Measurement of the inclusive e±p scattering cross section at high inelasticity y and of the structure function FL


Experiment H1 located at electron/positron-proton (ep) collider HERA in DESY Hamburg has collected currently the most precise measurements of cross-section of electron/positron scattered on protons. The scatter results informs us about the inner structure of protons. This scattering experiment is similar to famous alfa particle scattering on Au foil in 1909 which led to discovery of atomic nuclei (Rutherford's experiment). In case of electron-proton scattering we study the properties of point-like particles in proton, partons (quarks and gluons). Our specific measurement was performed in kinematic range 1,5 GeV2 < Q2 < 120 GeV2 and 2,9x10-5 < x < 0,01 where x stands for relative momentum of parton in proton and Q2 is a negative value of four-momentum transferred between positron and proton squared and can be interpreted as a spatial resolution of the measurement. Measurements was conducted for four energies of proton beam Ep = 920, 820, 575 a 460 GeV. That allowed us to directly determine the so-called longitudinal structure function of proton FL for the first time. The structure function is proportional to gluon density in proton. Measurements showed tha FL is significantly non-zero. This result is important for theoretical description of gluon emissions from quarks inside proton. Because these emissions are the only source of gluons in proton this experimental result is significant instrument for verification of predictions originating in quantum chromodynamics, theory that describes gluon emissions.

Proton longitudinal structure function FL as a function of Q2 variable. Color belts representpredictions based on models stated in bottom of the image. This result is a verification of quantum chromodynamics predictions in new kinematic range.