IndoFrench High Energy Physics Collaboration

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====(QGP) Extreme QCD Working Group ====
Jean-Yves Ollitrault(Conv.), Sourendu Gupta, Rajiv Gavai, Rajeev Bhalerao, Rishi Sharma, Jean-Paul Blaizot, Francois Gelis, Edmond Iancu, Gregory Soyez, Chandrodoy Chattopadhyay, Amaresh Jaiswal

2018 Progress Report

Rajeev Bhalerao from IISER Pune and Jean-Yves Ollitrault from IPhT Saclay
have started a new project on non-Gaussian fluctuations in heavy-ion collisions in collaboration with Giuliano Giacalone, a PhD student at IPhT Saclay.  
The initial motivation was to understand recent experimental data [arXiv:1604.07663] published by the ALICE collaboration at LHC. 
In this article, ALICE presents analyses of a new set of cumulants of 4-particle correlations, dubbed "symmetric cumulants". 
Cumulants vanish beyond order 2 for Gaussian fluctuations, hence cumulants of order 4 are direct signatures of non-Gaussianity.
Non-Gaussian fluctuations are a commonly studied in the context of cosmology.
They might be present in the very early Universe ("primordial" non-Gaussianity) and they are also generated through the expansion. 
The discussion can be carried over to heavy-ion collisions, but there is a notable difference. 
In a large system, such as the Universe, one typically expects almost Gaussian fluctuations due to the central limit theorem.
By contrast, in a very small system such as a nucleus-nucleus collisions, primordial non-Gaussianities are fairly large (typically at the 10% level) and easy to see.
Our study has focused on non-Gaussianities which are not generated by the expansion, but present initially.
We have finalized a first article [arXiv:1811.00837, submitted to Physical Review C] where we show how the kurtosis of elliptic flow fluctuations can be measured experimentally.
We extract it using preliminary data from the CMS collaboration and show that it is positive, in contrast with the kurtosis of triangular flow fluctuations, which was shown to be negative in 2011.
We show that both signs are naturally predicted by hydrodynamic calculations. 
We are preparing a second article which is more general.
The goal of this ongoing work is twofold.
First, we point out that different observables which are typically studied separately have the same physical origin: primordial non-Gaussianities.
Second, we express these "non-Gaussian observables" in terms of the statistical properties (n-point functions) of the initial density field, by through a general expansion in powers of the fluctuations.
We thus relate experimental observables to the state produced right after the collision which, in turn, can be studied via first-principles calculations (high-density QCD).
IndoFrench High Energy Physics Collaboration

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