# Collectivity from interference

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## Abstract

In hadronic collisions, interference between different production channels affects momentum distributions of multi-particle final states. As this QCD interference does not depend on the strong coupling constant *α*_{ s }, it is part of the no-interaction baseline that needs to be controlled prior to searching for other manifestations of collective dynamics, e.g., in the analysis of azimuthal anisostropy coefficients *v*_{ n } at the LHC. Here, we introduce a model that is based on the QCD theory of multi-parton interactions and that allows one to study interference effects in the production of *m* particles in hadronic collisions with *N* parton-parton interactions (“sources”). In an expansion in powers of 1/(*N* _{ c } ^{2} − 1) and to leading order in the number of sources *N* , we calculate interference effects in the *m*-particle spectra and we determine from them the second and fourth order cumulant momentum anisotropies *v*_{ n }{2} and *v*_{ n }{4}. Without invoking any azimuthal asymmetry and any density dependent non-linear dynamics in the incoming state, and without invoking any interaction in the final state, we find that QCD interference alone can give rise to values for *v*_{ n }{2} and *v*_{ n }{4}, *n* even, that persist unattenuated for increasing number of sources, that may increase with increasing multiplicity and that agree with measurements in proton-proton (pp) collisions in terms of the order of magnitude of the signal and the approximate shape of the transverse momentum dependence. We further find that the non-abelian features of QCD interference can give rise to odd harmonic anisotropies. These findings indicate that the no-interaction baseline including QCD interference effects can make a sizeable if not dominant contribution to the measured *v*_{ n } coefficients in pp collisions. Prospects for analyzing QCD interference contributions further and their possible relevance for proton-nucleus and nucleus-nucleus collisions are discussed shortly.

## Keywords

Heavy Ion Phenomenology## Notes

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