The LHC has opened a new window for studies of strong interactions which today are described remarkably well in the bulk of the phase space by Quantum Chromo Dynamics (QCD). However, there are also a number of corners in the phase space which challenge our understanding of QCD and its predictions.
At the LHC, protons are collided - composite objects consisting of quarks and gluons each carrying a momentum fraction x of their mother proton. The behavior of protons in the medium-x range is well understood and has been studied extensively e.g. at HERA and the Tevatron; here, the dynamics of the partons inside the proton are well described by the DGLAP evolution equations which are also at the heart of most modern Monte Carlo (MC) generators. But, there are regions of phase space, which are not well covered by the standard theoretical approaches: the region of very low x and the region of very large x (or very large pt).
QCD at the Extremes is covering these areas - we first cover the area of medium x, where we believe we have the proper theory and we compare measurements with those theory predictions. Then, the very low x region, where we have no well established theory to describe the measurements will be investigated. And finally, with the high luminosity and high energy reached at the LHC run II we can investigate the up to now unexplored region of very high pt and very large x.
QCD at the Extremes - at very small and very large values of x - is far from being understood; QCD is NOT ``bread-and-butter physics’’, serving only to describe backgrounds for searches of new physics! The LHC offers the unique possibility to test QCD at the extremes and to gain new insight into very fundamental (but by no means simple) physics.