Measurement of the Proton Structure Function F2 at Low Q2 and Very Low x with the ZEUS Beam Pipe Calorimeter at HERA

Abstract

Results on measurements of the proton structure function from the two HERA experiments H1 and ZEUS in 1994 show that perturbative QCD (pQCD) allows to describe the observed rapid rise of with decreasing x down to Q2 = 1.5GeV2. Whereas the pQCD region exhibits a strong rise of σγ*p tot with increasing W2, the total cross-section for real photon-proton scattering σγp tot shows only a modest rise with W2. The proton structure function F2 and the total virtual photon-proton (γ*p) cross-section σγ*p tot have been measured in inelastic neutral current scattering, e+p → e+X, at HERA in the previously unexplored transition region of non-perturbative to perturbative QCD at low Q2. To study this kinematic region, a small electromagnetic Beam Pipe Calorimeter (BPC) has been designed, constructed and installed in the ZEUS experiment in 1995. The accuracy of the measurement of F2 in this kinematic region relies mainly on a precise energy calibration and detector alignment. An energy calibration with a precision of 0.5% and a detector alignment of 0.5mm have been achieved. Results on a measurement of F2 and σγp tot are obtained for 0.11 ≤ Q2≤ 0.65 GeV2 and 1.7 ċ 10−6 ≤ x ≤ 6.0 ċ 10−5, corresponding to a range in the γ*p center-of-mass energy of 104 ≤ W ≤ 251 GeV. The results on F2 and σγp tot indicate a smooth transition between the pQCD region and the photoproduction region. A detailed phenomenological analysis as well as a QCD analysis has been carried out to investigate the limitations of a non-perturbative as well as a perturbative QCD description.