A Measurement of the Diffractive Proton Structure Function F

A measurement is presented of the diffractive proton structure function describing the process ep → eXY , where X and Y represent the hadronic final states into which the exchanged virtual photon and the proton dissociate respectively. The data, with an integrated luminosity of 3.74 pb−1, were collected with unbiased triggers using the H1 detector at the HERA positron proton collider in 1999. The measurement is presented in the form of a 3-dimensional structure function F D(3) 2 (β,Q , x IP ), integrated over the region MY < 1.6 GeV and |t| < 1 GeV. The kinematic range covered is 1.5 GeV < Q < 45 GeV, 0.001 < β < 0.9 and 10−4 x IP < 0.05. Previous published H1 results for F D(3) 2 (β,Q , x IP ) only exist for Q ≥ 4.5 GeV and β ≥ 0.01 hence not only does this analysis provide a new high precision measurement of F D(3) 2 (β,Q , x IP ) but it also provides kinematic coverage at lowerQ and β values than has previously been possible. The results were extracted with full statistical and systematic errors and a good agreement is generally observed in the region of overlap with previous H1 measurements. The Q dependence at fixed values of x IP exhibits rising scaling violations for most of the β range, which start to fall only at the highest values of β. This behaviour is expected for the exchange of a gluondominated pomeron which evolves according to the DGLAP equations. Comparisons with different models of diffraction were made and a value for the effective pomeron intercept αIP(0) = 1.107± 0.026 (stat.) was obtained from a Regge-motivated fit.