QCD Evolution Studies through Mulijets at HERA
Tom Danielson
PhD Student: University of Wisconsin, Madison, WI
Given an initial measurement, the structure of the proton can be determined for all values of x and Q2 via perturbative QCD (pQCD) equations describing the “evolution” of the proton’s constituent partons. Currently, the most commonly used set of pQCD equations is DGLAP (Dokshitzer, Gribov, Lipatov, Altarelli, Parisi). These equations have been tested thoroughly at HERA, and in general the equations describe experimental results well. In a kinematic range in which certain approximations used in DGLAP-based calculations are not expected to be applicable, another set of QCD evolution equations, BFKL (Balitsky, Fadin, Kuraev, Lipatov) may better describe parton evolution. The different predictions of the two models for parton evolution provide can be tested in this kinematic region by examining the angular relations of the jets. DGLAP predicts a strong angular correlation of the emitted jets (“back-to-back” in azimuthal angle and similar in polar angle); whereas BFKL predicts that the jet angles are not necessarily strongly correlated. BFKL also predicts an excess of jets with large transverse momentum emitted at a small polar angle (“forward region”). By examining the angular correlations of jets and comparing the results with leading-order Monte Carlo models Ariadne and Lepto (which mimic BFKL and DGLAP, respectively) and also with DGLAP-based NLO calculations, the kinematic regions in which these evolution equations are applicable may be better understood. An excess of data where jets have a small azimuthal separation could signal the onset of BFKL effects
Figure: Differential cross-section as a function of exchange-photon 4-momenta squared. The data are corrected to the hadron level and compared to previously published results. The previous results showed good agreement with DGLAP-based NLO calculations in a phase space where DGLAP was expected to be applicable. The inner lines indicate agreement to within 1% while the outer lines indicate agreement to within 5%.