In the largest currently operating heavy-ion collider, the LHC, sufficient collision energy is reached to create a new form of nuclear matter called the Quark-Gluon Plasma (QGP). This new state of matter is believed to be formed at the early stage of the ultra-relativistic Pb-Pb collision, but cannot be directly detected. The hot and dense but rapidly expanding system of quarks and gluons quickly freezes into final state hadrons, which we measure in our detectors. How then can we obtain information on this system? Insight into the early stage of the nucleus-nucleus collision can be provided by the analysis ofevent-by-event correlations and fluctuations.
In this talk, new data on forward-backward charged particle correlations and multiplicity fluctuations in Pb-Pb collisions from the ALICE experiment at the LHC will be presented.
I will focus on the "standard" and the "partial" correlation coefficient, as well as the strongly-intensive quantity Σ. All these observables will be studied as a function of (1) the distance between forward and backward pseudorapidity intervals (the η gap), (2) the centrality of the collision and (3) the width of the centrality bin. Two different centrality estimators will be used: charged particle multiplicity and spectator nucleon energy.
At the present moment, the strong dependence of the measured "standard" forward-backward correlation coefficient (bcorr) on the applied centrality estimator, and as a function of the width of the centrality bin, are well-established experimental facts. However, in this presentation, I will demonstrate that the "standard" correlation strength is dominated by volume fluctuations for wider centrality windows. This effect decreases significantly when narrowing the width of the centrality bin.
On the other hand, my more recent studies show that the analysis of partial forward-backward correlations and of the strongly intensive quantity Σ, both give a chance to reduce the trivial (non-dynamical) effect induced by spurious system volume fluctuations. Therefore the analysis of both observables provides new information on the dynamics at the early stage of the collision, which is far more direct than that obtained from earlier studies.