Measurement of Leading Neutron Production in Deep Inelastic ep Scattering at HERA

Deep{inelastic scattering events with a leading neutron have been observed by the H1 experiment at HERA using a forward neutron calorimeter. The calorimeter has been installed in 1996 and covers scattering angles between 0 mrad and < 0:6 mrad. For the rst time at HERA, the semi{inclusive cross section d(ep ! enX)=(dxdQ 2 dE 0) for the production of leading neutrons with transverse momenta p T 200 MeV has been measured in the kinematic region 2 G e V 2 Q 2 50 GeV 2 , 6 10 ;5 x 6 10 ;3 and neutron energy 160 GeV E 0 E p , where E p = 820 GeV is the proton beam energy. The data are used to test diierent production models. The neutron data are compared to a similar measurement of leading proton production. For fractional energies 0:7 E 0 =E p 0:9, both semi{inclusive cross sections can be described by a Regge model of leading baryon production which consists of pion, pomeron and reggeon exchanges. The leading neutron data are used to estimate for the rst time the structure function of the pion at small Bjorken{x. Animadverti jam ante aliquot annos quu am multa, ineunte aetate, falsa pro veris admiserim, & quu am dubia sint quaecunque istis postea superextruxi, ac proinde funditus omnia semel in vit^ a esse evertenda ...] Several years have now elapsed since I rst became aware that I had accepted, even from my youth, many false opinions for true, and that consequently what I afterward based on such principles was highly doubtfull and from that time I was convinced of the necessity of undertaking once in my life to rid myself of all the opinions I had adopted ...]