| It is increasingly important to understand, in details the space and momentum observables in elementary particle collisions (e.g. p + p collisions), as they should serve as a reference to the same observables in heavy-ion collisions. Such a comparison is crucial to claim a discovery of new phenomena in the big system. However, in low-multiplicity systems, global conservation laws generate significant N-body correlations in addition to other physics effects. We discuss a formalism to analytically calculate these effects on single-particle distributions and multi-particle correlation functions.;Transverse mass distributions in relativistic heavy ion collisions provide valuable information about the dynamics of the system. The comparison of the spectra from big systems with analogous distribution from p + p collisions led to a claims of discovery of strong collective flow dominating the low momentum part of the spectra in heavy ion collisions. However, we question such a comparison by pointing out the risk of ignoring conservation laws when comparing high- (e.g. Au + Au) and low-multiplicity (e.g. p + p) collisions. Then, we argue that a correct treatment of the effects due to energy and momentum conservation may account for most of the difference between spectra in small and big system. As a result, we show that after this effect is considered, p + p collisions have similar amount of radial flow as Au + Au collisions at RHIC.;The effect of phase-space constraints due to energy and momentum conservation project onto two-particle space in a non-trivial way, affecting the shape of the two-particle correlation functions, and therefore, complicating the femtoscopic analysis.;We also present results from p + p collisions at s =200 GeV, d + Au collisions at sNN =200 GeV and Au + Au collisions at sNN =19.6 GeV from the STAR Experiment at RHIC. The sizes of homogeneity regions are extracted through femtoscopic analysis of the pion correlations. In small system, we see a significant effect of phase-space constraints due to the energy and momentum conservations and we use our formalism to treat these non-femtoscopic correlations.;For the first time, we compare RHIC femtoscopic results from Au + Au collisions at sNN =19.6 GeV with previously published results from SPS experiments at very similar energy of the collision. We put STAR results from small systems in the context of world data from femtoscopic studies in elementary particle collisions and observe trends seen in the data. We also directly compare STAR results from heavy-ion and p + p collisions, under identical analysis, detector acceptance and performance. We identify that the multiplicity and the transverse mass dependence of femtoscopic radii in small systems is surprisingly similar to what is seen in heavy ion collisions.;Based on these similarities between spectra and femtoscopic results from small and big systems, we speculate that there is as strong radial flow in p + p collisions as observed in Au + Au collisions. |
