Selected Results on the Transition From Short to Long Distance Physics at Hera

Abstract

Abstract. Recent results from the HERA ep collider are discussed with emphasis on the transition from short to long distance phenomena in QCD. The results cover inclusive ep scattering, inclusive diffractive scattering, vector meson production, and deeply virtual Compton scattering (DVCS).

Small-x behavior of the structure function F_2 and its slope partial ln(F_2)/partial ln(1/x) for "frozen" and analytic strong-coupling constants

“…We studied the Q 2 dependence of the structure function F 2 and the slope λ eff F 2 = ∂ ln F 2 /∂ ln(1/x) at small x values in the framework of perturbative QCD. Our twist-two results are in very good agreement with HERA data [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18] at Q 2 ∼ > 2.5 GeV 2 , where perturbation theory is applicable. The applications of the frozen and analytic versions of the strong-coupling constants, α MS fr (Q 2 ) and α MS an (Q 2 ), significantly improve the agreement with the HERA data [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18] for both the structure function F 2 and its slope λ eff…”

“…Using the results of the previous sections, we analyze HERA data for the structure function F 2 and its slope ∂ ln F 2 /∂ ln(1/x) at small x values from the H1 and ZEUS Collaborations [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18] The experimental results for the x dependence of F 2 in bins of Q 2 are shown in Fig. 1, while the Q 2 dependence of λ eff F 2 (x, Q 2 ) for an average small-x value of 10 −3 are shown in Figs.…”

“…The experimental data from DESY HERA on the structure function F 2 of deep-inelastic scattering (DIS) [1,2,3,4,5,6,7,8,9,10,11,12,13,14] and its derivatives ∂F 2 /∂ ln Q 2 [4,6,15] and ∂ ln F 2 /∂ ln(1/x) [15,16,17,18] bring us into a very interesting kinematic range for testing theoretical ideas on the behavior of quarks and gluons carrying a very small fraction of the proton's momentum, the so-called small-x region. In this limit, one expects that the conventional treatment based on the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) equations [19,20,21,22] does not account for contributions to the cross section which are leading in α s ln(1/x); moreover, the parton density functions (PDFs), in particular the one of the gluon, become large, and the need arises to apply a high-density formulation of QCD.…”

“…and its slope ∂ ln F 2 /∂ ln(1/x) from the generalized DAS approach with H1 and ZEUS experimental data [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. Detailed inspection of the H1 data points [4,6,16] reveals that, in the ranges x < 0.01 and Q 2 ∼ > 2 GeV 2 , they exhibit a power-like behaviour of the form…”

“…The H1 and ZEUS Collaborations [15,17,18] also presented new data for λ(Q 2 ) at quite small values of Q 2 . As may be seen from Fig.…”