HERA collider physics

Abstract

HERA, the first electron-proton collider, has been delivering luminosity since 1992. It is the natural extension of an impressive series of fixed-target leptonnucleon scattering experiments. The increase of a factor ten in center-of-mass energy over that available for fixed-target experiments has allowed the discovery of several important results, such as the large number of slow partons in the proton, and the sizeable diffractive cross section at large Q 2 . Recent data point to a possible deviation from Standard Model expectations at very high Q 2 , highlighting the physics potential of HERA for new effects. The HERA program is currently in a transition period. The first six years of data taking have primarily elucidated the structure of the proton, allowed detailed QCD studies and had a strong impact on the understanding of QCD dynamics. The coming years will bring the era of electroweak studies and high Q 2 measurements. This is therefore an appropriate juncture at which to review HERA results.

Exclusive diffractive processes in electron-ion collisions

“…However, these measurements are integral parts of the physics programs of future facilities such as the EIC [17] and the LHeC [16]. We show results for J/ψ and φ production.…”

“…At HERA, an unexpected discovery was that approximately 10% of the ep cross-section is from diffractive final states [16] and that this fraction is fairly independent of W and Q 2 . What characterizes these events experimentally is the presence of a rapidity gap, a region in the angular coverage which exhibits no hadronic activity.…”

Decomposition of the QCD string into dipoles and unintegrated gluon distributions

“…12, the integrated gluon distribution of the proton xG p (x, Q 2 ) is shown as a function of Bjorken -x at photon virtualities of Q 2 = 1, 5, and 20 GeV 2 . Recall that the parameter x 0 = 2.4 · 10 −3 has been adjusted in the previous section such that the experimental data of xG p (x, Q 2 ) at Q 2 = 1 GeV 2 [37] are reproduced. For x > ∼ 10 −3 , xG p (x, Q 2 ) is mainly determined by non-perturbative physics as can be seen from Figs.…”

“…where the values of the exponents P = 0.73 and NP = 0.125 are adopted from [1] and x 0 = 2.4 · 10 −3 is adjusted to reproduce at Q 2 = 1 GeV 2 the integrated gluon distribution of the proton xG p (x, Q 2 ) extracted from the HERA data [37]. The small-χ limit (2.24) considered here is applicable only for x = Q 2 /s ≥ 10 −4 .…”

Diffractive hadroproduction of dijets andW’s at the Fermilab Tevatron collider and the Pomeron structure function

“…Difficulties arise when one tries to obtain a unified description for diffractive processes starting with both electron-or positron-proton (ep) and antiproton-proton (pp) collisions, respectively, studied at the DESY ep HERA and Tevatron colliders. Although several theoretical approaches have successfuly been employed to describe different aspects of hard diffraction revealed by the ep HERA reactions [3], some of them based on Regge theory, diffractive hadroproduction continues to be one of the most challenging topics in hadron dynamics.…”