Abstract :
Collective, explosive flow in central heavy ion collisions manifests itself in the mass dependence of pT distributions and femtoscopic length scales, measured in the soft sector (pT≲1 GeV/c). Measured pT distributions from proton-proton collisions differ significantly from those from heavy ion collisions. This has been taken as evidence that p+p collisions generate little collective flow, a conclusion in line with naive expectations. We point out possible hazards of ignoring conservation laws when comparing high- and low-multiplicity final states. We argue that the effects of energy and momentum conservation actually dominate the observed systematics, and that p+p collisions may be much more similar to heavy ion collisions than generally thought.
