with the powers following a linear function of t: .
The linear function can be interpreted in the framework of Regge
Acknowledgments
We thank the DESY Directorate for their strong support and encouragement, and the HERA machine group for their diligent efforts. We are grateful for the support of the DESY computing and network services. The design, construction and installation of the ZEUS detector have been made possible owing to the ingenuity and effort of many people who are not listed as authors. This study was only made possible by the physics insight and work of G. Levman, to whom we are greatly indebted.
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Measurement of dijet photoproduction for events with a leading neutron at HERA
2010, Nuclear Physics B
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Supported by the US Department of Energy.
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Also affiliated with University College London, London, UK.
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Supported by the Italian National Institute for Nuclear Physics (INFN).