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September 15, 2017 | 3:30 p.m. - 4:30 p.m.
Category: Seminar
Location: Physics #312 | Map
666 W. Hancock
Detroit, MI 48201
Cost: Free!
Audience: Academic Staff, Current Graduate Students, Current Undergraduate Students, Faculty

Title: Determination of η/s with pT correlations from Pb-Pb collisions at √sNN = 2.76 and 5.04 TeV

Speaker: Víctor González (Universidad Complutense de Madrid)


QGP existence and its production in heavy ion collisions have been confirmed by experiments at RHIC and at LHC.  The observed QGP behaves like a strongly interacting fluid that undergoes an explosive expansion, featuring a very low shear viscosity to entropy density ratio, η/s, i.e. exhibiting strong collectivity, but with near perfect liquid fluidity.

Experimentally, the determination of the shear viscosity is conventionally carried out by comparing flow-coefficients, with predictions of hydrodynamical simulations involving shear viscosity to entropy density.  Such comparisons feature many sources of theoretical uncertainty: initial conditions, event-by-event fluctuations, etc, but have been steadily improving making a 10% precision estimate of η/s now reachable.

In the line of possible methods to determine η/s directly from measured data, a new theoretical approach, based on two-particle transverse momentum correlation, G2, was proposed by Gavin et al.  The method relies on the existence of friction forces between neighboring fluid cells determined by the fluid viscosity. Friction produces drag that broaden the correlation function of particle pairs. The model of Gavin et al. is based on correlations between momentum currents which, experimentally, are measured with pT pair correlator G2.

The challenge of obtaining η/s from LHC data at sNN = 2.76 and 5.04 TeV energies, within the reachable precision, will not only validate the proposed method but pave the way for extensions which will enable the extraction of the system relaxation time τπ from the evolution of the G2 rapidity profile showing the transition from a wavelike transport scenario to a diffusive one.

This type of analyses, which extract results from the shape variation of, in this case, two-particle transverse momentum correlation, through different collision centralities, are very challenging, specially at the addressed energies. All the detector effects have to be identified and their impact minimized. But also the different pile-up scenarios, facilitated by the diverse time response of the involved detector components, have to be addressed and the potentially involved events discarded.

We report on our progress in extracting η/s from LHC data at the ALICE detector as well as on the methods we are devising to cope with the detector inefficiencies.

For more information about this event, please contact Matt Barrett at +1 313 5770750 or