Elsevier

Physics Letters B

Volume 610, Issues 3–4, 24 March 2005, Pages 199-211
Physics Letters B

Study of the pion trajectory in the photoproduction of leading neutrons at HERA

https://doi.org/10.1016/j.physletb.2005.01.101Get rights and content

Abstract

Energetic neutrons produced in ep collisions at HERA have been studied with the ZEUS detector in the photoproduction regime at a mean photon–proton center-of-mass energy of 220 GeV. The neutrons carry a large fraction 0.64<xL<0.925 of the incoming proton energy, and the four-momentum transfer squared at the proton–neutron vertex is small, |t|<0.425GeV2. The xL distribution of the neutrons is measured in bins of t. The (1xL) distributions in the t bins studied satisfy a power law dN/dxL(1xL)a(t), with the powers a(t) following a linear function of t: a(t)=0.88±0.09(stat.)−0.39+0.34(syst.)(2.81±0.42(stat.)−0.62+1.13(syst.)GeV−2)t. This result is consistent with the expectations of pion-exchange models, in which the incoming proton fluctuates to a neutron–pion state, and the electron interacts with the pion.

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 xL, 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, γpnX, 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 (1xL) distribution is presented as a function of t for large xL (0.64<xL<0.925) and small |t| (|t|<0.425GeV2). 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 (θ>127°) of the CAL. In addition, the trigger required an energy deposit in the FNC corresponding to xL>0.2. The trigger efficiency of the FNC was close to 100% for the xL range under consideration in this Letter (xL>0.64).

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 xL and t. The bins were chosen to be well within the acceptance in the xLt plane and contained 12 523 events. The corrected xL distributions of leading neutrons as a function of t are shown in Fig. 2. The distributions are consistent with a power-law dependence in (1xL) of the form dN/dxL(1xL)a(t). For each t bin the power a(t) 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 a(t).

The pion “flux”, the splitting function of a proton to a neutron and pion (pnπ+), can be written [15] as fπ/p(xL,t)=14πgnπp24πt(mπ2t)2(1

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 (1xL) distributions in bins of t are described by a power law, dN/dxL(1xL)a(t), with the powers a(t) following a linear function of t: a(t)=0.88±0.09(stat.)−0.39+0.34(syst.)(2.81±0.42(stat.)−0.62+1.13(syst.)GeV−2)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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28

Supported by the US Department of Energy.

1

Also affiliated with University College London, London, UK.

29

Supported by the Italian National Institute for Nuclear Physics (INFN).

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