Abstract
A measurement of the inclusive ep scattering cross section is presented in the region of low momentum transfers, 0.2 GeV^2 < Q^2 < 12 GeV^2, and low Bjorken x, 5x10^-6 < x < 0.02. The result is based on two data sets collected in dedicated runs by the H1 Collaboration at HERA at beam energies of 27.6 GeV and 920 GeV for positrons and protons, respectively. A combination with data previously published by H1 leads to a cross section measurement of a few percent accuracy. A kinematic reconstruction method exploiting radiative ep events extends the measurement to lower Q^2 and larger x. The data are compared with theoretical models which apply to the transition region from photoproduction to deep inelastic scattering.
Highlights
Deep inelastic lepton-hadron scattering (DIS) is pivotal for the understanding of the structure of the nucleon and of the dynamics of parton interactions
For the spaghetti calorimeter (SpaCal), this extra activity can be induced by the scattered electron, with some energy leaking outside the isolation cone or by a radiated photon emitted at a large angle
In all these studies the relative shifts of the energy scale in the data versus the simulation are consistent within 1% which is taken as a systematic uncertainty of the energy determination at Ee = 2 GeV
Summary
Deep inelastic lepton-hadron scattering (DIS) is pivotal for the understanding of the structure of the nucleon and of the dynamics of parton interactions. The measurement region is further extended towards lower Q2 and higher x values by exploiting events with hard photons emitted collinearly to the electron beam (Initial State Radiation or ISR). Such events are treated as ep events at an effectively reduced centre-of-mass energy. Unlike in the previous H1 ISR analysis [36], the emitted photons are not explicitly detected, but the missing momentum is determined using momentum conservation For this method the BST charged particle validation of the scattered electron is important to reduce the physics background from photoproduction events, in which the scattered electron escapes undetected in the electron beam direction.
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