"Ridge" in proton-proton scattering at 7 TeV.

One of the most important experimental results for proton-proton scattering at the LHC is the observation of a so-called "ridge" structure in the two-particle correlation function versus the pseudorapidity difference Δη and the azimuthal angle difference Δφ. One finds a strong correlation around Δφ=0, extended over many units in Δη. We show that a hydrodynamical expansion based on flux tube initial conditions leads in a natural way to the observed structure. To get this result, we have to perform an event-by-event calculation, because the effect is due to statistical fluctuations of the initial conditions, together with a subsequent collective expansion. This is a strong point in favor of a fluidlike behavior even in pp scattering, where we have to deal with length scales of the order of 0.1 fm.