The "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 favour of a fluid-like behavior even in $pp$ scattering, where we have to deal with length scales of the order of 0.1 fm.

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