Abstract
For the first time a principle-component analysis is used to separate out different orthogonal modes of the two-particle correlation matrix from heavy ion collisions. The analysis uses data from sqrt(s[NN]) = 2.76 TeV PbPb and sqrt(s[NN]) = 5.02 TeV pPb collisions collected by the CMS experiment at the LHC. Two-particle azimuthal correlations have been extensively used to study hydrodynamic flow in heavy ion collisions. Recently it has been shown that the expected factorization of two-particle results into a product of the constituent single-particle anisotropies is broken. The new information provided by these modes may shed light on the breakdown of flow factorization in heavy ion collisions. The first two modes ("leading" and "subleading") of two-particle correlations are presented for elliptical and triangular anisotropies in PbPb and pPb collisions as a function of pt over a wide range of event activity. The leading mode is found to be essentially equivalent to the anisotropy harmonic previously extracted from two-particle correlation methods. The subleading mode represents a new experimental observable and is shown to account for a large fraction of the factorization breaking recently observed at high transverse momentum. The principle-component analysis technique has also been applied to multiplicity fluctuations. These also show a subleading mode. The connection of these new results to previous studies of factorization is discussed.
Highlights
The primary goal of experiments with heavy ion collisions at ultrarelativistic energies is to study nuclear matter under extreme conditions
Quantum chromodynamics on the lattice predicts the formation of a quark-gluon plasma (QGP) at energy densities that are attainable in relativistic heavy ion collisions
Using the method introduced in Ref. [22], this paper presents the first experimental use of applying Principal-component analysis (PCA) to two-particle correlations in order to study factorization breaking as a function of pT
Summary
The primary goal of experiments with heavy ion collisions at ultrarelativistic energies is to study nuclear matter under extreme conditions. Quantum chromodynamics on the lattice predicts the formation of a quark-gluon plasma (QGP) at energy densities that are attainable in relativistic heavy ion collisions. In the context of these models, the azimuthal anisotropy of hadron emission is the response to the initial density profile of the overlap region of the colliding nuclei. Such anisotropic emission, for a given event, can be quantified through a Fourier decomposition of the single-particle distribution dN dp
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